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At least 1,189 records · Page 66Linked to original sources
USGS2000

Hydrogeology and the distribution of salinity in the Floridan Aquifer system, southwestern Florida

A study was conducted to establish a detailed hydrogeologic framework in the complex Floridan aquifer system of southwestern Florida, and to evaluate and relate the distribution of salinity found in this system. The Floridan aquifer system consists of the Upper Floridan aquifer, middle confining unit, and Lower Floridan aquifer. The Upper Floridan aquifer extends into a basal unit of the Hawthorn Group; however, a regional unconformity present at the base of this unit generally marks the top of the Floridan aquifer system, as it does in the rest of southern Florida. The basal Hawthorn unit, which is defined at its top by a correlative marker unit, ranges in thickness from 120 to 460 feet. Paleotopography present prior to deposition of the basal Hawthorn unit, which resulted at least in part from erosion, is believed to have caused some of this variation in thickness. However, in some areas where the basal Hawthorn unit is thick, particularly in Lee County, depositional buildup created paleotopographic highs at the top of the unit. In these areas, permeable limestone zones are present in the unit, giving the unit a high transmissivity. In most of the study area, the Floridan aquifer system can be divided into a brackish-water zone, a salinity transition zone, and a saline-water zone. The brackish-water zone contains water with a dissolved-solids concentration of less than 10,000 milligrams per liter. The saline-water zone has a dissolved-solids concentration of at least 35,000 milligrams per liter and a salinity similar to that of seawater. The salinity transition zone that separates these two zones is usually 150 feet or less in thickness. The altitude of the base of the brackish-water zone was mapped primarily using geophysical logs; it ranges from as shallow as 565 feet below sea level along the coast to almost 2,200 feet below sea level inland. This mapping indicated that the boundary represents a salinity interface, the depth of which is controlled by head in the brackish-water zone. Chloride concentrations in the upper part of the brackish-water zone range from 400 to 4,000 milligrams per liter. A large area of relatively low salinity in north-central Collier County and to the northwest, as defined by a 1,200-milligram-per-liter chloride-concentration line, coincides with a high area on the basal contact of the Hawthorn Group. As this contact dips away from this high area to central Hendry and southwestern Collier Counties, chloride concentration increases to 2,000 milligrams per liter or greater. However, the increase in salinity in these areas occurs only in the basal Hawthorn unit or Suwannee Limestone, but not in deeper units. In central Hendry County, the increase occurs only in the basal Hawthorn unit in an area where the unit is well developed and thick. These areas of higher salinity could have resulted from the influx of seawater from southwestern Collier County into zones of higher permeability in the Upper Floridan aquifer during high sea-level stands. The influx may only have occurred in structurally low areas and may have experienced incomplete flushing subsequently by the modern freshwater flow system. In an area in north-central Collier County, the altitude of the base of the brackish-water zone is anomalously deep given the position of this area relative to the coast. In this area, the base extends as deep as 2,090 feet below sea level, and the salinity transition zone is not present or is poorly defined. The origin of this anomalous area is interpreted to be related to the development of a unit containing thick dolomite and evaporite beds high in the middle confining unit of the Floridan aquifer system. The top of this dolomite-evaporite unit, which probably has very low permeability, occurs at the base of the brackish-water zone in this area. The axis of a high area mapped at the top of the unit trends to the northwest from central Collier County into north-central Lee County. This axis parallels and lies just to the west of the anomalous area, and it could have acted as an impermeable sill, preventing saline water from moving in laterally from the coast to the southwest and up from the Lower Floridan aquifer. Locating a Floridan aquifer system well field in or near this anomalous area could be optimal because of the lack of a salinity interface at depth.

Water-Resources Investigations Report↗
USGS2002

Fault structure and mechanics of the Hayward Fault, California from double-difference earthquake locations

The relationship between small-magnitude seismicity and large-scale crustal faulting along the Hayward Fault, California, is investigated using a double-difference (DD) earthquake location algorithm. We used the DD method to determine high-resolution hypocenter locations of the seismicity that occurred between 1967 and 1998. The DD technique incorporates catalog travel time data and relative P and S wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events. The relocated seismicity reveals a narrow, near-vertical fault zone at most locations. This zone follows the Hayward Fault along its northern half and then diverges from it to the east near San Leandro, forming the Mission trend. The relocated seismicity is consistent with the idea that slip from the Calaveras Fault is transferred over the Mission trend onto the northern Hayward Fault. The Mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the Calaveras Fault at Calaveras Reservoir. In some locations, discrete structures adjacent to the main trace are seen, features that were previously hidden in the uncertainty of the network locations. The fine structure of the seismicity suggests that the fault surface on the northern Hayward Fault is curved or that the events occur on several substructures. Near San Leandro, where the more westerly striking trend of the Mission seismicity intersects with the surface trace of the (aseismic) southern Hayward Fault, the seismicity remains diffuse after relocation, with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation. The seismicity is highly organized in space, especially on the northern Hayward Fault, where it forms horizontal, slip-parallel streaks of hypocenters of only a few tens of meters width, bounded by areas almost absent of seismic activity. During the interval from 1984 to 1998, when digital waveforms are available, we find that fewer than 6.5% of the earthquakes can be classified as repeating earthquakes, events that rupture the same fault patch more than one time. These most commonly are located in the shallow creeping part of the fault, or within the streaks at greater depth. The slow repeat rate of 2–3 times within the 15-year observation period for events with magnitudes around M = 1.5 is indicative of a low slip rate or a high stress drop. The absence of microearthquakes over large, contiguous areas of the northern Hayward Fault plane in the depth interval from ∼5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large-magnitude earthquakes.

California↗
USGS2004

Hydrogeology, water quality, and distribution and sources of salinity in the Floridan aquifer system, Martin and St. Lucie Counties, Florida

The Floridan aquifer system is considered to be a valuable source for agricultural and municipal water supply in Martin and St. Lucie Counties, despite its brackish water. Increased withdrawals, however, could increase salinity and threaten the quality of withdrawn water. The Floridan aquifer system consists of limestone, dolomitic limestone, and dolomite and is divided into three hydrogeologic units: the Upper Floridan aquifer, a middle confining unit, and the Lower Floridan aquifer. An informal geologic unit at the top of the Upper Floridan aquifer, referred to as the basal Hawthorn/Suwannee unit, is bound above by a marker unit in the Hawthorn Group and at its base by the Ocala Limestone; a map of this unit shows an area where substantial eastward thickening begins near the coast. This change in thickness is used to divide the study area into inland and coastal areas. In the Upper Floridan aquifer, an area of elevated chloride concentration greater than 1,000 milligrams per liter and water temperature greater than 28 degrees Celsius exists in the inland area and trends northwest through north-central Martin County and western St. Lucie County. A structural feature coincides with this area of greater salinity and water temperature; this feature is marked by a previously mapped northwest-trending basement fault and, based on detailed mapping in this study of the structure at the top of the basal Hawthorn/Suwannee unit, an apparent southeast-trending trough. Higher hydraulic head also has been mapped in this northwest-trending area. Another area of high chloride concentration in the Upper Floridan aquifer occurs in the southern part of the coastal area (in eastern Martin County and northeastern Palm Beach County); chloride concentration in this area is more than 2,000 milligrams per liter and is as great as 8,000 milligrams per liter. A dissolved-solids concentration of less than 10,000 milligrams per liter defines the brackish-water zone in the Floridan aquifer system; the top and base of this zone are present at the top of the aquifer system and within the Lower Floridan aquifer, respectively. The base of the brackish-water zone, which can approximate a brackish-water/saltwater interface, was determined in 13 wells, mostly using resistivity geophysical logs. The depth to the saltwater interface was calculated using the Ghyben-Herzberg approximation and estimated predevelopment hydraulic heads in the Upper Floridan aquifer. In five of six inland area wells, the depth to the base of the brackish-water zone was substantially shallower than the estimated predevelopment interface (260 feet or greater), whereas in five of seven coastal area wells, the difference was not large (less than about 140 feet). Confining units in the inland area, such as dense dolomite, may prevent an interface from forming at its equilibrium position. Because of head decline, the calculated interface using recent (May 2001) water levels is as much as 640 ft above the base of the brackish water zone (in the northern part of the coastal area). Isotopic data collected during this study, including deuterium and oxygen-18 (18O/16O), the ratio of strontium-87 to strontium-86, and carbon-13 (13C/12C) and carbon-14, provide evidence for differences in the Floridan aquifer system ground-water geochemistry and its evolution between inland and coastal areas. Ground water from the inland area tends to be older than water from the coastal area, particularly where inland area water temperature is elevated. Isotopic data together with an anomalous vertical distribution of salinity in the coastal area indicate that the coastal area was invaded with seawater in relatively recent geologic time, and this water has not been completely flushed out by the modern-day flow system. Upward leakage from the Lower to Upper Floridan aquifer of high salinity water occurs through structural deformities, such as faults or fracture zones or associated dissolution features in the inland area. An upward trend in salinity is indicated in 16 monitoring wells in the inland area, and agricultural withdrawals are probably causing these increases. Most of these wells are located in areas of elevated Upper Floridan aquifer ground-water temperature. Areas of higher water temperature could represent areas of greater potential for increases in salinity. More detailed mapping of the structure of the uppermost geologic units in the aquifer system could better define areas of deformation. Additionally, high potential exists in much of the study area for upward or lateral movement of the saltwater interface because of large declines in hydraulic head since predevelopment. The northern part of the coastal area has the greatest potential for movement; however, upward movement of the interface in the coastal area could be retarded by low vertical permeability. The potential for upward or lateral movement of the interface in the southern part of the coastal area seems to be low, but structural deformation could be present in northeastern Palm Beach County, allowing for localized upward leakage of saltwater.

Florida↗
USGS2011

Geologic map of the Ganiki Planitia quadrangle (V-14), Venus

The Ganiki Planitia (V-14) quadrangle on Venus, which extends from 25° N. to 50° N. and from 180° E. to 210° E., derives its name from the extensive suite of plains that dominates the geology of the northern part of the region. With a surface area of nearly 6.5 x 10 6 km 2 (roughly two-thirds that of the United States), the quadrangle is located northwest of the Beta-Atla-Themis volcanic zone and southeast of the Atalanta Planitia lowlands, areas proposed to be the result of large scale mantle upwelling and downwelling, respectively. The region immediately south of Ganiki Planitia is dominated by Atla Regio, a major volcanic rise beneath which localized upwelling appears to be ongoing, whereas the area just to the north is dominated by the orderly system of north-trending deformation belts that characterize Vinmara Planitia. The Ganiki Planitia quadrangle thus lies at the intersection between several physiographic regions where extensive mantle flow-induced tectonic and volcanic processes are thought to have occurred. The geology of the V-14 quadrangle is characterized by a complex array of volcanic, tectonic, and impact-derived features. There are eleven impact craters with diameters from 4 to 64 km, as well as four diffuse 'splotch' features interpreted to be the product of near-surface bolide explosions. Tectonic activity has produced heavily deformed tesserae, belts of complex deformation and rifts as well as a distributed system of fractures and wrinkle ridges. Volcanic activity has produced extensive regional plains deposits, and in the northwest corner of the quadrangle these plains host the initial (or terminal) 700 km of the Baltis Vallis canali, an enigmatic volcanic feature with a net length of ~7,000 km that is the longest channel on Venus. Major volcanic centers in V-14 include eight large volcanoes and eight coronae; all but one of these sixteen features was noted during a previous global survey. The V-14 quadrangle contains an abundance of minor volcanic features including individual shield volcanoes and localized fissure eruptions as well as many small annular structures and domes, which often serve as the source for local lava flows. The topographic and geophysical characteristics of the Ganiki Planitia quadrangle are less complex than the surface geology, but they yield equally valuable information about the region’s formation and evolution. Referenced to the mean planetary radius of 6051.84 km, the average elevation in the quadrangle is -0.26±0.86 km (2σ) with a full range of -2.58 km to 1.85 km. The highest 2.5 percent of elevations in the quadrangle (above 0.60 km) are associated primarily with the major tessera blocks and the peaks of a few volcanic edifices, whereas the lowest 2.5 percent (below -1.12 km) mostly occur within corona interiors and in the northwest corner of the quadrangle where the plains begin to merge into the Atalanta Planitia lowlands. At the ~4.6 km/pixel scale of the topography data, the mean point-to-point topographic slope is 0.63° and topographic slopes greater than 2° cover less than 5 percent of the region. Overall, the topography of the Ganiki Planitia quadrangle can be characterized as flat, low lying, and nearly devoid of abrupt topographic variation. Complementing this gentle topography, the geoid anomaly has a generally linear gradient that decreases north-northwest from a high of ~20 m at the southern edge of the quadrangle (the northern border of the Atla Regio anomaly) to a low of -30 to -40 m along the northern edge (Konopliv and others, 1999). The vertical component of the gravity anomaly varies from ~50 mGal to -40 mGal, and integrated analysis of the gravity and topography data indicates that dynamically supported regions and areas of thickened crust are both present within the quadrangle. Because the Ganiki Planitia quadrangle is a plains-dominated lowland area that lies between several major physiographic provinces (namely, Atla Regio, Atalanta Planitia, and Vinmara Planitia), a geologic map of the region may yield insight into a wide array of important problems in Venusian geology. The current mapping effort and analysis complements previous efforts to characterize aspects of the region’s geology, for example stratigraphy near parabolic halo crater sites, volcanic plains emplacement, wrinkle ridges, volcanic feature distribution, volcano deformation, coronae characteristics, lithospheric flexure, and various features along a 30±7.58° N. geotraverse. Our current research focuses on addressing four specific questions. Has the dominant style of volcanic expression within the quadrangle varied in a systematic fashion over time? Does the tectonic deformation within the quadrangle record significant regional patterns that vary spatially or temporally, and if so what are the scales, orientations and sources of the stress fields driving this deformation? If mantle upwelling and downwelling have played a significant role in the formation of Atla Regio and Atalanta Planitia as has been proposed, does the geology of Ganiki Planitia record evidence of northwest-directed lateral mantle flow connecting the two sites? Finally, can integration of the tectonic and volcanic histories preserved within the quadrangle help constrain competing resurfacing models for Venus?

Scientific Investigations Map↗
USGS1998

Geohydrology and distribution of volatile organic compounds in ground water in the Casey Village area, Bucks County, Pennsylvania

Casey Village and the adjoining part of the U.S. Naval Air Warfare Center (NAWC) are underlain by the Late Triassic-age Stockton Formation, which consists of a dipping series of siltstones and sandstones. The direction of vertical ground-water gradients in the Stockton Formation varies among well locations and sometimes with time. Vertical gradients can be substantial; the difference in water levels at one well pair (two wells screened at different depths) was 7.1 ft (feet) over a 32-ft vertical section of the aquifer. Potentiometric-surface maps show a groundwater divide that bisects the Casey Village area. For wells screened between 18 and 64 ft below land surface (bls), the general ground-water gradient is to the east and northeast on the east side of the divide and to the south and southwest on the west side of the divide. For wells screened between 48 and 106 ft bls, the general ground-water gradient is to the northeast on the east side of the divide and to the southwest and northwest on the west side of the divide. An aquifer test at one well in Casey Village caused drawdown in wells on the opposite side of the ground-water divide on the NAWC and shifted the ground-water divide in the deeper potentiometric surface to the west. Drawdowns formed an elliptical pattern, which indicates anisotropy; however, anisotropy is not aligned with strike or dip. Hydraulic stress caused by pumping crosses stratigraphic boundaries. Between 1993 and 1996, the trichloroethylene (TCE) concentration in water samples collected from wells in Casey Village decreased. The highest concentration of TCE measured in water from one well decreased from 1,200 mg/L (micrograms per liter) in 1993 when domestic wells were pumped in Casey Village to 140 mg/L in 1996, 3 years after the installation of public water and the cessation of domestic pumping. This suggests that pumping of domestic wells may have contributed to TCE migration. Between 1993 and 1996, the tetrachloroethylene (PCE) concentration in water samples collected from wells in Casey Village decreased only slightly. The highest concentration of PCE measured in water from one well decreased from 720 mg/L in 1993 to 630 mg/L in 1996. The distribution of TCE and PCE in ground water indicates the presence of separate PCE and TCE plumes, each with a different source area. The TCE plume appears to be moving in two directions away from the ground-water divide area. The pumping of a domestic well may have caused TCE migration into the ground-water divide area. From the divide area, the TCE plume appears to be moving both to the east and the west under the natural hydraulic gradient. Aquifer-isolation tests conducted in the well with the highest TCE concentrations showed that concentrations of TCE in water samples from the isolated intervals were similar but slightly lower in the deeper isolated zones than in the shallower isolated zones. Upward flow was measured in this well during geophysical logging. If the source of TCE to the well was from shallow fractures, upward flow of less contaminated water could be flushing TCE from the immediate vicinity of this well. This may help explain why the concentration of TCE in water from this well decreased an order of magnitude between 1993 and 1996.

Water-Resources Investigations Report↗
USGS1999

Ground-water system, estimation of aquifer hydraulic properties, and effects of pumping on ground-water flow in Triassic sedimentary rocks in and near Lansdale, Pennsylvania

Ground water in Triassic-age sedimentary fractured-rock aquifers in the area of Lansdale, Pa., is used as drinking water and for industrial supply. In 1979, ground water in the Lansdale area was found to be contaminated with trichloroethylene, tetrachloroethylene, and other man-made organic compounds, and in 1989, the area was placed on the U.S. Environmental Protection Agency's (USEPA) National Priority List as the North Penn Area 6 site. To assist the USEPA in the hydrogeological assessment of the site, the U.S. Geological Survey began a study in 1995 to describe the ground-water system and to determine the effects of changes in the well pumping patterns on the direction of ground-water flow in the Lansdale area. This determination is based on hydrologic and geophysical data collected from 1995-98 and on results of the simulation of the regional ground-water-flow system by use of a numerical model. Correlation of natural-gamma logs indicate that the sedimentary rock beds strike generally northeast and dip at angles less than 30 degrees to the northwest. The ground-water system is confined or semi-confined, even at shallow depths; depth to bedrock commonly is less than 20 feet (6 meters); and depth to water commonly is about 15 to 60 feet (5 to 18 meters) below land surface. Single-well, aquifer-interval-isolation (packer) tests indicate that vertical permeability of the sedimentary rocks is low. Multiple-well aquifer tests indicate that the system is heterogeneous and that flow appears primarily in discrete zones parallel to bedding. Preferred horizontal flow along strike was not observed in the aquifer tests for wells open to the pumped interval. Water levels in wells that are open to the pumped interval, as projected along the dipping stratigraphy, are drawn down more than water levels in wells that do not intersect the pumped interval. A regional potentiometric map based on measured water levels indicates that ground water flows from Lansdale towards discharge areas in three drainages, the Wissahickon, Towamencin, and Neshaminy Creeks. Ground-water flow was simulated for different pumping patterns representing past and current conditions. The three-dimensional numerical flow model (MODFLOW) was automatically calibrated by use of a parameter estimation program (MODFLOWP). Steady-state conditions were assumed for the calibration period of 1996. Model calibration indicates that estimated recharge is 8.2 inches (208 millimeters) and the regional anisotropy ratio for the sedimentary-rock aquifer is about 11 to 1, with permeability greatest along strike. The regional anisotropy is caused by up- and down-dip termination of high-permeability bed-oriented features, which were not explicitly simulated in the regional-scale model. The calibrated flow model was used to compare flow directions and capture zones in Lansdale for conditions corresponding to relatively high pumping rates in 1994 and to lower pumping rates in 1997. Comparison of the 1994 and 1997 simulations indicates that wells pumped at the lower 1997 rates captured less ground water from known sites of contamination than wells pumped at the 1994 rates. Ground-water flow rates away from Lansdale increased as pumpage decreased in 1997. A preliminary evaluation of the relation between ground-water chemistry and conditions favorable for the degradation of chlorinated solvents was based on measurements of dissolved-oxygen concentration and other chemical constituents in water samples from 92 wells. About 18 percent of the samples contained less than or equal to 5 milligrams per liter dissolved oxygen, a concentration that indicates reducing conditions favorable for degradation of chlorinated solvents.

Water-Resources Investigations Report↗
USGS2001

Hydrogeologic framework and geochemistry of the intermediate aquifer system in parts of Charlotte, De Soto, and Sarasota counties, Florida

The hydrogeologic framework underlying the 600-square-mile study area in Charlotte, De Soto, and Sarasota Counties, Florida, consists of the surficial aquifer system, the intermediate aquifer system, and the Upper Floridan aquifer. The hydrogeologic framework and the geochemical processes controlling ground-water composition were evaluated for the study area. Particular emphasis was given to the analysis of hydrogeologic and geochemical data for the intermediate aquifer system. Flow regimes are not well understood in the intermediate aquifer system; therefore, hydrogeologic and geochemical information were used to evaluate connections between permeable zones within the intermediate aquifer system and between overlying and underlying aquifer systems. Knowledge of these connections will ultimately help to protect ground-water quality in the intermediate aquifer system. The hydrogeology was interpreted from lithologic and geophysical logs, water levels, hydraulic properties, and water quality from six separate well sites. Water-quality samples were collected from wells located along six ground-water flow paths and finished at different depth intervals. The selection of flow paths was based on current potentiometric-surface maps. Ground-water samples were analyzed for major ions; field parameters (temperature, pH, specific conductance, and alkalinity); stable isotopes (deuterium, oxygen-18, and carbon-13); and radioactive isotopes (tritium and carbon-14). The surficial aquifer system is the uppermost aquifer, is unconfined, relatively thin, and consists of unconsolidated sand, shell, and limestone. The intermediate aquifer system underlies the surficial aquifer system and is composed of clastic sediments interbedded with carbonate rocks. The intermediate aquifer system is divided into three permeable zones, the Tamiami/Peace River zone (PZ1), the Upper Arcadia zone (PZ2), and the Lower Arcadia zone (PZ3). The Tamiami/Peace River zone (PZ1) is the uppermost zone and is the thinnest and generally, the least productive zone in the intermediate aquifer system. The Upper Arcadia zone (PZ2) is the middle zone and productivity is generally higher than the overlying permeable zone. The Lower Arcadia zone (PZ3) is the lowermost permeable zone and is the most productive zone in the intermediate aquifer system. The intermediate aquifer system is underlain by the Upper Floridan aquifer, which consists of a thick, stratified sequence of limestone and dolomite. The Upper Floridan aquifer is the most productive aquifer in the study area; however, its use is generally restricted because of poor water quality. Interbedded clays and fine-grained clastics separate the aquifer systems and permeable zones. The hydraulic properties of the three aquifer systems are spatially variable. Estimated trans-missivity and horizontal hydraulic conductivity varies from 752 to 32,900 feet squared per day and from 33 to 1,490 feet per day, respectively, for the surficial aquifer system; from 47 to 5,420 feet squared per day and from 2 to 102 feet per day, respectively, for the Tamiami/Peace River zone (PZ1); from 258 to 24,633 feet squared per day and from 2 to 14 feet per day, respectively, for the Upper Arcadia zone (PZ2); from 766 to 44,900 feet squared per day and from 10 to 201 feet per day, respectively, for the Lower Arcadia zone (PZ3); and from 2,350 to 7,640 feet squared per day and from 10 to 41 feet per day, respectively, for the Upper Floridan aquifer. Confining units separating the aquifer systems have leakance coefficients estimated to range from 2.3 x 10-5 to 5.6 x 10-3 feet per day per foot. Strata composing the confining unit separating the Upper Floridan aquifer from the intermediate aquifer system are substantially more permeable than confining units separating the permeable zones in the intermediate aquifer system or separating the surficial aquifer and intermediate aquifer systems. In Charlotte, Sarasota, and western De Soto Counties, hydraulic

Water-Resources Investigations Report↗
USGS2004

Hydrogeologic characteristics of four public drinking-water supply springs in northern Arkansas

In October 2000, a study was undertaken by the U.S. Geological Survey (USGS) in cooperation with the Arkansas Department of Health to determine the hydrogeologic characteristics, including the extent of the recharge areas, for Hughes Spring, Stark Spring, Evening Shade Spring, and Roaring Spring, which are used for public-water supply in northern Arkansas. Information pertaining to each spring can be used to enable development of effective management plans to protect these water resources and public health. An integrated approach to determine the ground-water characteristics and the extent of the local recharge areas of the four springs incorporated tools and methods of hydrology, structural geology, geomorphology, geophysics, and geochemistry. Analyses of discharge, temperature, and water quality were completed to describe ground-water flow characteristics, source-water characteristics, and connectivity of the ground-water system with surface runoff. Water-level contour maps were constructed to determine ground-water flow directions and ground-water tracer tests were conducted to determine the extent of the recharge areas and ground-water flow velocities. Hughes Spring supplies water for the city of Marshall, Arkansas, and the surrounding area. The mean annual discharge for Hughes Spring was 2.9 and 5.2 cubic feet per second for water years 2001 and 2002, respectively. Recharge to the spring occurs mainly from the Boone Formation (Springfield Plateau aquifer). Ground-water tracer tests indicate the recharge area for Hughes Spring generally coincides with the surface drainage area (15.8 square miles) and that Hughes Spring is connected directly to the surface flow in Brush Creek. The geochemistry of Hughes Spring demonstrated variations with flow conditions and the influence of surface-runoff in the recharge area. Calcite saturation indices, total dissolved solids concentrations, and hardness demonstrate noticeable differences with flow conditions reflecting the reduced residence time and interaction of water with the source rock within the ground-water system at higher discharges for Hughes Spring. Concentrations of fecal indicator bacteria also demonstrated a substantial increase during high-flow conditions, suggesting that a non-point source of bacteria possibly from livestock may enter the system. Conversely, nutrient concentrations did not vary with flow and were similar to concentrations reported for undeveloped sites in the Springfield Plateau and Ozark aquifers in northern Arkansas and southern Missouri. Deuterium and oxygen-18 data show that the Hughes Spring discharge is representative of direct precipitation and not influenced by water enriched in oxygen-18 through evaporation. Discharge data show that Hughes Spring is dominated by conduit type ground-water flow, but a considerable component of diffuse flow also exists in the ground-water system. Carbon-13 data indicate a substantial component of the recharge water interacts with the surface material (soil and regolith) in the recharge area before entering the ground-water system for Hughes Spring. Tritium data for Hughes Spring indicate that the discharge water is a mixture of recent recharge and sub-modern water (recharged prior to 1952). Stark Spring supplies water for the city of Cushman, Arkansas, and the surrounding area. 2 Hydrogeologic Characteristics of Four Public Drinking-Water Supply Springs in Northern Arkansas The mean annual discharge for Stark Spring was 0.5 and 1.5 cubic feet per second for water years 2001 and 2002, respectively. The discharge and water-quality data show the ground-water system for Stark Spring is dominated by rapid recharge from surface runoff and mainly consists of a conduit- type flow system with little diffuse-type flow. Analyses of discharge data show that the estimated recharge area (0.79 square mile) is larger than the surface drainage area (0.34 square mile). Ground-water tracer tests and the outcrop of the

Water-Resources Investigations Report↗
USGS2018

Geohydrology, geochemistry, and numerical simulation of groundwater flow and land subsidence in the Bicycle Basin, Fort Irwin National Training Center, California

Groundwater pumping from Bicycle Groundwater Basin (referred to as Bicycle Basin) in the Fort Irwin National Training Center, California, began in 1967. From 1967 to December 2010, about 46,000 acre-feet of water had been pumped from the basin and transported to the Irwin Basin. During this time, not only did water levels in the basin decline by as much as 100 feet, the quality of the groundwater pumped from the basin also deteriorated in some wells. The U.S. Geological Survey collected geohydrologic data from existing wells, test holes, and 16 additional monitoring wells installed at 6 sites in Bicycle Basin during 1992–2011 to determine the quantity and quality of groundwater available in the basin. Geophysical surveys, including electrical, gravity, and seismic refraction surveys, were completed to help determine the geometry of the structural basin, delineate depths to the interface between Quaternary and Tertiary rocks, map the depth to the water table, and used to develop a geohydrologic framework and groundwater-flow model for Bicycle Basin. Water samples were used to determine the groundwater quality in the basin and to delineate potential sources of poor-quality groundwater. Analysis of stable isotopes of oxygen and hydrogen in groundwater indicated that presentday precipitation is not a major source of recharge to the basin. Tritium and carbon-14 data indicated that most of the groundwater in the basin was recharged prior to 1952 and had an apparent age of 15,625–39,350 years. Natural recharge to the basin was not sufficient to replenish the groundwater pumped from the basin. Interferograms from synthetic aperture radar data (InSAR), analyzed to evaluate land-surface subsidence between 1993 and 2010, showed 0.23 to 1.1 feet of subsidence during this period near one production well north of Bicycle Lake (dry) playa. A groundwater-flow model of Bicycle Basin was developed and calibrated using groundwater levels for 1964– 2010, and a subsidence model using land-surface deformation data for 1993–2010. Between January 1967 and December 2010, the simulated total recharge from precipitation runoff and underflow from adjacent basins was about 5,100 acre-feet and pumpage from the Bicycle Basin was about 47,000 acrefeet of water. Total outflows exceeded natural recharge during this period, resulting in a net loss of about 42,100 acre-feet of groundwater storage in the basin. The Fort Irwin National Training Center is considering various groundwater-management options in the Bicycle Basin. The groundwater-flow model was used to (1) evaluate changes in groundwater levels and subsidence with the addition of capture and recharge of simulated runoff in retention basins (scenario 1) for predevelopment through 2010; (2) simulate a base case (scenario 2) for reference; and (3) compare projections of alternative future pumping strategies for 2011–60 (scenarios 3–5). Model results from the runoff-capture simulation (scenario 1) indicated that total recharge, including runoff captured using retention basins, locally increased water levels, which partially offset, but did not mitigate, groundwater depletion associated with pumping. Groundwater-storage depletion in scenario 1 was about 14 percent less than without runoff capture. Simulated-drawdown results in model layer 1 in the eastern part of the basin indicated that, because of the captured runoff, simulated heads were as much as 100 feet higher in December 2010 than prior to the onset of development in 1967. In contrast, simulated drawdown for model without runoff capture indicated that, without captured runoff, simulated heads for December 2010 in this area were 80–90 feet lower than during the predevelopment period. Subsidence was mitigated slightly in scenario 1 compared to without runoff capture; the largest decrease in subsidence at observation sites was about 0.07 feet.

California↗
USGS2015

Aquifer geometry, lithology, and water levels in the Anza–Terwilliger area—2013, Riverside and San Diego Counties, California

The population of the Anza–Terwilliger area relies solely on groundwater pumped from the alluvial deposits and surrounding bedrock formations for water supply. The size, characteristics, and current conditions of the aquifer system in the Anza–Terwilliger area are poorly understood, however. In response to these concerns, the U.S. Geological Survey, in cooperation with the High Country Conservancy and Rancho California Water District, undertook a study to (1) improve mapping of groundwater basin geometry and lithology and (2) to resume groundwater-level monitoring last done during 2004–07 in the Anza–Terwilliger area. Inversion of gravity data, including new data collected for this study, was done to estimate the thickness of the alluvial deposits that form the Cahuilla and Terwilliger groundwater basins and to understand the geometry of the underlying basement complex. After processing of the gravity data, the thickness of the alluvial aquifer materials was modeled by using all available lithology, density, and geophysical data. The thickest alluvial deposits (greater than 500 feet) are in the northern part of the study area along the south side of the San Jacinto fault zone, in the southern part of the Cahuilla groundwater basin, and in the western part of the Terwilliger groundwater basin. Through most of the area of alluvial materials, the thickness of the alluvium estimated from gravity data is less than 400 feet. Analysis of more than 900 drillers’ logs indicated that in areas having relatively thick alluvium, particularly along the San Jacinto fault zone and in the Terwilliger Valley, the alluvium is predominantly composed of sands and gravels. Fine-textured sediments appeared to be discontinuous rather than forming laterally extensive, low-permeability layers. More than 500 drillers’ logs indicated only bedrock is present, indicating that the fractured bedrock is an important source of groundwater, primarily for domestic use, in the study area. The depths of the holes drilled into the bedrock indicated that fractures potentially supplying water to wells persist in the upper few hundred feet and that the permeable zone of the fractured bedrock extends to depths greater than weathered zones in the upper part of the basement complex. Water-level data were collected from 59 wells during fall 2013. These data indicated that hydraulic head did not vary substantially with well depth and that the measured water levels in bedrock and alluvium were similar. Large offsets in groundwater altitude across the San Jacinto fault zone indicated that the fault zone is a barrier to groundwater flow in the northeastern part of the Anza Valley. On the basis of data from 33 wells, water levels mostly declined between the fall of 2006 and the fall of 2013; the median decline was 5.1 feet during this period, for a median rate of decline of about 0.7 feet/year. Based on data from 40 wells, water-level changes between fall 2004 and fall 2013 were variable in magnitude and trend, but had a median decline of 2.4 feet and a median rate of decline of about 0.3 feet/ year. These differences in apparent rates of groundwater-level change highlight the value of ongoing water-level measurements to distinguish decadal, or longer term, trends in groundwater storage often associated with climatic variability and trends. Fifty-four long-term hydrographs indicated the sensitivity of groundwater levels to climatic conditions; they also showed a general decline in water levels across the study area since 1986 and, in some cases, dating back to the 1950s.

California↗
USGS2011

Groundwater conditions and studies in the Augusta–Richmond County area, Georgia, 2008–2009

Groundwater studies and monitoring efforts conducted during 2008–2009, as part of the U.S. Geological Survey (USGS) Cooperative Water Program with the City of Augusta in Richmond County, Georgia, provided data for the effective management of local water resources. During 2008–2009 the USGS completed: (1) installation of three monitoring wells and the collection of lithologic and geophysical logging data to determine the extent of hydrogeologic units, (2) collection of continuous groundwater-level data from wells near Well Fields 2 and 3, (3) collection of synoptic groundwater-level measurements and construction of potentiometric-surface maps in Richmond County to establish flow gradients and groundwater-flow directions in the Dublin and Midville aquifer systems, (4) completion of a 24-hour aquifer test to determine hydraulic characteristics of the lower Dublin aquifer, and upper and lower Midville aquifers in Well Field 2, and (5) collection of groundwater samples from selected wells in Well Field 2 for laboratory analysis of volatile organic compounds and groundwater tracers to assess groundwater quality and estimate the time of groundwater recharge. Potentiometric-surface maps of the Dublin and Midville aquifer systems for 2008–2009 indicate that the general groundwater flow direction within Richmond County is eastward toward the Savannah River, with the exception of the area around Well Field 2, where pumping interrupts the eastward flow of water toward the Savannah River and causes flow lines to bend toward the center of pumping. Results from a 24-hour aquifer test conducted in 2009 within the upper and lower Midville aquifers at Well Field 2 indicated a transmissivity and storativity for the upper and lower Midville aquifers, combined, of 4,000 feet-squared per day and 2x10 -4 , respectively. The upper and lower Midville aquifers and the middle lower Midville confining unit, which is 85-feet thick in this area, yielded horizontal hydraulic conductivity and specific storage values of about 45 feet per day and 2x10 -6 ft -1 , respectively. Results from the 24-hour aquifer test also indicate a low horizontal hydraulic conductivity for the lower Dublin aquifer of less than 1 foot per day. Of the 35 volatile organic compounds (VOCs) analyzed in 23 groundwater samples during 2008–2009, only six were detected above laboratory reporting limits in samples from eight wells. No concentration in groundwater samples collected during 2008–2009 exceeded drinking water standards. Trichloroethene had the maximum VOC concentration (1.9 micrograms per liter) collected from a water sample during 2008–2009. Water-quality sampling of several wells near Well Field 2 indicate that, while in operation, the northernmost production well might have diverted groundwater, containing low levels of trichloroethene from at least two other production wells. Analysis of sulfur hexafluoride data indicate the average year of recharge ranges between 1981 and 1984 for water samples from five wells open to the upper and lower Midville aquifers, and 1991 for a water sample from one shallow well open to the lower Dublin aquifer. All of these ages suggest a short flow path and nearby source of contamination. The actual source of low levels of VOCs at Well Field 2 remains unknown. Three newly installed monitoring wells indicate that hydrogeologic units beneath Well Fields 2 and 3 are composed of sand and clay layers. Hydrogeologic units, encountered at Well Field 2, in order of increasing depth are the lower Dublin confining unit, lower Dublin aquifer, upper Midville confining unit, upper Midville aquifer, lower Midville confining unit, and lower Midville aquifer. West of Well Field 3, hydrogeologic units, in order of increasing depth are the Upper Three Runs aquifer, Gordon confining unit, Gordon aquifer, lower Dublin confining unit, lower Dublin aquifer, upper Midville confining unit, upper Midville aquifer, lower Midville confining unit, and lower Midville aquifer.

Georgia↗
USGS2000

Hydrogeology of the regional aquifer near Flagstaff, Arizona, 1994-97

Sandstones, siltstones, and limestones that are Pennsylvanian to Permian in age underlie the southern part of the Colorado Plateau near Flagstaff, Arizona, and contain a complex regional aquifer that has become increasingly important as a source of water for domestic, municipal, and recreational uses. Ground-water flow in the regional aquifer is poorly understood in this area because (1) depth of the aquifer limits exploratory drilling and testing and (2) the geologic structure increases the complexity of the aquifer characteristics and the ground-water flow system. Four methods were used to improve the understanding of the hydrogeology of the regional aquifer near Flagstaff. • Remote-sensing techniques and geologic mapping provided data to identify many structural features that indicate a more complex structural environment and history than previously realized. • Data from surface-geophysical techniques that included ground-penetrating radar, seismic reflection and seismic refraction, and square-array resistivity, verified that some of the geologic structures expressed at land surface propagate deep into the subsurface and through the principal water-bearing zones of the regional aquifer at near-vertical angles. • A well and spring inventory, borehole-geophysical methods, and well and aquifer tests provided additional information relating aquifer and ground-water flow characteristics to geologic structure. • Water-chemistry data, which included major ion, nutrient, trace-element, and radioactive and stable-isotope analyses, provided an independent means of verifying the hydrogeologic characteristics of the aquifer and were used to determine recharge and discharge areas, groundwater movement, and ground-water age. Ground-water recharge occurs throughout the area but is greatest at higher altitudes where precipitation is greater and in areas where heavily fractured rock units of the aquifer are exposed. The estimated annual average recharge to the regional aquifer in the study area is about 290,000 acre-feet. Ground water flows laterally and vertically through pore spaces in the rock and along faults and other fractures from high-altitude areas in the southern part of the study area to regional drains north of the study area along the Little Colorado and Colorado Rivers, and to drains south of the study area along Oak Creek and the Verde Valley. Ground-water discharge in these areas—about 400,000 acre-feet per year—exceeds the annual recharge to the aquifer in the Flagstaff area, but ground water from areas outside the study area contributes to this discharge as well. The saturated thickness of the regional aquifer averages about 1,200 feet, and the amount of water in storage could be as much as 4,800,000 acrefeet, or about 10 percent of the total volume of the aquifer. The quality of water in the regional aquifer in terms of dissolved-solids concentrations is good for most uses throughout the area. Dissolvedsolids concentrations generally are less than 500 milligrams per liter. Water in the regional aquifer is primarily a calcium magnesium bicarbonate type. In some areas near the Rio de Flag, the water has significant nitrate and chloride components, which indicate direct recharge in these areas from the Rio de Flag. Oxygen and deuterium data indicate a common recharge source for water in the aquifer and that some sites receive recharge from surface waters where evaporation has occurred. Estimated carbon-14 ages and tritium activities indicate ground-water ages from less than 200 years in the Lake Mary area to more than 5,000 years in the Wupatki area. The regional aquifer is heterogeneous and anisotrophic and has a complex ground-water flow system. The most productive water-bearing material tends to be fine- to medium-grained sandstones, and ground-water flow and potential well yields are related to geologic structure. Fracturing associated with structural deformation increases recharge locally and also increases the potential for high well yields. Surface-geophysical techniques provided information on the orientation of high-angle, deep-seated structure in the saturated zone. Borehole-geophysical data identified horizontal to near-horizontal fractures as significant components of the fracture-flow system not apparent in the surface-geophysical data. Structural features that strike northwest appear to be areas that have the greatest potential for high well yields. A north-northeastward-striking structure may be just as promising, but additional data are needed to verify this relation.

Arizona↗
USGS2009

Sand resources, regional geology, and coastal processes of the Chandeleur Islands Coastal System: An evaluation of the Breton National Wildlife Refuge

Breton National Wildlife Refuge, the Chandeleur Islands chain in Louisiana, provides habitat and nesting areas for wildlife and is an initial barrier protecting New Orleans from storms. The U.S. Geological Survey (USGS) in partnership with the University of New Orleans Pontchartrain Institute for Environmental Sciences undertook an intensive study that included (1) an analysis of island change based on historical maps and remotely sensed shoreline and topographic data; (2) a series of lidar surveys at 3- to 4-month intervals after Hurricane Katrina to determine barrier island recovery potential; (3) a discussion of sea level rise and effects on the islands; (4) an analysis of sea floor evolution and sediment dynamics in the refuge over the past 150 years; (5) an assessment of the local sediment transport and sediment resource availability based on the bathymetric and subbottom data; (6) a carefully selected core collection effort to groundtruth the geophysical data and more fully characterize the sediments composing the islands and surrounds; (7) an additional survey of the St. Bernard Shoals to assess their potential as a sand resource; and (8) a modeling study to numerically simulate the potential response of the islands to the low-intensity, intermediate, and extreme events likely to affect the refuge over the next 50 years. Results indicate that the islands have become fragmented and greatly diminished in subaerial extent over time: the southern islands retreating landward as they reorganize into subaerial features, the northern islands remaining in place. Breton Island, because maintenance of the Mississippi River-Gulf Outlet (MRGO) outer bar channel requires dredging, is deprived of sand sufficient to sustain itself. Regional sediment transport trends indicate that large storms are extremely effective in transporting sand and controlling the shoreline development and barrier island geometry. Sand is transported north and south from a divergent zone near Monkey Bayou at the southern end of the Chandeleur Islands. Numerical simulation of waves and sediment transport supports the geophysical results and indicates that vast areas of the lower shoreface are affected and are undergoing erosion during storm events, that there is little or no fair weather mechanism to rework material into the littoral system, and that as a result, there is a net loss of sediment from the system. Lidar surveys revealed that the island chain immediately after Hurricane Katrina lost about 84 percent of its area and about 92 percent of its prestorm volume. Marsh platforms that supported the islands’ sand prior to the storm were reduced in width by more than one-half. Repeated lidar surveys document that in places the shoreline has retreated about 100 m under the relatively low-energy waves since Hurricanes Katrina and Rita; however, this retreat is nonuniform. Recent high-resolution geophysical surveys of the sea floor and subsurface within 5–6 km of the Chandeleur Islands during 2006 and 2007 show that, in addition to the sand that is rebuilding portions of the island chain, a large volume of sand is contained in Hewes Point, in an extensive subtidal spit platform that has formed at the northern end of the Chandeleur Islands. Hewes Point appears to be the depositional terminus of the alongshore transport system. In the southern Chandeleurs, sand is being deposited in a broad tabular deposit near Breton Island called the southern offshore sand sheet. These two depocenters account for approximately 70 percent of the estimated sediment volume located in potential borrow sites. An additional large potential source of sand for restoration lies in the St. Bernard Shoals, which are estimated to contain approximately 200 × 10 6 m 3 of sand. Successful restoration planning for the Breton National Wildlife Refuge should mimic the natural processes of early stages of barrier island evolution including lateral transport to the flanks of the island chain from a centralized sand source that will ultimately enhance the ability of the islands to naturally build backbarrier marsh, dunes, and a continuous sandy shoreline. Barrier island sediment nourishment should be executed with the understanding that gulf shoreline erosion is inevitable but that island area can be maintained and enhanced during retreat (thus significantly prolonging the life of the island chain) with strategic sand placement.

Louisiana↗
USGS2003

Ground-water conditions and studies in Georgia, 2001

The U.S. Geological Survey (USGS) collects ground-water data and conducts studies to monitor hydrologic conditions, to better define ground-water resources, and address problems related to water supply and water quality. Data collected as part of ground-water studies include geologic, geophysical, hydraulic property, water level, and water quality. A ground-water-level network has been established throughout most of the State of Georgia, and ground-water-quality networks have been established in the cities of Albany, Savannah, and Brunswick and in Camden County, Georgia. Ground-water levels are monitored continuously in a network of wells completed in major aquifers of the State. This network includes 17 wells in the surficial aquifer, 12 wells in the upper and lower Brunswick aquifers, 73 wells in the Upper Floridan aquifer, 10 wells in the Lower Floridan aquifer and underlying units, 12 wells in the Claiborne aquifer, 1 well in the Gordon aquifer, 11 wells in the Clayton aquifer, 11 wells in the Cretaceous aquifer system, 2 wells in Paleozoic-rock aquifers, and 7 wells in crystalline-rock aquifers. In this report, data from these 156 wells were evaluated to determine whether mean-annual ground-water levels were within, below, or above the normal range during 2001, based on summary statistics for the period of record. Information from these summaries indicates that water levels during 2001 were below normal in almost all aquifers monitored, largely reflecting climatic effects from drought and pumping. In addition, water-level hydrographs for selected wells indicate that water levels have declined during the past 5 years (since 1997) in almost all aquifers monitored, with water levels in some wells falling below historical lows. In addition to continuous water-level data, periodic measurements taken in 52 wells in the Camden County-Charlton County area, and 65 wells in the city of Albany-Dougherty County area were used to construct potentiometric-surface maps for the Upper Floridan aquifer. Ground-water quality in the Upper Floridan aquifer is monitored in the cities of Albany, Savannah, and Brunswick and in Camden County; and monitored in the Lower Floridan aquifer in the Savannah and Brunswick areas. In the Albany area since 1998, nitrate concentrations in the Upper Floridan aquifer have increased in 4 of the 11 wells monitored, and in 1 well, concentrations were above the U.S. Environmental Protection Agency's (USEPA) 10 milligrams per liter (mg/L) drinking-water standard. In the Savannah area, chloride concentration in water from four wells in the Upper Floridan aquifer showed no appreciable change during 2001, remaining within the USEPA 250 mg/L drinking-water standard; in seven wells completed in the Lower Floridan aquifer and in underlying zones, the chloride concentration remained above the drinking-water standard, with one well showing an increase over previous years. In the Brunswick area, water samples from 66 wells completed in the Upper or Lower Floridan aquifers were collected during June 2001 and analyzed for chloride. A map showing chloride concentrations in the Upper Floridan aquifer during June 2001 indicates that concentrations remained above USEPA drinking-water standards across a 2-square-mile area. In the north Brunswick area, chloride concentrations in the Upper Floridan aquifer continued to increase, whereas in the south Brunswick area, concentrations continued to decrease. In the Camden County area, chloride concentrations in six wells completed in the Upper Floridan aquifer remained within drinking-water standards. With the exception of one well, concentrations remained the same and were below 40 mg/L. In one well, concentrations showed a sharp decline during 2001, but remained above 130 mg/L. Ongoing studies during 2001 include evaluation of agricultural chemicals in shallow ground water in southwestern Georgia; evaluation of saltwater intrusion and water-level and water-quality m

Georgia↗
USGS2004

Stratigraphy and tectonic history of the Tucson Basin, Pima County, Arizona, based on the Exxon state (32)-1 well

The Tucson Basin is a relatively large late Cenozoic extensional basin developed in the upper plate of the Catalina detachment fault in the southern Basin and Range Province, southeastern Arizona. In 1972, Exxon Company, U.S.A., drilled an exploration well (Exxon State (32)-1) near the center of the Tucson Basin that penetrated 3,658 m (12,001 ft) of sedimentary and volcanic rocks above granitoid basement. Detailed study of cuttings and geophysical logs of the Exxon State well has led to revision of the previously reported subsurface stratigraphy for the basin and provided new insight into its depositional and tectonic history. There is evidence that detachment faulting and uplift of the adjacent Catalina core complex on the north have affected the subsurface geometry of the basin. The gravity anomaly map of the Tucson Basin indicates that the locations of subbasins along the north-trending axis of the main basin coincide with the intersection of this axis with west-southwest projections of synforms in the adjacent core complex. In other words, the subbasins overlie synforms and the ridges between subbasins overlie antiforms. The Exxon State well was drilled near the center of one of the subbasins. The Exxon well was drilled to a total depth of 3,827 m (12,556 ft), and penetrated the following stratigraphic section: Pleistocene(?) to middle(?) Miocene upper basin-fill sedimentary rocks (0-908 m [0-2,980 ft]) lower basin-fill sedimentary rocks (908-1,880 m [2,980-6,170 ft]) lower Miocene and upper Oligocene Pantano Formation (1,880-2,516 m [6,170-8,256 ft]) upper Oligocene to Paleocene(?) volcanic and sedimentary rocks (2,516-3,056 m [8,256-10,026 ft]) Lower Cretaceous to Upper Jurassic Bisbee Group (3,056-3,658 m [10,026-12,001 ft]) pre-Late Jurassic granitoid plutonic rock (3,658-3,827 m [12,001- 12,556 ft]). Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well The 1,880 m (6,170 ft) of basin-fill sedimentary rocks consist of alluvial-fan, alluvial-plain, and playa facies. The uppermost unit, a 341-m-thick (1,120-ft) lower Pleistocene and upper Pliocene alluvial-fan deposit (named the Cienega Creek fan in this study), is an important aquifer in the Tucson basin. The facies change at the base of the alluvial fan may prove to be recognizable in well data throughout much of the basin. The well data show that a sharp boundary at 908 m (2,980 ft) separates relatively unconsolidated and undeformed upper basin fill from denser, significantly faulted lower basin fill, indicating that there were two stages of basin filling in the Tucson basin as in other basins of the region. The two stages apparently occurred during times of differing tectonic style in the region. In the Tucson area the Pantano Formation, which contains an andesite flow dated at about 25 Ma, fills a syntectonic basin in the hanging wall of the Catalina detachment fault, reflecting middle Tertiary extension on the fault. The formation in the well is 636 m thick (2,086 ft) and consists of alluvial-fan, playa, and lacustrine sedimentary facies, a lava flow, and rock- avalanche deposits. Analysis of the geophysical logs indicates that a K-Ar date of 23.4 Ma reported previously for the Pantano interval of the well was obtained on selected cuttings collected from a rock-avalanche deposit near the base of the unit and, thus, does not date the Pantano Formation. The middle Tertiary volcanic and sedimentary rocks have an aggregate thickness of 540 m (1,770 ft). We obtained a new 40Ar/ 39Ar age of 26.91+0.18 Ma on biotite sampled at a depth of 2,584-2,609 m (8,478-8,560 ft) from a 169-m-thick (554-ft) silicic tuff in this interval. The volcanic rocks probably correlate with other middle Tertiary volcanic rocks of the area, and the sedimentary rocks may correlate with the Cloudburst and Mineta Formations exposed on the flanks of the San Pedro Basin to the northeast. The Bisbee Group in the Exxon well is 602 m (1,975 f

Scientific Investigations Report↗
USGS2014

Origins and delineation of saltwater intrusion in the Biscayne aquifer and changes in the distribution of saltwater in Miami-Dade County, Florida

Intrusion of saltwater into parts of the shallow karst Biscayne aquifer is a major concern for the 2.5 million residents of Miami-Dade County that rely on this aquifer as their primary drinking water supply. Saltwater intrusion of this aquifer began when the Everglades were drained to provide dry land for urban development and agriculture. The reduction in water levels caused by this drainage, combined with periodic droughts, allowed saltwater to flow inland along the base of the aquifer and to seep directly into the aquifer from the canals. The approximate inland extent of saltwater was last mapped in 1995. An examination of the inland extent of saltwater and the sources of saltwater in the aquifer was completed during 2008–2011 by using (1) all available salinity information, (2) time-series electromagnetic induction log datasets from 35 wells, (3) time-domain electromagnetic soundings collected at 79 locations, (4) a helicopter electromagnetic survey done during 2001 that was processed, calibrated, and published during the study, (5) cores and geophysical logs collected from 8 sites for stratigraphic analysis, (6) 8 new water-quality monitoring wells, and (7) analyses of 69 geochemical samples. The results of the study indicate that as of 2011 approximately 1,200 square kilometers (km 2 ) of the mainland part of the Biscayne aquifer were intruded by saltwater. The saltwater front was mapped farther inland than it was in 1995 in eight areas totaling about 24.1 km 2 . In many of these areas, analyses indicated that saltwater had encroached along the base of the aquifer. The saltwater front was mapped closer to the coast than it was in 1995 in four areas totaling approximately 6.2 km 2 . The changes in the mapped extent of saltwater resulted from improved spatial information, actual movement of the saltwater front, or a combination of both. Salinity monitoring in some of the canals in Miami-Dade County between 1988 and 2010 indicated influxes of saltwater, with maximum salinities ranging from 1.4 to 32 practical salinity units (PSU) upstream of the salinity control structures. Time-series electromagnetic induction log data from monitoring wells G–3601, G–3608, and G–3701, located adjacent to the Biscayne, Snapper Creek, and Black Creek Canals, respectively, and upstream of the salinity control structures, indicated shallow influxes of conductive water in the aquifer that likely resulted from leakage of brackish water or saltwater from these canals. The determination that saltwater influxes were recent is supported by the similarity in the oxygen and hydrogen stable isotope composition in samples from the Snapper Creek Canal, 1.6 kilometers (km) inland of a salinity control structure, and in samples from well G–3608, which is adjacent to the canal, as well as by the relative ages of the water sampled from well G–3608 and other wells open to the aquifer below the saltwater interface. Historical and recent salinity information from the Card Sound Road Canal, monitoring well FKS8 located adjacent to the canal, and the 2001 helicopter electromagnetic survey indicated that saltwater may occasionally leak from this canal as far inland as 15 km. This leakage may be prevented or reduced by a salinity control structure that was installed in May 2010. Saltwater also may have leaked from the Princeton Canal. Results of geochemical sampling and analysis indicate a close correspondence between droughts and saltwater intrusion. Tritium/helium-3 apparent (piston-flow) ages determined from samples of saltwater with chloride concentrations of about 1,000 milligrams per liter (mg/L) or greater generally corresponded to a period during which droughts were frequent. Comparison of average daily air temperatures in Miami, Florida, with estimates of recharge temperatures determined from the dissolved gas composition in water samples indicated that saltwater likely entered the aquifer in April or early May when water levels are typically at their lowest during the year. Conversely, most of the samples of freshwater with chloride concentrations less than about 1,000 mg/L indicate recharge temperatures corresponding to air temperatures in mid to late May when rainfall and water levels in the aquifer increase, and the piston-flow ages of these samples correspond to wet years. The piston-flow ages of freshwater samples generally were younger than ages of samples of saltwater. Saltwater samples that were depleted in boron, magnesium, potassium, sodium, and sulfate, and enriched in calcium relative to the concentrations theoretically produced by freshwater/seawater mixing, generally were found to be associated with areas where saltwater had recently intruded. The calcium to (bicarbonate + sulfate) molar ratios (Ca/(HCO 3 +SO 4 )) of these samples generally were greater than 1. Saltwater samples from some of the monitoring wells, however, indicated little or no enrichment or depletion of these ions relative to the theoretical freshwater/seawater mixing line, and the Ca/(HCO 3 +SO 4 ) molar ratios of these samples generally were less than 1. Results indicated that aquifer materials are approaching equilibrium with seawater at these well locations.

Florida↗
USGS2007

Relation of Lake-Floor Characteristics to the Distribution of Variable Leaf Water-Milfoil in Moultonborough Bay, Lake Winnipesaukee, New Hampshire, 2005

Geophysical, water, and sediment surveys were done to characterize the effects of surficial geology, water and sediment chemistry, and surficial-sediment composition on the distribution of variable leaf water-milfoil in Moultonborough Bay, Lake Winnipesaukee, New Hampshire. Geophysical surveys were conducted in a 180-square-kilometer area, and water-quality and sediment samples were collected from 24 sites in the survey area during July 2005. Swath-bathymetric data revealed that Moultonborough Bay ranged in depth from less than 1 meter (m) to about 15 m and contained three embayments. Seismic-reflection profiles revealed erosion of the underlying bedrock and subsequent deposition of glaciolacustrine and Holocene lacustrine sediments within the survey area. Sediment thickness ranged from 5 m along the shoreward margins to more than 15 m in the embayments. Data from sidescan sonar, surficial-sediment samples, bottom photographs, and video revealed three distinct lake-floor environments: rocky nearshore, mixed nearshore, and muddy basin. Rocky nearshore environments were found in shallow water (less than 5 m deep) and contained sediments ranging from coarse silt to very coarse sand. Mixed nearshore environments also were found in shallow water and contained sediments ranging from silt to coarse sand with different densities of aquatic vegetation. Muddy basin environments contained the finest-grained sediments, ranging from fine to medium silt, and were in the deepest waters of the bay. Acoustic Ground Discrimination Systems (AGDS) survey data revealed that 86 percent of the littoral zone (the area along the margins of the bay and islands that extends from 0 to 4.3 m in water depth) contained submerged aquatic vegetation (SAV) in varying densities: approximately 36 percent contained SAV bottom cover of 25 percent or less, 43 percent contained SAV bottom cover of more than 25 and less than 75 percent, and approximately 7 percent contained SAV bottom cover of more than 75 percent. SAV included variable leaf water-milfoil, native milfoil, bassweed, pipewort, and other species, which were predominantly found near shoreward margins and at depths ranging from less than 1 to 4 m. AGDS data were used in a Geographic Information System to generate an interpolated map that distinguished variable leaf water-milfoil from other SAV. Furthermore, these data were used to isolate areas susceptible to variable leaf water-milfoil growth. Approximately 21 percent of the littoral zone contained dense beds (more than 59 percent bottom cover) of variable leaf water-milfoil, and an additional 44 percent was determined to be susceptible to variable leaf water-milfoil infestation. Depths differed significantly between sites with variable leaf water-milfoil and sites with other SAV (p = 0.04). Variable leaf water-milfoil was found at depths that ranged from 1 to 4 m, and other SAV had a depth range of 1 to 2 m. Although variable leaf water-milfoil was observed at greater depths than other SAV, it was not observed below the photic zone. Analysis of constituent concentrations from the water column, interstitial pore water, and sediment showed little correlation with the presence of variable leaf water-milfoil, with two exceptions. Iron concentrations were significantly lower at variable leaf water-milfoil sites than at other sampling sites (p = 0.04). Similarly, the percentage of total organic carbon also was significantly lower at the variable leaf water-milfoil sites than at other sampling sites (p = 0.04). Surficial-sediment-grain size had the greatest correlation to the presence of variable leaf water-milfoil. Variable leaf water-milfoil was predominantly growing in areas of coarse sand (median grain-size 0.62 millimeters). Surficial-sediment-grain size was also correlated with total ammonia plus organic nitrogen (Rho = 0.47; p = 0.02) and with total phosphorus (Rho = 0.44; p = 0.05) concentrations in interstitial pore-water samples.

Scientific Investigations Report↗
USGS2001

Mars Global Surveyor Thermal Emission Spectrometer experiment: Investigation description and surface science results

The Thermal Emission Spectrometer (TES) investigation on Mars Global Surveyor (MGS) is aimed at determining (1) the composition of surface minerals, rocks, and ices; (2) the temperature and dynamics of the atmosphere; (3) the properties of the atmospheric aerosols and clouds; (4) the nature of the polar regions; and (5) the thermophysical properties of the surface materials. These objectives are met using an infrared (5.8- to 50-μm) interferometric spectrometer, along with broadband thermal (5.1- to 150-μm) and visible/near-IR (0.3- to 2.9-μm) radiometers. The MGS TES instrument weighs 14.47 kg, consumes 10.6 W when operating, and is 23.6×35.5×40.0 cm in size. The TES data are calibrated to a 1-σ precision of 2.5 −6 ×10 −8 W cm −2 sr −1 /cm −1 , 1.6×10 −6 W cm −2 sr −1 , and ∼0.5 K in the spectrometer, visible/near-IR bolometer, and IR bolometer, respectively. These instrument subsections are calibrated to an absolute accuracy of ∼4×10 −8 W cm −2 sr −1 /cm −1 (0.5 K at 280 K), 1–2%, and ∼1–2 K, respectively. Global mapping of surface mineralogy at a spatial resolution of 3 km has shown the following: (1) The mineralogic composition of dark regions varies from basaltic, primarily plagioclase feldspar and clinopyroxene, in the ancient, southern highlands to andesitic, dominated by plagioclase feldspar and volcanic glass, in the younger northern plains. (2) Aqueous mineralization has produced gray, crystalline hematite in limited regions under ambient or hydrothermal conditions; these deposits are interpreted to be in-place sedimentary rock formations and indicate that liquid water was stable near the surface for a long period of time. (3) There is no evidence for large-scale (tens of kilometers) occurrences of moderate-grained (>50-μm) carbonates exposed at the surface at a detection limit of ∼10%. (4) Unweathered volcanic minerals dominate the spectral properties of dark regions, and weathering products, such as clays, have not been observed anywhere above a detection limit of ∼10%; this lack of evidence for chemical weathering indicates a geologic history dominated by a cold, dry climate in which mechanical, rather than chemical, weathering was the significant form of erosion and sediment production. (5) There is no conclusive evidence for sulfate minerals at a detection limit of ∼15%. The polar region has been studied with the following major conclusions: (1) Condensed CO 2 has three distinct end-members, from fine-grained crystals to slab ice. (2) The growth and retreat of the polar caps observed by MGS is virtually the same as observed by Viking 12 Martian years ago. (3) Unique regions have been identified that appear to differ primarily in the grain size of CO 2 ; one south polar region appears to remain as black slab CO 2 ice throughout its sublimation. (4) Regional atmospheric dust is common in localized and regional dust storms around the margin and interior of the southern cap. Analysis of the thermophysical properties of the surface shows that (1) the spatial pattern of albedo has changed since Viking observations, (2) a unique cluster of surface materials with intermediate inertia and albedo occurs that is distinct from the previously identified low-inertia/bright and high-inertia/dark surfaces, and (3) localized patches of high-inertia material have been found in topographic lows and may have been formed by a unique set of aeolian, fluvial, or erosional processes or may be exposed bedrock.

Journal of Geophysical Research E: Planets↗