Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Interfaces”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,567 records · Page 87Linked to original sources
USGS2003

Simulation of the Ground-Water Flow System in 1992, and Simulated Effects of Projected Ground-Water Withdrawals in 2020 in the New Jersey Coastal Plain

In 1992, ground-water withdrawals from the unconfined and confined aquifers in the New Jersey Coastal Plain totaled about 300 million gallons per day, and about 70 percent (200 million galllons per day) of this water was pumped from confined aquifers. The withdrawals have created large cones of depression in several Coastal Plain aquifers near populated areas, particularly in Camden and Ocean Counties. The continued decline of water levels in confined aquifers could cause saltwater intrusion, reduction of stream discharge near the outcrop areas of these aquifers, and depletion of the ground-water supply. Because of this, withdrawals from wells located within these critical areas have been reduced in the Potomac-Raritan-Magothy aquifer system, the Englishtown aquifer system, and the Wenonah-Mount Laurel aquifer. A computer-based model that simulates freshwater and saltwater flow was used to simulate transient ground-water flow conditions and the location of the freshwater-saltwater interface during 1989-92 in the New Jersey Coastal Plain. This simulation was used as the baseline for comparison of water levels and flow budgets. Four hypothetical withdrawal scenarios were simulated in which ground-water withdrawals were either increased or decreased. In scenario 1, withdrawals from wells located within critical area 2 in the Potomac-Raritan-Magothy aquifer system were reduced by amounts ranging from 0 to 35 percent of withdrawals prior to 1992. Critical area 2 is mainly located in Camden County, and most of Burlington and Gloucester Counties. With the reductions, water levels recovered about 30 feet in the regional cone of depression centered in Camden County in the Upper Potomac-Raritan-Magothy aquifer and by 20 ft in the Lower and Middle Potomac-Raritan-Magothy aquifers. In scenarios 2 to 4, withdrawals projected for 2020 were input to the model. In scenario 2, withdrawal restrictions within the critical areas were imposed in the Potomac-Raritan-Magothy aquifer system, the Englishtown aquifer system, and the Wenonah-Mount Laurel aquifer, but withdrawals were increased outside the critical areas to the projected 2020 demand. With withdrawals restrictions in critical areas, water levels recovered about 20 feet at the center of the regional cone of depression in the Upper Potomac-Raritan Magothy aquifer. Water levels recovered by about 20 feet at the center of a regional cone of depression in the Englishtown aquifer system in Ocean County, and by about 20 feet in the Wenonah-Mount Laurel aquifer in the same area. In scenario 3, withdrawals were increased to the projected 2020 demand inside and outside the critical areas. As a result, water levels declined as much as 20 feet at the center of a regional cone of depression in the Englishtown aquifer system in Ocean County, and as much as 10 feet in the Wenonah-Mounty Laurel aquifer near this area. The Englishtown aquifer system and the Wenonah-Mount Laurel aquifer are particularly sensitive to increases and decreases in withdrawals because in certain areas the transmissivities of these aquifers are lower than the transmissivities of other confined aquifers of the New Jersey Coastal Plain, and because these aquifers are hydraulically connected. Simulated water levels declined by as much as 10 ft at the center of the regional cone of depression in Atlantic County. In scenario 4, withdrawal amounts were equal to that in scenario 2, except an additional 13.2 million gallons per day was withdrawn from hypothetical wells located outside the critical areas in the Upper Potomac-Raritan-Magothy aquifer, Englishtown aquifer system, and the Wenonah-Mount Laurel aquifer. The additional withdrawals resulted in increased leakage from overlying aquifers to the Wenonah-Mount Laurel aquifer and subsequently to the Englishtown aquifer system.

Water-Resources Investigations Report↗
USGS1976

Nitrogen and phosphorus uptake in the Everglades Conservation Areas, Florida, with special reference to the effects of backpumping runoff

In much of the water pumped into the northern Everglades, Florida, concentrations of inorganic nitrogen and phosphorus are relatively high. These nutrients are transported in the canals or into the peripheral marshes. Concentrations decrease sharply within 330 feet or less of the canals, whereas specific conductance remains essentially unchanged within this distance. The sharp decrease in inorganic nitrogen and phosphorus near the canal edge indicates net uptake in these shallow waters. Concentrations of nitrogen and phosphorus also decrease as water moves through the conservation areas in canals. This decrease is due partly to dilution by rainfall and runoff, and partly to net uptake in the canals and their peripheral marsh. The large canals of the northern and eastern parts of the conservation areas often have relatively low concentrations of dissolved oxygen which show little fluctuation within 24 hours. Backpumping 50 percent of the total annual canal runoff in southeast Florida would add from 990 to 6,160 tons of nitrogen and from 10 to 62 tons of phosphorus to the conservation areas. The bottom sediments of the Everglades are a sink for nitrogen and phosphorus. They can, however, be a source of these nutrients when anaerobic conditions exist at the water-sediment interface or when bottom material becomes resuspended. (Woodard-USGS)

Florida↗
USGS1978

Ground-water resources of the Cape Lookout National Seashore, North Carolina

Fresh groundwater in the Cape Lookout National Seashore in North Carolina occurs in the unconfined aquifer, an upper confined aquifer, and a lower limestome aquifer. The unconfined aquifer beneath dunes on the barrier islands is estimated to yield as much as 30 gallons per minute of freshwater to a horizontal well, but this aquifer is subject to periodic overwash by the ocean, thus temporarily contaminating the aquifer with saltwater. The upper confined aquifer is about 90 to 150 feet deep, but is known to contain freshwater only in the Drum Inlet area. The potential yield of this aquifer where it is fresh is unknown because of the possibility of saltwater encroachment. The lower limestone aquifer (150 to 550 feet deep) contains freshwater southeast of Drum Inlet; potential yields is estimated to be as great as 500 gallons per minute per well. The estimated yields from all aquifers depend on the position of the saltwater interface at any given site. Maps of the Seashore show areas where freshwater may occur in the unconfined aquifer; each map is accompanied by a description of the availability of freshwater.

North Carolina↗
USGS1979

Urban stormwater data management system with applications to south Florida studies

A data management system was developed to store and retrieve urban stormwater data collected from four small urban basins in south Florida. The system is event oriented in that all data from one storm are stored together on the computer file. The data include descriptive information about the storm and data on rainfall, stage, discharge, and water quality. The data management system, composed of about 20 Fortran programs, was developed to store data, retrieve tables for publication, calculate flow discharges and constituent loads, and provide for interfacing with statistical and deterministic model application programs. (Kosco-USGS)

Water-Resources Investigations Report↗
USGS1983

Appraisal of the interconnection between the St Johns River and the surficial aquifer, east-central Duval County, Florida

The proposed deepening of the navigation channel in the St. Johns River to about 46-48 feet below sea level (45 feet below mean low water) may breach up to 11 feet of limestone unit of the lower part of the surficial aquifer along a 25-mile channel. The limestone supplies water to numerous domestic wells along the river in the Jacksonville area. Recent channel improvements to about 39 feet below sea level have already breached the limestone at many locations. Where breaching has not yet occurred, less than 1 to 6 feet of undifferentiated sediments overlie the limestone. These sediments, consisting predominantly of sand, are generally too permeable to form an effective confining layer. Chloride concentrations determined from water in wells tapping the limestone unit range from 8 to 6,600 milligrams per liter. However, chloride concentrations in water from wells located more than 500 feet from the river, were generally less than 30 milligrams per liter. The proposed dredging operation is not expected to alter significantly the present hydrologic system. The current position of the interface most likely represents conditions that will be present after future channel improvements. (USGS)

Florida↗
USGS1984

Freshwater runoff and salinity distribution in the Loxahatchee River estuary, southeastern Florida, 1980-82

Freshwater mixed with seawater over a distance of 5 to 10 river miles in the Loxahatchee River estuary during a recent study. Large freshwater inflows vertically stratified the estuary and shifted the mixing zone seaward. In the northwest fork of the estuary, the saltwater-freshwater interface moved daily about 0.5 to 1.5 river miles as a result of tides, and annually about 3 to 5 miles as a result of seasonal changes in freshwater inflow. In the southwest fork, saltwater movement upstream was blocked by a gate and dam structure in Canal-18, 4.7 miles upstream from the Atlantic Ocean. Although Canal-18 discharged about one-third of the total freshwater tributary inflow to the estuary, the effects of canal discharge on salinity were limited to relatively brief periods. Much of the time, no freshwater was discharged. (USGS)

Florida↗
USGS1984

Low-flow transport models for conservative and sorbed solutes — Uvas Creek, near Morgan Hill, California

Models describing low-flow transport of conservative (nonreactive) and reactive solutes, which adsorb on the streambed, are developed and tested. Temporary storage within the bed plays an important role in solute movement. Three different models of bed-storage processes are developed for conservative solutes. One model assumes the bed is a well-mixed, nondiffusing, nonreacting zone. Solute flux into the bed is then proportional to the difference between stream and bed-solute concentrations. A second model assumes that solute is transported within the bed by a vertical diffusion process. The bed-solute concentration, which matches the stream concentration at the interface, varies with depth in the bed according to Fick 's law. A third model assumes convection in the downstream direction occurs in certain parts of the bed, while the mechanism of the first model functions elsewhere. Storage of absorbing species is assumed to occur by equilibrium adsorption within streambed particles. Uptake rate is described by an intraparticle diffusion process. Model equations were solved using finite element numerical methods. Models were calibrated using data from a 24-hour injection of conservative chloride and adsorptive Sr ions at Uvas Creek near Morgan Hill, California. All models predict well except for some overestimation by the adsorption model during dieaway.

California↗
USGS1984

Reconnaissance evaluation of contamination in the alluvial aquifer in the East Poplar oil field, Roosevelt County, Montana

Water moving from north to south in the alluvial aquifer of the Poplar River valley becomes contaminated with sodium chloride in the area underlain by the East Poplar oil fields. Four types of ground water were identified in the study area. Type 1 is sodium bicarbonate water. Type 2 is sodium chloride water with varying quantities of calcium and magnesium. Type 3 water contains sodium and chloride in significantly larger concentrations than Type 2. Type 4 water is the brine being injected into brine-disposal wells. Contamination of the alluvial aquifer is indicated by a brine-freshwater interface in the alluvium, by downstream increase in chloride concentration of the Poplar River, and by downstream change in water type of the Poplar River. Contamination also may be indicated by the distribution of iron and manganese concentrations in water from wells near a brine-disposal well. Possible sources of sodium chloride contamination in the alluvium are brine-disposal wells, pipelines, and storage or evaporation pits. The contamination can occur from leaks in the casing of disposal wells or in pipelines caused by the corrosive nature of the brine or from storage or evaporation pits that have been improperly sealed or have sustained tears in the sealing material. (USGS)

Water-Resources Investigations Report↗
USGS1984

Application of the U.S. Geological Survey's precipitation-runoff modeling system to the Prairie Dog Creek basin, southeastern Montana

The U.S. Geological Survey 's precipitation-runoff modeling system was tested using 2 year 's data for the daily mode and 17 storms for the storm mode from a basin in southeastern Montana. Two hydrologic response unit delineations were studied. The more complex delineation did not provide superior results. In this application, the optimum numbers of hydrologic response units were 16 and 18 for the two alternatives. The first alternative with 16 units was modified to facilitate interfacing with the storm mode. A parameter subset was defined for the daily mode using sensitivity analysis. Following optimization, the simulated hydrographs approximated the observed hydrograph during the first year, a year of large snowfall. More runoff was simulated than observed during the second year. There was reasonable correspondence between the observed snowpack and the simulated snowpack the first season but poor the second. More soil moisture was withdrawn than was indicated by soil moisture observations. Optimization of parameters in the storm mode resulted in much larger values than originally estimated, commonly larger than published values of the Green and Ampt parameters. Following optimization, variable results were obtained. The results obtained are probably related to inadequate representation of basin infiltration characteristics and to precipitation variability. (USGS)

Water-Resources Investigations Report↗
USGS1984

Planning report for the Gulf Coast Regional Aquifer-System Analysis in the Gulf of Mexico coastal plain, United States

Large quantities of water for municipal, industrial and agriculture use are supplied from the aquifers in Tertiary and younger sediments over an area of about 225,000 square miles in the Coastal Plain of Alabama, Arkansas, Florida, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Tennessee, and Texas. Three regional aquifer systems, the Mississippi Embayment aquifer system, the Coastal Lowlands aquifer system, and the Texas Coastal Uplands aquifer system have been developed to varying degrees throughout the area. A variety of problems has resulted from development such as movement of the saline-freshwater interface into parts of aquifers that were previously fresh, lowering of the potentiometric surface with resulting increases in pumping lift, and land-surface subsidence due to the compaction of clays within the aquifer. Increased demand for ground water is anticipated to meet the needs of urban growth, expanded energy development, and growth of irrigated agriculture. The U. S. Geological Survey initiated an eightyear study in 1981 to define the geohydrologic framework, describe the chemistry of the ground water, and to analyze the regional ground-water flow patterns. The objectives, plan, and organization of the study are described in this report and the major tasks to be undertaken are outlined.

Water-Resources Investigations Report↗
USGS1986

Ground-water resource assessment of the Montauk area, Long Island, New York

The water resources of the Montauk area were investigated from October 1980 through September 1983 to assess the availability of fresh groundwater. The principal aquifer, which consists of fine- to coarse-grained stratified glacial drift, is the sole source of freshwater. The freshwater/saltwater interface lies as much as 150 ft below sea level, but the till unit and the marine-clay unit limit the thickness of the zone from which freshwater may easily be withdrawn to < 100 ft in most places. Precipitation, the sole source of freshwater in the Montauk area , averages about 42 in/yr. Direct runoff is approximately 1 in/yr, and evapotranspiration about 20 in/yr; the remaining 21 inches is the net recharge to the principal aquifer. The horizontal hydraulic conductivity, as calculated from specific-capacity data, ranges from 130 to 350 ft/d. Analysis of an aquifer pumping test indicates that horizontal hydraulic conductivity of the principal aquifer is approximately 275 ft/d and the vertical hydraulic conductivity is approximately 90 ft/d; the storage coefficient ranges from 1.1 x .001 to 2.2 x .001. A numerical two-dimensional flow model that simulates the areal flow of fresh- and saltwater was developed to evaluate the effects of the present level of groundwater development and the potential effects of increased future withdrawals. Model results indicate that the principal aquifer is capable of producing several times more than the 0.6 mil gal/d now being withdrawn for public supply. Examination of well design and pumping rates, and analysis of analog-model studies of saltwater upconing to wells, indicate that the peak pumping rate of public supply wells exceeds the limit necessary to avoid contamination from saltwater. With proper well design and distribution of pumping stress, the aquifer system would be capable of supplying enough water to meet the needs of the population projected through 1995.

New York↗
USGS1986

Distribution and altitude of the top of saline ground water in the southeastern Coastal Plain

A map prepared by the U.S. Geological Survey shows the approximate distribution of saline water (greater than 10,000 mg/L as NaCl) in the Southeastern Coastal Plain. The primary distribution of saline water is in Cretaceous sediments and may be characterized as an extensive body of fluid whose upper surface generally slopes upward from inland toward coastal areas. Some freshwater appears to occupy parts of the deeper water-bearing zones in Georgia. A small amount of saline water appears to occupy a narrow upper zone of limited extent in lower Tertiary sediments in Georgia and parts of South Carolina. The freshwater-saline water interface for the lower zone lies offshore, east of the South Carolina coast. Consequently, the saline water distribution is inferred offshore.

southeastern Coastal Plain↗
USGS1985

Projected ground-water development, ground-water levels, and stream-aquifer leakage in the South Fork Solomon River Valley between Webster Reservoir and Waconda Lake, north-central Kansas, 1979-2020

A two-dimensional finite difference computer model was used to project changes in the potentiometric surface, saturated thickness, and stream aquifer leakage in an alluvial aquifer resulting from four instances of projected groundwater development. The alluvial aquifer occurs in the South Fork Solomon River valley between Webster Reservoir and Waconda Lake in north-central Kansas. In the first two projections, pumpage for irrigation was held constant at 1978 rates throughout the projection period (1979-2020). In the second two projections, the 1978 pumpage was progressively increased each yr through 2020. In the second and fourth projections, surface water diversions in the Osborne Irrigation Canal were decreased by 50 %. For the third and fourth projections, each grid-block in the modeled area was classified initially as one of six types according to whether it represented irrigable or nonirrigable land, to its saturated thickness, to its location inside or outside the canal-river area, and to its pumping rate. The projected base-flow rates (leakage from the aquifer to the river) were lower during the irrigation season (June, July, and August) than during the other months of the yr because of the decline in hydraulic head produced by groundwater pumpage. Stream depletion, calculated as a decrease below the average (1970-78) estimated winter base-flow rate of 16.5 cu ft/sec, varied inversely with base flow. For the first two projections, a constant annual cycle of well pumpage and recharge was used throughout the projection period. Aquifer leakage to the river was nearly constant by the mid-to-late 1990's, implying that flow conditions had attained a stabilized annual cycle. The third and fourth projections never attained an annual stabilized cycle because the irrigation pumpage rate was increased each year. By the early 1980's, the hydraulic head had fallen below river stage, reversing the hydraulic gradient at the stream-aquifer interface and resulting in net leakage from the river to the aquifer during the summer months. By the early 1990 's, the projected potentiometric surface of the aquifer was lower than the river stage even during the winter and spring months. (Author 's abstract)

Water-Resources Investigations Report↗
USGS1986

Tests for injecting, storing, and recovering freshwater in a saline artesian aquifer, Lee County, Florida

An investigation was made of the suitability of a saline, artesian limestone aquifer for the injection, storage, and recovery of freshwater from the Caloosahatchee River. The tests were conducted on a well tapping a leaky artesian system that has a transmissivity of 800 square feet per day, a storage of 1 x 10-4, and a leakance of 0.01 per day. The specific capacity of the injection well was increased through acidizing and was decreased as a result of well clogging during injection. Three injection tests were made wherein the amounts of freshwater injected, the storage duration, and the quality of water injected varied. Analysis of the test data showed that freshwater recoverability ranged from 9.7 to 38.7 percent of the total injected. Differences were attributed principally to differences in the quality of water injected and storage duration. Repeated injection-recovery cycles probably would result in greater recoverability. Head buildup, nearly 200 feet in one test, was a prime problem related chiefly to clogging from suspended material in the injected water and to bacterial growth at the wellbore-limestone interface. Regular backflushing was required. Total head buildup decreased as a result of acidizing the injection well. No coliforms or fecal streptococcus were noted in the recovered water. Growth of anaerobic bacteria occurred. Changes in the quality of the recovered water included decreases in concentration of dissolved organic carbon by as much as 15 mg/L (milligrams per liter), organic nitrogen by as much as 0.80 mg/L, and nitrate by as much as 0.50 mg/L. Increases were noted in ammonia by 0.40 mg/L, and iron by as much as 0.60 mg/L. These changes are consistent with the presence of an anaerobic bacterial ecosystem.

Florida↗
USGS1986

Cluster analysis of phytoplankton data collected from the National Stream Quality Accounting Network in the Tennessee River basin, 1974-81

A computer program, Numerical Taxonomy System of Multivariate Statistical Programs (NTSYS), was used with interfacing software to perform cluster analyses of phytoplankton data stored in the biological files of the U.S. Geological Survey. The NTSYS software performs various types of statistical analyses and is capable of handling a large matrix of data. Cluster analyses were done on phytoplankton data collected from 1974 to 1981 at four national Stream Quality Accounting Network stations in the Tennessee River basin. Analysis of the changes in clusters of phytoplankton genera indicated possible changes in the water quality of the French Broad River near Knoxville, Tennessee. At this station, the most common diatom groups indicated a shift in dominant forms with some of the less common diatoms being replaced by green and blue-green algae. There was a reduction in genera variability between 1974-77 and 1979-81 sampling periods. Statistical analysis of chloride and dissolved solids confirmed that concentrations of these substances were smaller in 1974-77 than in 1979-81. At Pickwick Landing Dam, the furthest downstream station used in the study, there was an increase in the number of genera of ' rare ' organisms with time. The appearance of two groups of green and blue-green algae indicated that an increase in temperature or nutrient concentrations occurred from 1974 to 1981, but this could not be confirmed using available water quality data. Associations of genera forming the phytoplankton communities at three stations on the Tennessee River were found to be seasonal. Nodal analysis of combined data from all four stations used in the study did not identify any seasonal or temporal patterns during 1974-81. Cluster analysis using the NYSYS programs was effective in reducing the large phytoplankton data set to a manageable size and provided considerable insight into the structure of phytoplankton communities in the Tennessee River basin. Problems encountered using cluster analysis were the subjectivity introduced in the definition of meaningful clusters, and the lack of taxonomic identification to the species level. (Author 's abstract)

Tennessee↗
USGS1989

Hydrogeology and water quality near a solid- and hazardous-waste landfill, Northwood, Ohio

Hydrogeology and water quality of ground water and selected streams were evaluated near a landfill in northwestern Ohio. The landfill is used for codisposal of solid and hazardous waste. Water-level and geologic data were collected from 36 wells and 3 surface-water sites during the period November 1983 to November 1985. Water-quality samples were collected from 18 wells and 3 surface-water sites this during this same period. The primary aquifers in the area are the Greenfield Dolomite and underlying Lockport Dolomite of Silurian age. These bedrock carbonates are overlain by two clay tills of Wisconsin age. The tills are capped by a glacial lake clay. The tills generally are saturated, but do not yield sufficient water to be considered an aquifer. Two wells in the study area yield water, in part, from discontinuous deposits of outwash sand and gravel at the lower till-bedrock interface. Regional ground-water flow is from southwest to northeast; local flow is influenced by a ground-water mound centered under the northernmost cells of the landfill. Water levels in wells penetrating refuse within the landfill and the presence of leachate seeps indicate that the refuse is saturated. Head relations among the landfill, till, and dolomite aquifer indicate a vertical component of flow downward from the landfill to the dolomite aquifer. Water levels near the landfill fluctuate as much as 14 feet per year, in contrast to fluctuations of less than 3 feet per year in wells upgradient landfill. Ground waters from wells completed in the dolomite aquifer and glacial till were found to have major-iron concentrations controlled, in large part, by reaction with calcite, dolomite, and other minerals in the aquifer. Only minor departures from equilibrium mineral saturation were noted for ground water, except in wells affected by cement/grout contamination. Molal ratios of calcuim:magnesium in ground water suggest a similar chemical evolution of waters throughout the dolomite aquifer in the study area. Stable-isotope ratios of oxygen and hydrogen indicate the source of water in the till unit and dolomite aquifer is atmospheric precipitation. Elevated levels of total dissolved solids, boron, ammonia, and iron in the leachate and in wells downgradient of the landfill may indicate mixing of ground water with leachate. Oxygen and hydrogen stable-isotope ratios were used to differentiate waters from the glacial till and dolomite aquifer. Isotope ratios also show a shift off the local mixing line for leachate and for a well just downgradient from the landfill. The shift to heavier values of o D in the well water may be indicative of leachate mixing with ground water. The effect of this mixing denoted by hydrologic, isotopic, and chemical-quality data is limited mostly to elevated levels of the common ions. Analysis did not indicate significant levels of toxic metals or organic contaminants except phenol, which was present at concentrations of from 1 to 5 micrograms per liter in six wells. Analysis of water-quality data from nearby streams suggest that surface leaching from the landfill does not significantly affect stream-water quality, but may contribute to higher level of trace metals in the streambed sediments.

Water-Resources Investigations Report↗
USGS1993

Application of ground-penetrating radar methods in determining hydrogeologic conditions in a karst area, west-central Florida

Ground-penetrating radar (GPR) is useful as a surface geophysical method for exploring geology and subsurface features in karst settings. Interpretation of GPR data was used to infer lithology and hydrogeologic conditions in west-central Florida. This study demonstrates how GPR methods can be used to investigate the hydrogeology of an area. GPR transmits radio- frequency electromagnetic waves into the ground and receives reflected energy waves from subsurface interfaces. Subsurface profiles showing sediment thickness, depth to water table and clay beds, karst development, buried objects, and lake-bottom structure were produced from GPR traverses obtained during December 1987 and March 1990 in Pinellas, Hillsborough, and Hardee Counties in west-central Florida. Performance of the GPR method is site specific, and data collected are principally affected by the sediment and pore fluids, conductances and dielectric constants. Effective exploration depths of the GPR surveys through predominately unsaturated and saturated sand and clay sediments at five study sites ranged from a few feet to greater than 50 feet below land surface. Exploration depths were limited when high conductivity clay was encountered, whereas greater exploration depths were possible in material composed of sand. Application of GPR is useful in profiling subsurface conditions, but proper interpretation depends upon the user's knowledge of the equipment and the local hydrogeological setting, as well as the ability to interpret the graphic profile.

Water-Resources Investigations Report↗
USGS1994

Saltwater intrusion and quality of water in the Floridan aquifer system, northeastern Florida

Saltwater intrusion into freshwater aquifers has resulted in increased chloride concentrations in water from some wells in northeastern Florida. The principal areas of saltwater intrusion in the Floridan aquifer system in the study area are east-central Duval County, the southern two-thirds of St. Johns County, and along the coast. At least five possible mechanisms of saltwater movement, some more plausible than others, could explain the observed increases in chloride concentration in the upper freshwater zones of the Floridan aquifer. They are (1) the presence of unflushed pockets of relict seawater; (2) lateral movement of the freshwater-saltwater interface off the northeastern Florida coast; (3) upconing of saltwater from deeper, salty zones below pumped wells; (4) upward leakage from deeper, salty water-bearing zones through failed, uncased, or improperly plugged or constructed wells; and (5) upward leakage from salty water-bearing zones through semiconfining units that are thin, or are breached by joints, faults, or collapse features. Ground-water withdrawals in Duval, Nassau, and St. Johns Counties increased from about 183 to 254 million gallons per day from 1965 to 1988. Approximately 90 percent of the total withdrawal is from the Floridan aquifer system, resulting in long-term declines in the potentiometric surface of the Upper Floridan aquifer of about one-third to three-fourths foot per year. Hydraulic heads in the lower part of the aquifer system are naturally higher than in the upper parts. Declines in head in the upper part have further increased the vertical head difference between zones, increasing the potential for vertical ground-water flow from lower zones of higher head upward through structural deformities, leaky confining beds, and wells, to higher zones of lower head. Lower zones typically have higher chloride concentrations than do upper zones. Concentrations of chemical constituents in water from the Floridan aquifer system vary both areally and with depth. Chloride concentrations in water in the Upper Floridan aquifer in the study area range from about 4.6 to 3,600 milligrams per liter. Data indicate that in much of the study area, water in the upper zone of the Lower Floridan aquifer generally is slightly more mineralized than water from the overlying Upper Floridan aquifer. Water from the Fernandina permeable zone varies in quality from fresh to saline. Chemical analyses of water from five monitoring wells tapping this zone indicate maximum chloride concentrations of 16,800 milligrams per liter. The potential for saltwater contamination of the freshwater-bearing zones probably will continue to increase in northeastern Florida as artesian pressure in the upper freshwater zones continues to decline. Implementation of wise water-management strategies could, however, reduce the potential for saltwater intrusion.

Florida↗