Geology topics

Phillip D. Hays

Publications and source records attributed to Phillip D. Hays.

At least 19 recordsLinked to original sources

USGS2023

Potentiometric surfaces (2013, 2015), groundwater quality (2010–15), and water-level changes (2011–13, 2013–15) in the Sparta-Memphis aquifer in Arkansas

The Sparta-Memphis aquifer, present across much of eastern Arkansas, is the second most used groundwater resource in the State, with the Mississippi River Valley alluvial aquifer being the primary groundwater resource. The U.S. Geological Survey, in cooperation with Arkansas Department of Agriculture-Natural Resources Division, Arkansas Geological Survey, Natural Resources Conservation Service, Union County Water Conservation Board, and the Union County Conservation District, collects groundwater data across the Sparta-Memphis aquifer extent in Arkansas. This report presents water-level data for measurements conducted during two time periods, January–May 2013 and January–June 2015, and discusses water-level altitude changes for the 2011–13 and 2013–15 periods in the Sparta-Memphis aquifer. Accompanying water-level data in this report include groundwater-quality data for the period 2010–15 in the Sparta-Memphis aquifer. Groundwater data can guide ongoing and future groundwater-monitoring efforts and inform management of the aquifers in Arkansas. Water levels measured at 306 wells from January to May 2013 and 273 wells from January to June 2015 are graphically presented as potentiometric-surface maps. Measurements from 2011, 2013, and 2015 were used in the construction of 2011–13 and 2013–15 water-level change maps. Select long-term hydrographs are included in the report to illustrate water-level changes at the local scale. Water-level data show the influence of climate, pumping, and conservation and management efforts on groundwater levels. With respect to climate, the study area experienced extreme drought conditions between January 2011 and December 2012. The proximate effects of drought—increased evapotranspiration, decreased recharge, and increased irrigation needs—resulted in water-level declines that were particularly notable in the northern and central portions of the study area. Groundwater sampled in 2010–15 from 148 wells completed in the Sparta-Memphis aquifer was analyzed for specific conductance, pH, chloride (Cl) concentration, and bromide (Br) concentration. In 2015, groundwater-quality data from 103 wells completed in the Sparta-Memphis aquifer had a median specific conductance of 356 microsiemens per centimeter at 25 degrees Celsius and a median Cl concentration of 9.5 milligrams per liter (mg/L). The data show two areas of higher Cl (greater than 10 mg/L) and higher Br (greater than 0.5 mg/L) concentrations in Union, Calhoun, and Bradley Counties in southern Arkansas and Monroe and Phillips Counties in eastern-central Arkansas. A Cl and Br mixing model indicates the two regions of wells may have different sources of higher salinity. In the greater Union County area, water in most wells may be a mixture of recharge or precipitation and higher salinity groundwater from the Nacatoch aquifer. Water in wells in eastern-central Arkansas may be sourced from aquifers having a higher Cl concentration (and thus, also a higher Cl-to-Br ratio).

Arkansas↗

USGS2023

Hydrogeologic framework of the Red River alluvial aquifer and Carrizo-Wilcox aquifer in northwestern Louisiana

Groundwater in northwestern Louisiana is a valuable resource needed for expanding public-supply needs as well as possible energy development needs arising from Haynesville Formation natural-gas production. The Red River alluvial and the Carrizo-Wilcox aquifers are two of the most important and heavily pumped aquifers in northwestern Louisiana; however, little documentation of the regional hydrogeologic framework is available. The U.S. Geological Survey and the Louisiana Department of Transportation and Development have consolidated information from, and built upon, previous studies of the Red River alluvial and the Carrizo-Wilcox aquifers to characterize and document the regional hydrogeologic framework of northwestern Louisiana. The study area has been tectonically modified and includes abundant structural features such as salt domes and areally extensive faulting in addition to minor folding related to these features, all of which impact the sedimentological and hydraulic characteristics of the freshwater-bearing strata. The hydrogeologic framework of northwestern Louisiana comprises a sequence of structurally modifi ed, complexly interbedded, varyingly interconnected, clayey, sandy, and gravelly alluvial sediments. The important freshwater hydrogeologic units include the Quaternary Red River alluvial and upland terrace aquifers, and the underlying Tertiary Sparta, Cane River, and Carrizo-Wilcox aquifers. The Midway confining unit underlies the Carrizo-Wilcox aquifer throughout the study area. No freshwater is present in or below the Midway Group. Tertiary-age formations exposed at land surface in the study area have been incised by the Red River and are hydraulically connected to the Quaternary Red River alluvium in the Red River valley. In 2010, 7.73 million gallons per day (Mgal/d) of water were withdrawn from the Red River alluvial aquifer in the study area, representing an increase of 2.00 Mgal/d, or about 35 percent, over 2005 withdrawal rates. The Tertiary Carrizo Sand and Wilcox Group crop out across much of the study area. The two units are hydraulically connected and function as a single hydrologic unit referred to as the Carrizo-Wilcox aquifer. In 2010, 19.33 Mgal/d of water were withdrawn from the Carrizo-Wilcox aquifer in the study area, representing an increase of nearly 1.8 Mgal/d, or about 10 percent, over 2005 withdrawal rates. Any expansion in energy development, as well as water needs of an increasing population, could result in an increased demand on groundwater in northwestern Louisiana.

Louisiana↗

USGS2021

Effects of climate and land-use change on thermal springs recharge—A system-based coupled surface-water and groundwater-flow model for Hot Springs National Park, Arkansas

A three-dimensional hydrogeologic framework of the Hot Springs anticlinorium beneath Hot Springs National Park, Arkansas, was constructed to represent the complex hydrogeology of the park and surrounding areas to depths exceeding 9,000 feet below ground surface. The framework, composed of 6 rock formations and 1 vertical fault emplaced beneath the thermal springs, was discretized into 19 layers, 429 rows, and 576 columns and incorporated into a 3-dimensional steady-state groundwater-flow model constructed in MODFLOW-2005. Historical daily mean thermal spring flows were simulated for one stress period of approximately 34 years (1980–2014), chosen to represent the period of record for historical climate data used in the quantification of the boundary conditions. The groundwater-flow model was manually calibrated to historical daily mean thermal spring flows of 88,000 cubic feet per day observed over a 12-year period of record (1990–1995 and 1998–2005) at the thermal springs collection system. Calibration was achieved by calculating starting heads and general head boundary conditions from the Bernoulli equation and then adjusting the horizontal and vertical hydraulic conductivities of the rock formations and vertical fault and the hydraulic conductance of head-dependent flux boundaries. The groundwater-flow model was coupled to a surface-water model developed in the Precipitation-Runoff Modeling System (PRMS) by using PRMS-simulated gravity drainage as a specified flux recharge boundary condition in the groundwater-flow model. Together, the coupled models were used to (1) locate the areas of groundwater recharge to the thermal springs in the discretized hydrogeologic framework by using forward and reverse particle-tracking capabilities of MODPATH, (2) simulate the effects of variable recharge rates on the spring flows at the thermal springs, and (3) assess possible effects of climate and land-use change on the long-term variability of spring flows at the thermal springs. Forward and backward particle-tracking maps indicated that the most prevalent areas of recharge in the discretized hydrogeologic framework used in this study were within about 0.6–0.9 mile of the thermal springs. Forward particle tracking indicated a recharge area southwest of the thermal springs that corresponded to a location where the predominant lithologies are the Arkansas Novaculite, Hot Springs Sandstone, and Bigfork Chert. Backward particle tracking indicated a second localized area of recharge to the northeast of the thermal springs that corresponded to a location where the dominant lithology is the Bigfork Chert. The groundwater-flow model indicated that the most probable recharge formations are the Arkansas Novaculite, Bigfork Chert, and Hot Springs Sandstone. The simulated effects of climate and land-use changes on the variability of the spring-flow rates at the thermal springs generally resulted in reductions of thermal spring flow attributed to urban development and more extreme climates characterized by elevated mean surface air temperatures. The groundwater-flow model predicted a linear relation between the thermal spring discharge and the cumulative recharge volume applied to the hydrogeologic framework, and the positive slope of the predicted relation between recharge and simulated thermal spring flow indicates that more extreme precipitation events that supply more recharge may in fact increase the thermal spring-flow rates.

Arkansas↗

USGS2021

Stable isotope and geochemical characterization of nutrient sources and surface water near a confined animal feeding operation in the Big Creek watershed of northwest Arkansas

A concentrated animal feeding operation (CAFO) established in Newton County, Arkansas, near Big Creek, a tributary of the Buffalo National River, raised concern about potential degradation of water quality in the karst watershed. In this study, isotopic tools were combined with standard geochemical approaches to characterize nutrient sources and dynamics in the Big Creek watershed. An isotopic and geochemical reference database of potential nutrient sources in the Big Creek watershed was constructed based on samples collected from representative potential sources. Nutrient sources and stream samples were analyzed for delta (δ) 15 N-NO 3 , δ 18 O NO 3 , and a suite of selected dissolved ions. Data provide evidence of modification of potential local nutrient source signatures by nitrification, atmospheric deposition, evaporation, and denitrification. Samples taken from the CAFO waste pond, a septic system, field and parking lot runoff, fertilizer, and hog manure exhibited different δ 15 N-NO 3 and δ 18 O-NO 3 values as compared to stream samples. Stream δ 15 N-NO 3 and δ 18 O-NO 3 values cannot be explained by direct input of any one of these potential sources without modification of the isotopic composition by mixing or fractionation. Big Creek nitrate isotope values (-3.4 per mil [‰] to 6.7‰ δ 15 N-NO 3 and -7.6 to 9.1‰ δ 18 O-NO 3 ) were similar to values expected from nitrification of nitrogen stored in soils sampled in the watershed (2.8 to 7.6‰ δ 15 N-NO 3 and 3.4 to 4.8‰ δ 18 O-NO 3 ).

Arkansas↗

USGS2020

Phosphorus runoff risk assessment in karstic regions of the U.S.

The Phosphorus (P) Index risk assessment tool has been widely adopted across the U.S. to identify and rank site vulnerability to P runoff as part of the Natural Resources Conservation Service (NRCS) nutrient management planning (NMP) process. However, limited success has been achieved in addressing the risk of P loss by subsurface flow pathways, despite its relative importance in certain areas of the U.S., particularly in those States dominated by karst terrain. Here we review how States with varying land areas classified as having karst features address the risk of P runoff during the NMP process. Indices adopted in Illinois and Indiana require setbacks (15 – 72 m widths) around surface karst features. The remaining States with karst, address the risk of P loss in NMP development rather than the application of a P Index. Given the spatially variable hydrogeologic properties of karst, technically rigorous field‐scale factors are unlikely to be developed in the near future.

Agricultural & Environmental Letters↗

USGS2019

Potentiometric surface of groundwater-level altitudes near the planned Highway 270 bypass, east of Hot Springs, Arkansas, July–August 2017

The Ouachita Mountains aquifer system potentiometric-surface map is one component of the Hot Springs Bypass Groundwater Monitoring Project. The potentiometric-surface map provides a baseline assessment of shallow groundwater levels and flow directions before the construction of the Arkansas Department of Transportation planned extension of the Highway 270 bypass, east of Hot Springs, Arkansas. The map provides data regarding status of groundwater levels and potential effects on the recharge area in the Hot Springs National Park and to groundwater that supplies water to domestic users near the Highway 270 bypass. Groundwater levels from 66 wells were measured in July–August 2017. Fifty nine of the 66 groundwater-level altitudes measured, along with select surface-water features and springs, were used to construct the Ouachita Mountains aquifer system potentiometric-surface map. The potentiometric surface, a two-dimensional representation, shows groundwater-level altitudes ranging from a maximum of 766 ft above the North American Vertical Datum of 1988 (NAVD 88) to a minimum of 443 ft NAVD 88. The spring altitudes on the potentiometric-surface map range from 534 ft to 927 ft above NAVD 88. The study area, located in the Ouachita Mountains physiographic section of the Ouachita physiographic province, comprises narrow valleys and high ridges of Stanley Shale, Hot Springs Sandstone, Arkansas Novaculite, Missouri Mountain-Polk Creek Shale, and Bigfork Chert. The highest groundwater-level altitudes observed were in the Hot Springs Sandstone, Arkansas Novaculite, and Missouri Mountain-Polk Creek Shale. The springs discharge in outcrop areas of the Stanley Shale, Bigfork Chert, and Arkansas novaculite. The planned Highway 270 bypass will cut across ridges and valleys comprising these formations and, very importantly, across areas with elevations above 660 ft above NAVD 88 that define the hot springs recharge zone. This potentiometric-surface map defines the status of the shallow groundwater potentiometric surface near the Highway 270 bypass prior to initiation of construction activities. A post-construction potentiometric map is planned. It must be noted that shallow groundwater levels are also subject to climatic effects including changes in amount and timing of precipitation and changes in temperature.

Arkansas↗

USGS2019

Encylopedia of Caves

For many people, a visit to a cave is a wondrous event directing our minds to ponder the mysteries presented by these unique places and inspiring questions: How old is the cave? What was the role of water in forming the cave and where did the water come from? How is the cave connected to the surface environment? These are intriguing questions to ask, and karst scientists use isotope geochemistry to help solve these mysteries. Isotopes are atoms of the same chemical element that have the same number of protons but vary in their number of neutrons. As a result of their atomic mass difference, isotopes of a single element may exhibit slightly different chemical behavior. Radioisotopes are unstable and the nucleus of a parent isotope will spontaneously break apart (decay), releasing energy and changing into another element (daughter product) by loss or gain of protons, neutrons, or electrons. Stable isotopes, as implied by the name, are stable and do not spontaneously decay. These two types of isotopes are used widely by scientists to understand ancient and modern karst systems. Generally, radioisotopes are used for absolute dating karst water, cave sediments, cave formations (speleothems), and other material preserved in caves (such as bones). Stable isotopes can provide information about relative ages of cave water and speleothems. An absolute age provides a numeric date—such as 100,000 years old—whereas a relative age provides information that something is older or younger than something else—such as cave art is younger than a speleothem found in the same cave. Stable isotopes are used to study ancient karst systems because isotopic signals of past climate (paleoclimate) and environmental conditions (paleoenvironment) are preserved in speleothems and sediments. Stable isotopes are also used to understand modern systems, primarily through studies that distinguish sources of karst water, cave air, or contaminants, mixing of those sources, and biologic or chemical reactions that process compounds, such as breakdown of contaminants or organic matter. The variation in mass among isotopes is small, and isotope abundances are measured as a ratio of a common isotope to its less common isotopic counterparts (stable isotopes) or the abundance of a parent compared to daughter isotope (radioisotopes). As an example, hydrogen (H) has three naturally occurring isotopes. Most H comprises one proton (1H), but the rarer stable isotope (called deuterium) comprises one proton and one neutron (2H), and the radioisotope (called tritium) comprises one proton and two neutrons (3H). Because scientists use the minute differences in isotopes to test hypotheses, a scientist must understand the accuracy, precision, and error of the available methods, the assumptions about the chemical conditions of interest, and the limitations of the isotopic method being used. Many isotopic studies employ multiple isotopic tracers to better leverage the strengths and offset the limitations of using a single isotope. This article focuses on isotopes used to study the geology and hydrology of caves, but much additional isotopic work has been used to characterize the biology of caves.

Book chapter↗

USGS2018

Coupling high-frequency stream metabolism and nutrient monitoring to explore biogeochemical controls on downstream nitrate delivery

Instream biogeochemical process measurements are often short-term and localized. Here we use in situ sensors to quantify the net effects of biogeochemical processes on seasonal patterns in baseflow nitrate retention at the river-reach scale. Dual-station high-frequency in situ nitrate measurements, were coupled with high-frequency measurements of stream metabolism and dissolved inorganic carbon, in a tributary of the Buffalo National River, Arkansas. Nitrate assimilation was calculated from net primary production, and combined with mass-balance measurements, to estimate net nitrification and denitrification. The combined net effects of these instream processes (assimilation, denitrification, and nitrification) removed >30–90% of the baseflow nitrate load along a 6.5 km reach. Assimilation of nitrate by photoautotrophs during spring and early summer was buffered by net nitrification. Net nitrification peaked during the spring. After midsummer, there was a pronounced switch from assimilatory nitrate uptake to denitrification. There was clear synchronicity between the switch from nitrate assimilation to denitrification, a reduction in river baseflows, and a shift in stream metabolism from autotrophy to heterotrophy. The results show how instream nitrate retention and downstream delivery is driven by seasonal shifts in metabolic pathways; and how continuous in situ stream sensor networks offer new opportunities for quantifying the role of stream biota in the dynamics, fate, and transport of nitrogen in fluvial systems.

Arkansas↗

USGS2017

Carbon cycling in the mantled karst of the Ozark Plateaus, central United States

The nature of carbon (C) cycling in the unsaturated zone where groundwater is in contact with abundant gas-filled voids is poorly understood. The objective of this study was to trace inorganic-C cycling in a karst landscape using stable-C isotopes, with emphasis on a shallow groundwater flow path through the soil, to an underlying cave, and to the spring outlet of a cave stream in the Ozark Plateaus of northwestern Arkansas. Carbon dioxide (CO 2 ) concentration and isotopic composition (δ 13 C-CO 2 ) in gas and dissolved inorganic carbon (DIC) concentration and isotopic composition (δ 13 C-DIC) in water were measured in samples collected from two suction-cup soil samplers above the cave, three sites in the cave, and at the spring outlet of the cave stream. Soil-gas CO 2 concentration (median 2,578 ppm) and δ 13 C-CO 2 (median − 21.5‰) were seasonally variable, reflecting the effects of surface temperature changes on soil-CO 2 production via respiration and organic-matter decomposition. Cave-air CO 2 (median 1,026 ppm) was sourced from the soil zone and the surface atmosphere, with seasonally changing proportions of each source controlled by surface temperature-driven air density gradients. Soil-DIC concentration (median 1.7 mg L − 1 ) was lower and soil-δ 13 C-DIC (median − 19.5‰) was lighter compared to the cave (median 23.3 mg L − 1 and − 14.3‰, respectively) because carbonate-bedrock dissolution provided an inorganic source of C to the cave. Carbon species in the soil had a unique, light stable-C isotopic signature compared to the cave. Discrimination of soil-C sources to karst groundwater was achieved, which is critical for developing hydrologic budgets using environmental tracers such as C.

Geoderma Regional↗

USGS2016

Hydrogeology and hydrologic conditions of the Ozark Plateaus aquifer system

The hydrogeology and hydrologic characteristics of the Ozark Plateaus aquifer system were characterized as part of ongoing U.S. Geological Survey efforts to assess groundwater availability across the Nation. The need for such a study in the Ozark Plateaus physiographic province (Ozark Plateaus) is highlighted by increasing demand on groundwater resources by the 5.3 million people of the Ozark Plateaus, water-level declines in some areas, and potential impacts of climate change on groundwater availability. The subject study integrates knowledge gained through local investigation within a regional perspective to develop a regional conceptual model of groundwater flow in the Ozark Plateaus aquifer system (Ozark system), a key phase of groundwater availability assessment. The Ozark system extends across much of southern Missouri and northwestern and north-central Arkansas and smaller areas of southeastern Kansas and northeastern Oklahoma. The region is one of the major karst landscapes in the United States, and karst aquifers are predominant in the Ozark system. Groundwater flow is ultimately controlled by aquifer and confining unit lithologies and stratigraphic relations, geologic structure, karst development, and the character of surficial lithologies and regolith mantle. The regolith mantle is a defining element of Ozark Plateaus karst, affecting recharge, karst development, and vulnerability to surface-derived contaminants. Karst development is more advanced—as evidenced by larger springs, hydraulic characteristics, and higher well yields—in the Salem Plateau and in the northern part of the Springfield Plateau (generally north of the Arkansas-Missouri border) as compared with the southern part of the Springfield Plateau in Arkansas, largely due to thinner, less extensive regolith and purer carbonate lithology. Precipitation is the ultimate source of all water to the Ozark system, and the hydrologic budget for the Ozark system includes inputs from recharge, losing-stream sections, and groundwater inflows and losses of water to gaining-stream sections, groundwater withdrawals, and surface-water and groundwater outflows to neighboring systems. Groundwater recharge, estimated by a soil-water-balance model, represents about 24 percent, or 11 inches, of 43.9 inches annual precipitation. Recharge is spatially variable, being greater in the northern Springfield Plateau and Salem Plateau than in the southern Springfield Plateau (generally south of the Arkansas border) because of differences in regolith mantle extent and thickness and carbonate lithology and hydraulic properties. Increased precipitation and decreased agricultural land use during the period 1951 through 2011 increased recharge by approximately 5 percent. Although all Ozark streams have losing, neutral, and gaining sections, they are dominantly gaining and are a net sink for groundwater with nearly 90 percent of groundwater recharge returned to springs and streams. Groundwater pumping is a small but important loss of water in the Ozark system hydrologic budget; water-level declines and local cones of depression have been observed around pumping centers and strong concerns exist over potential effects on stream and spring flow. Data indicate that societal needs for freshwater resources in the Ozark Plateaus will continue to increase and will do so in the context of changing climate and hydrology. Groundwater will continue to be an important part of supporting these societal needs and also local ecosystems. The unique character and hydrogeologic variability across the Ozark system will control how the system responds to future stress. Groundwater of the Ozark system in the northern study area is more dynamic, has greater storage and larger flux, and has greater potential for further development than in the part of the study area south of the Arkansas-Missouri border. Further south in Arkansas, a line exists, roughly defined as 5 miles south of the Springfield Plateau-Boston Mountains boundary, beyond which further extensive municipal or commercial development appears unlikely under current economic and resource-need conditions. A small part of the Ozark system groundwater budget is currently drafted for use, leaving an apparently large component available for further development and use—particularly in the northern Springfield Plateau and Salem Plateau; however, the effects of increased pumping on groundwater’s role in maintaining ecosystems and ecosystem services are not quantitatively well understood, and the close relation between groundwater and surface water highlights the importance of further quantitative assessment.

Arkansas, Kansas, Missouri, Oklahoma↗

USGS2016

Altitudes and thicknesses of hydrogeologic units of the Ozark Plateaus aquifer system in Arkansas, Kansas, Missouri, and Oklahoma

A hydrogeologic framework was constructed to represent the altitudes and thicknesses of hydrogeologic units within the Ozark Plateaus aquifer system as part of a regional groundwater-flow model supported by the U.S. Geological Survey Water Availability and Use Science Program. The Ozark Plateaus aquifer system study area is nearly 70,000 square miles and includes parts of Arkansas, Kansas, Missouri, and Oklahoma. Nine hydrogeologic units were selected for delineation within the aquifer system and include the Western Interior Plains confining system, the Springfield Plateau aquifer, the Ozark confining unit, the Ozark aquifer, which was divided into the upper, middle, and lower Ozark aquifers to better capture the spatial variation in the hydrologic properties, the St. Francois confining unit, the St. Francois aquifer, and the basement confining unit. Geophysical and well-cutting logs, along with lithologic descriptions by well drillers, were compiled and interpreted to create hydrologic altitudes for each unit. The final compiled dataset included more than 23,000 individual altitude points (excluding synthetic points) representing the nine hydrogeologic units within the Ozark Plateaus aquifer system.

Arkansas, Kansas, Missouri, Oklahoma↗

USGS2015

Trace-metal and organic constituent concentrations in bed sediment at Big Base and Little Base Lakes, Little Rock Air Force Base, Arkansas—Comparisons to sediment-quality guidelines and indications for timing of exposure

This report compares concentrations for a wide range of inorganic and organic constituents in bed sediment from Big Base Lake and Little Base Lake, which are located on Little Rock Air Force Base, Arkansas, to sediment-quality guidelines. This report also compares trace-metal concentrations in a bed-sediment core sample to sediment age to determine when the highest concentrations of trace metals were deposited in Big Base Lake. Trace-metal results often were higher than background concentrations in the surrounding Pulaski County area, and concentrations of arsenic, cadmium, cobalt, copper, lead, manganese, mercury, nickel, and zinc at one or more of three study sites were higher than median concentrations for a study involving 98 urban streams in seven metropolitan areas of the United States. Concentrations for most polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and organochlorine pesticides in all three bed-sediment samples were less than the laboratory reporting limit or were detected at low concentrations. Some contaminants were detected at concentrations that are potentially toxic to sediment-dwelling biota; however, in general, the analyses suggest that the risk of sediment toxicity may be relatively low. Threshold effect concentrations were exceeded for 14 constituents—arsenic, copper, lead, nickel, and zinc, five polycyclic aromatic hydrocarbons compounds, chlordane, and all three dichlorodiphenyltrichloroethane (DDT) congeners—which suggests potential toxicity to some sediment-dwelling biota. Only two constituents had concentrations that exceeded published probable effect concentrations—arsenic (at the deepest site in Big Base Lake, NS6) and p,p’-dichlorodiphenyldichloroethylene (p,p’-DDE; at both sites in Big Base Lake, NS5 and NS6). Regarding highest concentrations and associated timing of exposure, trace metals analyzed in the sediment core seem to indicate three fairly distinct exposure patterns. For 11 trace metals that had the highest concentration measured in the shallowest and most recently deposited sediment, the most likely explanation is recent exposure by anthropogenic activities. Most of the 11 trace metals with highest concentrations in shallow sediment are relatively innocuous; however, arsenic, copper, selenium, and zinc are among the U.S. Environmental Protection Agency’s 126 priority pollutants. For three trace metals (cadmium, lead, and mercury), for which concentrations were highest in sediments that were 16–20 centimeters down the core, it is likely that a source associated with those contaminants during the period when those sediments were deposited, was reduced or eliminated. The eight remaining trace metals, for which concentrations were highest in sediments that were just below the prereservoir surface, likely had sources that were eliminated soon after lake construction or occurred at relatively high background concentrations in soils in the area around Little Rock Air Force Base.

Arkansas↗

USGS2015

Using stable isotopes of carbon to investigate the seasonal variation of carbon transfer in a northwestern Arkansas cave

Stable-isotope analyses are valuable in karst settings, where characterizing biogeochemical cycling of carbon along groundwater flow paths is critical for understanding and protecting sensitive cave and karst water resources. This study quantified the seasonal changes in concentration and isotopic composition ( δ 13C) of aqueous and gaseous carbon species—dissolved inorganic carbon (DIC) and gaseous carbon dioxide (CO 2 )—to characterize sources and transfer of these species along a karst flow path, with emphasis on a cave environment. Gas and water samples were collected from the soil and a cave in northwestern Arkansas approximately once a month for one year to characterize carbon cycling along a conceptual groundwater flow path. In the soil, as the DIC concentration increased, the isotopic composition of the DIC became relatively lighter, indicating an organic carbon source for a component of the DIC and corroborating soil DIC as a proxy for soil respiration. In the cave, a positive correlation between DIC and surface temperature was due to increased soil respiration as the organic carbon signal from the soil was transferred to the cave environment via the aqueous phase. CO 2 concentration was lowest in the cave during colder months and increased exponentially with increasing surface temperature, presumably due to higher rates of soil respiration during warmer periods and changing ventilation patterns between the surface and cave atmosphere. Isotopic disequilibrium between CO 2 and DIC in the cave was greatest when CO 2 concentration was changing during November/ December and March/April, presumably due to the rapid addition or removal of gaseous CO 2 . The isotopic disequilibrium between DIC and CO 2 provided evidence that cave CO 2 was a mixture of carbon from several sources, which was mostly constrained by mixture between atmospheric CO 2 and soil CO 2 . The concentration and isotopic composition of gaseous and aqueous carbon species were controlled by month-to-month variations in temperature and precipitation and provided insight into the sources of carbon in the cave. Stable carbon isotope ratios provided an effective tool to explore carbon transfer from the soil zone and into the cave, identify carbon sources in the cave, and investigate how seasonality affected the transfer of carbon in a shallow karst system.

Journal of Cave and Karst Studies↗

USGS2014

Aquifers of Arkansas: protection, management, and hydrologic and geochemical characteristics of groundwater resources in Arkansas

Sixteen aquifers in Arkansas that currently serve or have served as sources of water supply are described with respect to existing groundwater protection and management programs, geology, hydrologic characteristics, water use, water levels, deductive analysis, projections of hydrologic conditions, and water quality. State and Federal protection and management programs are described according to regulatory oversight, management strategies, and ambient groundwater-monitoring programs that currently (2013) are in place for assessing and protecting groundwater resources throughout the State. Physical attributes, groundwater geochemistry, and groundwater quality are described for each of the 16 aquifers of the State. Information in regard to the hydrology and geochemistry of each of the aquifers is summarized from about 550 historical and recent publications. Additionally, more than 8,000 sites with groundwater-quality data were obtained from the U.S. Geological Survey National Water Information System and the Arkansas Department of Environmental Quality databases and entered into a spatial database to investigate distribution and trends in chemical constituents for each of the aquifers. The 16 aquifers of the State were divided into two major physiographic regions of the State: the Coastal Plain Province (referred to as Coastal Plain) of eastern and southern Arkansas, which includes 11 of the 16 aquifers, and the Interior Highlands Division (referred to as Interior Highlands) of western Arkansas, which includes the remaining 5 aquifers. The 11 aquifers in the Coastal Plain consist of various geologic units that are Cenozoic in age and consist primarily of Cretaceous, Tertiary, and Quaternary sands, gravels, silts, and clays. Groundwater in the Coastal Plain represents one of the most valuable natural resources in the State, driving the economic engines of agriculture, while also supplying abundant water for commercial, industrial, and public-supply use. In terms of age from youngest to oldest, the aquifers of the Coastal Plain include Quaternary alluvial aquifers, including the Mississippi River Valley alluvial aquifer (the most important aquifer in Arkansas in terms of volume of use and economic benefits), the Jackson Group (a regional confining unit that served for decades as an important source of domestic supply), and the Cockfield, Sparta, Cane River, Carrizo, Wilcox, Nacatoch, Ozan, Tokio, and Trinity aquifers. The Mississippi River Valley alluvial aquifer accounts for approximately 94 percent of all groundwater used in the State, and the aquifer is used primarily for irrigation purposes. The Sparta aquifer is the second most important aquifer in terms of use, and the aquifer was used in the past dominantly as a source of public and industrial supply, although increasing irrigation use is occurring because of critically declining water levels in the Mississippi River Valley alluvial aquifer. Other aquifers of the Coastal Plain generally are used as important local sources of domestic, industrial, and public supply, in addition to other minor uses. Water quality generally is good for all aquifers of the Coastal Plain, except for elevated iron concentrations and localized areas of high salinity. The high salinity results from intrusion from underlying formations, evapotranspiration processes in areas of low recharge, and inadequate flushing in downgradient areas of residual salinity from deposition in marine environments. Trends in the spatial distribution of individual chemical constituents are related to position along the flow path for most aquifers of the Coastal Plain. These trends include elevated iron and nitrate concentrations with lower pH values and dissolved solids in groundwater from the outcrop areas, transitioning to lower iron and nitrate (related to changes in redox) and higher pH and dissolved solids (dominantly from the dissolution of carbonate minerals) in groundwater downgradient from outcrop areas. Groundwater generally trended from a calcium- to a sodium-bicarbonate water type with increasing cation exchange along the flow path. The Interior Highlands of western Arkansas has less reported groundwater use than other areas of the State, reflecting a combination of factors. These factors include prevalent and increasing use of surface water, less intensive agricultural uses, lower population and industry densities, lesser potential yield of the resource, and lack of detailed reporting. The overall low yields of aquifers of the Interior Highlands result in domestic supply as the dominant use, with minor industrial, public, and commercial-supply use. Where greater volumes are required for growth of population and industry, surface water is the greatest supplier of water needs in the Interior Highlands. The various aquifers of the Interior Highlands generally occur in shallow, fractured, well-indurated, structurally modified bedrock of this mountainous region of the State, as compared to the relatively flat-lying, unconsolidated sediments of the Coastal Plain. In terms of age from youngest to oldest, the aquifers of the Interior Highlands include: the Arkansas River Valley alluvial aquifer, the Ouachita Mountains aquifer, the Western Interior Plains confining system, the Springfield Plateau aquifer, and the Ozark aquifer. Spatial trends in groundwater geochemistry in the Interior Highlands differ greatly from trends noted for aquifers of the Coastal Plain. In the Coastal Plain, the prevalence of long regional flow paths results in regionally predictable and mappable geochemical changes along the flow paths. In the Interior Highlands, short, topographically controlled flow paths (from hilltops to valleys) within small watersheds represent the predominant groundwater-flow system. As such, dense data coverage from numerous wells would be required to effectively characterize these groundwater basins and define small-scale geochemical changes along any given flow path for aquifers of the Interior Highlands. Changes in geochemistry generally were related to rock type and residence time along individual flow paths. Dominant changes in geochemistry for the Ouachita Mountains aquifer and the Western Interior Plains confining system are attributed to rock/water interaction and changes in redox zonation along the flow path. In these areas, groundwater evolves along flow paths from a calcium- to a sodium-bicarbonate water type with increasing reducing conditions resulting in denitrification, elevated iron and manganese concentrations, and production of methane in the more geochemically evolved and strongest reducing conditions. In the Ozark and Springfield Plateau aquifers, rapid influx of surface-derived contaminants, especially nitrogen, coupled with few to no attenuation processes was attributed to the karst landscape developed on Mississippian- and Ordovician-age carbonate rocks of the Ozark Plateaus. Increasing nitrate concentrations are related to increasing agricultural land use, and areas of mature karst development result in higher nitrate concentrations than areas with less karst features.

Arkasas↗

USGS2013

Using isotopes of dissolved inorganic carbon species and water to separate sources of recharge in a cave spring, northwestern Arkansas, USA Blowing Spring Cave

Blowing Spring Cave in northwestern Arkansas is representative of cave systems in the karst of the Ozark Plateaus, and stable isotopes of water (δ18O and δ2H) and inorganic carbon (δ13C) were used to quantify soil-water, bedrock-matrix water, and precipitation contributions to cave-spring flow during storm events to understand controls on cave water quality. Water samples from recharge-zone soils and the cave were collected from March to May 2012 to implement a multicomponent hydrograph separation approach using δ18O and δ2H of water and dissolved inorganic carbon (δ13C–DIC). During baseflow, median δ2H and δ18O compositions were –41.6&permil; and –6.2&permil; for soil water and were –37.2&permil; and –5.9&permil; for cave water, respectively. Median DIC concentrations for soil and cave waters were 1.8 mg/L and 25.0 mg/L, respectively, and median δ13C–DIC compositions were –19.9&permil; and –14.3&permil;, respectively. During a March storm event, 12.2 cm of precipitation fell over 82 h and discharge increased from 0.01 to 0.59 m3/s. The isotopic composition of precipitation varied throughout the storm event because of rainout, a change of 50&permil; and 10&permil; for δ2H and δ18O was observed, respectively. Although, at the spring, δ2H and δ18O only changed by approximately 3&permil; and 1&permil;, respectively. The isotopic compositions of precipitation and pre-event (i.e., soil and bedrock matrix) water were isotopically similar and the two-component hydrograph separation was inaccurate, either overestimating (>100%) or underestimating (<0%) the precipitation contribution to the spring. During the storm event, spring DIC and δ13C–DIC decreased to a minimum of 8.6 mg/L and –16.2&permil;, respectively. If the contribution from precipitation was assumed to be zero, soil water was found to contribute between 23 to 72% of the total volume of discharge. Although the assumption of negligible contributions from precipitation is unrealistic, especially in karst systems where rapid flow through conduits occurs, the hydrograph separation using inorganic carbon highlights the importance of considering vadose-zone soil water when analyzing storm chemohydrographs.

Arkansas↗

USGS2013

Geochemical and isotopic variations in shallow groundwater in areas of the Fayetteville Shale development, north-central Arkansas

Exploration of unconventional natural gas reservoirs such as impermeable shale basins through the use of horizontal drilling and hydraulic fracturing has changed the energy landscape in the USA providing a vast new energy source. The accelerated production of natural gas has triggered a debate concerning the safety and possible environmental impacts of these operations. This study investigates one of the critical aspects of the environmental effects; the possible degradation of water quality in shallow aquifers overlying producing shale formations. The geochemistry of domestic groundwater wells was investigated in aquifers overlying the Fayetteville Shale in north-central Arkansas, where approximately 4000 wells have been drilled since 2004 to extract unconventional natural gas. Monitoring was performed on 127 drinking water wells and the geochemistry of major ions, trace metals, CH 4 gas content and its C isotopes (δ 13 C CH4 ), and select isotope tracers (δ 11 B, 87 Sr/ 86 Sr, δ 2 H, δ 18 O, δ 13 C DIC ) compared to the composition of flowback-water samples directly from Fayetteville Shale gas wells. Dissolved CH 4 was detected in 63% of the drinking-water wells (32 of 51 samples), but only six wells exceeded concentrations of 0.5 mg CH 4 /L. The δ 13 C CH4 of dissolved CH 4 ranged from −42.3&permil; to −74.7&permil;, with the most negative values characteristic of a biogenic source also associated with the highest observed CH 4 concentrations, with a possible minor contribution of trace amounts of thermogenic CH 4 . The majority of these values are distinct from the reported thermogenic composition of the Fayetteville Shale gas (δ 13 C CH4 = −35.4&permil; to −41.9&permil;). Based on major element chemistry, four shallow groundwater types were identified: (1) low (<100 mg/L) total dissolved solids (TDS), (2) TDS > 100 mg/L and Ca–HCO 3 dominated, (3) TDS > 100 mg/L and Na–HCO 3 dominated, and (4) slightly saline groundwater with TDS > 100 mg/L and Cl > 20 mg/L with elevated Br/Cl ratios (>0.001). The Sr ( 87 Sr/ 86 Sr = 0.7097–0.7166), C (δ 13 C DIC = −21.3&permil; to −4.7&permil;), and B (δ 11 B = 3.9–32.9&permil;) isotopes clearly reflect water–rock interactions within the aquifer rocks, while the stable O and H isotopic composition mimics the local meteoric water composition. Overall, there was a geochemical gradient from low-mineralized recharge water to more evolved Ca–HCO 3 , and higher-mineralized Na–HCO 3 composition generated by a combination of carbonate dissolution, silicate weathering, and reverse base-exchange reactions. The chemical and isotopic compositions of the bulk shallow groundwater samples were distinct from the Na–Cl type Fayetteville flowback/produced waters (TDS &sim;10,000–20,000 mg/L). Yet, the high Br/Cl variations in a small subset of saline shallow groundwater suggest that they were derived from dilution of saline water similar to the brine in the Fayetteville Shale. Nonetheless, no spatial relationship was found between CH 4 and salinity occurrences in shallow drinking water wells with proximity to shale-gas drilling sites. The integration of multiple geochemical and isotopic proxies shows no direct evidence of contamination in shallow drinking-water aquifers associated with natural gas extraction from the Fayetteville Shale.

Applied Geochemistry↗

USGS2012

Shallow groundwater quality and geochemistry in the Fayetteville Shale gas-production area, north-central Arkansas, 2011

The Mississippian Fayetteville Shale serves as an unconventional gas reservoir across north-central Arkansas, ranging in thickness from approximately 50 to 550 feet and varying in depth from approximately 1,500 to 6,500 feet below the ground surface. Primary permeability in the Fayetteville Shale is severely limited, and successful extraction of the gas reservoir is the result of advances in horizontal drilling techniques and hydraulic fracturing to enhance and develop secondary fracture porosity and permeability. Drilling and production of gas wells began in 2004, with a steady increase in production thereafter. As of April 2012, approximately 4,000 producing wells had been completed in the Fayetteville Shale. In Van Buren and Faulkner Counties, 127 domestic water wells were sampled and analyzed for major ions and trace metals, with a subset of the samples analyzed for methane and carbon isotopes to describe general water quality and geochemistry and to investigate the potential effects of gas-production activities on shallow groundwater in the study area. Water-quality analyses from this study were compared to historical (pregas development) shallow groundwater quality collected in the gas-production area. An additional comparison was made using analyses from this study of groundwater quality in similar geologic and topographic areas for well sites less than and greater than 2 miles from active gas-production wells. Chloride concentrations for the 127 groundwater samples collected for this study ranged from approximately 1.0 milligram per liter (mg/L) to 70 mg/L, with a median concentration of 3.7 mg/L, as compared to maximum and median concentrations for the historical data of 378 mg/L and 20 mg/L, respectively. Statistical analysis of the data sets revealed statistically larger chloride concentrations (p-value <0.001) in the historical data compared to data collected for this study. Chloride serves as an important indicator parameter based on its conservative transport characteristics and relatively elevated concentrations in production waters associated with gas extraction activities. Major ions and trace metals additionally had lower concentrations in data gathered for this study than in the historical analyses. Additionally, no statistical difference existed between chloride concentrations from water-quality data collected for this study from 94 wells located less than 2 miles from a gas-production well and 33 wells located 2 miles or more from a gas-production well; a Wilcoxon rank-sum test showed a p-value of 0.71. Major ion chemistry was investigated to understand the effects of geochemical and reduction-oxidation (redox) processes on the shallow groundwater in the study area along a continuum of increased rock-water interaction represented by increases in dissolved solids concentration. Groundwater in sandstone formations is represented by a low dissolved solids concentration (less than 30 mg/L) and slightly acidic water type. Shallow shale aquifers were represented by dissolved solids concentrations ranging upward to 686 mg/L, and water types evolving from a dominantly mixed-bicarbonate and calcium-bicarbonate to a strongly sodium-bicarbonate water type. Methane concentration and carbon isotopic composition were analyzed in 51 of the 127 samples collected for this study. Methane occurred above a detection limit of 0.0002 mg/L in 32 of the 51 samples, with concentrations ranging upward to 28.5 mg/L. Seven samples had methane concentrations greater than or equal to 0.5 mg/L. The carbon isotopic composition of these higher concentration samples, including the highest concentration of 28.5 mg/L, shows the methane was likely biogenic in origin with carbon isotope ratio values ranging from -57.6 to -74.7 per mil. Methane concentrations increased with increases in dissolved solids concentrations, indicating more strongly reducing conditions with increasing rock-water interaction in the aquifer. As such, groundwater-quality data collected for this study indicate that groundwater chemistry in the shallow aquifer system in the study area is a result of natural processes, beginning with recharge of dilute atmospheric precipitation and evolution of observed groundwater chemistry through rock-water interaction and redox processes.

Arkansas↗

USGS2009

Geochemistry, Comparative Analysis, and Physical and Chemical Characteristics of the Thermal Waters East of Hot Springs National Park, Arkansas, 2006-09

A study was conducted by the U.S Geological Survey in cooperation with the Arkansas State Highway and Transportation Department to characterize the source and hydrogeologic conditions responsible for thermal water in a domestic well 5.5 miles east of Hot Springs National Park, Hot Springs, Arkansas, and to determine the degree of hydraulic connectivity between the thermal water in the well and the hot springs in Hot Springs National Park. The water temperature in the well, which was completed in the Stanley Shale, measured 33.9 degrees Celsius, March 1, 2006, and dropped to 21.7 degrees Celsius after 2 hours of pumping - still more than 4 degrees above typical local groundwater temperature. A second domestic well located 3 miles from the hot springs in Hot Springs National Park was discovered to have a thermal water component during a reconnaissance of the area. This second well was completed in the Bigfork Chert and field measurement of well water revealed a maximum temperature of 26.6 degrees Celsius. Mean temperature for shallow groundwater in the area is approximately 17 degrees Celsius. The occurrence of thermal water in these wells raised questions and concerns with regard to the timing for the appearance of the thermal water, which appeared to coincide with construction (including blasting activities) of the Highway 270 bypass-Highway 70 interchange. These concerns were heightened by the planned extension of the Highway 270 bypass to the north - a corridor that takes the highway across a section of the eroded anticlinal complex responsible for recharge to the hot springs of Hot Springs National Park. Concerns regarding the possible effects of blasting associated with highway construction near the first thermal well necessitated a technical review on the effects of blasting on shallow groundwater systems. Results from available studies suggested that propagation of new fractures near blasting sites is of limited extent. Vibrations from blasting can result in rock collapse for uncased wells completed in highly fractured rock. However, the propagation of newly formed large fractures that potentially could damage well structures or result in pirating of water from production wells appears to be of limited possibility based on review of relevant studies. Characteristics of hydraulic conductivity, storage, and fracture porosity were interpreted from flow rates observed in individual wells completed in the Bigfork Chert and Stanley Shale; from hydrographs produced from continuous measurements of water levels in wells completed in the Arkansas Novaculite, the Bigfork Chert, and Stanley Shale; and from a potentiometric-surface map constructed using water levels in wells throughout the study area. Data gathered from these three separate exercises showed that fracture porosity is much greater in the Bigfork Chert relative to that in the Stanley Shale, shallow groundwater flows from elevated recharge areas with exposures of Bigfork Chert along and into streams within the valleys formed on exposures of the Stanley Shale, and there was no evidence of interbasin transfer of groundwater within the shallow flow system. Fifteen shallow wells and two cold-water springs were sampled from the various exposed formations in the study area to characterize the water quality and geochemistry for the shallow groundwater system and for comparison to the geochemistry of the hot springs in Hot Springs National Park. For the quartz formations (novaculite, chert, and sandstone formations), total dissolved solids concentrations were very low with a median concentration of 23 milligrams per liter, whereas the median concentration for groundwater from the shale formations was 184 milligrams per liter. Ten hot springs in Hot Springs National Park were sampled for the study. Several chemical constituents for the hot springs, including pH, total dissolved solids, major cations and anions, and trace metals, show similarity with the shale formations

Scientific Investigations Report↗