For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://store.usgs.gov.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. DatumVertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88).Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).Altitude, as used in this report, refers to distance above the vertical datum. vii Supplemental InformationSpecific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C).Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). Abbreviations AbstractThe hydrogeology and hydrologic characteristics of the Ozark Plateaus aquifer system were characterized as part of ongoing U

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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 (d 13 C) 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.

The Mississippi Alluvial Plain hosts one of the most prolific shallow aquifer systems in the United States but is experiencing chronic groundwater decline. The Reelfoot rift and New Madrid seismic zone underlie the region and represent an important and poorly understood seismic hazard. Despite its societal and economic importance, the shallow subsurface architecture has not been mapped with the spatial resolution needed for effective management. Here, we present airborne electromagnetic, magnetic, and radiometric observations, measured over more than 43,000 flight-line-kilometers, which collectively provide a system-scale snapshot of the entire region. We develop detailed maps of aquifer connectivity and shallow geologic structure, infer relationships between structure and groundwater age, and identify previously unseen paleochannels and shallow fault structures. This dataset demonstrates how regional-scale airborne geophysics can close a scale gap in Earth observation by providing observational data at suitable scales and resolutions to improve our understanding of subsurface structures.

In northwestern Arkansas, karst aquifers have experienced degraded water quality due to impacts of landuse changes, such as increased urbanization or intensification of agriculture. Water quality at a karst spring was characterized by quantifying the variability of fecal-indicator bacteria, specifically Escherichia coli (E. coli), throughout base-flow periods and storm events. Waterquality change over time was assessed using an intermittent record of nitrate and chloride data since the 1990s. A Geographic Information System was used to quantify changes in land use over time and identify housing subdivisions using on-site septic systems. E. coli was significantly greater during storm events (649 CFU/100 mL or MP/ 100 mL) than base-flow periods (41 CFU/100 mL or MPN/ 100 mL). The increase in E. coli at the spring following storm events was interpreted to result from flushing of bacteria from the land surface, through the epikarst and karst conduits, and into groundwater. Nitrate and chloride have increased significantly since the 1990s, following the general pattern of increased urbanization in the recharge area. Septic-tank effluent may be degrading the water quality of the karst spring based on the dominance of on-site septic tank usage in the recharge area, unsuitable topography and soil type for septic tank absorption fields, increased nitrate and chloride concentrations concomitant with increased urbanization, and increase of the fecal-indicator bacteria E. coli following storm events.