Patterns of Heterogeneity within Phreatic Karst Aquifers of the Great Valley, Virginia and West Virginia: Evidence from Time Series Hydrologic Monitoring, Groundwater Chemistry, and Stygobite Site Occupancy

Phreatic karst waters in the Central Appalachian Great Valley are the subject of an ongoing habitat monitoring project across the geographic range of the federally threatened Madison Cave Isopod (MCI, Antrolana lira), a stygobitic crustacean previously documented at each monitoring site. Eight caves and five wells were monitored, via instrumentation, hourly from June, 2016 thru October, 2017 for water level (WL), temperature (T), and specific conductivity (Cs.) Sites were visited quarterly to download data, collect water samples, and deploy baited traps to assess occupancy by stygobitic fauna. Samples were analyzed for major ions, inorganic carbon, and stable water isotopes. Precipitation data from National Climate Data Center stations were used to evaluate response to precipitation events. There is significant variation between sites in water chemistry, in baseline WL, Cs, and T values, and in the response of these parameters to precipitation. Median temperatures varied from 11.6 to 13.8 °C, with ranges within sites of 0.7 to 15.5 °C. Median Cs values varied from 442 to 726 uS/cm at 25 °C, with ranges within sites of 16 to 573 uS/cm. Ranges of WL within sites varied from 0.9 to 9.6 m. Ten sites exhibited rapid WL increases following precipitation, accompanied by spikes and/or dips in Cs. Six of these sites exhibited significant T changes. Sites with negative Cs response typically exhibited T changes reflecting precipitation temperature, with several exhibiting a compound response to larger events. Samples analyzed as calcium-magnesium-bicarbonate type waters, with Ca:Mg ratios from 9 to 5.5:1 and Ca + Mg values from 1.8 to 4.1 mmol/l. Geochemical parameters grouped more by site than by season. Chloride, nitrate, sulfate and sodium levels at some sites suggest significant land use influence from agricultural and stormwater management practices. Average δ2H and δ18O compositions suggest winter-dominated (Nov-April) recharge. Over the course of the study, site occupancy of these known MCI sites ranged from 0 to 100%, both for MCI and other stygobitic invertebrates. Occupancy rates appeared unrelated to geochemical or hydrodynamic patterns. Data are consistent with a complex, compartmentalized phreatic aquifer, reflecting the folded, faulted, and fractured bedrock structure. Water levels determine inter-compartment connectivity, facilitating episodic migration of stygobitic species. Characterization of such a system to support groundwater and habitat management decisions would require a much higher spatial density of monitoring stations than presently exists in the Great Valley.