State Water Resources Research Institute Program (WRRI)

Details for Project ID 2009TX335G, 2009

The Role of Epikarst in Controlling Recharge, Water Quality and Biodiversity in Karst Aquifers: A Comparative Study between Virginia and Texas

Institute: Texas
USGS Grant Number:
Start Date: 2009-08-01 End Date: 2011-07-31
Total Federal Funds: $59,300 Total Non-Federal Funds: $59,299

Principal Investigators: Benjamin Schwartz

Abstract: The epikarst, often called the “skin” of karst aquifers (Bakalowicz, 2004), is a critical zone that significantly influences hydrology, water quality, and ecosystems in karst aquifer systems. The epikarst regulates both the quantity and quality of autogenic recharge to karst aquifers and, as a result, is perhaps the most important component of the system. However, due to its extreme heterogeneity, the epikarst is notoriously difficult to characterize. Our primary research objective is to use hydrologic, geochemical and biological methods in a ‘holistic’ approach to study the epikarst in diverse geologic and climatic settings. Research questions we address are: 1. What are the factors that determine thresholds of water excess (precipitation-evapotranspiration) that must be crossed in order to generate recharge to karst aquifers? 2. How do water quality and biological community diversity vary under different conditions of recharge? 3. How do responses of epikarst recharge scale up to responses in ground water flow and aquifer response on a catchment scale? To answer these questions, we have already installed instrumentation at four research sites in a shallow Virginia cave and at five research sites in four different shallow Texas caves and have collected preliminary hydrologic, geochemical and biologic data. Each station is instrumented to allow continuous measurements and periodic sampling of hydrologic (precipitation and drip rates), geochemical (pH, temperature, redox potential, specific conductance, dissolved oxygen, major ions, inorganic and organic carbon) and biological (copepod and other invertebrates) parameters. In-cave drip sites have been chosen where cave ceilings are within 15 m of the surface to ensure proximity to the hydrogeologically complex epikarst zone. This is within the known vertical range (15m) of the epikarst observed in most karst regions (Klimchouk, 2004). Our sites were selected to have high transferability of research results. In Virginia, the selected site (James Cave) is located in a low-relief, autogenically recharged karst region in Pulaski County, which is representative of much of the Shenandoah Valley, a sub-province of the Appalachian Great Valley. The Great Valley is the largest contiguous karst region in the Appalachians, extending from Alabama to New York, and is a regionally important carbonate aquifer. In Texas, the four caves are located in the eastern edge of the Edwards Plateau between Austin and San Antonio and are in autogenic portions of the sensitive recharge zone for the Edwards Aquifer. The Edwards Aquifer is one of the most productive carbonate aquifers in the world. The work we propose here will expand on our existing studies to include nearby groundwater wells, springs, and cave streams so that we can scale-up our results of recharge monitoring to compare with real, synchronous hydrologic measurements of subsurface zero-order cave streams, catchment springs, and water table responses. Our results will also be applicable to paleoclimate studies using speleothems by determining how epikarst processes affect the composition of drip waters forming speleothems. From a biological perspective, very little work has been done investigating the epikarstic fauna in North America and our results will make a significant contribution to knowledge about epikarstic fauna in the regions.