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Description of Research A) Research Objectives The epikarst, often called the "skin" of karst aquifers, is a critical zone which controls hydrological fluxes, water quality of recharge, and ecosystem function. It is a variably weathered transition zone between the soil and un- weathered bedrock in the vadose zone that has enhanced porosity and functions to store and direct percolating water towards vertical drains or springs in the karst (Jones et al. 2004). Traditionally, the study of epikarst has been the purview of hydrogeologists primarily concerned with epikarst evolution and hydrological characterization. However, in recent years, the epikarst was "discovered" by ecologists with an interest in understanding the unique ecology of karst regions. Studies at the interface of hydrogeology and ecology are important and timely because of growing concerns over groundwater quantity and quality in regions simultaneously threatened by vegetation change (e.g. woody encroachment), urbanization and climate change. According to the Karst Waters Institute, the Edwards Aquifer of central Texas is among the top ten most threatened karst aquifers in the world. Fortunately, the Edwards Plateau has also become a focal point for conducting multidisciplinary research into karst processes. This proposal will make an important contribution to these efforts by addressing two of the major uncertainties in epikarst hydroecology:1) Where in the soil /epikarst system are the roots of the major water consumers (trees) located and how do trees interact with groundwater recharge flows? While the first studies on root ecology of the region placed roots deep in the fractured rock, tapping directly into pooled cave water (Jackson et al. 1999), more recent studies questioned whether the "deep root' scenario can be uncritically generalized across the karst landscape (Wilcox, 2005). 2) What is the fate of contaminants applied to the surface of the soil /epikarst system? Nitrogen contamination of karst aquifers through the encroachment of cities into aquifer recharge zones is a worldwide problem (Legrand, 1973) but the processes of nitrogen transport, storage and removal through biological processes are not well understood. Epikarst is notoriously heterogeneous, related to geologic properties and the circumstances of epikarst evolution. While this presents challenges to karst research, it is also an opportunity for exploring the structural determinants of epikarst hydrological and ecological function. Our approach here is to capitalize on epikarst heterogeneity by examining hydro - ecological processes as a function of soil and epikarst thickness and structure. At six sites spanning a range of soil depths, epikarst thickness, and bedrock characteristics, we will address the following questions: 1) How does the structure of the soil /epikarst system affect the dynamics of water flow? 2) How does it affect the dynamics and location of water uptake in trees? 3) Using ammonium nitrate as a model contaminant, how does soil /epikarst structure affect contaminant transport, retention and removal? Our working hypothesis is that a positive relationship exists between the residence time of water in the soil/epikarst system, and both utilization of epikarst water by trees and the capacity of plants and soil biota to intercept ammonium nitrate inputs. While it may be obvious that thicker soil cover will have these effects, it is less obvious whether thicker epikarst, which could also increase the residence time of water and contaminants, may have a similar effect and could thus compensate for a lack of soil cover. The research proposed here is not funded through any other source and is the first collaboration between Schwartz and Schwinning. However, the proposed research builds upon the independent research experiences of both PIs. Schwartz, with collaborators from Virginia Tech, U.S.G.S., American University, and the Karst Research Institute in Slovenia, is currently conducting a study in Virginia to investigate epikarst controls on recharge quantity and quality under an agricultural field by monitoring and analyzing cave drip - waters. This study is the first to integrate long -term high - resolution monitoring of cave drip rates and geochemistry, and surface environmental conditions, for the purposes of characterizing epikarst flow and transport. The study also employs copepod analysis of drip waters, an emerging technique in the study of soil/epikarst flow regimes. Copepods are small crustaceans that live in