TRINITY AQUIFER EPIKARST STUDY USING D18O AND DD STABLE ISOTOPE ANALYSIS, CAVE WITHOUT A NAME, SOUTH-CENTRAL TEXAS

Our research uses stable isotope analysis to investigate infiltration and recharge in the epikarst at Cave Without a Name (CWAN) in Kendall County, South-central Texas, in the karstic Lower Glen Rose formation. Surface water and groundwater resources in the region are currently being impacted by pumping and drought conditions; quantifying recharge rates at a variety of scales is required before groundwater systems can be accurately modeled. We employed several techniques, including the use of δ18O and δ2D in rainwater and groundwater to estimate and model infiltration, storage capacity, recharge, and evaporative losses in an epikarst where soil depths average 10-30 cm.

A weather station and precipitation sampler at the site record environmental parameters at 10-minute intervals and collect rainwater. Groundwater samples are collected periodically at several drip sites where drip rate and geochemical parameters are also logged at 10-minute intervals. Water sampling and stable isotope analyses were performed by graduate students and researchers at Texas State University, San Marcos using a Model 908-0008 Liquid Water Stable Isotope Analyzer (Los Gatos Research, Inc.). δ18O and δ2D data from three drip sites were compared with the Global (GMWL) and Local Meteroic Water Lines (LMWL), and samples were analyzed to identify potential differences between sites.

Preliminary results reveal a LMWL of y=6.7252x+7.8262 (n = 24), with an average un-weighted precipitation isotope composition of δ2D= -20.1 and δ18O=-4.15. A slope of <8x generally indicates an evaporative environment. The amount-weighted mean of precipitation is δ2D=-35.3 and δ18O=-6.06, and the average groundwater composition is δ2D= -21.9 and δ18O= -4.26. Groundwater samples plot along a trend of y=2.7871x-10.132 (n = 111), which suggests additional evaporative enrichment after precipitation has infiltrated through soils. The difference between amount-weighted precipitation values and groundwater values implies that a significant amount of evaporative enrichment is occurring in the shallow epikarst before soil water mixes with fresh precipitation and contributes to recharge. Current work is focused on developing a model that describes the evaporative evolution of stable isotope composition from precipitation.