A NEW METHODOLOGY FOR COLLECTING ISOTOPIC PRECIPITATION DATA TO BETTER UNDERSTAND RECHARGE IN SHALLOW KARST AQUIFERS

Increased application of oxygen (O) and hydrogen (H) isotope data to aid groundwater studies often is hampered by the age of the data collected and the collection methodology. Precipitation analyzed in earlier studies may not be representative of current climatic conditions and the monthly-composite rainfall samples collected in previous data sets to construct Local Meteoric Water Lines may not provide adequate detail to partition rainfall into meaningful hydrogeologic analyses. Sporadic and rapid recharge events in shallow karst aquifers may need data from events and at a daily scale. The new method proposed for collecting O and H isotopic precipitation data strives to increase applicability of O and H isotope distributions in recharge studies of shallow karst aquifer systems. The focus on shallow karst aquifers results from the time scale of recharge events in karst aquifers where interpretations may be more limited by older, monthly-composite data. In addition, the low porosity and high vertical hydraulic conductivity found in many shallow karst aquifers results in rapid recharge with less mixing of stored vadose water. The proposed methodology emphasizes the variation in isotopic composition related to the amount, intensity, and temperature of precipitation events but also considers data accuracy and cost of collection. Early results helped refine techniques and confirmed expected isotopic variability among events. Additional review is sought to substantiate or modify methodological concepts and interpretations regarding the new data collection methodology.