USING DEUTERIUM EXCESS TO ASSESS SEASONAL TIMING OF GROUNDWATER RECHARGE TO AQUIFERS IN ARKANSAS, USA

Arkansas ranks fourth in the United States for groundwater use. Characterizing groundwater recharge to providing aquifers is paramount to quantifying and managing resources. The nature, timing, and rates of groundwater recharge are difficult to assess. Direct and indirect assessment results often have errors in excess of 30% and may provide little information on timing; however, isotopic data—particularly isotopic compositions of water (i.e. δ²H and δ¹⁸O) and deuterium excess (d) among local precipitation, streams, and groundwater—can greatly aid in estimating the timing of groundwater recharge as well as better constrain recharge rates. To date, no studies have analyzed δ²H, δ ¹⁸O, and d-values for precipitation, streams, and groundwater across Arkansas with regard to groundwater recharge. Precipitation isotope data and calculated d-values provided by USNIP and the University of Arkansas Stable Isotope Facility were used in conjunction with modeled isoscapes (www.waterisotopes.org) and compared with stream and groundwater isotopic compositions downloaded from the National Water Information System (NWIS). Bimodal d-values distributions were observed for both measured and modeled precipitation isotopic data. Moreover, while inter-annual δ²H and δ¹⁸O variation was relatively low, d-values exhibited a range of approximately 7 ‰, annually. Groundwater d-values were found to be most similar to winter and spring precipitation d-values (i.e. November through April); groundwater and winter/spring precipitation values being about 11 ‰. Streams with small catchment areas are also were noted to exhibit d-values similar to groundwater values. Most groundwater recharge in Arkansas occurs during winter and spring time periods when evapotranspiration is lower and precipitation amounts are higher. A small number of d-value outliers of groundwater were observed and may represent areas of faster flow paths; however, a much higher temporal resolution dataset is needed to evaluate this. This assessment of recharge timing to groundwater in Arkansas and could aid modeling of local aquifers by comparing isotope results with existing numerical recharge models. In areas of relatively low inter-annual δ²H and δ¹⁸O variation, d-values may provide an efficient method to characterize groundwater recharge timing.