CONNECTING PRECIPITATION AND SHALLOW GROUNDWATER IN NORTH WEST ARKANSAS: GEOCHEMICAL AND ISOTOPIC APPROACHES

Compilation of water isotope data is a valuable tool for understanding moisture sources, groundwater recharge periods and connectivity of groundwater to springs and streams. Public water supplies in northwest Arkansas primarily draw on Beaver Lake, a reservoir lying in a karst region where intimate connection between surface water and groundwater strongly influences watershed hydrology; however, studies characterizing linkage between groundwater and surface water systems in the watershed are limited. For this study, modeled and measured precipitation isotope data were compared with groundwater isotope data in Northwest Arkansas. Modeled average monthly precipitation δ²H and δ¹⁸O values show strong seasonal trends, becoming progressively less negative from January to May (with δ²H values going from -104 to -14 and δ¹⁸O values going from -13.8 to -2.2) and more negative from June to December (with δ²H values going from -29 to -106 and δ¹⁸O values going from -4.2 to -14.2). Groundwater values showed little variability with δ²H values ranging from -33 to -41.6 and δ¹⁸O values ranging from -5.8 to -6.9. Precipitation values for March-April and August-October were most similar to the median groundwater values of -36.25‰ and -6.15‰, for δ²H and δ¹⁸O, respectively. This suggests these time periods as likely being the predominant groundwater recharge periods. Alternatively, groundwater values may represent an averaging of annual precipitation values. Spring data were similar, but tended to show more positive δ²H and δ¹⁸O values, which may indicate greater exposure to evaporation for the shallow spring flow paths. Further isotopic data collection for the region would greatly aid in watershed budgeting methods as well as contribute to knowledge of the overall hydrologic system.