IMPLICATIONS OF δ 18O AND δ2H STABLE ISOTOPES FOR RECHARGE TO THE HIGH PLAINS AQUIFER, NORTHWESTERN KANSAS
A complete core profile was obtained through the 65 m thick unsaturated zone above the HPA using a hollow stem auger. Core tubes were carefully sealed to retain moisture. A variety of techniques are being utilized to extract water from the core to construct water content, Cl, NO3 and (δ2H/δ18O) profiles. The geochemical and isotopic data will be compared with that for the underlying saturated zone. Water samples collected from a monitoring well and five irrigation wells in the area were analyzed for chemistry and stable isotopes (δ2H/δ18O). Isotopic δ18O values were between -11.4 ‰ to -11.2 ‰ VSMOW and δ2H values between -74 ‰ to -79 ‰ VSMOW. These data plot near the Global Meteoric Water Line, with no evidence of evaporation, and are isotopically lighter than water collected from southern areas of the HPA. This is consistent with an overall northward trend of heavy isotope depletion within the aquifer. This north-south geographical trend could be related to variations in average annual temperatures throughout the High Plains region, or a rain out effect related to distance from the moisture source affecting isotopic compositions within precipitation. The lack of evaporative isotopic signatures at the field site indicates a precipitation source for HPA water that has not been significantly affected by recent anthropogenic activities.
Comparison of geochemical and isotopic data between the saturated and unsaturated zone will aid in the characterization and identification of the unknown recharge. Source identification will help in the assessment of continued use of this area as a water source for irrigated agriculture.