IMPLICATIONS OF δ 18O AND δ2H STABLE ISOTOPES FOR RECHARGE TO THE HIGH PLAINS AQUIFER, NORTHWESTERN KANSAS

The High Plains aquifer (HPA) is one of the largest aquifers in the world and is critical for agricultural production in both the United States and Kansas. Due to extensive groundwater mining, water levels have been declining in the western part of the Kansas HPA. A recent study by the Kansas Geologic Survey revealed unexpected increases in water levels in the northwestern Kansas portion of the HPA not associated with precipitation or pumping, indicating a previously unknown source of inflow to the aquifer. This investigation uses physical and geochemical methods to identify the source, and quantify the amount, of recharge to the HPA in 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.