Long-term (1970s–2016) changes in groundwater geochemistry in the High Plains aquifer in south-central Kansas, USA

Changes in groundwater chemistry in the Great Bend Prairie aquifer, a portion of the High Plains aquifer in south-central Kansas (USA), were studied in order to better understand factors influencing groundwater quality and aquifer sustainability. To assess changes, groundwater samples from 22 monitoring wells were analyzed during 2016. Results were then compared to data obtained previously from the same wells in the 1970s and 1980s. Of the wells sampled, 13 wells were screened near the water table (average depth 22 m) and 9 wells were screened near the aquifer base (average depth 41 m). Nitrate levels in 2016 were higher for 20 of 21 wells with data available for comparison. The average increase for shallow-aquifer and aquifer-base samples was 9.5 (standard deviation, SD, 12.9) and 3.4 (SD 3.1) mg/L as N, respectively. Nitrate isotope ratios (δ15N-NO3 and δ18O-NO3) of the 2016 samples are consistent with nitrification of ammonium-based fertilizers as the nitrate source with potential contributions from animal waste. Total dissolved solute levels were also higher in samples from nine of 12 shallow-aquifer wells and four of eight aquifer-base wells, with average increases of 191 (SD 238) and 194 (SD 133) mg/L, respectively. Taken together, the results demonstrate that water quality has decreased considerably over the past 40 years primarily because of fertilizer use, but that groundwater mixing, evapotranspiration, and potentially animal waste inputs also affected groundwater chemistry. These findings help identify the scale of water-quality degradation in the High Plains aquifer.