SPATIAL AND TEMPORAL VARIATION IN GROUNDWATER GEOCHEMISTRY IN THE HIGH PLAINS AQUIFER SYSTEM, SOUTHCENTRAL KANSAS

Groundwater from the High Plains aquifer system supplies nearly one-third of the irrigation water used in the U.S. and drinking water for millions of people. Understanding how groundwater quality is changing in response human activities and natural processes is critical to effectively managing this important resource. Our study considers variation in groundwater geochemistry in the Great Bend Prairie aquifer, a portion of the High Plains aquifer system composed of unconsolidated Quaternary deposits in southcentral Kansas. We collected samples during summer 2016 from 24 monitoring wells and compare our results to data collected previously from the same wells as early as the late 1970s. Of the wells we sampled, 13 wells were screened in the upper portion of the aquifer (avg. depth 22 m), 10 wells were screened near the aquifer base (avg. depth 43 m), and one well was screened in underlying bedrock. Samples from the upper aquifer tended to have lower temperature, pH, and total dissolved solids (TDS) content than samples from the aquifer base. Values averaged 16.4°C, 7.30, and 611 mg/L, respectively, for upper aquifer samples and 17.1°C, 7.42, and 4,415 mg/L for aquifer base samples. The bedrock groundwater sample had a temperature of 22.4°C, a pH of 7.58, and 282 mg/L TDS content, although much higher salinities are observed in groundwater from the bedrock in other portions of the field area. Compared to samples collected previously, samples we collected tended to have higher TDS and nitrate content, particularly those we collected from the upper aquifer. TDS was 211 mg/L higher in samples we collected from the upper aquifer and 12 mg/L lower in samples we collected from the aquifer base on average compared to previous samples. Nitrate concentrations were 9.5 and 3.9 mg/L as N higher on average in samples we collected from the upper aquifer and the aquifer base, respectively, compared to previous samples. Overall, nitrate concentrations exceeded the U.S. standard for drinking water (10 mg/L as N) in seven of the samples we collected, compared to only two of the samples collected previously. Our results indicate that water quality in the aquifer has degraded over the past four decades and that surface inputs from human activities and natural processes are the primary drivers.