Nitrate Concentration Patterns Over Space and Time in a Regionally Sloping Sedimentary Aquifer, Texas, USA

Nitrate concentrations in a regionally sloping sedimentary aquifer were compiled every 4 years from 1999 to 2015. Sampled wells ranged from 14.6 m deep in the outcrop zone to 1031.4 m deep in the confined zone, with a median depth of 192.1 m. Approximately 3.6% of 138 samples collected in 2015 exceeded the drinking water standard of 44.3 mg/L; while low, this percentage was highest among five sampling years. All observed exceedances were in relatively shallow wells in the outcrop zone. Generally, the spatial pattern of nitrate occurrence persisted through time. Shallow wells tended to have higher nitrate concentrations, in addition to larger increases or decreases in concentration over time. Maximum concentrations ranged from 30.5 mg/L in 1999 to 100.5 mg/L in 2015. Over any 4-year period, the maximum concentration increase was 42.1 mg/L, and the maximum decrease was 41.0 mg/L, both occurring in wells in the outcrop zone. This study has land management implications for sloping sedimentary aquifers. Practices that alter nitrogen inputs and dilution processes in unconfined zones may produce large fluctuations in nitrate concentration over short time periods.