Impacts of agricultural irrigation recharge on groundwater quality in a basalt aquifer system (Washington, USA): A multi-tracer approach

Irrigation in semi-arid agricultural regions can have profound effects on recharge rates and the quality of shallow groundwater. This study coupled stable isotopes ( ², ¹⁸O), age-tracers ( ³H, CFCs, ¹⁴C), ⁸⁷Sr/ ⁸⁶Sr ratios, and elemental chemistry to determine the sources, residence times, and flowpaths of groundwater and agricultural contaminants (e. g. NO ₃ ^-) in the Saddle Mountains Basalt Aquifer in central Washington, USA, where over 80% of the population depend on groundwater for domestic use. Results demonstrate the presence of two distinct types of water: contaminated irrigation water and pristine regional groundwater. Contaminated irrigation water has high NO ₃ ^-concentrations (11-116 mg/l), ⁸⁷Sr/ ⁸⁶Sr ratios (0.70659-0.71078) within range of nitrogen-based fertilizers, detectable tritium (2.8-13.4 TU), CFC ages 20-40 years, high δ ¹⁸O values (-16.9 to -13.5‰), and ~100 percent modern ¹⁴C. Pristine regional groundwater has low NO ₃ ^-concentrations (1-5 mg/l), no detectable tritium (≤0.8 TU), low δ ¹⁸O values (-18.9 to -17.3‰) and ¹⁴C ages from ~15 to 33 ky BP. Nitrogen and oxygen isotopes of NO ₃ ^-,combined with high dissolved oxygen values, show that denitrification is not an important process in the organic-poor basalt aquifers resulting in transport of high NO ₃ ^- irrigation water to depths greater than 40 m in less than 30 years.