Abstract
Irrigation in semi-arid agricultural regions can have profound effects on recharge rates and the quality of shallow groundwater. This study coupled stable isotopes (2Η, 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. NO3–) 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 NO3– 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 δ18O values (−16.9 to −13.5‰), and ∼100 percent modern 14C. Pristine regional groundwater has low NO3– concentrations (1–5 mg/l), no detectable tritium (≤0.8 TU), low δ18O values (−18.9 to −17.3‰) and 14C ages from ∼15 to 33 ky BP. Nitrogen and oxygen isotopes of NO3–, 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 NO3– irrigation water to depths greater than 40 m in less than 30 years.
Published Version
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