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)Iu, O-18), age-tracers (H-3, CFCs, C-14), Sr-87/Sr-86 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), Sr-87/Sr-86 ratios (0.70659-0.71078) within range of nitrogen-based fertilizers, detectable tritium (2.8-13.4 TU), CFC ages 20-40 years, high delta O-18 values (-16.9 to -13.5aEuro degrees), and similar to 100 percent modern C-14. Pristine regional groundwater has low NO (3) (-) concentrations (1-5 mg/l), no detectable tritium (a parts per thousand currency sign0.8 TU), low delta O-18 values (-18.9 to -17.3aEuro degrees) and C-14 ages from similar to 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.