A MULTI-TRACER APPROACH TO DETERMINE THE IMPACTS OF AGRICULTURAL IRRIGATION RECHARGE ON GROUNDWATER QUALITY IN THE SADDLE MOUNTAINS BASALT AQUIFER, COLUMBIA BASIN, WASHINGTON

Irrigation in semi-arid agricultural regions has profound effects on recharge rates and water quality of shallow groundwater. In the case of oxic groundwater systems, such as the Flood Basalt aquifers of the western U.S., high nitrate (NO₃^-) concentrations resulting from extensive fertilizer use may persist for decades or longer owing to the absence of denitrification. Stable isotope signatures (δ²H, δ¹⁸O) were used in conjunction with age-tracers (³H, CFCs, ¹⁴C), strontium isotopes (⁸⁷Sr/⁸⁶Sr), and elemental chemistry to determine the sources, residence times, and flowpaths of groundwaters and agricultural contaminants (e.g. NO₃^-) in the Saddle Mountains Basalt Aquifer in central Washington. Results demonstrate the presence of two distinct waters: contaminated irrigation waters and pristine regional groundwaters. Contaminated irrigation waters have 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 to 40 yrs, high δ¹⁸O values (-13.5‰ to -16.9‰), and ~100 percent modern carbon (pmc). Pristine regional groundwaters have low NO₃^- concentrations (1-5 mg/l), no tritium, low δ¹⁸O values (-17.3‰ to -18.9‰) and ¹⁴C ages from ~15 to 33 k yrs BP. Nitrogen and oxygen isotopes of NO₃^-, combined with high dissolved oxygen values, confirm that denitrification is not an important process in the organic-poor basalt aquifers resulting in transport of high NO₃^- irrigation waters to depths greater than 40 m in less than 30 years. The persistence of agricultural nitrates in the Saddle Mountains Basalt Aquifer is of water quality concern as populations in the region are dependent on groundwater resources for domestic water supply.