Rapid Denitrification in a Deep Basalt Aquifer: Implications for Aquifer Storage and Recovery

摘要

Aquifer storage and recovery (ASR), typically utilized as a municipal water management technique, also can provide a means of storing water for irrigation in agricultural areas where water availability is limited. The Umatilla Basin in north central Oregon receives fewer than 20 inches of annual precipitation. The basin is underlain by lavas of the Columbia River Basalt Group (CRBG), highly transmissive confined aquifers with low recharge rates. Years of overdrafting aquifers to irrigate agriculture land have depleted the CRBG and further groundwater use has been restricted and locally curtailed. ASR operations in the Umatilla Basin can provide needed irrigation water and supply recharge to the depleted aquifer. Two large agricultural operations in the Umatilla Basin began using ASR in 2006 to provide water for irrigation. Source water for ASR storage is pumped from a seasonally available shallow alluvial aquifer, which has variable nitrate concentrations ranging between <3 mg/L to >9 mg/L. Per Oregon ASR rules, nitrate concentrations in injection source water are limited to a maximum of 7 mg/L. Nitrate in the recovered water, however, decreases quickly to < 1 mg/L, suggesting that nitrate may not persist within the CRBG aquifer used for ASR storage. In contrast to nitrate, other constituents in the recovered water (e.g., calcium, sodium, chloride, and potassium) show little variation, inconsistent with either migration or mixing as an explanation of the nitrate decrease. Nitrogen isotopic ratios (?~(15)N) increase markedly, ranging from +3.5 to > +50, and correlate inversely with nitrate concentrations. This variation occurs within a period of < 3 weeks and recovery of < 10% of the originally injected volume. These observations are consistent with rapid denitrification, suggesting that nitrate, and therefore ASR, pose little risk to water quality in the CRBG aquifer.