title: Denitrification in a deep basalt aquifer: implications for aquifer storage and recovery.
abstract: Aquifer storage and recovery (ASR) can provide a means of storing water for irrigation in agricultural areas where water availability is limited. A concern, however, is that the injected water may lead to a degradation of groundwater quality. In many agricultural areas, nitrate is a limiting factor. In the Umatilla Basin in north central Oregon, shallow alluvial groundwater with elevated nitrate-nitrogen of <3 mg/L to >9 mg/L is injected into the Columbia River Basalt Group (CRBG), a transmissive confined aquifer(s) with low natural recharge rates. Once recovery of the injected water begins, however, NO3 -N in the recovered water decreases quickly to <3 mg/L (Eaton et al. 2009), suggesting that NO3 -N may not persist within the CRBG during ASR storage. In contrast to NO3 -N, other constituents in the recovered water show little variation, inconsistent with migration or simple mixing as an explanation of the NO3 -N decrease. Nitrogen isotopic ratios (δ(15) N) increase markedly, ranging from +3.5 to > +50, and correlate inversely with NO3 -N concentrations. This variation occurs in <3 weeks and recovery of <10% of the originally injected volume. TOC is low in the basalt aquifer, averaging <1.5 mg/L, but high in the injected source water, averaging >3.0 mg/L. Similar to nitrate concentrations, TOC drops in the recovered water, consistent with this component contributing to the denitrification of nitrate during storage.
authors.affiliation: GSI Water Solutions, Inc., Portland, OR 97204.
authors.forename: Dennis
authors.initials: D
authors.lastname: Nelson
authors.forename: Jason
authors.initials: J
authors.lastname: Melady
mesh_terms: D057914:Agricultural Irrigation; D002244:Carbon / Q000032:analysis / Q000737:chemistry; D058440:Denitrification; D060587:Groundwater / Q000737:chemistry*; D009566:Nitrates / Q000032:analysis / Q000737:chemistry; D009922:Oregon; D009930:Organic Chemicals; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry; D060753:Water Quality
chemical_list: D009566:Nitrates; D009930:Organic Chemicals; D014874:Water Pollutants, Chemical; D002244:Carbon