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.
Basalt flows of the Columbia River Basalt Group (CRBG) host a series of regionally extensive aquifers between western Idaho and the Pacific Ocean that serve as an important source for domestic, municipal, agricultural, and industrial water supply throughout much of this area, and are the sole source for some communities in the Willamette Valley. Rapid growth and increased pumping have resulted in significant water level declines in some locales in the Willamette Valley, forcing some communities to develop other water sources, and/or develop aquifer storage and recovery projects to store water in CRBG aquifers.
The CRBG generally consists of multiple concordant, tabular sheet flows. The primary water-bearing horizons within the CRBG are associated the vesicular and/ or brecciated flow top and flow bottom (pillow/hyaloclastite) structures that form the interflow zone between two flows. The interiors of the CRBG flows typically have limited vertical permeability and act as aquitards, creating a series of layered confined aquifers. The dominant groundwater flow pathway in the CRBG aquifer system is along these individual, laterally extensive, interflow zones. Tectonic structures may modify the dominant flow regime in the CRBG by offsetting or otherwise disturbing originally laterally continuous interflow zones. Faults result in a wide spectrum of effects on flow in the CRBG aquifers depending on the nature of the fault.
The hydraulic properties inherent to CRBG aquifers, including high degree of confinement, low bulk permeability and limited recharge have led to overdraft conditions in many areas. Conversely, these characteristics create favorable conditions for aquifer storage and recovery system development in the central Willamette Valley and Tualatin Basin.
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