An evaluation of artificial groundwater recharge as a conjunctive use approach to mitigating surface water depletions in the Bitterroot Valley, Montana

Increasing dependence on groundwater in unconfined and semi-confined aquifers has the potential to produce significant surface-water depletion impacts that can extend well beyond the irrigation season (off-season). A conjunctive use approach that uses artificial groundwater recharge to mitigate off-season surface-water depletions is one potential method for resolving some water use impacts that arise from changing the patterns of water use that are widespread in many western states.;Squaw Creek is a small stream in the Bitterroot Valley, MT, that separates the undeveloped Bitterroot River flood plain from the adjacent upland area where increasing development is taking place. Consequently Squaw Creek is susceptible to depletion from groundwater withdrawals in this rapidly developing area. The small size of the stream and the site characteristics make this an ideal location to investigate the feasibility of artificial recharge as a means to facilitate off-season mitigation within an intermontane basin.;This investigation involved field studies to establish the hydrogeologic characteristics of the site, a field test to monitor the response of the groundwater and surface-water systems to recharge, and a numerical modeling analysis to assess the timing and magnitude of recharge effects under a range of recharge scenarios.;Groundwater levels were monitored in piezometers, and surface-water stage and flows were monitored in Squaw Creek to document existing flow conditions and evaluate groundwater and surface-water interactions. Aquifer transmissivity and streambed permeability were established through grain-size analyses and permeameter tests. An artificial recharge pilot test was conducted over a two-week period to assess the amount of water the aquifer was capable of storing and the ability of Squaw Creek to receive the infiltrated water. Results from the field investigation and pilot test indicate that the shallow aquifer in the vicinity of the infiltration pond is able to receive a significant amount of water; however the amount of water that can be stored was found to be limited due to the highly transmissive characteristics of the aquifer.;As a result of these characteristics the majority of the injected water discharged back to Squaw Creek within hours of its infiltration. The close proximity of the infiltration area used in the pilot test caused piping through the streambed, resulting in significantly more groundwater being discharged to the aquifer than what was recharged. This piping affect suggests the proximity of the recharge area must be considered in evaluating recharge areas. A numerical model was developed to evaluate the effects of artificial groundwater recharge over a longer time period under a range of recharge scenarios and assess the capability of artificial recharge to facilitate off-season mitigation on Squaw Creek. The modeling results indicate that the aquifer is capable of receiving more than 300 gpm of artificial recharge over an irrigation season; however the transport of the water to the stream is rapid due to the high transmissivity of the shallow aquifer. As a consequence, induced recharge could only offset a portion an average of 202 gpm of the 300 gpm depletion effects from off-season use. Induced groundwater storage, however, has the ability to facilitate off-season mitigation in areas where there is a significant distance to the nearest surface-water body, or when the off-season depletions are of limited duration (i.e. residual off-season drawdown effects due to irrigation season pumping). Artificial recharge in highly transmissive aquifers (typical of those found in intermontane basins) may be successful in scenarios where depletions are relatively low, significantly more recharge is applied than is depleted by groundwater use, or when there is a large distance between the stream and the recharge area.;Keywords: Conjunctive use, groundwater/surface water interaction, artificial recharge, off-season mitigation.