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Surface infiltration and unsaturated zone characterization in support of managed aquifer recharge in Bedell Flat, Washoe County, Nevada

December 30, 2021

Aquifer storage and recovery (ASR) expands the portfolio of public water supply and improves resiliency to drought and future water demand. This study investigated the feasibility of ASR in the Bedell Flat Hydrographic Area using land-based methods including in-channel managed aquifer recharge (MAR) and rapid infiltration basins (RIB). Bedell Flat, one of two flow-through groundwater basins near Reno, Nevada, was a likely candidate for ASR because of its deep basin fill, proximity to supplemental water sources and infrastructure, and lack of development. In-channel MAR feasibility was determined from seepage losses along the Bird Springs ephemeral channel measured using Parshall flumes and heat-as-a-tracer inverse modeling. The feasibility of RIB was evaluated by characterizing vadose zone boreholes installed with roto-sonic drilling to water table. Field characterization of sediment and lithologic descriptions was accomplished at 1-foot (ft) increments. Bulk sediment samples were collected every 5 ft and cores from a split spoon were sampled at 10, 20, 30, 40, 60 and 100 ft below land surface (bls). Collected samples were analyzed for texture, moisture content, and geochemistry.

Infiltration rates in Bird Springs channel increased downgradient with the hydraulic conductivity of the upper reaches ranging from 0.002 to 0.14 meter per hour (m/h) and the lower reaches from 0.5 to 1.5 m/h. Differences in discharge measurements indicate that seepage losses also increase down channel. When normalized to 1 ft of channel stage, modeled seepage loss rates ranged from 0.02 to 5.34 cubic feet per second (ft3/s) per mile (mi). Perched zones of soil moisture residing on top of dry fine-textured, clay-rich layers were prevalent in the Bird Springs drainage, indicating complicated flow paths for any supplement recharge water. Characterization of boreholes in Bird Springs drainage indicates low permeability clay layers 1–10 ft thick interbedded within extensive, grussy sands of high permeability. The presence of low permeability clay layers (1–10 ft thick) prompted a shift in analysis to the adjacent Sand Hills drainage where four additional boreholes indicated fewer perched water zones and at greater depths. Nitrates in the sediment pore-water (a condition that would discourage ASR) were integrated with depth to the aquifer 180 ft bls. Wells BF-MW-04 (Bird Springs) and BF-MW-07 (Sand Hills) contained 4,270 and 2,436 kilograms per hectare of nitrogen, respectively, which could potentially load excessive nitrogen to a receiving aquifer.

An economically viable ASR project requires a minimum input of 2 million gallons per day (approximately 3 ft3/s), which either channel appears to have the sufficient capacity to infiltrate such a volume before reaching the valley bottom of Bedell Flat. However, the trajectory of the infiltrated water is complicated by the lithology and lateral transmissivity of the underlying sediments. There is also concern that this volume of infiltrated water may cause undesirable groundwater levels at the outflow in less than 5 years.