Gaining streams can provide an integrated signal of relatively large groundwater capture areas. In contrast to the point-specific nature of monitoring wells, gaining streams coalesce multiple flow paths. Impacts on groundwater quality from unconventional gas development may be evaluated at the watershed scale by the sampling of dissolved methane (CH4 ) along such streams. This paper describes a method for using stream CH4 concentrations, along with measurements of groundwater inflow and gas transfer velocity interpreted by 1-D stream transport modeling, to determine groundwater methane fluxes. While dissolved ionic tracers remain in the stream for long distances, the persistence of methane is not well documented. To test this method and evaluate CH4 persistence in a stream, a combined bromide (Br) and CH4 tracer injection was conducted on Nine-Mile Creek, a gaining stream in a gas development area in central Utah. A 35% gain in streamflow was determined from dilution of the Br tracer. The injected CH4 resulted in a fivefold increase in stream CH4 immediately below the injection site. CH4 and δ(13) CCH4 sampling showed it was not immediately lost to the atmosphere, but remained in the stream for more than 2000 m. A 1-D stream transport model simulating the decline in CH4 yielded an apparent gas transfer velocity of 4.5 m/d, describing the rate of loss to the atmosphere (possibly including some microbial consumption). The transport model was then calibrated to background stream CH4 in Nine-Mile Creek (prior to CH4 injection) in order to evaluate groundwater CH4 contributions. The total estimated CH4 load discharging to the stream along the study reach was 190 g/d, although using geochemical fingerprinting to determine its source was beyond the scope of the current study. This demonstrates the utility of stream-gas sampling as a reconnaissance tool for evaluating both natural and anthropogenic CH4 leakage from gas reservoirs into groundwater and surface water.

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The southwestern United States (US) relies on the Colorado River to sustain its ecosystems, communities, and economies. The Colorado River provides irrigation water to nearly 4.5 million acres of land, generates over 4,200 megawatts of hydroelectric power, and supplies water to over 35 million people in the United States and 3.3 million people in Mexico each year (U.S. Bureau of Reclamation, 2012Reclamation, , 2017. Furthermore, it is estimated that the Colorado River supports over 16 million jobs, with an annual economic benefit of over $1.4 trillion (James et al., 2014).

These reports also contain monitoring-well and production-well completion information, as well as historical water-quality and precipitation data. The objectives of this report are to present and interpret (1) groundwater levels, reservoir altitude, well withdrawals, drain discharge, meteorological data, reservoir water temperature, and inflows/outflows from March 2002 through December 2011 for estimating monthly amounts of managed aquifer recharge from Sand Hollow Reservoir to the Navajo Sandstone, and (2) groundwater and surface water chemical data collected prior to the construction of the reservoir through March 2012 for evaluating groundwater flow paths and travel times of this managed aquifer recharge. This study is a cooperative effort by the Washington County Water Conservancy District (WCWCD) and the U.S. Geological Survey (USGS). Support for this work was provided by both the USGS and the WCWCD. From 2002 through 2011, total surface-water diversions of about 199,000 acre-ft to Sand Hollow Reservoir have allowed the reservoir to remain nearly full since 2006. Groundwater levels in monitoring wells near the reservoir rose through 2006 and then fluctuated more recently because of variations in reservoir water-level altitude and nearby pumping from production wells. Between 2004 and 2011, a total of about 19,000 acre-ft of groundwater was withdrawn by these wells for municipal supply. In addition, a total of about 21,000 acre-ft of shallow seepage was captured by French drains adjacent to the North and West Dams (fig. 2) and used for municipal supply, irrigation, or returned to the reservoir.