Aquifer storage and recovery (ASR) is the artificial recharge of water into an aquifer, where it is stored for later withdrawal. Water is stored during relatively “wet” periods and is used later during periods of drought and high water demand. Artificial recharge is generally achieved either by ponding water in surface basins, where it can seep into the soil and infiltrate into the aquifer (surface spreading), or by injecting the water directly into the aquifer through a well (injection). Storing water in the subsurface prevents evaporative loss common with surface reservoirs, and allows for fewer infrastructure and space requirements than surface reservoirs.
Many entities in Utah, including the Weber Basin Water Conservancy District, Leamington Town, Washington County Water Conservancy District, Brigham City, and the Jordan Valley Water Conservancy District have active ASR projects. Using experience from previous collaborative ASR projects, the UGS has recently evaluated the potential for ASR in Cache Valley.
Cache County expressed interest in ASR projects to provide the county with increased flexibility in managing groundwater resources. Although Cache Valley has an abundance of water resources relative to other areas in Utah, agricultural activities and a growing population create a demand for a well-managed hydrologic system. Excess spring runoff can augment groundwater resources through artificial groundwater recharge as part of one or more ASR projects. A provision by the Utah Division of Water Rights requires the forfeiture of a water right if not used for seven years. Due to redevelopment of agricultural lands, groundwater rights for those lands are forfeited due to nonuse. ASR would allow Cache County to secure the water rights through water banking before they are forfeited to other entities. The UGS proposed to establish an ASR project in multiple phases to (1) locate areas that would likely be suitable for ASR, (2) determine the suitability of selected sites via more in-depth investigation, (3) design and implement an ASR pilot project, and (4) conduct a post-project investigation.
In 2011 the UGS, in cooperation with Utah State University emeritus professor Robert Oaks, Jr., documented the initial phase (site assessment) of the ASR investigation in UGS Open-File Report 579. Potentially suitable areas had a downward hydrologic gradient, lack of laterally continuous clay confining units, available land for use, and available water source(s) for infiltration. A clay confining layer above the principal aquifer, coupled with the near-surface extent of the relatively low-permeability Salt Lake Formation, limits the potential surface-spreading sites to a narrow band along the eastern mountain front of Cache Valley, between Smithfield and Hyrum. Three sites were identified as adequate for ASR, including a surface-infiltration sand pit east of Providence, a surface-infiltration gravel pit east of North Logan at the mouth of Green Canyon, and a possible injection well in Logan.
The 2011 water year was relatively wet, and melting of the winter snowpack produced a significant flow of surface water from the usually dry mouth of Green Canyon. During June 2011, North Logan City diverted water into impoundmentponds created in the gravel pit area adjacent to the mouth of Green Canyon to accommodate excess flow. The water diversion provided an opportunity for us to monitor how well the site worked for ASR.
In August, the UGS investigated the infiltration at Green Canyon using a high-precision gravimeter to measure movement of the mass of infiltrating water in the subsurface. High-precision gravity data comprise an important tool that helps document the movement of groundwater that has infiltrated in recharge areas. Paul Gettings of the University of Utah helped create a radial network of gravity stations around the area of infiltration (gravity pit), at which the UGS measured the gravity during several measuring campaigns. These measuring campaigns were conducted in late August 2011, late September 2011, late October 2011, and early March 2012, measuring gravity at the same sites each time.
Computer software created by Paul Gettings enabled us to calculate changes in mass based on the very small changes in gravity observed between measurements. All changes in mass determined from the gravity changes were likely changes in the total mass of water at each gravity station.
We compared the gravity measurements to hydrologic measurements, including water levels in two wells, discharge of a spring, flow into the infiltration area, and elevation of the water in the infiltration area, to estimate the volume of infiltrated water. At least 2000 acre-feet (about 650 million gallons) of water infiltrated into the gravel pit from mid–June 2011 to late September 2011. Well water levels and gravity changes indicate groundwater recharge included contributions from regional infiltration,
subsurface flow, and infiltration into the gravel pit.
This study is an excellent opportunity to observe how the aquifer system near the mouth of Green Canyon reacts to large amounts of infiltrating water. The area is hydrologically complex, with a large number of inputs and outputs of water (see diagram). The ASR study in Cache Valley is ongoing, but we are confident that artificial recharge was occurring as a result of the water being diverted into the impoundment ponds. Cache County has interest in further developing this site as an area for ASR, which would consist of adding more flow control, a monitoring well, measuring water chemistry, and continued gravity measurements. Overall, if the county pursues ASR, they will have another tool to help them better manage water resources.