Monitoring aquifer-storage change at Tucson Water's Southeast Houghton Area Recharge Project

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The Arizona Water Science Center is monitoring aquifer-storage changes at an artificial recharge facility operated by Tucson Water in southeast Tucson. Aquifer-storage change is monitored by measuring changes in gravity over time at the same network of benchmarks. As water is added or removed from the aquifer, there is a change in mass and a corresponding measurable change in gravity.

Tucson Water currently operates several artificial recharge projects in the Tucson Active Management Area, recharging both Colorado River water delivered via the Central Arizona Project (CAP) canal, and reclaimed wastewater. A new recharge facility, the Southeast Houghton Area Recharge Project (SHARP) has been constructed to permit storage of an additional 4,000 acre-feet per year of reclaimed water by the utility. Tracking the movement of recharged water is necessary to assess the dispersal of that water within specific target boundaries, and to enable resource managers to plan and develop locations of water supply wells that will utilize the recharged water.

The SHARP facility is located in a largely undeveloped area currently used for recreation, with relatively few wells nearby (figure 1). Installing additional monitoring wells would be expensive, and previous studies in the Southwest have shown that water levels in wells alone do not always provide a complete estimate of aquifer-storage change due to aquifer heterogeneity, well construction, and proximity to recharge sources (Pool, 2008). Repeat microgravity is the only geophysical method that overcomes the limitations of groundwater-level monitoring by providing a direct, quantitative measurement of changes in aquifer storage.

Map showing location of the Southeast Houghton Area Recharge Project (SHARP) in the Tucson Active Manage Area, AZ.

Figure 1. Location of the Southeast Houghton Area Recharge Project (SHARP) facility and surrounding area within the Tucson Active Management Area, Arizona.

The repeat microgravity method, developed primarily at the University of Arizona and the U.S. Geological Survey (USGS) Arizona Water Science Center, is an established method for monitoring aquifer-storage changes in alluvial basins. The method has successfully been used to track the dispersal of water recharged through the channel of an ephemeral stream in Tucson, Arizona (Pool, 1997). Gravity monitoring of a spreading-basin artificial recharge facility in Avra Valley, Arizona, showed preferential groundwater flow, as storage accumulated to the west of three recharge basins but not to the east (Kennedy and Ferré, 2016). Also, at that facility, gravity data proved useful for identifying hydraulic conductivity and storage parameters in a MODFLOW groundwater model (Kennedy and others, 2016). The method is presently in use by the USGS Southwest Gravity Program for several projects across New Mexico, Arizona, and California. Maps of storage change from microgravity measurements can help water managers track the storage, movement and dispersal of recharged water in time. The spatial and temporal resolution of the mapped storage changes can be largely controlled by monitoring station density/distribution, and by the timing of repeat measurements.

This project is designed to provide information about the magnitude and location of aquifer-storage changes associated with artificial recharge at Tucson Water's SHARP facility. The proposed monitoring network and timing of surveys is designed to identify storage changes occurring during active recharge cycles and over several years of seasonal recharge within the focus area (figure 2).This  project will provide Tucson Water managers with an improved temporal and spatial understanding of aquifer-storage changes occurring near the SHARP facility. This information is useful for quantifying movement of recharged water, and for evaluating the performance of the SHARP facility.

Map showing location of the Southeast Houghton Area Recharge Project (SHARP) in the Tucson Active Manage Area, AZ.

Figure 2. Proposed gravity network within focus/study area. Final locations dependent on existing infrastructure and access to undeveloped areas. Proposed gravity locations that partially overlap wells would be located on well pads, where possible. Two of the wells within the facility (NW and SE corners) would be located on the well pads of existing monitoring wells.


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