Aquifer-storage monitoring at Tucson Water’s Heritage Project
Tucson Water’s Santa Cruz River Heritage Project releases up to 3,150 acre-feet a year of reclaimed water into the Santa Cruz River channel near downtown Tucson, AZ, much of which is expected to recharge to the subsurface through the channel bottom. Tracking the movement of recharged water is necessary to limit the dispersal of that water within specific target boundaries, and to enable resource managers to plan for and understand the impacts of the recharge.
Although the volume of water being stored can be readily estimated, the distribution of that water, once recharged, depends on the underlying characteristics of the aquifer and on the pattern of recharge. An equivalent water-level change in different wells may represent a large change in stored water if the specific yield is large, or a small change in stored water if the specific yield is small. 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 both 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. Also, in contrast to water-level measurements, which are measured at a single point that may or may not be representative of the aquifer, the region of sensitivity of a gravity measurement is well defined by Newton’s Law and encompasses a large volume of the subsurface. This enables improved interpolation between measurement locations and improved maps of aquifer storage change. These maps can help water managers track the storage, movement and dispersal of recharged water in time.
Gravity data at approximately 60 locations (exact number varies as stations are lost or replaced) along the Santa Cruz River channel from about 22nd street to Grant (see illustration below) are monitored annually to assess aquifer-storage change resulting from in-channel recharge. Preliminary results from this project are published in a data release, which includes gravity values for several surveys from March 2020 to March 2021. An interpretive report including results to March 2022 is scheduled to be published in 2023.
Tucson Water’s Santa Cruz River Heritage Project releases up to 3,150 acre-feet a year of reclaimed water into the Santa Cruz River channel near downtown Tucson, AZ, much of which is expected to recharge to the subsurface through the channel bottom. Tracking the movement of recharged water is necessary to limit the dispersal of that water within specific target boundaries, and to enable resource managers to plan for and understand the impacts of the recharge.
Although the volume of water being stored can be readily estimated, the distribution of that water, once recharged, depends on the underlying characteristics of the aquifer and on the pattern of recharge. An equivalent water-level change in different wells may represent a large change in stored water if the specific yield is large, or a small change in stored water if the specific yield is small. 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 both 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. Also, in contrast to water-level measurements, which are measured at a single point that may or may not be representative of the aquifer, the region of sensitivity of a gravity measurement is well defined by Newton’s Law and encompasses a large volume of the subsurface. This enables improved interpolation between measurement locations and improved maps of aquifer storage change. These maps can help water managers track the storage, movement and dispersal of recharged water in time.
Gravity data at approximately 60 locations (exact number varies as stations are lost or replaced) along the Santa Cruz River channel from about 22nd street to Grant (see illustration below) are monitored annually to assess aquifer-storage change resulting from in-channel recharge. Preliminary results from this project are published in a data release, which includes gravity values for several surveys from March 2020 to March 2021. An interpretive report including results to March 2022 is scheduled to be published in 2023.