Modeling a Recharge Mound and Identifying Groundwater-Flow Parameters at a Managed Aquifer Recharge Facility Using Repeat Microgravity Data

To better understand the distribution and fate of recharged water at an artificial recharge facility, a network of 16 repeat microgravity stations was established at the South Houghton Artificial Recharge Project in Tucson, Arizona. The Project consists of 3 basins, each approximately 9,000 m2, collectively permitted to recharge up to 4.6 million m3 per year of treated effluent at a previously undeveloped desert location. This was the first known project where gravity data were collected prior to the initial recharge cycle, rather than after recharge had begun. Repeat microgravity is an efficient method to monitor recharge because measurements are made non-invasively at the land surface. Groundwater-storage changes measured by this method are useful on their own, using the Bouguer slab assumption to convert the change in gravitational acceleration to the change in groundwater storage, but additional information can be extracted through three-dimensional modeling of recharge through the unsaturated zone. We use Hydrus 3D to simulate the gravity response from alternative conceptual models that incorporate perching layers and various storage and conductivity properties. Pre-recharge estimates based on borehole logs suggested there were no perching layers and infiltrating water would move straight down, but the results of this study show horizontal dispersal and uneven infiltration among the 3 basins, possibly related to both recharge application patterns and spatial heterogeneity of subsurface properties.