65 Comparison of Microgravity Geodetic Observations with Hydrologic Observations in Salt Lake Valley

Faculty Mentor: Tonie van Dam (Geology and Geophysics, University of Utah)

Like many other population centers in the Western U.S., the Salt Lake Valley is experiencing increased water stress due to population growth and climate change. As the population of Salt Lake County is projected to increase from 1.2 million to 1.33 million by 2060 (Thiriot, 2020) and climate warming is expected to continue (IPCC, 2023), a better understanding of groundwater storage and availability in the Wasatch Mountain Block is critical for developing effective water management strategies.

Groundwater storage typically exhibits a delayed response to anthropogenic stressors and climatic factors, with observable trends varying in length from seasonal to decadal (Gyawali et al., 2022). As a result, quantifying this storage can prove challenging. Groundwater monitoring is generally carried out via two primary approaches. Traditional hydrological measurements (e.g. stream discharge, depth to groundwater, precipitation) are effective surface water metrics but are prone to large amounts of error when used to indirectly estimate groundwater storage. Groundwater quantities can also be estimated based on water budget residuals, but these results tend to be unreliable as they depend on compounding assumptions and insufficient data (Safeeq et al., 2021). In this study, we suggest that microgravity presents an opportunity to directly measure and quantify groundwater storage changes in the Wasatch Mountain Block.

During this study, microgravity measurements were taken at 7 locations in Red Butte Canyon, a snowmelt-dominated headwater encatchment in the Salt Lake Valley (see Figure 1). Sites were chosen based on co-location with previously existing instrumentation in the canyon such as NSF NEON wells, passive seismic nodes, discharge measurement sites, and flow presence sensors. These measurements were taken using a Scintrex CG-6 Gravimeter, providing μGal precision. Concrete pads were installed at observation sites to ensure consistency between measurements. Data was corrected for tidal anomaly using onboard estimations, as well as instrumental drift using two base stations located at the University of Utah, measured before and after each survey. Before plotting, the mean was removed from both datasets.

Figure 2 depicts normalized and corrected gravity data, as well as Red Butte Creek discharge measurements obtained by the USGS at a monitoring site near Fort Douglas. Precipitation events are also indicated on this chart. Gravity measurements across all stations show a strong temporal correlation with stream discharge. Both datasets show peaks correlating with precipitation events, with gravity showing a longer temporal lag after these occurrences. These increases disrupt the standard decaying discharge curve that is expected during summer months where snowmelt is almost complete and contributes little input to surface and groundwater. Variability between gravity stations exhibits a correlation with elevation in the canyon. This may suggest a spatial lag in hydrological response or a variation in gravitational acceleration due to a lack of terrain or elevation corrections.

The observed temporal correlation between gravity observations and discharge measurements supports the viability of gravity as a means of monitoring groundwater storage and availability. Gravity and discharge both increase in response to precipitation events, but gravity data may capture seasonal or long-term trends that stream discharge alone does not reflect. To better assess the strengths of gravity for groundwater monitoring, we will continue to collect data over a longer temporal span.

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