North Phoenix aquifer monitoring with repeat microgravity

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The City of Phoenix has traditionally relied on surface-water supplies from the Salt, Verde, and Colorado River watersheds. To increase water-supply resiliency and flexibility, the City is expanding its Artificial Storage and Recovery (ASR) operations in the north Phoenix area. USGS is measuring small changes in gravity caused by groundwater-storage changes to better understand where and when recharged water is stored in the aquifer.


Photograph showing a relative gravity meter set up on a concrete slab.

Relative-gravity meter

Repeat microgravity surveys entail measuring how gravity changes over time. These surveys are useful for detecting gravity change caused by changes in groundwater storage as water is pumped from or recharged into an aquifer. The main advantages of the gravity method are that it provides a quantitative measurement of storage change, and applies in nearly all situations, regardless of depth to groundwater or aquifer porosity.

Two instruments are used for repeat microgravity surveys: the relative-gravity meter and the absolute-gravity meter. The highly portable relative-gravity meter is used to measure relative-gravity differences between stations. The absolute-gravity meter is used at a smaller number of stations to provide "benchmark values." These measurements are combined (post-processed), similar to height differences and elevations in a levelling survey. This combination of relative- and absolute-gravity meters provides the highest-possible accuracy and is a unique capability of the USGS Southwest Gravity Program.

Photograph showing an absolute gravity meter setup on a sidewalk.

Absolute-gravity meter

Beginning in 2020, approximately 50 gravity stations have been established and surveyed in the north Phoenix well field, in the area between Loop 101 and Carefree Highway, and Cave Creek and Scottsdale Roads. Additional stations will be established in winter 2021 and the entire network surveyed annually until 2023. During this period additional ASR wells will begin operation in the aquifer.

Based on the size of the study area and depth-to water (which determines the "sensing radius" of a repeat microgravity measurement), approximately 80 gravity stations will be established in total. Because measurements at each station cover a relatively large region (each station is sensitive to an area with a radius of about 10 times the depth to water), the monitoring network will effectively encompass the entire aquifer(s) within the study area. This allows the total volume of storage, in acre-feet, to be interpolated and estimated between each gravity survey.

Repeat microgravity provides an integrative measurement of all subsurface water content changes, including soil moisture changes that may not contribute to recharge. Therefore, a cosmic-ray soil moisture sensor (COSMOS) probe is used to monitor soil-moisture changes at absolute-gravity stations. These measurements, collected during each survey, are used to estimate and remove the gravity signal caused by soil-moisture change that doesn't lead to recharge.

Changes in land-surface elevation will be monitored using survey-grade GPS. Because gravity is affected by mass and distance, a change in elevation (that is, distance from the Earth's center of mass) will cause a change in gravity. A GPS campaign will be carried out during the first and last years of the monitoring and the data will complement remotely sensed elevation changes monitored using satellites (InSAR).

Map of micro gravity station network, Phoenix, AZ.

Map of Phoenix micro gravity station network.

Relevance and Benefits

Aquifer-storage monitoring is important for safe, efficient utilization of the groundwater-storage resource, yet is inherently difficult, often owing to the scarcity of monitoring wells. Repeat microgravity can provide information similar to that from wells, using passive instruments at the land surface, at a fraction of the cost and effort. Storage changes mapped using repeat microgravity provide water-resource managers an additional tool to plan storage and recovery. Upon the conclusion of each survey (starting with the second survey), a map will be produced showing changes in aquifer storage. This information is useful for siting new wells, for evaluating the performance of ASR facilities, and for improving groundwater-model simulation of ASR operations.

This study contributes to the USGS Water Science Strategy goal to “provide society the information it needs regarding the amount…of water in all components of the water cycle at high temporal and spatial resolution,” with the specific objective of advancing hydrologic monitoring networks and techniques (Evenson and others, 2013) and the Department of Interior priority to sustainably develop our energy and natural resources.

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