Demonstration of a Towed Geophysical System (tTEM) for the Evaluation of Potential Recharge Sites

The Place

The Central Valley, located in the heart of California, supplies 40% of the nation's food on less than 1% of the nation's farmland. While the arid, Mediterranean climate of the Central Valley is ideal for year-round agricultural production, groundwater extraction is necessary to meet the water needs of growers and municipalities. Excessive groundwater use over the past century has lowered the groundwater levels across the Central Valley, with deleterious effects on groundwater availability and quality. Managing and reversing the declining water levels requires a multifaceted approach, and one promising method is managed aquifer recharge (MAR).

The Problem

Managed aquifer recharge, where excess water is used to recharge the groundwater aquifers, has been proposed as a means of curbing groundwater depletion in the Central Valley. However, locating new fields for recharge is difficult. One type of MAR is AgMAR, where agricultural fields are flooded in the off-season to replenish the underlying aquifer. Since agricultural fields are typically linked to existing surface water conveyance infrastructure, they are an appealing first choice. However, depending on the geology in the area, water may sit stagnant on the surface and take long periods of time to infiltrate, or may infiltrate, only to move laterally away and not down to the water table. Our challenge is accurately mapping out the sediments in the subsurface to understand where the water might move, allowing water agencies to better prioritize and plan for recharge.

Our Approach

The towed time-domain electromagnetic (tTEM) system is specifically designed for imaging the upper 70-100 m of the subsurface with high lateral resolution, making it a perfect solution for recharge problems in the Central Valley. The tTEM images the electrical resistivity of the subsurface, which is controlled in part by the types of sediments in the ground. Combining the tTEM measurements with a small number of direct sediment type measurements (e.g. from cone penetrometer testing (CPT)) allows us to convert the tTEM resistivity models into 3D sediment type maps. These sediment type maps can then be used to determine the suitability of a field for recharge. .

Results to Date

In 2017, we acquired tTEM data at multiple almond and pistachio orchards as well as recharge sites outside Tulare, CA; the first demonstration of the use of this geophysical system in the United States. The resistivity models of the subsurface from one of the almond groves are shown below in Figure 1. In 2019, CPT data was collected at five locations within this almond grove, to acquire the site-specific information about sediment type needed to develop a transform between resistivity and sediment type. A methodology was developed for creating this transform, allowing up to convert the resistivity models derived from the tTEM to models of sediment type (coarse fraction). Figure 2 shows these sediment type models at the almond grove shown in Figure 1. Figure 3 shows one potential connected pathway through coarse dominated materials beneath this site.