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

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tTem System

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.

tTEm derived resistivity models
Figure 1: view of the one-dimensional (1D) resistivity model resulting from inversion of the towed transient electromagnetic (tTEM) data, underlain by a satellite image of the study area. Warm colors (reds) correspond to high resistivity values; cool colors (blues) correspond to low resistivity values. Models are shown from 10 meters below the ground surface [mbgs] to the depth of investigation (DOI, between 40–85 mbgs). Goebel and Knight (2021).
Sediment texture model
Figure 2: View of the sediment type models resulting from transformation of the inverted towed transient electromagnetic (tTEM) resistivity models, underlain by a satellite image of the study area. The color scale spans from 0% coarse-dominated material (blue) to 100% coarse-dominated material (red). Models are shown from 10 mbgs to the top of the saturated zone. Goebel and Knight (2021).
pathway through sediment model
Figure 3: A slice through the sediment type models, extending from the ground surface to the top of the saturated zone, showing a potential recharge pathway through interconnected coarse-dominated sediments.

Project Sponsors

The Almond Board of California

Stanford Woods Institute for the Environment

The Gordon and Betty Moore Foundation

Visualization using Leapfrog

 

Project Publications and Presentations

https://acsess.onlinelibrary.wiley.com/doi/10.1002/vzj2.20131

Project Leads / Contacts

Meredith Goebel

Rosemary Knight