Skip to main content Skip to secondary navigation

Identifying Potential Recharge Pathways Using a Towed EM System (tTEM)

Main content start
flood_irrigation_brian_van_der_brug_los_angeles_times_photo_credit

The Place

In much of California's Central Valley, groundwater managers are increasingly looking towards managed aquifer recharge (MAR) to meet the needs of sustainable groundwater management. Geophysical techniques provide one approach to characterizing the suitability of the subsurface of a site for MAR. This work focuses on a number of sites near Tulare, within the Central Valley, where geophysical data was collected using a towed electromagnetic system (tTEM).

The Problem

Geophysical imaging methods, in particular electrical and electromagnetic methods, can be used to map out changing subsurface sediment texture, information which is critical for determining how water recharged at the ground surface might move, and if it will make it down to the water table. This information is essential when the aim is to select recharge sites which can most effectively convey water to the water table, and minimize any potential damage associated with poor infiltration and percolation. The challenge lies in how exactly to use the geophysical data to develop quantitative evaluation metrics for recharge site suitability, especially in light of uncertainty associated with these data and their transformation to sediment texture.

Our Approach

Using geostatistical modeling, we can create many realizations of the subsurface sediments at each site, each of which honors the geophysical data, and our uncertainty in going from these data to a model of sediment texture. We then develop two metrics by which we can evaluate these realizations; depth the the shallowest no-flow unit, and preferential flow path length measured by finding the shortest distance through connected flow units between surface and the water table. Figure 1 shows a conceptual illustration of these two metrics.

Site Evaluation Metrics
Figure 1: Conceptual illustration of metrics used to assess the sediment type models. (a) Metric 1, the depth to the shallowest low-K unit beneath each point on the surface (DEPTHlow-K). (b) The normalized path lengths (NPL) from each point on the surface to the water table.

Results 

We applied our approach to seven sites in the Tulare area. We found this approach can be useful for quantitatively evaluating a single potential recharge site, and for comparing multiple sites. Figure 2 shows results for one of the seven sites; maps for each of the metrics, averaged across all of the realizations for this site. Within this figure, it is clear that some portions of the site would be suitable for recharge, and others less suitable. It is also apparent that site suitability is tied to the metric you use to evaluate the site; some portions of the site might appear good by one metric while poor by another. This highlights the importance of understanding the specific needs/concerns at a site, and designing metrics with which to evaluate the sediment realizations that are in light with those needs/concerns.

Results of pathway evaluation at an almond grove
Figure 2: Maps of each of the two metrics, averaged across all realizations, for one of the seven sites evaluated.

 

Project Sponsors

Stanford Woods Institute for the Environment, The Gordon and Betty Moore Foundation

Project Publications and Presentations

https://acsess.onlinelibrary.wiley.com/doi/full/10.1002/vzj2.20192

Project Leads / Contacts

Karissa Pepin

Rosemary Knight

Seogi Kang

Meredith Goebel