Searching for Recharge Pathways Along the Edge of the Central Valley

Link to the published paper 

Media Coverage:

From the air, scientists map ‘fast paths’ for recharging California’s groundwater Ian James, LA Times

Hidden “Paleo Valleys” Could Help California Survive Droughts Erica Gies, Scientific American

Stanford scientists uncover ancient “super highways” to increase groundwater recharge Jesse Vad, SJV Water

Scientists use imaging technology in quest to recharge state’s shrinking groundwater table Alastair Bland, Bay City News

Stanford researchers use airborne technology to spot groundwater recharge sites Dan Ashley and Tim Didion, ABC 7 News

California Groundwater Mapping Homes in on Optimal Replenish Sites  Pam McFarland, Engineering News-Record

Could California Capture its Destructive Floodwaters to Fight Drought Erica Giles, New York Times

The Problem

The Central Valley of California is one of the most productive agricultural areas in the world. Groundwater is an important part of the supply of freshwater required for irrigation; this is particularly true during times of drought. A commitment to sustainably manage groundwater throughout California, has highlighted the need to 1) understand the way in which the groundwater systems in the Central Valley receive water from – are recharged – by the Sierra Nevada Mountains and 2) locate areas where natural recharge can be supplemented with managed aquifer recharge. In managed aquifer recharge, excess flood water is directed to locations so as to augment the amount of natural recharge. The question of interest in our work: Where are the recharge pathways - the zones of coarse, permeable materials – along the eastern edge of the Central Valley and extending out into the valley, that can move water from the surface to greater depths in the groundwater systems of the valley? 

One focus of our study was a feature in the Kings River area referred to as an incised valley fill (IVF) deposit, previously described in 1999 by UC Davis researchers Graham Fogg and Gary Weissmann: a valley carved by high velocity meltwater at the end of the last glacial period and filled with very coarse-grained materials - cobbles, gravel, sand. IVF deposits are uniquely optimal locations for recharge and are likely associated with a number of the major rivers entering the Central Valley from the Sierra Nevada. The challenge – efficiently finding them. This study offered an important test of our ability to detect one of these strategic hydrogeologic deposits with the airborne electromagnetic AEM method. If successful here, we could search for them throughout the rest of the Central Valley.

Our Approach

We use the airborne electromagnetic (AEM) method to “see beneath the ground” to a depth of ~300 m. The AEM method is a helicopter-deployed system, which acquires data along planned flight lines to measure the electrical resistivity of the subsurface. We have successfully used the AEM method in the past in this area to identify areas of high resistivity, corresponding to regions where there are coarse-grained sands and gravels, and areas of low resistivity corresponding to regions where there are likely clays.

The Place

We focused on an area within Tulare and Fresno Counties, in the southeastern part of the Central Valley. The figure above shows our planned flight lines. Our study included a strip of land covering the transition from the Sierra foothills to the valley floor, starting in the south at the Kaweah River and going north to the San Joaquin River at the northern boundary of Fresno County. We acquired data over the alluvial fans associated with the rivers and smaller creeks coming into the valley from the mountains. These fans have built up over time, so can extend to significant depths beneath the ground surface. These are locations where natural recharge occurs and also optimal locations for managed recharge.

In one location, near the Kings River, we had a dense coverage of flight lines over the area that had been identified by researchers from UC Davis as an incised valley fill (IVF) deposit. This feature can act as a “fastpath” for moving water to deep beneath the valley floor, recharging the groundwater system. If we could image this valley feature with AEM, we could then search for more of these features in the rest of the Central Valley. These locations could be used as key, strategic locations for managed recharge – providing an efficient way to get water into the deeper parts of the groundwater system.

Results

The AEM data were used to develop a model of the electrical resistivity of the subsurface that has horizontal resolution along the flight lines of ~40 m, and vertical resolution that varies from meters near the surface to tens of meters at depth. By integrating the AEM data with information obtained through a review of existing driller’s logs, water table data and other ancillary information we obtained a new understanding about the large-scale geological structure of the region at the eastern edge of the valley. We found the IVF deposit mapped by UC Davis researchers in the AEM data, imaged as a massive, highly resistive, linear feature extending from the eastern edge of the Central Valley, 40 km out into the valley. This means that the AEM method can be used to search for other deposits like this in the Central Valley. They would provide the natural infrastructure needed to implement recharge in the valley.

Resistivity models derived from the inversion of AEM data acquired in this study (left) and in a prior study (right, Kang et al. 2022).

The People

This research is being led by Rosemary Knight (Professor of Geophysics at Stanford). Other researchers playing a key role are are Klara Steklova (prior post-doctoral fellow), Alex Miltenbuerger (prior Ph.D. student),  Seogi Kang (post-doctoral fellow in the Environmental Geophysics Research Group at Stanford University), Meredith Goebel (Research Scientist in the Environmental Geophysics Research Group at Stanford University), and Graham Fogg (Professor, UC Davis). We are collaborating in further work with these data with Hoori Ajami (Assistant Professor, UC Riverside).

The planning and execution of this project has benefitted greatly from collaboration with those working in the study area:

• Aaron Fukuda (General Manager, Tulare Irrigation District)
• Augustine Ramirez (Senior Engineer, Fresno County Department of Public Works and Planning)
• Bill Stretch (Manager, Fresno Irrigation District)
• Chad Wegley (Manager, Kings River East GSA)
• Eric Osterling (General Manager, Greater Kaweah GSA)
• Glenn Allen (Division Manager, Fresno County Department of Public Works and Planning)
• Josh Rogers (Technical Advisory Committee Chair, South Kings GSA)
• Kassy Chauhan (Executive Officer, North Kings GSA)
• Larry Dotson (Senior Engineer, Kaweah Delta Water Conservation District)
• Michael Hagman (General Manager, Lindmore Irrigation District)
• Phil Desatoff (General Manager, Consolidation Irrigation District)
• Ronnie Samuelian (Kings Subbasin Coordinator, Vice President, Provost & Pritchard)

The selection of the AEM system and the acquisition and processing of the AEM data was handled by Aqua Geo Frameworks.

Sponsor: The Gordon and Betty Moore Foundation (grant number GBMF6189)