Between December 1-4, 2020, 1053 line-km of airborne electromagnetic (AEM) data were successfully acquired.
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. The recognized need to sustainably manage groundwater throughout California, has highlighted the lack of understanding about the way in which the groundwater systems in the Central Valley receive water from – are recharged – by the Sierra Nevada Mountains. The specific 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 surface water and/or groundwater from the Sierras deep into the groundwater systems of the valley? These pathways are not only the locations where natural recharge is occurring but are also locations where “managed” recharge can take place. In managed recharge, excess flood water is directed to locations so as to augment the amount of natural recharge.
These uniquely optimal locations for recharge populate all the major alluvial fans emanating from the Sierra Nevada, yet locations of most of them are unknown. This survey offers an important test of our ability to detect these strategic hydrogeologic deposits with AEM here, and throughout the rest of the Central Valley.
We will use the airborne electromagnetic (AEM) method to “see beneath the ground” to an anticipated 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. The figure above shows the 3-D electrical resistivity model derived from data acquired in November 2018. The electrical resistivity data are displayed on top of the Google Earth map, but actually display the changes in electrical resistivity that start at the ground surface and go to a depth of about 350 m. These data reveal, in great detail, the variation in the types of materials in the subsurface, which affects the way in which water moves and is stored below the ground. The cool colors (blues) show where there are fine-grained materials, such as clay. These materials contain water, but act as impediments to flow. The warm colors show regions where there are coarse-grained sands and gravels. These materials can hold a large amount of water and can move a large amount of water. These are the colors we are looking for in the new study – hoping to find the sand and gravel pathways from the Sierras that carry water to great depths in the valley.
We focus 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 includes 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. Of specific interest are 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 have planned a dense coverage of flight lines over an area that has been identified by researchers from UC Davis as a buried valley, carved by a river and then subsequently filled with sand and gravel deposits during the most recent glacial melt period. This buried valley is likely a “fastpath” for moving water to deep beneath the valley floor, recharging the groundwater system. If we find that we can image this valley feature with AEM, we can then search for more of these features in the rest of the Central Valley, identifying critical locations where natural recharge is likely occurring. These locations could also be used as key, strategic locations for managed recharge – providing an efficient way to get water into the deeper parts of the groundwater system.
The AEM data will be used to develop a model of the electrical resistivity of the subsurface that will have horizontal resolution along the flight lines of ~40 m, and vertical resolution that will vary from meters near the surface to tens of meters at depth. By integrating the AEM data with information obtained through a review of existing drillers’ logs, water table data and other ancillary information we will obtain new understanding about the large-scale geological structure of the region at the eastern edge of the valley. These new findings will provide insights about the natural processes controlling recharge from the Sierras into the valley and will reveal optimal locations for enhancing recharge.
This research is being led by Klara Streklova (post-doctoral fellow in the Environmental Geophysics Group at Stanford University) and Rosemary Knight (Professor of Geophysics at Stanford). Other researchers playing a key role are Meredith Goebel (post-doctoral fellow in the Environmental Geophysics Group at Stanford University) and Alex Miltenbuerger (Ph.D. student, Geophysics Dept., Stanford University). We are collaborating in this research with Graham Fogg (Professor, UC Davis) and 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 is being handled by Aqua Geo Frameworks.
Sponsor: The Gordon and Betty Moore Foundation (grant number GBMF6189)