Mapping California's water resources

Using advanced geophysical imaging technologies in novel ways, we are mapping California's groundwater to enable sustainable groundwater management for the state's major agricultural areas and elsewhere. California's subsurface aquifers are being extensively pumped, especially during years of drought. In some areas, the subsidence, or compaction, caused by over-pumping means that even with abundant rain, the lost water-storage capacity may never be recovered. Negative changes in water quality are also being seen - the intrusion of saltwater into coastal aquifers and high levels of arsenic in wells in the Central Valley.

But using the airborne electromagnetic method, we can identify freshwater aquifers and manage them carefully. In the same way that medical imaging revolutionized our approach to managing human health, geophysical imaging can revolutionize our approach to managing groundwater, giving us a roadmap to the sustainable management of one of our most precious resources.

Working with scientists from Aarhus University, Aqua Geo Frameworks, I•GIS, and Ramboll; the state of California Department of Water Resources; the State Water Resources Control Board; three regional water agencies; and the Danish Environmental Protection Agency (EPA), Stanford scientists are leading a $2 million project -- The Stanford Groundwater Architecture Project (GAP) -- to map the state's groundwater systems. Although legislation is in place requiring sustainable groundwater management, there is a critical need for data to support the development of plans. Without these data, "it's like managing a bank account and not knowing how much is going in and how much is coming out," says Stanford geophysicist Rosemary Knight, the lead scientist on the project.

Building on previous research led by Knight and other scientists the project commenced in October 2018 and will continue for two years. The purpose of the GAP is to determine the optimal workflow required to use the geophysical imaging method as the basis for sustainable groundwater management. Knight’s vision: that the geophysical imaging method will be adopted for groundwater management throughout the state.

The Approach

At the core of our approach is the use of an airborne electromagnetic (AEM) method to map out the large-scale architecture of the groundwater system. The helicopter-deployed system can acquire data along ~300 km in a day, “seeing” up to ~400 m below the ground. The critical question motivating the Stanford Groundwater Architecture Project (GAP): “how to take this on in California where there has been no history of using the AEM method for groundwater management?”

Our goal is to determine the optimal workflow for the use of AEM data in California to support sustainable groundwater management, as required by SGMA. By working with our project partners, processes and methodologies will emerge that will be transferrable to other regions throughout California (and elsewhere in the world). The key elements of our approach are shown in the schematic below:

Our approach is described here as a 9-step workflow, links to results, recommendations, and useful materials for the various steps.

1. Engage with the local agency to clearly identify the groundwater management questions linked to SGMA implementation.
   See results, recommendations, and useful materials.
2. Develop the Data Management System that will enable data sharing among project participants and with stakeholders.
   See results, recommendations, and useful materials.
3. Compile existing data such as well completion reports and geophysical logs along with geologic cross-sections and subsurface models.
   See results, recommendations, and useful materials.
4. Design the AEM survey to fill in the data gaps so as to answer the groundwater questions.
   See results, recommendations, and useful materials.
5. Acquire the AEM data using the helicopter-deployed system, with outreach in the local area to explain the objectives and procedures of the survey, and to address any health and safety concerns.
   See results, recommendations, and useful materials.
6. Analyze the AEM data to obtain models of electrical resistivity, the geophysical property that is measured.
7. Interpret the resistivity models to extract the information that we need, e.g. mapping of aquifer and aquitard units.
8. Integrate all data to generate the hydrogeologic conceptual model using a computational framework that allows us to explore the range of possible models that are compatible with the data.
9. Answer the management questions while acknowledging uncertainty so as to inform decision-making for sustainable groundwater management.