Mapping California’s aquifer systems by air, land and water

California’s groundwater basins are a critical source of water for communities, agriculture, ecosystems, and industry. But decades of drought, rising demand, and increasingly variable climate conditions have placed growing pressure on these resources. To support more sustainable groundwater management, the California Department of Water Resources (DWR) selected Ramboll to lead a statewide effort to map medium- and high-priority groundwater basins designated under the Sustainable Groundwater Management Act (SGMA).

Creating a statewide view of the subsurface

Ramboll led a multidisciplinary team to collect, process, and interpret more than 25,000-line kilometers (15,500-line miles) of airborne electromagnetic (AEM) data across California. Working with SkyTEM and other project partners, the team combined geophysical survey data with well records, lithological logs, water levels and water quality measurements to give water managers a clearer picture of the state’s complex subsurface conditions.

The resulting statewide dataset provides practitioners with a consistent foundation for understanding subsurface electrical resistivity and translating that information into practical groundwater management insights. Ramboll converted AEM resistivity models into coarse fraction models, helping identify areas more likely to contain sands and gravels that can function as aquifer systems, as well as finer-grained zones that may act as aquitards or have lower permeability.

“AEM data gives a better, localized understanding of groundwater basins and provides datasets that support the overall implementation of California’s Sustainable Groundwater Management Act and the development of groundwater sustainability plans,” said Steven Springhorn, technical assistance manager of the SGMA for the California DWR.

Beyond aquifer mapping, the data helps illuminate priority groundwater management questions across California, including saltwater intrusion in coastal areas, depth to bedrock or volcanic layers, opportunities for managed aquifer recharge and groundwater banking, and the hydraulic connection between neighboring basins. The surveys also revealed geologic structures such as faults and basin boundaries where strong resistivity contrasts are present.

Advancing from statewide mapping to local decision support

Following completion of the statewide survey in November 2023, DWR launched the Basin Characterization Program to bring the data into more detailed 3D models and visualization tools while filling information gaps in areas with limited AEM coverage. This next phase is helping translate a statewide dataset into basin-scale insights that groundwater sustainability agencies can use to support planning, modeling, and implementation.

Ramboll continues to support these local investigations using towed transient electromagnetic (tTEM) surveys, a ground-based technology that applies similar principles to AEM at a finer scale. The approach supports site-specific evaluations for managed aquifer recharge, levee management and groundwater system characterization. Ramboll is also contributing to aquifer recharge potential mapping, the use of legacy oil and gas datasets for aquifer characterization, and continued refinement of complex hydrogeologic features such as the Corcoran Clay.

Using innovative technologies to connect surface water and groundwater

As part of the Basin Characterization Program, Ramboll conducted demonstration projects with DWR using floating transient electromagnetic (FloaTEM) technology, including California’s first FloaTEM surveys along sections of the Sacramento River and nearby sloughs and a subsequent survey along the Feather River. FloaTEM applies electromagnetic methods from the water surface to characterize streambeds and underlying sediments, providing a valuable complement to air- and land-based geophysical data.

Together, AEM, tTEM, and FloaTEM are helping California build a more complete, actionable understanding of its groundwater systems. By integrating innovative field methods, technical interpretation and practical modeling support, Ramboll is helping DWR and local agencies make more informed decisions about groundwater sustainability and long-term water resilience.

For more information and updates on the work Ramboll is participating in, visit DWR’s websites on AEM survey and basin characterization.