Category: Research

Replenishing Groundwater in the San Joaquin Valley

February 6, 2023 No Comments

The San Joaquin Valley—which has the biggest imbalance between groundwater pumping and replenishment in the state—is ground zero for implementing the 2014 Sustainable Groundwater Management Act (SGMA). Expanding groundwater recharge could help local water users bring their basins into balance and make a dent in the long-term deficit of nearly 2 million acre-feet per year.

The experience with recharge in 2017―the first wet year since the enactment of SGMA―offers valuable insights in how to expand recharge. A survey of valley water districts’ current recharge efforts revealed strong interest in the practice, and a number of constraints.

Merced River Flood-MAR Reconnaissance Study Technical Memorandum 1 Plan of Study – Draft

February 6, 2023 No Comments

DWR, in partnership with the Merced Irrigation District (MID), is conducting a preliminary study using flood waters for managed aquifer recharge that can reduce flood risk, increase surface and groundwater supply reliability, and enhance ecosystems in the Merced River Basin. This Merced River Reconnaissance Study (study) is exploring the potential, feasibility, and effectiveness of Flood-MAR concepts, testing theories, and assessing strategies in overcoming barriers and challenges to project planning and implementation. The study will assess current conditions of the Merced River watershed and the vulnerability of these watershed management characteristics to a range of potential climate change futures. The study will also describe the public and private benefits that may be achieved through Flood-MAR strategies and quantify a range of benefits that Flood-MAR could provide in or adjacent to the Merced River watershed.

Groundwater and Stream Interaction in California’s Central Valley: Insights for Sustainable Groundwater Management

February 2, 2023 No Comments

This report was undertaken to provide technical information on the state of streams and groundwater resources in the Central Valley. The findings of this report were used to support the need for what is now known as the Sustainable Groundwater Management Act (SGMA). The intent was to illustrate the physical inter-relationship between the surface and groundwater resources and the potential impacts that groundwater pumping has had and is currently having on our rivers and streams to demonstrate the need for sustainable groundwater management.

Based on the scale of the data used in this study, the findings contained herein should not be used as a definitive source in determining whether a particular stream or river reach should or should not be considered as an interconnected surface water for SGMA purposes. Further study at a finer scale would be necessary for such a determination.

Management Considerations for Protecting Groundwater Quality Under Agricultural Managed Aquifer Recharge

February 2, 2023 No Comments

Unsustainable groundwater use in California – due in large part to historical over-pumping of aquifer systems, growing reliance on groundwater to meet irrigation and urban water demands, and increasing frequency of drought – affects all water users and threatens agricultural viability into the future, but has disproportionately impacted disadvantaged communities and jeopardizes their access to safe, clean and affordable water. To secure the availability of groundwater for all uses, the state enacted the Sustainable Groundwater Management Act (SGMA) in 2014. Groundwater Sustainability Agencies (GSAs) were charged with developing Groundwater Sustainability Plans (GSPs) to avoid undesirable effects of ongoing groundwater depletion. To meet these goals, many GSPs include managed aquifer recharge (MAR) as one of several key tools to improve groundwater sustainability.

Agricultural Managed Aquifer Recharge (AgMAR) is the act of intentionally flooding fallow, dormant, or active cropland when excess surface water is available. AgMAR has the potential to be a cost-effective and high impact form of MAR due to the large acreage of cropland throughout California. As more farmers adopt AgMAR, there is greater urgency to understand the potential water quality risks and benefits associated with recharge. While pesticides and geogenic contaminants such as arsenic pose additional water quality concerns in MAR projects, this paper focuses specifically on water quality considerations for nitrate and salts related to AgMAR activities.

Nitrate contamination of groundwater is expected to worsen into the future. However, a combination of improved nutrient management and carefully implemented AgMAR projects could improve groundwater quality faster than under business as usual. Improvements in nitrogen management practices should be prioritized to reduce current and future nitrogen (N) loading to groundwater. Furthermore, relatively clean (nitrate free) recharge water (e.g. high magnitude flood flows) should be used during AgMAR events in order to dilute incoming and existing nitrate in groundwater. AgMAR programs should prioritize sites that can recharge in longer-duration single-flooding events, such as sandier sites, to capitalize on the dilution effect and reduce biologically mediated mineralization of organic N (the conversion of organic N to nitrate).

AgMAR alone will not lead to substantive improvement in groundwater quality with respect to nitrate without concomitant improvements in current agronomic nitrogen management and sufficient water for dilution. The development of transparent and easy-to-use tools that estimate the amount of residual nitrate at the end of a growing season, the amount of water needed to dilute nitrate under AgMAR, and time of travel to groundwater will help in the successful implementation of recharge projects to avoid negative water quality externalities. Current nitrogen loading maps and locations of drinking water supply wells can be used by GSAs to get a sense of regional nitrogen loading to groundwater and help in planning and prioritizing efforts on sites to target for AgMAR.

Managed aquifer recharge site assessment with electromagnetic imaging: Identification of recharge flow paths

January 31, 2023 No Comments

Surface spreading recharge, the intentional flooding of the ground surface to replenish a groundwater system, is one approach used to mitigate groundwater overdraft in California’s Central Valley (CV). Choosing appropriate sites for surface spreading recharge, in regard to the sites’ ability to convey water from the ground surface to the desired recharge depth, can be challenging because of limited knowledge of the properties of the subsurface. In this study, we present an approach for using a towed time-domain electromagnetic (tTEM) imaging method to develop three-dimensional (3D) models of sediment type, map potential flow paths through the subsurface, and evaluate sites for surface spreading. We began with tTEM data from seven sites in the CV along with an existing resistivity-to-sediment type transform. We leveraged geostatistical methods to generate multiple 3D models of binary (flow and no-flow) sediment type from the tTEM data. We then developed two metrics to assess the quality of sites for recharge: (a) the depth to the shallowest no-flow unit beneath each point at the surface and (b) preferential flow paths lengths measured by finding the shortest distance through connected flow units between surface points and the desired depth. We explored how these metrics can be used to identify optimal areas within a site, then developed a way to compare and assess the relative suitability of each site using the decay in the number of vertical flow paths as a function of depth. Our methods can be used to rapidly identify potential sites for surface spreading recharge.

Airborne geophysical method images fast paths for managed recharge of California’s groundwater

January 25, 2023 No Comments

Given the substantial groundwater level declines in the Central Valley of California, there is an urgent need to supplement the recharge of the groundwater systems by implementing managed aquifer recharge. With approximately 170 km3 (140 million acre-feet) of available groundwater storage space, water deemed to be excess during wet years could be spread on the ground surface at selected locations allowing it to move downward to recharge the underlying aquifer system. Along the eastern edge of the Central Valley there are large paleovalleys that can act as fast paths expediting the downward movement of water. These paleovalleys, incised and then filled with coarse-grained materials—sand, gravel, cobbles—at the end of the last glacial period, are referred to as incised valley fill (IVF) deposits. An IVF deposit has been mapped at one location in the Kings River alluvial fan, with others proposed to exist in the fans of major rivers. If located, these deposits would be optimal sites for managed recharge. In this study, we assessed the use of a helicopter deployed geophysical method to efficiently locate IVF deposits throughout the Central Valley. We acquired 542 line-kilometers of airborne electromagnetic (AEM) data in the Kings River alluvial fan, with dense line-spacing over the Kings River IVF deposit which had been mapped as ∼2 km wide, extending over 20 km into the Central Valley, from the ground surface to a depth of 30 m. The IVF deposit was unambiguously imaged in the AEM data as an extensive linear feature that was more electrically resistive than the surrounding materials due to the high percentage of coarse-grained sediments. This study provides the evidence to support the rapid adoption of the AEM method to locate IVF deposits along the eastern edge of the Central Valley. These deposits provide valuable natural infrastructure for recharging California’s groundwater.

Recharge site assessment through the integration of surface geophysics and cone penetrometer testing

January 20, 2023 No Comments

Paper Abstract:

The ability to identify, at potential managed aquifer recharge sites, the presence of connected pathways of hydraulically conductive sediments from the ground surface to the water table could help minimize costs and risks associated with recharge operations. A spatially dense dataset had previously been acquired in an almond [Prunus dulcis (Mill.) D.A. Webb] grove in Tulare, CA, using tTEM, a towed transient electromagnetic (tTEM) geophysical method. In order to interpret reliable information about sediment type from the tTEM data, a transform from the tTEM-derived property, electrical resistivity, to sediment type is required. The uncertainty associated with derived models of sediment type can be significantly reduced if a site- and dataset-specific transform is used. Cone penetrometer testing (CPT) was conducted at five locations, strategically selected based on a review of the tTEM data. Co-located measurements of sediment type, derived from the CPT, and electrical resistivity, derived from the tTEM data, were used to create a resistivity-to-sediment-type transform, with sediment type classified as either coarse-grain-dominated (sand and gravel) or fine-grain-dominated (silt and clay) material. The transform captured the uncertainty associated with variable water salinity and content, the resolution of the tTEM data, and other components of the tTEM measurement workflow. Using this transform, models of sediment type were generated for the unsaturated zone at the site. Within these models are features, which we interpret as potential recharge pathways, corresponding to high fractions of coarse-grain-dominated material amongst regions of fine-grain-dominated material. The workflow developed at this site can provide a framework for using tTEM and CPT for recharge site assessment.

Flood-MAR Research and Data Development Plan

September 15, 2022 No Comments

This Flood-MAR Research and Data Development Plan (R&DD Plan) presents the work of the Flood-MAR Research Advisory Committee (RAC), a multidisciplinary group of subject matter experts across 13 research themes. The RAC was tasked to identify the research, data, guidance, and tools necessary to support and expand the implementation of Flood-MAR projects. Well-formulated Flood-MAR projects can benefit Californians and the environment through improved water supply reliability, flood-risk reduction, drought preparedness, aquifer replenishment, ecosystem enhancement, subsidence mitigation, water quality improvement, working landscape preservation and stewardship, climate change adaptation, recreation, and aesthetics