Category: Soils, Geology, and Aquifer Characterization

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.

Introduction to Groundwater Recharge

February 6, 2023 No Comments

The Central Valley is one of the most productive agricultural regions in the United States. Growers and communities, however, are faced with a future of declining groundwater quality, quantity, and availability during drought, due to overdraft. In areas where soil conditions are suitable and excess water is available, ground water recharge represents one of the most cost-effective methods to increase storage, thereby ensuring water supply and improving water quality. Through this guide, California almond growers can begin evaluating their options for addressing local sub-basin overdraft through recharge, helping secure reliable, sufficient, and drought resilient groundwater supplies.

GAMA: Groundwater Ambient Monitoring and Assessment Program

February 2, 2023 No Comments

The State Water Board GAMA Program and the U.S. Geological Survey have created tools to help users understand groundwater quality in California. A range of online tools allow users to access data through web maps and data querying tools.

SGMA Data Viewer

February 2, 2023 No Comments

As part of DWR’s technical assistance to Groundwater Sustainability Agencies (GSAs), other water managers, and the public, DWR has developed the SGMA Data Viewer. The SGMA Data Viewer provides access to groundwater-related datasets that are organized by the requirements of SGMA and the Groundwater Sustainability Plan (GSP) regulations for the purposes of supporting GSP development and implementation.

Currently, regional and statewide groundwater information is publicly available, however this information is collected and disseminated through a variety of portals and applications. The SGMA Data Viewer provides centralized data access that will improve coordination across the State and help GSAs meet the requirements of SGMA and the GSP regulations.

This site includes historical and current data on:
Groundwater levels
Groundwater storage
Water quality
Land subsidence
Interconnected surface water
Water budgets
Hydrogeological conceptual models
Reference layers

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.

On the risk of pesticide residue leaching under agricultural managed aquifer recharge

January 25, 2023 No Comments

In collaboration with the California Department of Pesticide Regulation this project is assessing pesticide residue leaching in response to agricultural groundwater recharge. To evaluate the risk of pesticide residue leaching we conducted 1) a towed Transient ElectroMagnetic (towTEM) geophysical survey to characterize sediment types in the deep vadose zone underlying the recharge site; 2) we flooded an 8 acre section of a fallow field for 8 days and monitored pesticide residue concentrations in soil cores, groundwater, and soil pore water before, during, and after the recharge experiment; 3) we performed a potassium bromide tracer test during the recharge experiment to measure solute travel time and breakthrough in nearby monitoring wells; and 4) we are in the process of developing a pesticide fate and transport model for the soil root and deep vadose zones underlying the recharge site using data collected from the study. The experiment was conducted at Terranova Ranch near Helm, CA. The soil at the site is characterized by sandy loams and loamy sands with a cemented duripan at around 1 m depth in some areas (Bachand et al., 2014). The experimental site was flooded in February 2021 using pumped groundwater. In total 38,774 m3 recharge water was applied at a flow rate of ~885 gallon/min; the total infiltration was 1.2 m. Sensors were installed at 0.2, 0.6, 1, 1.75 and 2.5 m at three locations within the flooded area and two locations in the non-flooded (i.e., control). At each sensor profile soil moisture, EC, temperature, gaseous O2, and redox potential were measured and pore water samples were taken from suction cups. CO2 and N2O emissions, ponding levels, pore water, and sediments were sampled too.

In the flooded area, sensor data showed a fast increase to near-saturated conditions within about one day. During the recharge, the shallow vadose zone at the three profiles remained in oxic conditions, except at the shallowest depths of 0.2 and 0.6 m. Pore water results showed that legacy nitrate concentrations were above MCL before flooding, reaching up to 600 mg/L in profile #3 at 0.2 m. During flooding, a large fraction of the initial soil nitrate was leached below 2.5 m, but nitrate was not fully flushed from the first 2.5 m of the vadose zone since nitrate concentrations during- and post- flooding were still above zero in most locations in Terranova. Ammonium concentrations were generally very low and showed no significant changes pre- and post-flooding. Prior, during and after flooding detectable concentrations of several pesticide residues could be measured in soil and pore water samples including imidacloprid, Metolachlor, thiamethoxam, methoxyfenozide and azoxystrobin. Some of these residues showed a clear leaching trend over the course of the experiment, indicating mobilization and dilution. Analysis of the field-collected pesticide residue data is still ongoing.

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.

Soil Agricultural Groundwater Banking Index

January 20, 2023 No Comments

The Soil Agricultural Groundwater Banking Index (SAGBI) is a suitability index for groundwater recharge on agricultural land. The SAGBI is based on five major factors that are critical to successful agricultural groundwater banking: deep percolation, root zone residence time, topography, chemical limitations, and soil surface condition.

California DWR Airborne Electromagnetic Surveys

January 18, 2023 No Comments

The Department of Water Resources is conducting airborne electromagnetic (AEM) surveys in California’s high- and medium-priority groundwater basins, where data collection is feasible, to assist local water managers as they implement the Sustainable Groundwater Management Act (SGMA) to manage groundwater for long term sustainability.

The AEM project provides state and federal agencies, groundwater sustainability agencies (GSAs), stakeholders, and the public with basin-specific and cross-basin geophysical data, tools, and analyses.

The surveys are funded by voter-approved Proposition 68, Senate Bill 5, and from the general fund. More information can be found in the AEM Proposition 68 Fact Sheet.

During an AEM survey, a helicopter tows electronic equipment that sends signals into the ground which bounce back. The process has been compared to taking an MRI of the ground’s subsurface. The data collected is used to create continuous images that are interpreted for underground geology.

The resulting information will provide a standardized, statewide dataset that improves the understanding of aquifer structures. It can also help with the development or refinement of hydrogeologic conceptual models and can help identify areas for recharging groundwater.