Category: Topic Area

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.

California Sustainable Groundwater Management Act (SGMA)

January 20, 2023 No Comments

The historic passage of SGMA in 2014 set forth a statewide framework to help protect groundwater resources over the long-term. SGMA is comprised from a three-bill legislative package, including AB 1739 (Dickinson), SB 1168 (Pavley), and SB 1319 (Pavley), and subsequent statewide Regulations. In signing SGMA, then-Governor Jerry Brown emphasized that “groundwater management in California is best accomplished locally.”

SGMA requires local agencies to form groundwater sustainability agencies (GSAs) for the high and medium priority basins. GSAs develop and implement groundwater sustainability plans (GSPs) to avoid undesirable results and mitigate overdraft within 20 years.

DWR serves two roles to support local SGMA implementation:

1) Regulatory oversight through the evaluation and assessment of GSPs

2) Providing ongoing assistance to locals through the development of:

Best management practices and guidance
Planning assistance
Technical assistance
Financial assistance

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.

Coyote Valley Conservation Areas Master Plan

December 7, 2022 No Comments

ABOUT COYOTE VALLEY
Coyote Valley is a rural and natural area located at the southern edge of San Jose and is one of the last remaining undeveloped valley floors in the region. Approximately seven miles long and two miles wide, it is defined by the Diablo Range to the east and the Santa Cruz Mountains to the west. The valley is remarkable for its role connecting the ecosystems of the Santa Cruz Mountains with the rest of California, as well as its scenic beauty, rich biodiversity, prime farmland, and unique water resources. The Santa Clara Valley Open Space Authority (Authority) and its partners have made protection of Coyote Valley a top priority, and in recent years approximately 1,500 acres of valley-floor land have been permanently protected.

ABOUT THE MASTER PLAN
The Coyote Valley Conservation Areas Master Plan (CVCAMP) will create a roadmap for implementing wildlife linkages, restoring water resources and habitat, supporting climate-smart agriculture, and providing equitable public access on Coyote Valley’s protected lands. The CVCAMP is managed by Authority in close partnership with the Peninsula Open Space Trust (POST) and the City of San Jose and will be created via an integrated, science-based planning process shaped by robust and inclusive stakeholder and community engagement.

COYOTE VALLEY’S WATER RESOURCES
A key focus element of the CVCAMP will be restoration of the historic Laguna Seca wetlands—the largest freshwater marsh in the region—and rehabilitation of a heavily modified creek system in the Fisher Creek floodplain. Restoration of these water resources will entail retirement of constructed channels, levees, and an earthen dam, and restoring natural landscape processes that will help rebuild the landscape’s capacity to buffer surrounding and downstream areas from increasingly unpredictable flood and stormwater events resulting from climate change. Floodplain restoration will also result in increased climate resilience for the region, enhanced habitat for wildlife, and a suite of other co-benefits like flood-managed aquifer recharge and carbon sequestration.

PROJECT TEAM, SCHEDULE, & FUNDING
In late 2021, the Authority hired a team of technical experts led by the firm SWCA Environmental Consulting & Engineering, Inc. to work on the CVCAMP. The Authority anticipates a three-to-five-year integrated planning effort. The CVCAMP will identify early implementation projects to be constructed within the next five to ten years, as well as develop a larger programmatic vision with a 20-to-30-year time horizon. Planning work is funded by the Authority and POST with generous support from partners, including the California Wildlife Conservation Board, and from the County of Santa Clara Parks & Recreation Department.

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

Flood-MAR White Paper

September 15, 2022 No Comments

The California Department of Water Resources (DWR) prepared this white paper to explore opportunities to use flood water for managed aquifer recharge (Flood-MAR) because DWR recognizes the need to rehabilitate and modernize water and flood infrastructure in California. Large-scale implementation of Flood-MAR can fundamentally change how flood and groundwater management are integrated by using flood water resulting from, or in anticipation of, rainfall or snowmelt for groundwater recharge on agricultural lands and working landscapes, including but not limited to refuges, floodplains, and flood bypasses.