Tag: Ag-MAR

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.

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.

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.