Agricultural Managed Aquifer Recharge (AgMAR) has been identified in Groundwater Sustainability Plans as a tool to replenish groundwater. Increasing groundwater levels is a known benefit to recharge, but it can also help increase groundwater quality through dilution of contaminants.
Christa Harader of Sustainable Conservation said there is increased interest in groundwater recharge among growers.
“They have to experiment and stick with it to see results for specific properties. It takes time,” Harader said.
Both short- and long-term benefits and risks of AgMAR must be understood. There is also the possibility that aquifer recharge can negatively impact water quality.
According to Sustainable Conservation, nitrate contamination of groundwater affects more than 600,000 people served by public supply wells throughout California and is expected to worsen into the future, with or without AgMAR.
Legacy nitrogen is the result of historical land use and fertilizer applications over time. Depending on soil type and past irrigation practices, legacy N is either gradually making its way through the unsaturated soil zone or has already entered the groundwater.
Recharge activity may influence the timing of how legacy N enters the groundwater, but it won’t influence the total amount that will eventually enter the groundwater.
To guide field scale recharge decisions, Sustainable Conservation recommends the following field level nitrate considerations.
Current crop management: Assess current land use practices to estimate the amount of annual nitrate leaching below the root zone. Prioritize recharge sites with crops that have low N demand and N applications that are managed.
Crop suitability: Recharge on active crop land should occur where permanent crops can tolerate soil saturation. Saturated soil conditions reduce the ability to uptake N, leaving more N to leach below the root zone.
Soils: Sandy, coarser-textured surface soils are generally best for recharge sites, unless there are underlying restrictive layers. These sites may be suitable for recharging large amounts of water and result in effective dilution of nitrates.
Nitrogen management under recharge: Recharge is best done during dormant periods. If possible, longer-duration applications are preferred over short, pulsed events to decrease N mineralization potential and increase denitrification potential, decreasing overall nitrate leaching into groundwater.
Water availability: Duration and quantity of water available for recharge should be considered to determine the number and size of recharge sites that can receive sufficient water, which may help dilute legacy N.
Harader said that although prospects for groundwater recharge this coming winter may be slim, growers who are interested in this program should contact their local Groundwater Sustainability Agency or irrigation district. They can also contact Taylor Broadhead (tbroadhead@suscon.org) at Sustainable Conservation.
