Filter Irrigation Runoff to Protect Water Quality

irrigation-run-off-lettuce-FEATURE-Photo-by-Michael-Cahn

Photo by Michael Cahn

Though source control strategies can greatly reduce the offsite movement of pesticides, these practices may not completely remove water quality risks to nearby creeks and rivers. Mitigation practices that can remove pesticides from the run-off are also needed.

How to Reduce Suspended Sediment in Run-Off

You can remove hydrophobic pesticides that bind strongly to soil, such as pyrethroids, using practices that drop out suspended sediments in run-off.

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Polyacrylamide: One effective practice for sprinkler and furrow irrigation is to treat irrigation water with a low concentration of polyacrylamide (PAM) polymer, which has the ability to flocculate and settle out suspended sediments. Concentrations of PAM as low as 3 to 5 ppm can often eliminate more than 90% of the suspended sediment in run-off. The remaining suspended sediment can be removed by other practices.

Retention basins and weirs: Shallow retention basins can be located at strategic points on a farm to slow the flow of tail water, thereby settling out sand and large silt particles. The shallow depth of these basins allows the trapped sediment to be easily removed with a backhoe and returned to the field. Weirs also can be positioned in ditches to slow the flow of run-off and settle out suspended sediments.

ditch-with-water-photo-by-Cahn

Portions of ditches vegetated with dense-growing grass species will slowly filter irrigation run-off as well as settle out suspended sediments.
Photo by Michael Cahn

Vegetated ditches: Portions of ditches vegetated with dense-growing grass species will slowly filter irrigation run-off as well as settle out suspended sediments.

How to Reduce Water-Soluble Pesticides in Run-Off

It can be more challenging to eliminate water-soluble pesticides than hydrophobic pesticides from run-off since they tend to want to stay dissolved in the run-off water. However, carbon materials can be used to sequester many types of water-soluble pesticides including organophosphate and neonicotinoid pesticides.

Carbon-and-compost-sock-filtering-runoff-Photo-by-Michael-Cahn

Direct runoff to flow over a sock filled with biochar or some other product that will bind pesticides. Photo by Michael Cahn.

Carbon and compost socks: Directing run-off water to flow over construction socks filled with activated charcoal, biochar, or compost can trap many water-soluble pesticides.

Using vegetation: Pesticides can adhere to living carbon materials, such as vegetation planted in drainage ditches or in shallow ponds. Similarly, wetlands, constructed on marginal land, have been shown to effectively treat soluble pesticides in agricultural run-off.

Bioreactors: Denitrification bioreactors constructed from a wood chip substrate, and used for eliminating nitrate in drainage and run-off, may also be effective in trapping many types of soluble pesticides. Adding biochar or activated carbon can improve the ability of the reactors to trap pesticides.

Integration of Practices is Key to Mitigation

Similar to source control practices, an integrated approach to mitigation will be most effective for eliminating pesticide toxicity in run-off. The efficacy of mitigation practices will vary depending on the mixture of the pesticides in run-off. Practices that are effective for both water-soluble (hydrophilic) and water-insoluble (hydrophobic) pesticides will cover the range of insecticides used in vegetables. Used together, costs of maintaining these practices can be reduced. For example, a sediment trap situated upstream of a vegetated ditch can eliminate the risk of sand inundating the ditch during intense storm events. Pretreating run-off through a vegetated ditch will increase the life-span of carbon treatment socks by preventing them from becoming clogged with fine sediment.

Protecting water quality has become a significant challenge that growers must now face to assure that the pesticides needed to grow their crops will be available in the future. Unfortunately, there is no one silver bullet that will eliminate toxicity in run-off, and the best suite of management practices to implement will depend on the site-specific conditions of the farm. However, using an integrated approach of source control and mitigation practices will likely be the most comprehensive and effective strategy to protect water quality.