Waterways and agriculture are interconnected. Water runoff from farm fields from rain or irrigation can end up in nearby streams and lakes. Runoff can transport nitrogen, sediments and sediment-associated agricultural pollutants and impact aquatic life.
A lot of chemicals move with sediments, including phosphorus from fertilizers and various pesticides. Pyrethroid insecticides, in particular, are widely used on farms for insect pest control and are frequently detected in sediment within streams and irrigation ditches at levels toxic to vulnerable aquatic species (Long et al. 2010b;Weston et al. 2004, 2008; Phillips et al. 2006). Additionally, nitrogen is water soluble and readily moves through landscapes.
In recent years, researchers in the Sacramento Valley have examined ways to mitigate water pollution risks from agricultural chemicals. Rachael Long, UC Cooperative Extension Farm Advisor, Emeritus, looked at the capacity of vegetated ditches and hedgerows to improve water quality. She also conducted many studies on hedgerows as a tool for enhancing beneficial insects and biological control of pests on farms.
Vegetated drainage ditches reduced sediment loads and associated pyrethroid insecticides by 60%
Rachael examined the benefits of grass-lined ditches at the end of fields for trapping sediments and sediment associated pollutants. What she found was that vegetated drainage on farms help prevent chemicals from moving off site. Specifically, she found that vegetated drainage ditches reduced sediment loads and associated pyrethroid insecticides by 60% (Long et al. 2010b).
Hedgerows take up the nitrogen (Velas et al. 2014; Webster et al. 2018) and filter sediments and their associated pollutants out of the water, allowing them to be broken down on site through sunlight and microbial activity.
Once the pollutants go into streams and lakes, it’s very difficult for them to break down and can take decades. Rachael says the big takeaway from her studies is that hedgerows and vegetated ditches help keep agricultural chemicals away from aquatic life. “I think that the use of these hedgerows, especially these little riparian hedgerows, for trapping sediments, is critically needed to prevent agricultural chemicals from moving off site.”
Read more about Rachael Long’s Studies:
Long RF et al. 2010a. Mitigation techniques reduce sediment in runoff from furrow-irrigated cropland. California Agriculture, 64(3):135-40. https://calag.ucanr.edu/Archive/?article=ca.v064n03p135
Long RF et al. 2010b. Protecting surface water from sediment-associated pesticides in furrow-irrigated fields. UC ANR publication number 8403, https://anrcatalog.ucanr.edu/Details.aspx?itemNo=8403
Velas et al. 2014. Hedgerow Effects on Birds and Water Quality in California’s Central Valley. Audubon California.
Other studies cited:
Phillips BM, Anderson BS, Hunt JW, et al. 2006. Solid phase sediment toxicity identification evaluation in an agricultural stream. Env Toxicol Chem 25(6):1671–6.
Webster AJ et al. 2018. Controls on denitrification potential in nitrate-rich waterways and riparian zones of an irrigated agricultural setting. Ecological application, 28(4): 1055-1067.
Weston DP, You J, Lydy MJ. 2004. Distribution and toxicity of sediment-associated pesticides in agriculture dominated water bodies of California’s Central Valley. Env Sci Technol 38:2752–9.
Weston DP, Zang M, Lydy MJ. 2008. Identifying the cause and source of sediment toxicity in an agriculture influenced creek. Env Toxicol Chem 27(4):953–62.