Using Vegetation and Polymers to Control Sediment, Nutrients, and Bacteria In Irrigation Run-off From Vegetable Fields

Irrigation tail water from vegetable fields is a source of nutrients, sediments and bacteria in surface water on the central coast of California.  Because of concerns for microbial food safety and quality of surface water, effective measures that control run-off are needed for the cool season vegetable industry.  We conducted a 2-year study evaluating the effectiveness of vegetated ditches and polymers to reduce levels of sediment, nutrients, and E. coli  bacteria in run-off from  sprinkler-irrigated lettuce grown in the Salinas Valley.  The trials were begun after the thinning stage of a head lettuce crop.  Generic E. coli was introduced to the field by placing satchels of sand media containing the bacteria in the furrows for the first irrigation with overhead sprinklers. The field trial followed a randomized latin square design with 4 replications over time. Four 0.36 ha plots were sprinkler irrigated with water treated with either a 5 ppm concentration of polyacrylamide or water that was untreated.  Run-off from the plots was diverted through ditches of a 52-m length that were either bare or lined with grass vegetation.  Composite water samples were collected above and below the ditches and analyzed for suspended sediments, nutrients, and E. coli and coliform bacteria.  Applied water was measured using flow meters and total run-off from the plots was measured using trapezoidal flumes.  Polyacrylamide in the irrigation water reduced suspended sediment concentration and turbidity by more than 90% in the run-off.  Polyacrylamide also reduced the concentration of total P, and total N by as much as 70% in the run-off, and soluble P was also significantly reduced but by less than 50%.   The vegetated ditches did not consistently reduce the concentration of suspended sediments and nutrients in the run-off.  Neither vegetation nor polyacrylamide reduced the amount of run-off from the fields or reduced the concentration of coliform and E. coli bacteria in the run-off.