Microbial food safety risks of reusing tail water for leafy green production

The use of sprinklers and furrow irrigation frequently results in significant volumes of run-off, also referred to as tail water. Although vegetable growers have made much progress in reducing irrigation run-off by using drip lines, overhead sprinklers are needed for germinating and establishing crops, and for watering high-density leafy greens such as spinach and baby greens. Also, a significant number of acres of lettuce and other vegetables are irrigated by furrow after crop establishment. Many Central Coast ranches have infrastructure for reusing tail water for irrigating crops, including sediment basins, reservoirs, and pumping systems. Currently growers are reluctant to irrigate crops with tail water due to a lack of information on microbial food safety risks. Several options exist for reusing run-off water, which may minimize microbial food safety risks for produce. Tail water could be used for irrigation practices that do not result in direct contact with the crop, such as pre irrigation and germination, as well as for dust control of unpaved roads. Tail water could be reused for crops on drip. Other options include treating run-off by chlorination or other means to kill microbial pathogens, and blending tail water with a clean water source so that microbial levels meet industry or regulatory target levels for surface water. The objectives of this project will be to: 1) monitor, characterize and quantify microbial populations in run-off water from Central Coast vegetable fields; 2) evaluate the risk of using this water source for the production of lettuce and other leafy green crops by quantifying survival of microorganisms during reuse applications; and 3) evaluate economically-feasible methods to treat tail water that would minimize microbial food safety risks for a range of reuse applications (e.g., pre-irrigation, dust control, irrigation).The food safety risk of reusing run-off water needs to be evaluated in commercial vegetable fields under conventional irrigation practices. Our project would address this need, and develop information on the food safety risks associated with re-using run-off water for leafy green production.

The use of sprinklers and furrow irrigation for the production of vegetables frequently results in significant volumes of run-off water, also referred to as tail water. Although vegetable growers have made much progress in reducing irrigation run-off by using drip lines, overhead sprinklers are needed for germinating and establishing crops, and for watering high-density leafy greens such as spinach and baby greens. Also, a significant number of acres of lettuce and other vegetables are irrigated by furrow after crop establishment. Many Central Coast ranches have infrastructure for using tail water for irrigating crops, including sediment basins, reservoirs, and pumping systems. Currently, growers are reluctant to irrigate crops with tail water due to a lack of information on microbial food safety risks. There are several options for reusing tail water that may minimize microbial food safety risks for produce. Tail water could be used for irrigation practices that do not result in direct contact to the crop; for example, using tail water for pre-irrigation and germination or for dust control of unpaved roads. Reusing run-off water for drip irrigation may also reduce microbial food safety risks. Other options include treating tail water by chlorination or other means to kill microbial pathogens, and blending tail water with a clean water source so that microbial levels meet industry or regulatory target levels for surface water. The objectives of this project will be to: 1) monitor, characterize and quantify microbial populations in run-off water from central coast vegetable fields; 2) evaluate the risk of using this water source for the production of lettuce and other leafy green crops by quantifying survival of microorganisms during simulated reuse applications; and 3) evaluate economically feasible methods to treat tail water that would minimize microbial food safety risks for a range of reuse applications (e.g., pre-irrigation, dust control, irrigation). Our project will develop information on the food safety risks associated with reusing run-off water for leafy green production. This information will provide the scientific basis for developing guidance for the use of tail water in the Leafy Greens Marketing Agreement (LGMA).

1. Characterize chemical and physical and microbiological composition of tail water from coastal vegetable fields. 

2. Evaluate risks of using tail water in lettuce and leafy green production by quantifying microbial survival during simulated application to soils used to grow leafy greens. 

[3. Evaluate methods to pre-treat tail water that improve the efficacy of antimicrobial treatments aimed at minimizing microbial food safety risks for a range of reuse applications (e.g., preirrigation and dust control, basic irrigation).]

• E. coli concentration was low in five out of six tail-water reservoirs, with a geometric mean that remained below 126 CFU/100 ml. 

• Prevalence of Salmonella (1.8%), STEC (1.8%) and E. coli O157 (0.4%) was lower than previously described for the Central Coast watershed (Cooley et al., 2014) or California reservoirs (Partyka et al., 2018). 

• Elevated concentrations of phosphate and nitrate did not increase the survival of Salmonella, generic E. coli or E. coli O157:H7 in water. 

• Tail water did not increase the survival of Salmonella or E. coli O157:H7 in soil or on growing lettuce plants compared with well water. 

• Tail water that meets CLGMA metrics for application to edible crops could be used for pre-irrigation and germination of leafy green vegetables and would not be expected to increase food safety risks more than irrigating with well water.