Publication Summary

There has been a recent decline in the fisheries in the Potholes Reservoir. In an attempt to determine a possible cause, water samples were collected from the reservoir during the peak application period for agricultural pesticides in 1998. Much of the water entering the Potholes Reservoir is irrigation-return water.

There has been a recent decline in the fisheries in the Potholes Reservoir. In an attempt to determine a possible cause, water samples were collected from the reservoir during the peak application period for agricultural pesticides in 1998. Much of the water entering the Potholes Reservoir is irrigation-return water.

Four herbicides were detected in every sample: 2,4-D, eptam (EPTC), atrazine, and atrazine desethyl (a breakdown component of atrazine). Other pesticides detected were the herbicides, 4-nitrophenol, MCPP (Mecoprop), MCPA, bentazon, simazine, bromacil, hexazinone, as well as the insecticide, azinphos-methyl (Guthion?). Levels of pesticides detected in the reservoir were lower than found in previous studies that sampled surrounding irrigation-return water, but the detection frequency was higher (Embrey and Block 1995, Wagner et al. 1996, Williamson et al. 1998).

Water quality criteria exist for only six of the 12 pesticides detected. Of these six, azinphos-methyl was detected at levels that exceeded EPA (U.S. Environmental Protection Agency) chronic water quality criteria for the protection of aquatic organisms. This occurred at two sites in the northwest part of the reservoir in May. This study does not definitively link pesticides to declining fish levels; however, pesticides may play a part.

Link to EIM data for User Study ID DLR0001