Publication Summary

Implementation of best management practices (BMPs) on irrigated agricultural lands can reduce nonpoint source pollution and improve the quality of irrigation tailwater and receiving waters by reducing soil erosion. The North Yakima Conservation District developed a BMP Demonstration Project in a sub-basin of the Moxee Drain watershed in order to demonstrate the benefits of BMPs to water quality. Water quality monitoring was carried out before, during, and after BMPs were implemented within two drainages in the project area where furrow-irrigated hops and sprinkler-irrigated orchards were the dominant land use. The primary objective of the study was to associate water quality benefits to BMP implementation.

Implementation of best management practices (BMPs) on irrigated agricultural lands can reduce nonpoint source pollution and improve the quality of irrigation tailwater and receiving waters by reducing soil erosion. The North Yakima Conservation District developed a BMP Demonstration Project in a sub-basin of the Moxee Drain watershed in order to demonstrate the benefits of BMPs to water quality. Water quality monitoring was carried out before, during, and after BMPs were implemented within two drainages in the project area where furrow-irrigated hops and sprinkler-irrigated orchards were the dominant land use. The primary objective of the study was to associate water quality benefits to BMP implementation.

Approximately 18 samples per site were collected at multiple sites during each irrigation season in 1991 and 1992. Sample analyses included suspended and settleable solids, turbidity, and nutrients. Pesticides were characterized in samples taken from irrigation tailwater and tailwater sediment.

Findings suggest that project BMPs were effective in improving water quality. Climatic factors leading to a 42% reduction in irrigation water supply between 1991 and 1992 appear to have also contributed to improved water quality, which confounded the "before/after" BMP evaluation strategy used in this study. Large changes in pollutant concentrations and loads (generally greater than 50%) needed to occur before they could be deemed statistically significant. Best management practices were implemented on 917 acres, or about 87% of the project's irrigated lands. Runoff from the lower drainage contributed the most useful water quality information, where BMPs were implemented on 257 acres of furrow-irrigated hops (70% of the lower drainage).

Median pollutant reductions at the site (D1) draining the BMP demonstration area were realized for total suspended solids (TSS) - 86%, TSS load - 90%, Imhoff settleable solids (SS) - 99%, nephelometric turbidity - 63%, absorbtometric turbidity 75%, ammonia - 56%, and ammonia load - 60%. An 18% reduction in the median flow was not significant. Median pollutant reductions at a comparable site (M1) draining non-BMP croplands were realized for Imhoff SS - 93%, nephelometric turbidity - 52%, and absorbtometric turbidity - 48%. A 28% reduction in median flow at site M1 was statistically significant. Roza Canal (upper drainage) monitoring did not yield useful data for determining the effectiveness of BMPs.

Pesticides detected in tailwater and tailwater sediments were similar to those detected by other investigators in the Moxee Drain area. Concentrations of DDT compounds from this and other studies exceeded Washington State water quality standards for freshwater chronic effects on aquatic biota.

Link to EIM data for User Study ID KESE0001