Abstract
The objective of this research was to assess soil conservation practices for improving water quality of return flows from rill irrigation in the Yakima River Basin, Washington, by combining patch application of polyacrylamide (PAM) with an additional erosion control practice. A 2-year field study was conducted that combined PAM with (1) check dams, (2) surge irrigation, (3) surface drains, and (4) grass filter strips. The study was conducted at three sites: two vineyards (A and B) with silt loam soils at 1.2% slope and a cornfield with sandy loam soils at 0.2% slope. During irrigation events, water samples and flow records were taken at periodic intervals from each treatment to determine nutrient concentrations and loads (total nitrogen (TN), total Kjeldahl N (TKN), nitrate–N (NO 3–N), total phosphorus (TP), particulate P (PP), soluble phosphorus (SP), and sediment load (SL)). For all treatments and sites, TN and TP concentrations were compared to USEPA value concentrations in streams of the Xeric West for full support of aquatic life and drinking water standards. Results showed that TN exceeded the USEPA-reference condition of 0.36 mg TN/L in all samples, while 96% of the samples exceeded the USEPA-recommended TP concentration value of 0.1 mg/L. All samples showed NO 3–N concentrations below the USEPA drinking-water standard of 10 mg/L. The only nutrient component in irrigation runoff that was strongly related to SL was PP concentration ( r = 0.87). For TKN, significant concentration and load reduction between control and the other four erosion control practices ( P < 0.05) occurred only in vineyard A. As for PP, the four PAM integrated control practices showed statistically significant effects with respect to the PAM control in vineyard B only. Although PAM is an excellent practice to control soil erosion with widespread adoption, additional off-site treatment may be needed for nutrient concentrations in irrigation return flows to meet reference conditions that would fully support aquatic life.
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