Water quality in Walnut Creek Watershed: herbicides and nitrate in surface waters

• Published in: Journal of environmental quality 1999-01, Vol.28 (1), p.45-59
• Main Authors: Jaynes, D.B, Hatfield, J.L, Meek, D.W
• Format: Article
• Language: English
• Subjects: alachlor; contaminants; farming systems; herbicides; metolachlor; metribuzin; nitrate nitrogen; runoff; Surface water; USA, Iowa; USA, Iowa, Walnut Creek; water pollution; Water quality
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq1999.00472425002800010005x

There is a lack of quantitative information describing the impact of farming on water quality at the watershed scale. This study documents the surface water quality of Walnut Creek--a 5130-ha watershed with about 86% of the land used for crop production, Starring in 1990, flow and concentrations of NO3-N and four herbicide atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3, 5-triazine-2,4-diamine], alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1, 2,4-triazin-5(4H)-one], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2- methoxy-1-methylethyl)acetamide]-were measured at eight locations. Nitrate-N concentrations often exceeded 10 mg L-1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha-1 yr-1 and represented 6 to 115% of the N applied as fertilizer in any year. Atrazine and metolachlor were detected at concentrations >0.2 microgram L-1 in about half of all water samples, while alachlor and metribuzin were seldom detected, Median concentrations for atrazine and metolachlor were below 1 microgram L-1 for all locations within the watershed. During runoff events, herbicide concentrations in the stream increased while NO3-N concentrations decreased. Yearly losses from the watershed ranged from 0.2 to 7.5 g ha-1 for atrazine and from 0.3 to 6.7 g ha-1 for metolachlor. These losses represent 0.18 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year.