Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 8, 1998
Publication Date: N/A
Interpretive Summary: Nonpoint source pollution has been linked to various agricultural practices; however, there is a lack of quantitative information that describes the effect of farming practices on surface water quality at the watershed scale. This study documents the effect of farming on the water quality of Walnut Creek - a 5130 ha intensively cropped watershed in central Iowa. Over a 6-year period, starting in 1990, water flow and concentrations of nitrate-nitrogen and four commonly used herbicides; atrazine, alachlor, metribuzin, and metolachlor were measured at eight locations within the watershed. The findings document that while detectable, herbicides rarely exceeded the maximum contaminant level for drinking water in this stream. Herbicide losses from the watershed totaled less than about 1% of the herbicide applied in most years for weed control. Conversely, nitrate-nitrogen was frequently measured at concentrations exceeding the maximum contaminant level standard set for drinking water. Nitrate-nitrogen losses were equivalent from 6 to 115% of the nitrate-nitrogen applied as fertilizer in any one year. The study documents that nitrogen contamination of surface water is the primary nonpoint pollutant within this watershed and should be the focus for future best management recommendations. The study will help guide researcher's and regulator's efforts in mitigating agriculture's contribution to nonpoint pollution of surface waters.
Technical Abstract: Nonpoint source pollution has been linked to various agricultural practices; however, there is a lack of quantitative information that describes the effect of farming practices on surface water quality at the watershed scale. This study documents the effect of farming on the water quality of Walnut Creek - a 5130 ha intensively cropped watershed in Iowa. Starting in 1990, flow and concentrations of nitrate-N and four herbicides, atrazine, alachlor, metribuzin, and metolachlor were measured at three stream locations, three county drain outlets, and two field tile outlets. Nitrate-N was detected at concentrations greater than 1 mg L**-1 in virtually every water sample. Nitrate-N concentrations often exceeded the maximum contaminant level (MCL) for drinking water of 10 mg L**-1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha**-1 yr**-1. These losses represent 6 to 115% of the nitrogen applied as fertilizer in any year. Atrazine ad metolachlor were detected at concentrations greater than 0.2 ug 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 ug L**-1 for all locations within the watershed. Herbicide concentrations had a positive correlation with flow during individual storms. Total losses from the watershed ranged from 0.2 to 7.5 g ha**-1 yr**-1 for atrazine and from 0.3 to 6.7 g ha**-1 yr**-1 for metolachlor. These losses represent 0.28 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year. While present in surface water, atrazine and metolachlor rarely posed a health risk in Walnut Creek. Conversely, nitrate was a persistent chemical found at concentrations exceeding safe drinking water standards.