Agricultural land use : effects on the chemical quality of runoff

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Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University

This research is the first phase of the Blackstone Environmental Quality Program, a study to distinguish the water quality effects of agricultural practices. Th is research focused on changes in runoff quaIity when land is put under cultivation. The data will be used in later phases of the program to evaluate chemical transport models. The three study watersheds, located in Virginia's southern Piedmont, had not been under cultivation for 30 years. During the research, two of the watersheds were put into agricultural production while the third was left uncultivated and used as a control.

A grab sampling program was maintained on two watersheds for the full five-year period of study. Continuous streamflow gaging was maintained for the last four years. During the final three years, sampling and flow gaging were also maintained on a third watershed. For those three years, grab sampling was supplemented by automatic samplers for stormflow sample collection. Samples were analyzed .for nitrate, ammonium, and total Kjeldahl nitrogen, orthophosphate and total phosphorus, bicarbonate alkalinity, pH, and specific conductance.

Clear differences in streamflow characteristics were noted between the treated and the control watersheds. More surface runoff was observed in the treated watersheds than in the control, suggesting that increased surface runoff may have resulted from the cropping activity. All three watersheds exceeded the phosphorus criterion proposed by the Environmental Protection Agency, and a two-to-threefold increase in total phosphorus concentration was attributed to agricultural land use without employment of conservation practices. A twofold increase in total nitrogen concentration also was noted. Soluble inorganic nitrogen was present at a low concentration in the control watershed, but accounted for 20-30 percent of all nitrogen exported from the treated watersheds. The presence of nitrate nitrogen in the agricultural watersheds was the characteristic most attributable to agricultural activity. The research showed that serious enrichment problems could occur in Piedmont lakes or impoundments if the predominant land use of a watershed were agricultural cropping and if conservation practices were not employed.