Aug. 25, 2000 Blacksburg, Va., August 24, 2000) -- Growing tomatoes under plastic presents good news and bad news as far as the impact on water quality and aquatic life are concerned.
Andrea Dietrich and Daniel Gallagher, professors of civil and environmental engineering at Virginia Tech, presented their findings regarding "Agricultural copper: Transport and toxicity in runoff from tomato fields to estuaries" at the 220th national meeting of the American Chemical Society in Washington, D.C. yesterday (Wednesday, Aug. 23).
Growing tomatoes using plastic mulch, raised beds, and drip irrigation controls soil moisture and prevents weeds, which reduces the need for herbicides and allows prescriptive use of fertilizers, thus reducing contamination by these chemicals. However, farmers must still use biocides to control fungi, bacteria, and insects, and there is still runoff -- increased runoff, in fact, due to the impermeable plastic and compacted soil between the rows. The plasticulture runoff entering nearby waterways is the copper-based bactericide and fungicide used on the plants to prevent disease.
Dietrich and her colleagues and students in agriculture, biology, and engineering have been studying copper runoff and its impacts and in waters off Eastern Virginia since the mid 1990s, with funding by various state and federal agencies. They have found that concentrations of copper in creeks exceeded the reported mortality level for clams, shrimp, and many fish species.
Farmers have considered several strategies to reduce the runoff, such as sediment ponds and grass strips, and Dietrich and Gallagher have been studying the effectiveness of such measures. In a recent project supported by Virginia Sea Grant (NOAA) and the Virginia Department of Agricultural and Consumer Services (VDACS), the researchers created simulated estuaries (complete with tidal cycles), tomato fields, and a rain simulator in a green house on campus. The research estuaries sustain fish and shrimp, and marsh plant populations so that the impact of agricultural runoff and strategies and treatments can be evaluated. Measurements from both nonplasticulture fields and plastic-covered fields were also part of the study.
The research confirmed that a sedimentation basin does reduce copper concentrations (by up to 90 percent). Sadly, however, it does not reduce the copper concentration sufficiently to eliminate the mortality of fish and grass shrimp. The enclosed greenhouse-scale research environment enabled careful chemical analysis of the copper and the researchers learned that "a significant percent is in a form that is bioavailable to the organisms," according to the final report sent to VDACS in June. "Although sedimentation has the potential to lower the total concentration of copper, it does not effect the fraction of bioavailable copper."
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