Land Use Impacts of Plastic Mulch Tomato Production and Evaluation of Sorbent Control on Shellfish Hatchery Operations
Schmitt, Dorothy Reade
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During the 1990s, shellfish hatcheries operating on the Eastern Shore of Virginia noticed an increase in mortalities of their clam larvae; it was suspected that this was a result of the deteriorating quality of the estuarine water used by aquaculture hatcheries. Many hatcheries obtain their facility process water directly from nearby streams and estuaries. During the same time period, there was also an increase in the use of plastic mulch on some of the vegetable fields on the Eastern Shore. The increased runoff commonly associated with these plastic-covered fields often contains copper-based bactericides that are used on the crops. The plasticulture fields were often located adjacent to the same estuaries from which the shellfish hatcheries draw their water. High levels of copper were measured in multiple surface water locations near these fields. Runoff associated with the plasticulture fields contained up to 238 ug/L dissolved copper, well in excess of the copper concentration of 16.4 ug/L dissolved copper known to affect the mortality of larval clams. Surface water samples collected from nearby unimpacted water bodies contained less than 4 ug/L dissolved copper. Sorption studies conducted with synthetic estuary water quantified the dissolved copper sorption capacities of eight sorbents, including GACs, zeolites, a greensand, and an ion exchange resin, Amberlite IRC-718. These isotherm studies showed that all sorbents removed soluble copper, and that increased salinity and a greater contact time were shown to increase sorption. Utilizing the Freundlich isotherm, the capacity of the eight sorbents studies ranged from 25 to 221 ug/g with a 24-hour contact time. Column studies with an influent concentration of 300 ug/L dissolved copper showed that using 10 grams of a GAC sorbent, a flow rate of 5 mL/min maintained the target effluent concentration of less than 16.4 ug/L for approximately 100 hours. A similar column using 10 g of resin sorbent at a flow rate of 12.5 mL/min maintained the target effluent concentration for approximately 300 hours. The high removal capacity of the resin makes it a desirable treatment for controlling copper in estuarine water used for aquaculture. Utilizing the sorbent capacities obtained from the Freundlich isotherm and a batch treatment, a typical 500 gallon tank used for clam aquaculture would require approximately four pounds of a GAC sorbent, or two pounds of the resin sorbent to reduce a source water that contains 300 ug/L of dissolved copper to below the larval clam toxicity level. Both sorbents would require a 24-hour contact time.
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