Agricultural runoff impacts on total maximum daily loads and water quality
Agricultural runoff impacts on
total maximum daily loads and water quality
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Florida has a diversified aquaculture industry that is expanding rapidly throughout the state. In order to assure that such expansion does not negatively affect the environment, the Florida State Legislature has charged the Florida Department of Agriculture and Consumer Services (FDACS) with the responsibility of developing Best Management Practices (BMPs) whereby aquaculture will be governed. To this end, the Florida Three Rivers Resource Conservation and Development Council, Inc. (Three Rivers RC&D), the Escambia Soil and Water Conservation District, the USDA – Natural Resources Conservation Service (NRCS), the University of West Florida (UWF) Wetlands Research Laboratory (WRL), and the University of Florida have worked cooperatively with the FDACS to evaluate a Best Management Practices (BMPs) for catfish aquaculture ponds. Objectives
This project is to evaluate a grass filter strip BMP designed to improve the
water quality of storm-induced production water discharges from a commercial
open-pond fish culture system by reducing dissolved and suspended solids and
attenuating nutrients as overflow progresses toward the receiving
watershed. We hope to draw inferences that may be useful in applying filter
strip technology to problem solving for Florida’s aquaculture industry. The project consists of one 8-acre channel catfish pond designed to detain and discharge storm water through a trickle-flow control device—as described in the University of Florida Bulletin 334—to a vegetated filter strip built in accordance with design criteria. Runoff sample collection sites were the point of pond discharge into the filter strip and the point of filterstrip discharge into the receiving watershed by autosampler and specific locations within the filter strip itself manually, typically an exponential gradient of 1m, 5m, 10,m, 20m, 40m, 80m, and 150m from the pond discharge pool. Water samples were obtained by 60 cc syringes taking care not to disturb sediments to ensure samples were representative of the water flowing down the grass strip. Whole water, and water filtered through Whatman GF/F filters were used for dissolved and particulate analysis of water quality. Other parameters were recorded by a water quality multiprobe (Temperature, Total Dissolved Solids, pH, Dissolved Oxygen). Gradients of dissolved oxygen down the filter strip were also recorded on a linear gradient (every 5m) on several occasions. . In addition, we took periodic composite soil samples each consisting of seven sub-samples along a transect at right angles to the water flow down the filter strip at 1m from the discharge pipe and at 5m, 10m, 20m, 40m, 80m, and 150m from the outfall. The composite soil sample were homogenized in Whirlpaks® and transported to the laboratory on ice in a cooler. After a small portion was removed from the soil samples for microbiological analysis, both soil and water samples were stored at -20°C to be later analyzed for parameters required by the contract with the Three Rivers RC&D and additional parameters as described in Table 1. Table 1: Analytical Parameters for Water and Soil Samples from the Grass Filter Strip Aquacultre BMP
Data based on analysis of samples from the site are organized in Table 2 Sampling Activity by UWFEarly in 2001, the UWF team installed a recording rain gauge, a flow meter / data-logging device, and two refrigerated autosamplers at the site. One autosampler was set up to collect samples at the pond outfall and the second at 150 meters from the outfall close to the discharge point to the receiving watershed. The current meter was checked by periodic manual determination of the flow rate. Water quality sampling did not begin until mid-summer 2002 due to extremely low rainfall resulting in a net loss of pond volume/level with no discharge. A decision was made to pump the pond up to the discharge level, and our first sampling coincided with that event. The discharge hole had been plugged, and the pond level increased to above the discharge hole. Removal of the plug resulted in a sustained flow down the filterstrip as the pond level adjusted accordingly. Rain events provided additional sampling opportunities beginning around 25 July 2002. Our first sample set was collected manually on 26 July 2002; subsequent rainfall events allowed collection of numerous runoff samples from the site, both manually during and following rainfall events, as well as by the autosamplers, triggered by flow in the discharge pipe. For several rain events, the autosamplers provided continuous samples during subsequent discharge, documented by continuous current meter recordings, with periodic manual sampling during the discharge period. Sample volumes and treatment were in accord with the WRL Certified protocols for manual sampling.
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