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Volume balance and toxicity analysis of highway storm water discharge from Cross Lake Bridge.
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    Final report; May 2003-Aug. 2009.
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  • Abstract:
    The Cross Lake Bridge in Shreveport, Louisiana, spans Cross Lake that serves as the city’s water supply. Concern about accidents on the bridge contaminating the lake prompted the Louisiana Department of Transportation and Development (LADOTD) to construct a closed drainage system on the bridge to convey precipitation falling on the bridge deck to a concrete lined holding pond on the east side of the lake. Once there, bridge runoff is tested for oil and grease, pH, and lead before discharge to 12-mile bayou. This report describes the methodology and results of two studies dealing with runoff from the Cross Lake Bridge. The first examined the degree to which repairs to the Cross Lake Bridge’s closed drainage system were successful in capturing runoff from the bridge. An earlier study found that, because of leakage, only about one half of the precipitation falling on the bridge actually ended up in the holding pond. This is substantially less than would be expected from a catchment that is essentially impervious. To test the success of repairs to the drainage system, volume balances for 22 precipitation events were conducted by comparing the rainfall on the bridge to the subsequent flow into the pond. The ratio of the volume of runoff to the volume of precipitation was found to average 0.86, which when compared to values from the previous study as well as published values, suggested a successful repair of the drainage system. The second study examined the use of the Microtox toxicity analysis system manufactured by Azur Environmental for use as a screening tool to assess toxicity of the pond contents. Microtox protocols rely on measurement of light output from a specific strain of luminescent bacteria. Toxicity is indicated when light output from the bacteria decreases. Tests can be run in time periods ranging from 5 to 30 minutes. The standard endpoint of the tests occurs once light output has dropped 50 percent and is termed the EC50. Samples from the water column and sediment were collected and analyzed for toxicity using the basic test (liquid) and the basic solid phase test (sediment).
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