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Effectiveness of Current BMPs in Controlling Sediment Discharges from Small Construction Sites

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Stormwater runoff from construction sites has become an increasingly major contributor of fine inorganic sediment input into streams and rivers. The negative impacts of excess fine sediment loads extend to all segments of the aquatic ecosystems from microbes to fish. While large construction projects represent single major potential pollution sources and are usually more visible, smaller construction sites (<5 acres) are both more numerous and are less likely to employ adequate erosion control best management practices (BMPs). The most common BMPs employed at such sites are silt fences. Few field studies have been performed to evaluate the effectiveness of these BMPs, especially as affected by physical site and rainfall characteristics.

The objectives of this project were to evaluate the effectiveness of currently-employed best management practices (i.e., silt fences) in controlling sediment discharges from small construction sites, and to evaluate the extent to which the addition of vegetated buffer strips improved control. Additionally, the sensitivity of a number of biological metrics to stress associated with sedimentation effects was also investigated. The study was carried out in the upper Cahaba River watershed in north central Alabama.

The mean count of small particles below silt fences was approximately 54% less than that from areas with no erosion control measures. However, even though the fences appeared to be properly installed and in good order, the variability between samples was sufficiently great that the difference between these means was not statistically significant. The silt fences did not reduce particle counts to levels comparable to nearby undisturbed sites. For every variable measured, the mean values of samples taken below silt fences were significantly higher (p << 0.001, ANOVA on log transformed data) than samples collected from undisturbed vegetated control sites. Particle count analysis indicated that silt fences followed by high quality vegetated buffer zones of 5, 10 and 15 feet resulted, on average, in the removal of approximately 54%, 58% and 60% of total particles respectively.

Although large variation was noted, silt fences alone removed approximately 52% of total solids from small construction site runoff. Silt fences followed by high quality vegetated buffer zones 5, 10 and 15 feet in width removed on average 55%, 62% and 68% of the total solids respectively. Increased removal of total solids by vegetated buffer zones correlated weakly with a decrease in the slope of the buffer zone, but was not significantly correlated with amount or intensity of rainfall.

Analysis of the fish biota revealed strong negative associations between measures of sedimentation and measures of the biological integrity of the fish community, including the Index of Biotic Integrity, Percent Relative Abundance (PRA) of darters, PRA crevice-spawning minnows, and PRA sunfish. A number of characteristics of stream benthic macroinvertebrate communities differed greatly between control and sediment-impacted sites, and thus appeared to be sensitive to sedimentation. Some of the metrics that appeared to reflect sediment-associated stresses include the Hilsenhoff Biotic Index, a variation of the EPT index (%EPT minus Hydropsyche and Baetis), and the Sorensen Index of Similarity to a reference site.
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Document Type: Research Article

Publication date: 01 January 2004

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