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Measures to Reduce Erosion and Turbidity in Construction Site Runoff
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    Final Report
  • Abstract:
    An evaluation of polyacrylamides (PAM) for both erosion and turbidity control for construction sites was conducted in both the laboratory and the field. A laboratory screening was conducted for 11 PAMs on 13 sediment sources from North Carolina Department of Transportation (NC DOT) construction sites around North Carolina. In addition, a field test of two PAMs at two rates, with and without straw mulch and seeding, on a 2:1 fill slope, a 4:1 cut slope, and a 4:1 fill slope were performed. The results indicate that there is no one PAM that is effective for turbidity reduction on all sediment sources, but several are promising for many soils. Superfloc A-100 ranked among the top three flocculants for 10 of 13 sediment sources. Some PAMs are equally effective but at different doses, some as low as .075 mg/L, or a few grams per 1,000 ft3 of water. The differences between PAMs in reducing turbidity was clearest shortly after mixing the PAM and soil (30 sec). These turbidity differences were usually maintained 30-60 minutes after mixing, but allowing the soil/water mix to settle for 24 hours reduced or eliminated the differences. Tests of PAM with and without mulching on 2:1 slopes at NC DOT construction sites failed to show a significant reduction in turbidity or erosion. Erosion rates were 20 times greater on bare soil plots after the first seven events, with or without PAM, compared to those mulched with straw and seeded to grass. During the eighth and last event, in which over 6 cm of rain was recorded, rates of over 50 tons/ha were recorded for a single, intense storm event for the bare soil plots compared to 3-9 tons/ha on the mulched/seeded plots. PAM at the highest rate (11 kg/ha) was effective in reducing erosion and turbidity on the 4:1 cut slope with a clay loam texture but the effect declined with each storm event. On the sandy 4:1 fill slope, there was no evidence of PAM effects, even at 20 kg/ha. PAM was effective in flocculating turbid water pumped from a borrow pit but turbulence within the basin tended to keep the flocs from settling.

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