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 were 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.
Applying PAM to bare soil surfaces to reduce erosion appears to have two limitations. One is the slope steepness, since we found no evidence of PAM effects on a 2:1 slope but some erosion and turbidity reductions on a 4:1 slope. Since PAM is known to only influence the surface 1-2 mm of soil by maintaining structure, the erosive forces on steep slopes apparently remove this layer relatively quickly during the first storm. Exactly where the breaking point is for PAM to be effective will likely be site specific. The second limitation is in the longevity of treatment effects. There is some evidence that these disappeared in the field after several rainfall events. This was further corroborated in our rainfall simulation tests. Again, the longevity of PAM effectiveness will be specific to the application, site characteristics, and rainfall patterns. Higher rates of PAM may be more effective for longer periods. In most cases, the mulch and seed treatment was usually so effective in reducing erosion and turbidity that the PAM treatments did not provide any additional benefit to water quality.
PAM was found to be effective in reducing turbidity produced from a wide range of sediment sources. A number of reactions were observed, including several sediment sources which did not respond to the PAMs in our tests. However, most of the sediments had relatively straightforward, linear reductions in turbidity with increasing PAM concentrations. In most cases, 0.5-1.0 mg/L was sufficient to achieve maximum turbidity reduction. It appears that PAM is effective for turbidity caused by a majority of sediments from around North Carolina.
We have not developed clear relationships between sediment and PAM properties and turbidity reduction at this point. Work will continue on this topic with the goal to develop a guide to PAM selection based on one or more sediment properties.
Recommendations to reduce erosion and turbidity in construction site runoff include:
- Mulching and seeding is extremely effective in stopping erosion - this practice should be implemented after soil disturbance as quickly as feasible.
- PAM use for erosion control on bare soil has potential, but may be limited to low slopes for short periods.
- PAM is effective in reducing turbidity caused by most sediment sources tested and should be included as part of sediment control systems in the future.
- PAMs will need to be selected for site-specific conditions. However, some PAMs are useful for a wider array of sediments than others.
- PAMs may not be effective in some cases and may need to be augmented or substituted with inorganic salts, coagulants, or other materials.
- Flocs formed after PAM applications may require filtration or relatively calm water in order to settle. Where pumping is already in place, such as at borrow pits, filtering systems would be relatively simple to devise or obtain.
