NC State Study Finds Better Way to Protect Streams from Construction Runoff

Researchers at North Carolina State University have found an exponentially better way to protect streams and lakes from the muddy runoff associated with stormwater around road and other construction projects.

The alternative is lower or comparable in cost to commonly used best management practices (BMPs) around construction sites, yet much more effective at keeping streams and lakes free of runoff sediment that pollutes water and harms aquatic life.

In a study comparing BMPs against alternatives on road stormwater runoff in western North Carolina, the NC State researchers found the alternative method kept local streams that received the runoff cleaner, and helped reduce the amount of sediment loss inside ditches near roads. Sediment and muddy water are among the most common pollutants of streams and lakes.

Dr. Rich McLaughlin, associate professor of soil science at NC State and one of the researchers involved in the project, says that the current BMPs used in controlling erosion and sediment involve using so-called “sediment traps” along with rock check dams in ditches. Sediment traps collect water with the heavier sediment – like dirt and other larger, heavier particles – settling to the bottom and the “cleansed” water moving through rock check dams, or piles of rock that are intended to slow the flow of water through the ditch. Water then travels out of the ditch through a pipe to streams, rivers or lakes.

In the study, McLaughlin and NC State colleagues Scott King, extension associate in soil science, and Dr. Greg Jennings, professor and extension specialist in biological and agricultural engineering, found that the BMPs don’t hold a candle to the alternative – natural fiber check dams (FCDs) enhanced with polyacrylamide (PAM), a chemical that causes sediment to clump together. FCDs use natural fibers instead of rocks as a type of dam to slow the flow of water in ditches.

The researchers found, in a measure of the “muddiness” of road runoff, that the BMPs yielded 3,813 nephelometric turbidity units (NTUs) in testing, equating to some rather muddy water, McLaughlin says. Fiber check dams with PAM yielded averages of 34 NTUs, a veritable drink of Perrier in comparison, McLaughlin adds.

Further, the study showed that after a storm, sites that used standard BMPs lost an average of 944 pounds of sediment compared with only 1.8 pounds of sediment lost at sites utilizing FCDs with PAM.

McLaughlin says that these results are so convincing that North Carolina’s Department of Transportation is in the process of making FCDs with PAM the new best management practice around road and construction sites. McLaughlin’s group is also training engineers and installers around the state and nationally in the use of this system.

A paper showing the study results appears in the March/April edition of the Journal of Soil and Water Conservation.

– kulikowski –

Note: An abstract of the paper follows.

“Improving construction site runoff quality with fiber check dams and polyacrylamide”

Authors: Richard A. McLaughlin, Scott E. King and Greg D. Jennings, North Carolina State University

Published: March/April 2009 edition of the Journal of Soil and Water Conservation

Abstract: Sediment and turbidity are among the most common pollutants affecting surface waters, resulting in reduced reservoir capacity, degradation of aquatic organism habitat, and decreased aesthetic value. Construction activities, including roadway projects, can be significant contributors to sediment loading in streams and lakes. We studied water quality in stormwater runoff from three systems for erosion and sediment control on two roadway projects in the North Carolina mountains. The first roadway project was divided into three experimental sections, each with one the following treatments installed in the adjacent drainage ditch: (1) the standard best management practice (BMP) consisting of narrow sediment traps in the ditch along with rock check dams, (2) fiber check dams (FCDs) consisting of a mix of straw wattles and coir logs, or (3) FCDs with granulated, anionic polyacrylamide (PAM) added to each. The second project was smaller and included only two of the experimental sections described above: (1) the standard BMPs and (2) FCDs with PAM. Significant reductions in turbidity and total suspended solids were obtained using the FCDs, particularly those with PAM added. At site 1, from June 2006 to March 2007, the average turbidity values for the stormwater runoff were 3,813 nephelometric turbidity units (NTU) for the standard BMPs, 202 NTU for the FCDs-only, and 34 NTU for the FCDs with PAM. Average turbidity in discharges at site 2 was reduced from 867 NTU for the standard BMPs to 115 NTU for the FCDs with PAM. Sediment loading at both sites was similarly reduced with the use of FCDs. At site 1, the standard BMPs lost an average of 428 kg (944 lb) of sediment per storm event compared to just 2.1 kg (4.6 lb) for the FCDs-only and 0.9 kg (2.0 lb) for the FCDs with PAM. At site 2, the standard BMPs lost an average of 3.3 kg (7.3 lb) per storm event compared with 0.8 kg (1.8 lb) for the FCDs with PAM. A conservative economic analysis suggests that the costs of the FCDs are lower than the standard BMPs. This study suggests that the use of FCDs with PAM can bring discharges from similar linear construction projects much closer to the regulatory guidelines for non-point source discharges than the current standard practices.