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Managing Construction Stormwater Runoff

Managing Construction Stormwater Runoff

By: Michael A. Perez, Ph.D., CPESC, Assistant Professor at Auburn University

From Getting Into Soil and Water 2018

Since the implementation of the Clean Water Act in 1972, the Environmental Protection Agency has been able to curtail pollution to waterways from many point sources. However, pollution impacts from nonpoint source stormwater runoff continue to increase. Stormwater management has become an increasingly important topic in the state of Iowa with a large focus on finding ways of improving agricultural runoff, which pollutes streams and rivers with high nutrient and sediment loads. While the vast majority of land in Iowa is dedicated to agricultural production, there is another major culprit to non-point pollution sources: construction sites.

Construction activities generally involve heavy earthmoving activities that disturb several acres of land at a time. Due to the nature of construction activity, sediment is the predominant pollutant of concern during the clearing and grading stages, which typically exposes large un-vegetated and unstabilized land areas to erosive elements. The lack of ground cover during construction results in land areas being susceptible to increased rates of soil erosion. Other pollutants associated with land development include: fertilizers, pesticides, petrochemicals, construction chemicals, wash water, paper, garbage and sanitary waste. As stormwater runoff flows over unprotected areas on construction sites, it can suspend and transport pollutants causing significant physical, chemical and biological water quality impacts, and impairments to nearby receiving waters (1).

Soil Loss

By the Numbers Stormwater induced erosion occurs when rain droplets impact unprotected soil, creating dislodgement and transport. Raindrops fall at 20 miles per hour and with equivalent impact energy of 10,000 tons of T.N.T. per square mile (2). Poorly managed construction activities can create a major source of pollution. In fact, sediment runoff rates from construction sites have been estimated to be 10 to 20 times higher than those of agricultural lands and 1,000 to 2,000 times greater than forested lands (3). Construction sites have measured erosion rates of approximately 20 to 200 tons per acre per year (4). Construction generated sediment washes into waterways in the U.S. at a rate exceeding 80 million tons per year. To put it into perspective, that amount of soil would be able to load 4.9 million dump trucks; enough dump trucks to completely fill a nine lane highway between New York and Los Angeles.

Managing Construction Site Runoff

The state of Iowa manages construction generated pollution through the National Pollutant Discharge Elimination System (NPDES) General Permit. This permit requires all construction activities that disturb one or more acre of land at a time to implement stormwater management practices. These practices include providing measures to reduce erosion, capture sediment and minimize other pollutant sources.

The most effective way to minimize sediment loss from construction sites is to prevent soil from dislodging and washing away in the first place. This can be accomplished by reducing the amount of area disturbed at one time and by reducing the total amount of time that disturbed areas are left exposed. For example, an effective pollution prevention plan would limit the disturbance to a single residential lot at a time, rather than clearing and grubbing an entire subdivision plat as is common practice in home building.

Erosion control practices focus on providing cover to unvegetated areas and protecting areas from surface runoff. Cover practices include straw mulching, erosion control blankets, and temporary and permanent vegetation. These practices act to absorb the impact of raindrops, increase infiltration, reduce runoff and slow runoff velocities. Other erosion control practices focus on managing surface runoff to prevent dislodgement of soil caused by high velocity and shear forces flowing through ditches, swales and channels.

The last line of defense on construction sites are sediment control practices. These practices are designed to capture and promote sedimentation on-site. Common sediment control practices include: sediment barriers, inlet protection practices and sediment basins. These practices function by temporarily impounding and detaining runoff allowing large, rapidly settleable particles to fall out of suspension through gravitational forces.

Active treatment methods can also be used to further improve stormwater prior to offsite discharge. These treatment practices generally rely on applying flocculants to encourage particle settlement and can include advanced treatment methods such as sand filtration.

An effective stormwater pollution prevention plan implements a combination of erosion and sediment control practices along with effective communication and management of work activities to minimize environmental impacts.

Research

Researchers at land grant universities such as Iowa State University, Auburn University and North Carolina State University are developing ways to improve the tools and technologies used on construction sites to help manage stormwater runoff and minimize the impact to our nation’s water bodies. Researchers evaluate practices under large-scale conditions to simulate runoff and sediment loads that are typical to construction sites. These simulated storm conditions allow practices to be evaluated under the same conditions to which they would be subjected in the field. This type of large-scale testing allows researchers to document how practices fail and can then provide installation improvements. Technology and treatment methods developed by researchers are helping construction site operators implement more effective tools to help manage stormwater runoff and reduce the amount of pollutants entering our water bodies.

[1] Maxted, J. R., and E. Shaver. The Use of Retention Basins to Mitigate Stormwater Impacts to Aquatic Life. Presented at National Conference on Retrofit Opportunities for Water Resource Protection in Urban Environments, Chicago, IL, 1998. [2] Georgia Soil and Water Conservation Commission. Manual for Erosion and Sediment Control in Georgia, 2016. [3] United States Environmental Protection Agency. Effluent Limitations Guidelines and Standards for the Construction and Development Point Source Category. Publication EPA-HQ-OW-2008-0465, 2009. [4] Pitt, R., S. E. Clark, and D. W. Lake. Construction Site Erosion and Sediment Controls: Planning, Design and Performance. DEStech Publications, Lancaster, Pa., 2007.

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