Launch Slideshow

Image

Cutting Off Sediment at the Source

Cutting Off Sediment at the Source

  • Image

    http://www.pwmag.com/Images/tmpA2F%2Etmp_tcm111-1343521.jpg?width=300

    true

    Image

    300

    An example of soil erosion. Effective erosion control at construction sites helps maintain water quality while reducing erosion-related cost overruns and project delays.

  • Image

    http://www.pwmag.com/Images/tmpA30%2Etmp_tcm111-1343525.jpg?width=300

    true

    Image

    300

    EPA's new effluent limitations guidelines require larger construction sites to limit sediment in stormwater discharges to 280 nephelometric turbidity units (NTU). This image shows what stormwater samples look like at different NTU levels — from pollutant-free water to nearly opaque, polluted water. From left: 0 NTU, 20 NTU, 100 NTU, and 800 NTU. Photos: Brian Wanzenried

  • Image

    http://www.pwmag.com/Images/tmpA31%2Etmp_tcm111-1343528.jpg?width=300

    true

    Image

    300

    Sedimentation basins are used to capture coarse sediment from storm flows, and temporarily detain stormwater so that the velocity of storm flows is reduced and the sediment falls. Maintenance is required to clean out sedimentation basins, like the one shown below, after storms — otherwise they fill up with sediment, causing the walls to erode.

  • Image

    http://www.pwmag.com/Images/tmpA32%2Etmp_tcm111-1343532.jpg?width=475

    true

    Image

    475

    This table shows the EPA's hierarchy of pollution-prevention techniques from most to least preferred. The Pollution Prevention Act of 1990 also lists source reduction as a preferred method of pollution control over recycling, treatment, and disposal.

To reduce water pollution generated during construction, the U.S. EPA has issued effluent limitations guidelines requiring larger sites to limit sediment in stormwater discharges and implement control technologies. Approved November 2009, the regulation limits runoff to 280 nephelometric turbidity units (NTU) on 20-acre construction sites and eventually, depending upon the location, on 10-acre sites.

The regulation, which took effect in February, is being phased in over four years. This gives permitting authorities — EPA, state environmental agencies, and public works departments with stormwater enforcement authority — time to develop monitoring requirements, and construction site owners and site operators time to find new ways of complying.

EPA's preferred way to prevent runoff is to limit the amount of soil and sediment that can be washed away. But although it's generally less expensive and less time consuming than treating water that's already polluted, source reduction sometimes requires changing the way site owners and contractors approach projects. Thus, it's an underutilized cost-savings tool.

The concept is simple: reducing or eliminating the generation of sediment eliminates the need to control the sediment by installing and maintaining treatment methods such as silt fences and sedimentation basins. If you reduce the amount of pollutant generated at the source, you also reduce the material use, liability costs, potential regulatory violations, and the need to invest in pollution-control equipment.

In the past, the industry tended to focus on end-of-pipe solutions —treatment and disposal methods — to control pollutants, rather than using source reduction to prevent the pollution. These methods may be easier and less expensive to implement under stormwater regulations that don't have discharge limits. However, to meet the discharge limits of the new EPA guidelines, construction sites may also have to add more expensive end-of-pipe pollutant-control technologies such as flocculation, coagulation, and filtration — a process where chemicals are added to bind small particles together before they are passed through a filter that removes them from the water.

Source reduction generally requires less effort to implement, inspect, and maintain and will likely cost less than using the above combination of treatment methods. But it usually requires more up-front planning: changes in construction design, activities, operations, and material use that prevent or reduce the quantity of pollutant generated.

Source reduction techniques include:

Grading the jobsite over a period of time. This minimizes the area of exposed soil, which in turn minimizes the potential for erosion. Additionally, existing vegetation acts as a natural sediment filter while improving infiltration.

Although this option may require separate mobilizations for grading equipment, preconstruction planning can optimize equipment use and minimize mobilization issues.

Reducing the amount of impervious surface. Using less asphalt, concrete, brick, stone, and other materials that seal the soil surface, and installing pervious concrete, porous pavers, and other pavements that allow rainwater to pass through them promotes infiltration rather than runoff. These pervious materials also help reduce runoff during construction, eliminating potential stormwater pollution.

In some cities, where post-construction regulations require that construction site owners/operators minimize runoff from completed projects, there has been increased use of pervious materials. In those cities, completed construction sites may be required to have the same runoff as they did prior to construction.

Stabilizing the site as soon as possible. Seeding, mulching, or otherwise covering exposed soils as soon as practical limits the potential for erosion. Most general permits require stabilization within short time frames anyway.

When stabilization involves establishing vegetation, speed up the process by avoiding soil compaction or regrading compacted soil, using topsoil — for example, instead of fill dirt — and applying appropriate water and fertilizer.

Taking seasonal weather fluctuations into account. Minimize soil exposure by scheduling work for fairly predictable times of low precipitation or, conversely, to ensure rainfall frequency corresponds with the vegetative establishment.

Although the wet and dry seasons do not always correspond correctly, major grading activities should be conducted during the dry season. This will not only reduce the potential for stormwater pollution, but it also will save time and money spent on cleaning out and repairing silt fences and sediment basins. The end of the project, when the grass and trees are typically planted, should be scheduled for the wet or growing season. This will also save time and money spent on watering trucks.

Controlling run-on to the site. Divert streams, ditches, and sheet flow around the site through grading and landscaping. Of course, take care that diverting the stormwater run-on doesn't cause erosion elsewhere.

It may also fall to your operation to educate the regulated community on how to effectively meet the new guidelines.

Departments enforcing stormwater permit compliance (i.e., Municipal Separate Storm Sewer Systems or MS4s) can support source reduction by providing comments during reviews of construction site stormwater pollution prevention plans, sediment and erosion control plans, and construction plans, as well as by conducting public outreach activities.

— Wanzenried and Doan are environmental engineers with engineering consulting firm Terracon, which provides geo-technical, environmental, construction materials, and facilities services.



















Web Extra

For more on the EPA's Effluent Limitations Guidelines and Standards s for the Construction and Developmentment Point Source Category, click here.