Construction Site Pollution Prevention Construction site runoff has been identified as a significant source of sediment loading within the urban environment. Many strides have been made over the past two decades in the development and implementation of stormwater pollution prevention plans (SWPPPs). While most sites are applying for required permits and preparing SWPPPs, there appears to be room for improvement in installation and maintenance of adequate erosion and sediment best management practices (BMPs).

What is the Difference Between Erosion and Sediment Control?

Erosion control practices protect the surface of the ground from being displaced by the force of falling precipitation or flowing water. Sediment control practices are intended to collect polluted runoff for a period of time, allowing suspended pollutants to settle out of runoff before it is allowed to leave a construction site.

The following are generally not new requirements. Rather, they are points of emphasis to increase compliance with existing regulations. Improvements are recommended in implementation of erosion control practices:

• Consider stormwater management early in the site design process. Look for ways to minimize the footprint of disturbed areas, lessen grading volumes and reduce impervious surfaces.

• Develop and implement a Soil Management Plan (SMP), with the goal of providing healthy soils across all open space areas on developed landscapes before construction has been completed.

• Where upstream areas drain through a construction site, stage construction to avoid disturbance to the flow path or provide stabilized methods to divert stormwater around or through site construction.

• Increase the use of temporary seeding and mulches. Use of adequate temporary mulch has been shown to reduce surface erosion by up to 98% compared to sites with no erosion controls.(1) State law currently requires that disturbed areas where grading activities cease for a period of longer than 21

Why is Pollution from Construction Sites a Problem?

Construction activities create new development from farmland or other open spaces. These activities strip off any vegetation that is reducing the potential for surface erosion. Once this vegetation is gone, the surface of the soil is easily washed away by rainfall and flowing water. Soil can also be tracked onto roads and highways or dumped into waterways. All of these actions make it likely that soil will be carried off site and washed into downstream storm sewers, creeks and rivers. This eroded soil (sediment) can plug up storm sewers and fill in waterways, affecting their ability to convey runoff. Other impacts of sediment are listed in detail in Chapter 6 of this plan.

Without effective controls, sediment discharge from construction sites often will range between 35–45 tons per acre.(1) Compare this with farmland areas which usually have loading rates of less than two tons per acre. Lawns and other stabilized areas have far lower erosion rates.

Construction sites can also be sources of other pollutants such as fuels, oils, paints, concrete washout, construction debris and human waste (collected in temporary toilet facilities from workers).

Source: 1. Developing Your Stormwater Pollution Prevention Plan: A Guide for Construction Sites, page 2; USEPA, May 2007 Source: Dunne, T. and L. Leopold, 1978; NRCS, 2000; NRCS, 2006; ASCE and WEF, 1992

days shall have temporary stabilization (such as mulch with seed) applied within 14 days after the last grading activity. Many sites are currently not providing adequate temporary stabilization measures to comply with this requirement.

On steeper slope areas or in areas of concentrated fLow, increase the use of rolled erosion control products (RECPs) and turf reinforcement mats (TRMs) where temporary mulch may be insufficient to prevent erosion.

Recommended improvements for sediment control practices:

  • Prior to commencing land disturbing activities, install perimeter site controls (such as silt fences, filter socks, wattles and sediment basins), stabilized construction entrances, trash collection areas and temporary sanitary facilities for site workers
  • Install interior site controls as soon as allowed by grading or utility construction
  • Don’t overload controls. Refer to design guidelines for sizing and design. For example, where silt fence is installed, provide at least 100 feet of silt fence length for each quarter acre drained.
  • Silt fences should feature “J-hooks” or other methods to increase their storage capacity and prevent concentrated flow from larger areas being directed to a single low point in a long fence. Silt fences often fail when they “blow out” from collecting too much runoff or sediment, because the area they collect runoff from is too large. Silt fences should have these features placed at intervals of no greater than 200 feet.
  • Use soil logs or wattles to break up the length of steeper slopes. Reducing the flow length along steep slopes can significantly reduce surface erosion.
  • State law requires sediment basins to be installed where attainable, when runoff from more than 10 disturbed acres is routed to a common outlet. These basins are to be designed with floating outlets or devices that collect water from the surface of ponded water. As pollutants settle out by gravity, the surface of the ponded water tends to be less polluted than that discharged from the bottom of the basin. Few of these types of outlets are being utilized currently. Also, as properly sized basins are often most effective at removal of suspended sediment from constructed runoff, it is recommended that new local policies be implemented to require their use in smaller disturbed areas.
  • All site controls should be checked on a weekly basis and before rainfall is expected to make sure they are in good working order. Controls should be maintained and repaired promptly as needed. Trash and sanitary collection facilities need to be emptied routinely and collected materials disposed of properly. Stabilized entrances may need new surface aggregate provided is they are failing to prevent off-site tracking from occurring.
  • When dewatering excavations, divert discharge to a sediment basin or other collection area on-site. Do not directly discharge such water to the storm sewer system without treatment or filtration. Avoid releasing concentrated flows at the top of steep slopes where gully erosion may be caused.
  • Immediately following full establishment of permanent vegetation, all temporary controls such as silt fences, soil logs, inlet protection devices should be removed. Accumulated sediment should be properly disposed.

Tracking onto roadway at unprotected construction site entrance

Recommended improvements to SWPPPs:

  • The plan should be a “living document.” The plan should be amended in some fashion so that the site map reflects current site conditions. Inspection records and changes to the sequence of construction events should be made part of the SWPPP document.
  • The SWPPP and all site controls are to be maintained as necessary until full establishment of vegetation across all disturbed areas. Site inspections and maintenance of controls should continue until all areas are stabilized with permanent vegetation and the Notice of Discontinuation (NOD) has been filed with the Iowa Department of Natural Resources.

Sediment washing into storm inlet

Unprotected stockpiles near inlet

Recommended Source: RDG

improvements to municipal inspections:

  • Routinely check sites to assure that construction sites are in compliance with state and local standards.
  • Respond promptly when polluted site runoff or off-site tracking is observed, or citizen complaints are received.
  • When necessary, use “stop work orders” and other methods to bring sites back into compliance before work on other construction items can proceed.

Application The plan recommends ordinances and internal policies be implemented and enforced that would apply these standards to all sites requiring either a local grading permit or authorization under the State of Iowa’s NPDES General Permit No.2 (construction sites or common plans of development which will disturb at least one acre).

Expected Impacts:

• Successful implementation of these policies would significantly reduce sediment loadings from construction sites and annual sediment loadings within the Walnut Creek watershed.

Weeds can be seen growing in this area,

• Reduced sediment loading will slow the rate of deposition within the flood plain. indicating it was graded at least a few weeks ago.

This maintains the flood plain’s ability to convey and store runoff. This reduces No temporary mulch or seeding is present.

• Reduced deposition also lowers the potential for streambank erosion due to deposited soil pushing flows toward the outside bends of streams.

Walnut Creek Watershed stretches across 53,000 acres