Stormwater pollution prevention tips: Outdoor storage of raw materials

Improper storage of materials outdoors can lead to stormwater contamination when materials wash off or dissolve into water, or are added to runoff by spills and leaks.

The California Stormwater Quality Association's Industrial and Commercial Handbook offers the following tips for preventing pollution from outdoor storage of materials. For more information, visit

General pollution prevent strategies

  • Emphasize employee education for successful implementation of best management practices.
  • Minimize inventory of raw materials.
  • Keep an accurate, up-to-date inventory of the materials delivered and stored on-site.
  • Try to keep chemicals in their original containers and keep them well labeled.

Suggested protocols


  • Store all materials inside. If this is not feasible, then all outside storage areas should be covered with a roof and bermed or enclosed to prevent stormwater contact. At the very minimum, a temporary, waterproof covering made of polyethylene, polypropylene or hypalon should be used over all materials stored outside.
  • Cover and contain stockpiles of raw materials to prevent stormwater from running into the covered piles. The covers must be in place at all times when work with the stockpiles is not occurring (applicable to small stockpiles only).
  • Implement erosion control practices at the perimeter of your site and at any catch basins to prevent erosion of the stockpiled material off-site, if the stockpiles are so large that they cannot feasibly be covered and contained.
  • Keep liquids in a designated area on a paved, impervious surface within a secondary containment.
  • Keep outdoor storage containers in good condition.
  • Minimize stormwater run-on by enclosing the area or building a berm around it.
  • Keep storage areas clean and dry.
  • Slope paved areas should be sloped in a manner that minimizes pooling of water on the site, particularly with materials that may leach pollutants into stormwater and/or groundwater, such as compost, logs and wood chips. A minimum slope of 1.5 percent is recommended.
  • Secure drums stored in an area where unauthorized persons may gain access to prevent accidental spillage, pilferage or any unauthorized use.
  • Cover wood products treated with chromated copper arsenate, ammonical copper zinc arsenate, creosote or pentachlorophenol with tarps or store indoors.

Raw material containment

  • Curbing should be placed along the perimeter of the area to prevent the run-on of uncontaminated stormwater from adjacent areas as well as runoff of stormwater from the stockpile areas.
  • Tanks should be bermed or surrounded by a secondary containment system.
  • The area inside the curb should slope to a drain. Liquids should be drained to the sanitary sewer if available. The drain must have a positive control such as a lock, valve or plug to prevent release of contaminated liquids.
  • Accumulated stormwater in petroleum storage areas should be passed through an oil/water separator.


  • Conduct regular inspections of storage areas so that leaks and spills are detected as soon as possible.
  • Check berms, curbing and containment for repair and patching.


  • Train employees well in proper material storage.
  • Train employees and contractors in proper techniques for spill containment and cleanup.

This article originally appeared in the 04/01/2005 issue of Environmental Protection.