For obvious reasons concrete trucks should never be washed out into the street or into the storm drain. Solids that are washed out of the concrete trucks can clog storm drains, causing flooding and expensive clean-up. However, it is also important to contain the concrete wash water as well. The wash water is very alkaline, which means it has a very high pH. Water must have a pH in the range of 6.5-9.0 in order for aquatic life to survive. Concrete wash water typically has a pH of 12 or above. Furthermore, the pH scale is logarithmic, meaning that a pH of 12 is 10 times greater than a pH of 11, 100 times greater than a pH of 10, and 1000 times greater than a pH of 9.0, which is the highest level of alkalinity that aquatic life can tolerate. If allowed to escape the site, concrete wash water can have a severe effect on our streams and rivers.
Other significant pollutants in concrete wash water are heavy metals such as chromium. If wash water that is high in heavy metals leaches through the soil to the water table, it can contaminate our groundwater, which is where most cities in Nebraska get all of their drinking water.
Concrete washout is also high in suspended and dissolved solids. The average stream or river in the United States typically has a suspended solid count of no more than 60 parts per million (ppm). The average suspended solids in concrete wash water is 27,000 ppm, well above the range at which aquatic life can survive.
Luckily, it is not very difficult to control concrete washout on your construction site. There are several different ways to build a concrete washout facility, the most simple being to dig a pit and line it with plastic sheeting that is at least 10-mil thick. If you don’t have a good location on your site for a facility like this, you can also build a portable facility by building a box with a liner to contain the washout. There are also several products available for purchase that are effective, portable concrete washout containers.
Always locate concrete washouts as far away from storm drains as possible. Make sure they are clearly labeled and that drivers know where they are supposed to wash out. By doing this, we can make sure that one of the biggest pollutants from construction sites does not reach our waterways.
Inlet protection is the last line of defense in preventing sediment from entering storm drain inlets and reaching our waterways. It should never be used as the only Best Management Practice (BMP), and often, if it fails, it is because there is not enough stormwater controls above the inlet to keep sediment on site. One of the common misconceptions about inlet protection is that it is meant to completely block off the inlet. Inlet protection should NEVER completely block off an inlet; not only can this be a safety hazard by flooding the roadway, it is ineffective because the sediment-laden water will just pass by to the next inlet. Instead, inlet protection is meant to slow the water flowing in the curb, shallowly pooling it to allow some of the sediment to drop out. Inlet protection can be constructed from rock socks, sediment control logs, silt fence, block and rock socks, or other materials. There are also numerous proprietary products available. The type of inlet protection you choose should depend on where your inlet is located.
Inlet protection in sump conditions
For inlets located in a sump, that is, at the low point of an area or a curb, it is important that the inlet continue to function during larger runoff events. For curb inlets, the maximum height of the protective barrier should be lower than the top of the curb opening to allow overflow into the inlet during larger storms. For area inlets, silt fence or other materials can be used to keep sediment-laden water out of the inlet. Just remember that if there is no overflow to the inlet, your site will experience some localized flooding.
Inlet Protection on Grade
If you inlet is located on a sloped street, instead of blocking off the inlet, you might try using curb socks in the flow line of the gutter uphill from the inlet. This will slow down the stormwater and give the sediment a chance to drop out before it reaches the inlet without blocking off the inlet.
For more information on inlet protection, see design details here from the Urban Drainage and Flood Control District IP-1 Urban Storm Drainage Criteria Manual Volume 3
Silt fence can be a very effective best management practice if it is used correctly. However, it is also one of the most misused and overused BMPs. Using silt fence incorrectly is not only ineffective, it can also be expensive. Here are some tips for using silt fence appropriately:
The NDEQ and the EPA have provided guidelines and tips for proper elimination of pollutants from construction sites. Construction sites of one acre or more must of a Construction Stormwater Permit from the NDEQ. Continue reading Helpful Links for Contractors