About Turbidity and Construction Site Control Requirements

Association of Construction and Development
January 22, 2013 — 2,385 views  

Turbidity is a basic measurement of water clarity. Sediment is the major contributor to surface water turbidity. Too much sediment in surface water causes problems such as, killing aquatic life, increasing drinking water treatment costs, and interfering with recreation in the water.

Measuring Turbidity

The most accurate way of measuring turbidity is to use a nephelometer. The device measures the scattering of light in a water sample in Nephelometric Turbidity Units (NTU's). NTU's is a measure of total turbidity, which includes Total Suspended Solids (TSS) and Total Dissolved Solids. TDS's are smaller than TSS's, so they contribute less to the overall turbidity.

Storm water runoff from construction sites may enter street drains that eventually empty into water bodies, or run directly into the water. In light of the potential problems caused by sedimentation and other construction debris, The U.S. Environmental Protection Agency (EPA) set standards to assist site managers in containing these pollutants.

Do I Need A Permit?

According to the EPA:

"The NPDES storm water program requires construction site operators engaged in clearing, grading, and excavating activities that disturb 1 acre or more, including smaller sites in a larger common plan of development or sale, to obtain coverage under an NPDES permit for their storm water discharges"

Getting a Permit

In many regions, permits are handled at the state level unless the project is on Indian or federal property. Developing a Storm water Pollution Prevention Plan (SWPPP) is a requirement in order to obtain an NPDES permit. The EPA has an excellent guide to help site operators develop a SWPPP.

About Best Management Practices

Permits are based around requirements for site operators to establish and maintain a list of Best Management Practices (BMP's) for controlling erosion and storm water runoff. Some BMP's include setting up adequate soil retention structures, slope drains, gradient terraces and sediment traps. They will also include site stabilization requirements such as laying sod or seeding after completion. You must meet minimum inspection requirements for your site and the associated BMP's.

Some specific BMP's and associated inspections vary based on your specific project and the surrounding geography. An NPDES permit may contain additional requirements for waste disposal to protect water bodies. There are "low erosivity waivers" available for small projects in certain regions that meet specific criteria.

The EPA provides a list of state agency contacts to contact for more information.

Association of Construction and Development