Sediment Control & Environmental Management Commissions
Zero In on Turbidity Controls

May 2002

Perhaps you say the headlines of the News & Observer back in June of 1997: "Wake Runoff Muddies Neuse to New Bern." Rick Dove, the Neuse Riverkeeper, sounded the alarm when a giant reddish plume of clay appeared in the Neuse River in New Bern. Dove and Chris Roessler of DENR flew up the river and traced the plume about 180-miles upstream to Crabtree Creek. Hundreds of construction sites were found to be fueling the massive mud slick.

Later on April 22, 1998 the News & Observer said: "Silt Cops Dig for Dirt on Muddied Waters". "The inspections had all the intrigue of a police raid: an early-morning rendezvous at Raleigh-Durham International Airport, a list of targeted sites and government agents fanning out on a secret mission. Instead of breaking up drug rings or liquor stills, however, the inspectors hit construction sites to ensure they weren't spilling mud into Crabtree Creek, whose waters last year dirtied the Neuse River all the way to New Bern." "In what was described as the first sweep of its kind, two dozen state and local officials inspected about 330 construction sites in the Crabtree watershed Tuesday."

And in another feature article on March 18, 2001 the News & Observer said: "Runoff Rises as Threat." The paper documented over 180 projects with serious violations over the last year. Some of the worst violators are governments themselves. "It's not hard to find the sources of the problems." The red clay is everywhere. "Our program is not designed to create crystal-clear water," said John L. Holley Jr., regional engineer for the state land resources division. "There's not but so much that conventional sediment control practices can do."

On March 30, 2000, Judge Beecher R. Gray of North Carolina Office of Administrative Hearings ruled that the North Carolina turbidity standard in 15A NCAC 2B .0211(3) (k), which reads:

      (k) Turbidity: the turbidity in the receiving water shall not exceed 50 Nephelometric Turbidity Units (NTU) in streams not designated as trout waters and 10 NTU in streams, lakes or reservoirs designated as trout waters; for lakes and reservoirs not designated as trout waters, the turbidity shall not exceed 25 NTU; if turbidity exceeds these levels due to natural background conditions, the existing turbidity level cannot be increased. Compliance with this turbidity standard can be met when land management activities employ Best Management Practices (BMPs) (as defined by Rule .0202(6) of this Section) recommended by the Designated Nonpoint Source Agency (as defined by Rule .0202 of this Section). BMPs must be in full compliance with all specifications governing the proper design, installation, operation and maintenance of such BMPs;

as interpreted and applied in the case (Wallace Burt, Jr. et al v. DENR and Highlands Cove, 99 HER 0980,0989,1084) was void because it allowed the numerical state water quality standard for turbidity to be violated if best management practices for erosion and sedimentation control were being followed. The North Carolina turbidity standard provides for a numerical limit of 10 NTU's (Nephelometric Turbidity Units) for this the subject stream which is a Trout Water.

On October 12, 2000, the Environmental Management Commission (EMC) ruled that turbidity levels in a stream may not exceed the specified numerical water quality standard for that stream even if best management practices are being followed. At the December 14, 2000 meeting of the EMC's Water Quality Committee, staff of the Divisions of Water Quality, Land Resources, Soil and Water Conservation, and Forest Resources, said that as many as 90% of the land disturbing activities that are currently reviewed by these various divisions might not meet the numerical turbidity standard despite compliance with BMP's. Since that time, the staffs have been directed to study the relationship between turbidity standards and best management practices. And, the Sedimentation Control Commission has been studying various ways to implement turbidity standards on all land disturbing activities. The Technical Advisory Committee of the Sedimentation Control Commission will be meeting this year to determine if the "state of the science" will allow for achieving 10, 25, and 50 NTU's on construction sites.

Excessive levels of turbidity in our streams and waters causes a host of problems. First, the turbidity significantly reduces the amount of biological activity by restricting light penetration. Second, the suspended particles causing the turbidity are chemically active and typically contain high levels of phosphorus, metals, and organic chemicals. Third, when the particles do settle, they can destroy biotic habitats by accumulations of mud. And, lastly, excessive turbidity is frequently the unsightly condition that the public sees and frequently responds to in complaints. We are all part of the problem, but we can be part of the solution of one of the most pervasive water quality pollution problems facing North Carolina and our whole region.

Why do normal sediment controls simply not work?

Traditional soil erosion controls on construction sites can effectively remove sand and larger silt particles from stormwater, but can not effectively remove smaller silt particles, clay, and organic particles. Small colloidal particles of clay and organics are so small that they are difficult to remove from the water on construction sites.

Clay particles have a diameter of 2 microns or less (a micron is a millionth of a meter; there are over 90 billion clay particles per gram). Particles of this size are so small that their weight has little or no effect on them settling out of the stormwater. These fine particles are referred to as "colloidal particles", because they function at something like a molecular level in water.

Clay particles have negative electrical charges, and they have an enormous surface area per gram of clay. The negative charges on the particles attract and bond with cations (positively charged ions) in the stormwater. The negative charges and large surface area means that these particles are chemically active and can dominate stormwater chemistry.

One of the most important properties of colloidal clay particles is that certain cations will create a diffused suspension making it even more difficult to settle the particles from the water. Phosphorus ions will create this diffused suspension. In fact, every time you run your dishwasher you creating such a suspension to keep your glasses from spotting.

The suspension can be reversed with certain other chemicals. Traditionally, calcium ions have been added into the clay suspension to over power the chemical bonds causing the suspension. As this process occurs, the calcium ions will cause the clay particles to come together and form soft aggregates of many particles. The aggregates act as larger particles and begin to settle out of the water. This process is called "flocculation" and is used in water treatment and wastewater treatment plants to clarify the water. However, this process can take 12 to 24 hours, which means that all the runoff on a construction site would need to be impounded without any discharge, treated, and held in a stilling basin until the settling process is complete. Then the stormwater can be carefully drained avoiding agitation and re-suspend of the particles.

This flocculation process was used by the North Carolina Arboretum during the construction of the Fredrick Law Olmstead Parkway near Asheville, NC. O'Brien Atkins & Associates and the North Carolina Arboretum wanted to enhance the environmental protection of Bent Creek, a Mountain Trout Water running parallel to the parkway. We were able to significantly reduce the turbidity leaving the construction site using calcium in finely ground dolomitic limestone to power the flocculation process. This project was funded through NCDOT using federal inter-modal highway funds.

New Technologies Emerge

Now new modern polymer techniques are available that will revolutionize how we conduct site design, manage runoff, and protect the water quality of the receiving streams. The new techniques will allow us to treat, flocculate, and trap the colloidal particles in less than 1-minute. All of this is possible because of the use of polyacrylamides (PAM's). The PAM's are extremely powerful flocculating agents. They are commercially available from a number of distributors. The most useful approach is to purchase PAM imbedded into "polymer gel block". Some of the trade names are "Floc Log" by Applied Polymer Systems, Inc. (; and "Gel-Floc" by Natural Site Solutions, Inc. (http://www.natural The polymer blocks are then installed in the stormwater pipes just upstream of the settling basin. The flowing water activates, mixes, and reacts with the PAM before the stormwater flows into the settling basin.

Applied Polymer Systems has about 40 different types of Floc Logs. They select the best type of Floc Log for your particular soils and water chemistry. They prefer to do this on a project by project basis and ask the contractor to send in a sample of the soil and stormwater.

Dr. Richard McLaughlin of NCSU Department of Soil Science has been conducting research on soils in North Carolina using PAM's ( McLaughlin has used elongated sediment basins with multiple filter fence baffles crossing the basins. Beyond the baffles, a floating shimmer is used to allow only the water near the top of the pool to discharge from the basin. The discharge from the basin then runs through a dense grass buffer prior to collection and discharge. This approach has provided the best results for the removal of colloidal particles. If properly done it can produce a very clear discharge. The results will vary depending upon the type of clay minerals present, site and climatic characteristics, facility design, and degree of maintenance. This approach should be used after traditional sediment control has already removed the sand and larger silt particles.

NCSU has periodically provided continued education courses on Sediment & Erosion Control. These workshops include presentation of Dr. Richard McLaughlin's research; for more information go to

Regulatory Enforcement

North Carolina will likely decide to require some phased approach to achieve reductions in turbidity in stormwater leaving a construction sites. This will require a whole new level of design and management of construction sites. It may mean that water quality samples will need to be collected from rainfall events that discharge stormwater from the construction site. The State is allowed to assess fines up to $10,000 per day for water quality violations.

Currently, the Federal stormwater permits for construction sites requires compliance with erosion control standards and water quality standards. EPA treats every local notice of violation of the erosion and sedimentation control program, as a violation of the federal stormwater laws and can carry fines of up to $27,500 per day. EPA has recently cited a developer in Fairfax County, VA alleging damages due to observed plumes of turbid waters in a lake -mile downstream of a new residential subdivision. During the alleged violations, the stormwater from the project passed through a wet detention basin on the project site. The detention basin provided about 4-hours of detention at peak discharge as required by the county. While, the detention basin effectively removed in excess of 85% of the sedimentation in the stormwater, it did not remove colloidal clay particles which created the downstream turbidity. The County has also sued and is seeking monetary damages of up to $2,400,000 to fund corrective actions. For further details see Landis Web Article: August 2001 and Landis Web Article: October 2001.

Implementation of these new technologies will be required in order to make progress in achieving the current water quality standards in North Carolina. It will not be easy to achieve the standards. Countless changes in construction management, many unpopular will be required. It will require contractors to become environmentally sensitive, more than they might ever dream possible. The implementation will need to be phase into practice and the design of the techniques will need to be carefully fitted to each site, since no two sites will be the same. These new scientific technologies will most likely form the basis for the phased implementation of non-point source water quality management on construction sites in North Carolina?