Meeting the New EPA Effluent Limitations Guideline for Construction Site Discharge Turbidity: Effectiveness of Different Turbidity Control Systems and Monitoring Methods

Focus Area

Water Quality/Wetlands


Natural Resources






1-2 years

Research Idea Scope

On December 1, 2009, the United States Environmental Protection Agency (EPA) released their final rule on the effluent limitation guidelines for stormwater discharges from construction sites.  Effluent guidelines are national standards for wastewater discharges to surface water and publicly-owned treatment plants.  The standards for effluent guidelines are technology-based.  The final rule has impacts for highway transportation construction activities and it applies to all construction activities required to obtain a NDPES permit and all new source construction and development activities.
The rule expands the NPDES permit by requiring reporting and measuring of turbidity levels from stormwater systems outlets in areas where 20 or more acres of land are disturbed at one time, including noncontiguous land disturbances that take place at the same time and are part of a larger common plan of development or sale.  The limit will be lowered to 10 acres in February of 2014.  The average turbidity of any discharge for any day must not exceed 280 nephelometric turbidity units (NTU).  While this specific turbidity limit number has been challenged in court, it is likely any new number that EPA develops will still require treatment beyond current practices. 
Water quality agencies and State DOTs are trying to determine how to implement this new ruling.  The details of how and where to monitor has been left to the permitting authority, usually the state water quality agency.  Guidance and information is needed on the type, interval, and frequency of sampling for each project.  In addition, the monitoring plans must consider factors such as: safety, outfall location, and access, in order to get to these locations. In the final rule, EPA noted that the permitting authority should exercise discretion when determining the monitoring locations and monitoring frequency for linear construction projects.  In other words, the water quality agency can choose to use representative sampling at certain locations that would represent the discharge characteristics of other locations.  State DOTs need the ability to implement this option.  Additionally DOTs require monitoring data and protocols on how to effectively demonstrate the adequacy of reference locations to the permitting agencies. 
California, Georgia, Oregon, Vermont and Washington already require monitoring and include turbidity action levels as part of their Construction General Permit.  They all have different action levels in their permits and several states have been doing this for years.  These states could serve as a resource regarding how to effectively implement a monitoring program. Information is needed on the state of the current practice; what states are monitoring, how they are monitoring, what data, methods, procedures, processes and records, specifically what documentation is being kept in the field, along with what erosion and sediment control measures provide the best protection for meeting turbidity limits, to name some of the items that could be gathered to help all states provide monitoring plans for their permits.  Information is also needed on how these programs and monitoring requirements are working in order to ensure that monitoring requirements in the new construction general permits can reasonably be implemented in the limited linear environment of highways, by all states.
Research Objective
The objective of this study is to evaluate and develop cost-effective methods to both control on-site turbidity levels and to sample discharge turbidity levels in order for State DOT to comply with the EPA turbidity guidelines.
The original EPA regulation mandates that discharge from a construction site can not exceed 280 nephelometric turbidity units (NTU).  Although this ruling has been set aside for 18 months it is likely that a similar limit will be implement that can be supported by science.  Multiple methods of turbidity control are currently being used but in some cases the minimum turbidity requirement is still not being met.  The factors involved in the success or failure of turbidity control systems need to be determined in order to properly implement and manage them to consistently meet target turbidity levels.  This study should determine the effectiveness, failure rates, and costs of a range of turbidity control approaches, including passive and active treatment systems.
Compliance with the ELG turbidity limit requires monitoring construction site storm water discharges.  There is a wide range of sampling options, ranging from grab samples to dedicated, automatic samplers or water quality probes.  These need to be compared for cost, reliability, and ability to provide an accurate assessment of turbidity in discharges.   This research project will enable DOTs to prepare for the implementation of the ELG turbidity limit requirements.  As the current rule has been set aside for 18 months this study has impeccable timing.  This research will permit DOTs to easily implement the new rule upon release. 
 Specific Tasks
Task 1. Evaluate and compare methods to reduce turbidity in construction site storm water for their cost, reliability, and ease of use on active construction sites.  Examples are:

  • Solid forms of flocculants placed in water conveyances leading to sediment basins or other sediment trapping device.
  • Dissolved flocculants dispensed into runoff prior sediment basins using rainfall (e.g. Auckland Regional Council displacement tanks), flowing water (e.g. Wheeltreater), or devices remotely powered by battery and/or solar.
  • Active treatment systems which typically collect runoff then pump it through a treatment system including flocculation and filtration steps prior to discharge.

Task 2. Evaluate and compare storm water sampling methods for cost, reliability, and ability to provide an accurate assessment of turbidity in discharges.   These would preferably be compared on active construction sites at the same discharge point.  Examples include:

  • Grab: obtaining samples manually at discharge points.
  • Stage: installing sample bottles in a basin or outlet so that they fill as the water rises (stage) during storm events.
  • Simple automatic sampler: a device designed to obtain one or more samples after flow or rainfall is detected.
  • Fully automated sampler: a programmable device which obtains samples based on flow or stage and which records flow and rainfall.
  • Water quality probe: a device which records turbidity directly.

Suggested By

Ted Sherrod, NCDOT; Frannie Brindle, Chair Natural Systems and Ecological Communities Subcommittee

[email protected]