The Efficacy of Treating Highway Runoff to Meet Watershed TMDL Goals

Focus Area

Water Quality/Wetlands


Natural Resources






1-2 years

Research Idea Scope



Departments of Transportation (DOTs) are being named as Designated Management

Agencies in Total Maximum Daily Load (TMDL) management plans. This designation

often brings with it the obligation for a substantial amount of staff time,

reporting and construction of capital facilities for stormwater treatment,

potentially at a high cost. While highways are an important source of some

pollutants, for others highways may only be a minor contributor, or the

pollutant of concern may have its origins outside the highway right-of-way, unrelated

to the operation or maintenance of the road. Consequently, the DOT may have

little to no control over the source of the pollutant. The portion of the total

pollutant load attributable to the DOT may be small compared to the total load

reduction needed, resulting in a nominal improvement in the receiving water,

and primary pollutant sources remain unaddressed. Treating all stakeholders

equally for all pollutants may not result in the highest benefit for the

environment. Previous research has focused on how DOTs can respond to and meet

TMDLs, and substantial effort has been put into characterizing highway runoff

and identifying the sources of pollutants, but this information has not been

used to help develop strategies for developing cost-effective TMDL management


Research Objective:

The project

will first develop a general evaluation of the importance of the various

highway runoff pollutants in a TMDL context. This analysis will be based on the

concentrations of pollutants in highway runoff, the sources of the pollutants

in the runoff, and the relative and absolute contribution of the highway runoff

pollutant to receiving waters (taking geographic and traffic variations into

account). The objective of this portion of the project will be to provide watershed

managers with guidance to determine if it is appropriate to name the DOT as a

stakeholder in the TMDL.


the project will develop a protocol for evaluating the cost effectiveness of

progressively more stringent highway runoff treatment requirements with regards

to meeting watershed TMDL pollutant reduction goals. Developing a unit cost by

BMP and by constituent within a highway environment will allow for the

formulation of more efficient control strategies among the stakeholders.

Results from

this investigation can be used by both DOTs and regulatory agencies in the

development of TMDL management plans to help allocate responsibilities and

resources for the maximum overall benefit to the receiving waters and the

cost/benefit for the designated management agencies.


Specific Tasks:

Task 1 –

Literature Review, Compilation of Data and Survey of State Practices and



literature review and the data compilation will focus on the following areas


Highway characterization data, selected from a broad range of

geographic conditions and traffic levels;


Source studies for the most common TMDL pollutants, both in

highway runoff and watersheds as a whole;


Effectiveness of stormwater BMPs at removal of the most common

TMDL pollutants;


Costs associated with the stormwater BMPs constructed in the

highway environment; and


Specifically evaluate NCHRP Synthesis 20-05/Topic 43-06 that has

information useful to this topic.

Task 2 –

Evaluation of Highway Runoff Contribution to TDML Pollutant Loads

Using the

information collected in Task 1, the project will evaluate when and in what

circumstances highway runoff is a substantial or primary source of the most

common TMDL pollutants. This evaluation should include in the assessment,

geographic variables such as land use, local soils and geology, traffic

volumes, relative and absolute size of receiving waters compared to highway

area, and ultimate sources of the pollutants. Develop a decision tree to aid

the practitioner in the assessment to determine if the DOT should participate

as a stakeholder in the TMDL.

Task 3 –

Evaluation of Cost Effectiveness of Highway BMPs in Meeting TMDLs

Using the

information from Tasks 1 and 2, the project will develop a protocol for

evaluating the cost effectiveness of BMPs to meet overall pollutant reduction

goals for the TMDL watershed. This may include assessing the relative and

absolute contribution of highway runoff to the watershed pollutant load,

incremental costs associated with increasing BMP size, effectiveness and frequency,

and local conditions. Develop unit cost data by BMP and by constituent that can

be used to assess TMDL compliance options, such as using regional treatment or

source control programs.

Suggested By

Center for Environmental Excellence by AASHTO Stormwater Management Community of Practice