Contribution of Zinc from Highway Fixtures: Targeted Monitoring

Focus Area

Water Quality/Wetlands


Natural Resources







Research Idea Scope

Zinc is an important targeted constituent for Departments of Transportation (DOTs). The average concentration in highway runoff exceeds water quality objectives. For individual storms, the measured concentrations of total zinc in highway discharge characterization studies exceed standards more than 85 percent of the time. A number of receiving waters in the United States are listed as impaired for zinc and TMDLs have been adopted or being developed. These TMDLs currently require DOTs to construct and operate structural BMPs at substantial costs to treat the storm water at the end-of-pipe. A potentially more cost-effective approach for managing zinc in storm water would be to remove the sources from the highway environment. Unfortunately, little is known about the relative magnitude of the various sources. This project is designed to fill part of that knowledge gap. 

Urgency and Payoff

This study could include Laboratory Preliminary Evaluation, Manual grab samples will be collected from runoff leaving individual galvanized fixtures (portions of guardrails, signs, signposts, building roofs, etc.) during the dry season by spraying deionized water on the fixture and collecting the runoff. The purpose is to determine if the quantity of zinc leaching from fixtures is likely to be significant source of pollution in highway stormwater runoff. Detailed observations will be made of the fixture locations, fixture surface area exposed to stormwater, and the flow path for the runoff they create, so that estimates can be made of the proportion of this runoff that reaches the catchment monitoring location. The objective of this monitoring will be to determine the contribution of zinc from individual components in the watershed that is discharged from various sources.  For some cases, it is likely that zinc from appurtenances is not mobilized beyond the immediate area due to adsorption to sediments.

Product: Estimates of quality and quantity for runoff from individual fixtures.

A simple model, perhaps in spreadsheet form, will be developed to estimate the total mass of zinc derived from individual sources during the events monitored. The objective is to determine the relative contribution from each of these individual sources and then to make a mass balance comparison with the load observed at the catchment outlet to assess their contribution of zinc relative to all other unquantified sources (tires, rainfall, dry atmospheric deposition, etc.)

A statistical analysis also will be performed to determine whether coating galvanized fixtures with an inert substance reduces the concentration of zinc in runoff. 

Urgency –  Metals are a target constituent for DOTs and must control them in their discharge, to the MEP.  This research will assist DOTs in prioritizing the sources of zinc in their runoff, and allow them to fashion effective mitigation programs (BMPs). This work is particularly urgent in that metals TMDLs are becoming common across the country and this research will be an important component for use in the TMDL implementation process as well as for the traditional elements of the DOT stormwater program. 

Suggested By

AASHTO Stormwater Community of Practice - May 2011 State-of-the-Practice Report