Zinc Runoff: Understanding Its Impact on the Environment

Introduction

Galvanized steel can be used safely as zinc runoff from HDG materials does not pose an environmental problem under normal conditions.

Zinc runoff from hot-dip galvanized steel is sometimes misunderstood because zinc is often grouped with “heavy metals” without context. That label has led to concerns that galvanized structures require special mitigation measures or additional coatings to prevent zinc from entering nearby water bodies.

However, decades of research and field observations tell a different story. Zinc is a naturally occurring element that is essential for life, and the natural weathering of hot-dip galvanized steel over many decades to protect steel from corrosion do not pose a significant threat to aquatic organisms. Studies evaluating galvanized structures in real-world environments, along with more recent risk assessments, demonstrate zinc runoff from zinc-coated construction materials does not present a significant aquatic-life concern under normal service conditions.

Understanding the difference between perception and reality is important for engineers, owners, and specifiers. By examining the science behind zinc runoff, bioavailability, and water-quality criteria, it becomes clear why galvanized steel continues to be widely used in environmentally sensitive applications without requiring default mitigation measures or additional coatings.

Why does zinc runoff from galvanized steel get labeled "harmful" so often?

Zinc runoff is often labeled harmful because zinc is frequently discussed without the context needed to understand bioavailability. Simply detecting zinc in runoff does not mean harm is occurring.

Several common misconceptions continue to drive concern:

• Zinc is a "heavy metal," so any amount must be toxic.
• Any zinc released from a galvanized coating is immediately bioavailable.
• Any measurable zinc concentration means aquatic-life criteria are exceeded.
• Galvanized steel near water automatically requires mitigation.
• Painting over hot-dip galvanizing is a necessary mitigation.

These assumptions oversimplify the issue. Zinc is an essential nutrient required by humans, animals, and plants. Like many naturally occurring substances, potential impacts depend on natural backgrounds, site-specific risks, and water chemistry rather than simply whether the element is present.

Another reason the misconception persists is that older guidance, research studies, and simplified source-estimation methods are still referenced today. In some cases, concerns about zinc runoff were based on conservative assumptions that did not fully account for dilution, site conditions, water chemistry, or bioavailability. More recent studies and risk assessments have helped provide a more complete understanding.

Key Point: Zinc is essential for life, and environmental effects depend on site conditions and bioavailability, not simply the presence of zinc itself.

What does the latest evidence say about zinc runoff rates, bioavailability, and lifecycle contributions from HDG steel?

The available evidence shows that zinc runoff from hot-dip galvanized steel is generally a small and gradual process that occurs over decades of service.

• See AGA/IZA’s 2025 risk assessment: Runoff from Zinc-Coated Construction Materials: Risk Assessment Supports Aquatic Safety (2025).
• AGA published the white paper HDG Steel’s Contribution to Zinc Levels in the Water Environment which evaluated six case studies of galvanized bridges and docks in aquatic environments.
• AGA published HDG Steel’s Contribution to Zinc Levels in the Soil Environment (2013) which similarly examined zinc added to the environment over decades of service and determined it is not enough to harm any organisms or exceed criterion levels.

As galvanized steel weathers, the zinc coating naturally develops a stable patina. This patina slows further weathering and helps explain why hot-dip galvanized steel provides long-term corrosion protection with minimal maintenance requirements. Zinc release generally decreases as the coating ages and the patina becomes established.

There is a small amount of zinc lost over the life of the galvanized coating. That zinc loss is spread across decades of exposure while the galvanized coating continues to deliver corrosion protection.

Bioavailability is the portion of zinc present in a form that can be taken up by aquatic organisms. Zinc bioavailability is influenced by site-specific water chemistry, including hardness, pH, dissolved organic carbon, and other factors which influence the concentration of biologically available zinc species and their potential effects on aquatic organisms.

For more detailed information about relevant factors that influence zinc speciation, bioavailability, and toxicity, see Biotic Ligand Models for Predicting Acute and Chronic Zinc Toxicity to Freshwater Organisms (2022).

Recent risk assessments evaluating zinc-coated construction materials have further confirmed that zinc runoff from these materials is not expected to pose a significant threat to aquatic life, even when evaluated using modern water-quality criteria. Readers interested in additional technical background can review AGA's Zinc & Stormwater Runoff resource.

How do stormwater benchmarks actually apply to galvanized structures?

Confusion often arises regarding the topic of stormwater benchmarks for zinc and methods to avoid exceedances. According to the U.S. Environmental Protection Agency (EPA), a benchmark is a concentration value in parts per million (ppm) used to evaluate a pollutant of a regulated activity as either: not expected to have an impact on receiving water quality, or, adversely affecting the receiving water quality (and therefore control measures must be evaluated).

Industrial facilities have stormwater benchmarks as part of their national pollutant discharge and elimination systems (NPDES) permit, whereas galvanized structures in-use do not have permit limits. However, it should be recognized that stormwater runoff will convey zinc from these structures to nearby surface waters. The time course over which zinc would be carried from galvanized structures to surface waters could span the entire lifetime of the galvanized structure, which could be 75 years or more. Indeed, several case studies conducted by the AGA, as mentioned above, demonstrate zinc released from galvanized structures (e.g., bridges and docks) under stormwater runoff scenarios does not lead to exceedances of hardness-based zinc WQC. Therefore, individual galvanized structures are not likely to result in water quality impairments due to zinc. Additionally, it is recognized that current hardness-based WQC are generally overly conservative (i.e., too low) for zinc because water chemistry characteristics other than hardness affect zinc bioavailability in surface waters. Consequently, new and improved WQC are being developed by the US EPA.

Why additional zinc runoff mitigation is often unnecessary

For many years, concerns about zinc runoff have led some stakeholders to recommend galvanized coatings be avoided or that additional coatings are necessary for galvanized steel structures. However, the available research does not support the idea that such measures should be routinely specified.

Modern studies and risk assessments consistently demonstrate that zinc runoff from zinc-coated construction materials is not expected to create significant aquatic-life impacts under normal service conditions.

As a result, specifying paint over galvanizing solely to address zinc runoff concerns is often unnecessary. In many cases, doing so adds cost and complexity without providing a meaningful environmental benefit.

If questions arise regarding unusually sensitive environments or project-specific concerns, specifiers, owners, and regulators are encouraged to consult the American Galvanizers Association (AGA) and the International Zinc Association (IZA) for the most current technical information and guidance.

Putting zinc runoff in perspective for responsible specification

Bottom line: Zinc runoff from hot-dip galvanized steel is a widely misunderstood topic. Although zinc is often characterized as a harmful heavy metal, decades of research show that the small quantities released during normal weathering are generally not enough to threaten aquatic life or justify routine mitigation measures.

Hot-dip galvanized steel continues to provide durable, long-term corrosion protection for bridges, utility structures, transportation infrastructure, and countless other applications. The evidence demonstrates that engineers and specifiers can confidently select galvanized steel based on its proven durability and sustainability benefits without automatically assuming additional coatings or runoff mitigation are required.

References:

• American Institute of Steel Construction (AISC), Is runoff from galvanized steel an environmental issue?, Architecture Center, Newsletter Archives and FAQ, 2026.
• AGA White Paper: Runoff from Zinc Coated Construction Materials: Risk Assessment Supports Aquatic Life
• AGA White Paper: Hot-Dip Galvanized Steel's Contribution to Zinc Levels in the Soil Environment
• AGA White Paper: Hot-Dip Galvanized Steel's Contribution to Zinc Levels in the Water Environment
• E. Van Genderen, W. Adams, R. Cardwell, J. Volosin, R. Santore, P. Rodriguez, An Evaluation of the Bioavailability and Aquatic Toxicity Attributed to Ambient Zinc Concentrations in Fresh Waters from Several Parts of the World, Integrated Environmental Assessment and Management, 2009.
• A. Ryan, R. Santore, K. Schiff, An Updated Unified Zinc Biotic Ligand Model for Protection of Freshwater Aquatic Life and its Application for Site-Specific Water Quality Objectives, Integrated Environmental Assessment and Management, 2026.
• D. DeForest, A. Ryan, L. Tear, K. Brix, Comparison of Multiple Linear Regression and Biotic Ligand Models for Predicting Acute and Chronic Zinc Toxicity to Freshwater Organisms, Environmental Toxicology and Chemistry, 2023.
• Ryan, Adam. 2025. Desktop Risk Assessment of Stormwater Runoff from Selected Zn-coated and Zn-based Construction Materials in Washington State. Prepared for AGA. 11 February 2025.