What Is Stormwater Associated with Industrial Activity?

Understanding Industrial Stormwater and When You Need an NPDES Stormwater Permit

If you own or operate an industrial facility - whether that’s manufacturing, concrete, mining, marinas, metalworking, or transportation, you’ve probably heard the term “stormwater associated with industrial activity.”

It’s one of those phrases that sounds straightforward but carries a lot of regulatory weight. In short, it’s the legal term for rainwater or snowmelt runoff that comes into contact with industrial operations in a way that could carry pollutants into nearby waterways.

This type of runoff is regulated under the Federal Clean Water Act (CWA) through the National Pollutant Discharge Elimination System (NPDES) program. If you discharge stormwater associated with industrial activity from your site, you’re likely required to have an NPDES permit - and failure to do so can result in fines, enforcement actions, or worse.

Heads up, if you think you might need a hand with your operation's stormwater management or permitting, reach out to us here for help! Now, let's unpack what all this means, why it matters, and what kinds of activities typically fall under this definition.

Table of Contents

• The Official Definition of Stormwater
• How NPDES Regulations Apply to Stormwater
• Examples of Stormwater Associated with Industrial Activity
• What Stormwater Is Not
• Why Stormwater Permitting Matters
• Where to Get NPDES Permitting Help
• Additional NPDES Permitting Information

The Official Definition of Stormwater

The phrase “stormwater associated with industrial activity” comes directly from 40 CFR §122.26(b)(14) of the Federal Clean Water Act regulations. In plain English, that definition says: “Stormwater associated with industrial activity means stormwater runoff that is directly related to manufacturing, processing, or raw materials storage areas at an industrial plant, which are exposed to stormwater.” That’s the legal foundation for industrial stormwater permitting in the United States.

In practice, this means if precipitation - rain, snow, or even sleet - falls on industrial materials, products, byproducts, or waste at your site, the resulting runoff is considered “stormwater associated with industrial activity.” Why? Because once that water contacts those materials, it can pick up pollutants and carry them into storm drains, streams, or rivers, potentially impacting water quality.

And realize this: there is no mention in this definition of whether or not you (or anyone else) considers the runoff to be “polluted”. You might consider it to be clean, but if it came into contact with any materials or processes at your site that might conceivably cause that runoff to carry away any amount of pollutant, then its regulated. Whether you think that runoff is “clean” or not!

How NPDES Regulations Apply to Stormwater

Under the Clean Water Act, any discharge of pollutants to U.S. surface waters requires an NPDES permit. That includes not just wastewater but also stormwater if it’s connected to industrial activity.

Most states are delegated by the EPA to administer their own NPDES programs, meaning they issue permits, set monitoring requirements, and enforce compliance. These are typically called Industrial Stormwater General Permits, though exact names vary from state to state (for example, Pennsylvania’s PAG-03, North Carolina’s NCG08, and New Jersey’s R9).

If your facility discharges stormwater from any area associated with industrial activity, you likely need one of these permits. The key phrase is “associated with.” Even if your stormwater discharge is mostly clean, if it originates from an area used for:

• Fueling areas
• Equipment storage or maintenance
• Loading and unloading of materials
• Outdoor production or processing
• Waste or scrap storage
• Raw material handling

...then it’s considered “associated with industrial activity”, and it’s regulated.

Examples of Stormwater Associated with Industrial Activity

To make this more clear, here are a few real-world examples we commonly see at RMA:

1. A Concrete Plant

Rainwater falls on aggregate stockpiles, mixes with cement dust, and runs across the yard to a discharge location like a drain or nearby stream. That’s stormwater associated with industrial activity.

2. A Trucking Terminal

Stormwater runs off areas where trucks are parked, refueled, and maintained. Because it can pick up oil, grease, and debris, it’s regulated under industrial stormwater rules.

3. A Metal Fabrication Shop

Stormwater that runs off a yard where metal parts are stored or painted is considered stormwater associated with industrial activity due to potential metals or solvents.

4. A Recycling Yard

Rainwater that contacts scrap materials, containers, or fluids from vehicles is stormwater associated with industrial activity, and it must be managed under a stormwater permit.

In all these cases, the key factor is exposure: if stormwater comes into contact with materials or activities that could contaminate it, it’s regulated.

What Stormwater Is Not

It’s equally important to understand what stormwater isn’t. Stormwater does not include process water. Process water, or “non-stormwater,” is water that’s generated, handled, or used in industrial operations (like equipment wash water, cooling water, or leachate).

Unlike stormwater, process water usually cannot be discharged under a general stormwater permit unless that permit specifically allows it (for example, North Carolina’s NCG19 concrete permit). In most cases, process water requires its own individual NPDES permit.

If you’re ever unsure whether what’s leaving your site is stormwater or process water, RMA recommends performing what we call The Dry Day Test: if water is being discharged on a dry day, when it hasn’t rained - it’s probably process water, not stormwater.

Why Stormwater Permitting Matters

You might be thinking, “It’s just rainwater - how bad could it be?” But remember: when rainwater runs across industrial areas, it’s not just water anymore. It can carry:

• Oils and greases
• Heavy metals
• Sediment
• Chemicals
• Nutrients like nitrogen and phosphorus

If that runoff enters a storm drain or waterway untreated, it can contribute to pollution, harm aquatic life, and trigger regulatory action. That’s why industrial stormwater permits require facilities to implement Best Management Practices (BMPs) - things like covered storage, good housekeeping, spill prevention, and regular inspections - to minimize pollutants in runoff.

Where to Get NPDES Stormwater Permit Help

Stormwater associated with industrial activity is any precipitation runoff that comes into contact with industrial operations or materials - and it’s regulated under the Clean Water Act’s NPDES program.

Most states follow the federal definition closely, though specific permit structures may vary. If your operations involve outdoor materials, equipment, or waste, chances are you’re subject to these requirements.

Understanding what qualifies as industrial stormwater - and getting the right permit coverage - isn’t just a box to check. It’s a key part of protecting your business and staying in compliance.

At RMA, we help facilities across the country determine whether their discharges qualify, what permits apply, and how to stay compliant without unnecessary complexity.

If you’re unsure whether your site’s runoff qualifies as stormwater associated with industrial activity, don’t guess.
Reach out to us today here and we’ll help you get it right from the start.