Water Works: Passive Stormwater Control Measure (SCM) Effectiveness: Have We Reached Our Limits?

It’s easy to understand why we have predominantly relied on passive stormwater management infrastructure to date. Such stormwater control measures (SCMs) generally are small-scale, highly dispersed practices, installed on a wide variety of sites without convenient electrical connections, let alone the budget or qualified personnel that would be required to monitor and maintain more-advanced systems.

I’m of the opinion that we should be striving to solve our collective infrastructure challenges with the simplest and most cost-effective means possible. When viewed through the lens of the widespread historic neglect of stormwater runoff’s negative impacts on receiving waters, passive SCMs have been moving us closer to our collective goals of “fishable and swimmable.” In other words, “awesome,” this stuff seems to be working, and we don’t need to pay that much attention to it!

It’s Complicated

Predominantly, stormwater-management regulations have been written to take into account the level of performance that typically can be achieved with passive SCMs. For example, the target 80-percent reduction of the total suspended solids (TSS) load standard that’s found its way into stormwater policies throughout North America was originally adopted, in part, because decision makers recognized existing SCMs could meet such a standard at a reasonable cost. Old-school stormwater-quality stalwarts such as wet ponds, sand filters and constructed wetlands come to mind. As we’ve monitored these practices in the field, the resulting data indicates these practices often can reduce the TSS loads from runoff routed to them by 80 percent or more. So what’s the problem?

As we’ve learned more about stormwater runoff—namely what’s in it, how much of it occurs and how it impacts receiving waters—the stormwater story has become more complicated. While we have long known that TSS isn’t the sole source of the stormwater quality problem, we vastly overestimated how impactful reducing TSS loads alone would be at protecting and restoring receiving waters.

Heavy metals (zinc, copper, lead), nutrients (phosphorus and nitrogen) in various chemical states, oils and greases, trash and debris, pathogens, and several other substances also have negative effects on receiving waters, as can the sheer volume of runoff leaving impervious areas. As a result of such complexity, our policies and standards are also becoming increasingly complex and stringent.

Additional pollutants of concern now are commonly being added to regulatory requirements. For example, it’s now common to see numeric load-reduction targets for heavy metals and nutrients at the state, regional or watershed level. Pollutant load-reduction targets also are rising, with some programs now targeting 90-percent TSS load reduction, for example. Total phosphorus removal targets have risen from 30- to 50-percent removal to 65-percent or greater load-reduction targets in sensitive and impaired watersheds.

We’re asking for more performance from our SCMs across a broader suite of known pollutants, yet we’re still largely relying on the same SCMs and unit processes to do so. The question that keeps lodging in my mind is just how much can we reasonably expect from our predominantly unchanged suite of passive solutions? As someone with a background in product development in the manufactured treatment device (MTD) SCM space, I feel we’re starting to run up against pollutant-removal goals that may not be feasible for passive SCMs to achieve consistently or on their own.

Compounding Concerns

Total maximum daily loads (TMDLs) are shining a light on this issue at the watershed scale, particularly in older, urbanized areas with a substantial amount of impervious acreage that’s lacking modern stormwater controls. In these situations, receiving waters often are impaired to the extent that drastic load reductions are needed to have a fighting chance of restoring the impacted watersheds. In addition, we must deploy SCMs on constrained urban sites where the cost to retrofit is high and constraints are commonplace.

These watersheds also tend to be impaired by multiple pollutants, which amplifies the challenge. For example, we may have solutions capable of a high level of phosphorus load reduction that are largely ineffective for pathogens or nitrogen, the latter of which remains a challenge for most of our SCMs. Similarly, a growing suite of emerging pollutants that are not yet commonly regulated such as chlorides, PFAS and 6PPD-q will increase the complexity substantially if/when we opt to regulate them.

No Easy Answers

It doesn’t seem viable to routinely deploy automated treatment facilities at the site level given the cost and complexity. Does that mean more regional treatment facilities to address the most complicated and pressing impairments? Source control may help, but it seems unlikely we will eliminate road salts, fertilizers or persistent PFAS entirely any time soon. Obviously, efforts to infiltrate where feasible also are useful, but we must protect groundwater in doing so, and many sites simply don’t have sufficient infiltration capacity or are otherwise unsuitable.

One thing I’m certain about is that we have come an incredibly long way since my early days in the field working on solutions to remove even more solids from stormwater runoff. We’ve collectively learned an immense amount about what else is in stormwater runoff, ways we can reduce runoff volume and how to capture a variety of additional pollutants. However, when I look out into the future and think about what’s next and the breakthroughs needed to get there, the only thing I’m certain of is that we have even more work to do.

Derek Berg Water Works