Early in my career in stormwater management, I designed storm sewer networks as isolated infrastructure—after all, MS4 is an abbreviation for Municipal Separate Storm Sewer System. Since then, I’ve gained enough experience to challenge that notion, particularly when it comes to green stormwater infrastructure (GSI). I don’t think I’m alone, so let me explain.

GSI inherently elevates principles of interconnectedness by recognizing that stormwater isn’t simply an isolated engineering problem, but part of a larger social-ecological system. Rather than moving runoff away from developed land as fast as possible, we recognize the benefits of mimicking natural processes onsite to minimize flood risk and enhance the quality of discharged water. We’ve also learned that when we leverage nature to minimize the impact of development, we reap social and economic benefits—from improved community health to increased local jobs and higher neighborhood property values.

A recent study published in the Journal of Hydrology by Kathryn Boening-Ulman et al. (doi.org/10.1016/j.jhydrol.2026.135633) reminded me that we may be overlooking other connections in our current use of GSI. This study identified several instances in which the presence of urban karst phenomena led to unintended behaviors of the installed GSI when compared to the design’s intent. I think this scenario is far more common than we might expect and has me wondering if we should be giving more consideration to the interconnectedness of our infrastructure beneath the surface.

Unintended Interactions

In urban development, buried infrastructure is everywhere: storm sewers, sanitary sewers, water mains and other utilities installed at various depths and often surrounded by porous backfill. These backfill materials often are far more hydraulically conductive than the native surrounding soils. Much like natural karst systems, these anthropogenic networks can rapidly convey flow over long distances, bypassing natural soil processes and creating hidden hydrologic connections, hence the term “urban karst.”

Situating stormwater infrastructure near these subsurface networks can unintentionally result in an entirely different water balance than envisioned during design. Water follows the path of least resistance, so highly conductive backfill easily becomes the preferential pathway for subsurface flow. This phenomenon is particularly relevant for GSI, because these practices are intentionally designed to interact with the surrounding soils. Instead of infiltrating captured stormwater, a bioretention cell may inadvertently discharge that water into adjacent utility trenches or other preferential pathways. While the practice may appear to perform as intended from the surface, its subsurface behavior may tell a very different story, contributing to a host of unintended and unseen consequences.

The Hidden Impact

The consequences of urban karst often remain invisible until they manifest as infrastructure failures or degraded environmental performance. Infiltrated runoff that would otherwise contribute to pollutant removal may instead reach receiving waters with minimal treatment. This could take the form of exporting nutrients from unstable or poorly established bioretention systems conveyed through these pathways or reoxygenation of internal water storage areas intended for denitrification due to unintended drain down.

The same connectivity also can exacerbate sewer intrusion and inflow when infiltrated stormwater finds its way into sanitary backfill envelopes, increasing wastewater treatment costs and reducing available capacity during wet-weather events.

Urban karst also alters the hydrologic response of developed watersheds in ways that are difficult to predict using conventional design assumptions. Water transported through preferential pathways is likely to emerge in unexpected locations, contributing to localized flooding or nuisance seepage well beyond the project boundary. In addition, these hydraulic conduits can intercept and redirect water that would otherwise move slowly through the native soil matrix, diminishing groundwater recharge (one of the primary objectives of many infiltration-based GSI practices).

These impacts matter, because they often occur outside the footprint of the stormwater practice itself. GSI is a preferred stormwater-management solution for many areas across the country, so long as the practices we install provide the benefits intended by the design. However, the broader subsurface network may ultimately determine where infiltrated water travels and how effectively the practice operates. As cities continue to densify and rely more heavily on GSI, understanding these hidden connections will become increasingly important for protecting water quality and maintaining infrastructure resilience.

A Paradigm Shift

Recognizing urban karst requires a shift in how engineers conceptualize the urban subsurface. It requires us to be aware that there’s a dynamic network of connectivity amongst all the additional infrastructure required to bring a project from concept to reality. This requirement for awareness is especially pronounced on redevelopment projects, where legacy infrastructure may be poorly mapped, undocumented or perhaps abandoned. Consideration of these uncertainties and accounting for them in design and construction can go a long way in providing long-term stormwater management.

Addressing urban karst doesn’t require abandoning infiltration-based GSI. More comprehensive utility mapping, geotechnical and hydrogeologic characterization; deployment of anti-seepage collars where appropriate; and careful construction practices all can minimize urban karst interactions. In addition, we should be strategic in our selection and placement of GSI practices. Where site conditions or existing infrastructure make infiltration unlikely to perform as intended, practices that filter, detain or harvest stormwater may provide greater long-term benefits than forcing infiltration into an unsuitable subsurface environment.

Urban karst is a reminder that successful design depends on understanding the entire environment in which we’re building. As engineers, we have become adept at recognizing the interconnectedness in watersheds above ground. The next step is to extend that same type of thinking below the surface. By acknowledging the hidden network of subterranean preferential flow paths created by urban development, we can move beyond treating the subsurface as a design assumption and begin treating it as a design consideration.