Best Practices for Stormwater Facility Selection and Design at Sites with Impacted Soil and Groundwater

Stormwater management facilities are an important function of managing stormwater runoff quantity and quality for new and existing developments. However, some facilities can have negative consequences if special consideration is not taken during the preliminary design. Site conditions (i.e., soil, groundwater, etc.) must be considered during location selection and design to ensure the best management practice (BMP) selected to address stormwater management at the facility will function properly, as the design intends.

TRC has worked on a variety of stormwater management facilities, ranging from small-scale vegetated filter strips and infiltration galleries to large-scale industrial stormwater management facilities equipped with multiple detention basins and manufactured treatment devices (MTD). Through this history of working in stormwater management design and compliance, we have seen a wide range of different infrastructure techniques used on sites with impacted soils and/or groundwater. Stormwater management design on impacted sites warrants expertise in fate and transport, going beyond typical stormwater quantity and quality design. Which approaches work in different types of geologic environments and which BMPs have potential to cause future issues once implemented? We have found design elements of four common types of BMPs that are critical to consider during the selection process.

Permeable Pavements

Infiltrating permeable pavement BMPs have grown in popularity as a viable option for stormwater management with the technological advancements of porous aggregate mixes and regulatory agency acceptance. The porous pavement can replace existing impervious material, such as asphalt, for site impervious reduction or be proposed in new designs. This BMP is most commonly used as a method of managing runoff quantity rather than quality, typically infiltrating the stormwater that interfaces with the permeable pavement into the soil column below the pavement.

Permeable pavements may not be a suitable option at facilities where the runoff infiltrates through impacted soils or in areas where naturally attenuated soils and constituents of concern are present. This BMP can assist in water quantity management but may reduce the control of water quality management. Beyond the measurement of the hydraulic conductivity of the soil beneath the BMP, the subsoil should be reviewed and tested to evaluate the risk of infiltrating stormwater runoff through that soil. In this situation the BMP for the site, to keep runoff from parking and vehicular traffic areas, includes conveying the stormwater to a surface BMP for management. Ideally, a location for the receiving facility can be found outside of the environmentally impacted area.

Detention & Retention Facilities

Stormwater detention (dry basin) and retention (wet basin) BMPs are common selections used for site stormwater management where larger areas direct concentrated flow to a single location. In some areas of the United States groundwater levels are high, which can effectively eliminate the viability of these BMPs. A typical requirement set by regulators for these types of BMPs is ensuring that the bottom of the basin is between two and four feet above seasonal high groundwater level. This requirement is not always feasible to achieve and is an important consideration when discussing design BMPs. Groundwater flow can also have an impact on these BMPs, for example, if a facility is located where it will interface with groundwater with a known impact, this can create a scenario for pollution of stormwater and transport of the pollutant via stormwater discharge.

Underground structures that may interface with the groundwater table are likely to allow inflow and infiltration (I&I) into these structures from the lack of watertightness, making a potential pathway for contaminants that may be present within the groundwater or surrounding soils. This type of BMP may be determined to entail higher risks for a facility with known groundwater (or even soil) contamination issues, but liners are a potential design solution to provide an impermeable barrier to groundwater and/or contaminated soil to isolate the stormwater BMP. During construction, quality assurance and quality control measures should be in place to verify structures are installed as designed and provide the watertightness by testing or other measures to confirm I&I are not present in your stormwater conveyance design.

Conveyance Channels

Conveyance swales and channels are BMPs that are typically selected to divert runoff or convey runoff from one point of the site to another where a detention facility, such as a basin, is provided to attenuate storm events. Swales and channels can also be used to improve stormwater quality, reducing suspended particles through filtration and settling. Sites often consider using swales or bio-swales if stormwater across the site is not being naturally directed to on-site basins or other attenuation BMPs. Conveyance swales and channels are useful tools that can greatly increase the effectiveness of a site’s stormwater management system, but they are not universally applicable.

Conveyance BMPs such as these are often excavated to achieve target grades and slopes. If a site has a shallow contaminant plume or the site has historical impacts to the soil, there can be risk in contacting the plume when excavating near or adjacent to the swale or channel. Excavation raises concerns of exposing impacted soil and groundwater, directly or indirectly, to stormwater. If contaminants from the plume are introduced to the conveyance BMP, they may be transported to other stormwater BMPs downstream and ultimately, be discharged from the site. Reviewing available environmental investigation reports is a key planning step to designing conveyance systems. Solutions could be to consider another location for the swale or channel where excavation near the impacted area can be avoided with acceptable confidence or to consider developing a diversion berm placed on grade to avoid excavation altogether.

Mulch Layers

Mulches, particularly hardwood mulches, are specified in numerous stormwater BMPs that either have an infiltration design element or are vegetated with plant material. Mulch placement is a common technique for moisture control to promote plant survival and as an adsorbent of heavy metals and nutrients in stormwater runoff. While hardwood mulches are selected for their density to avoid floatation and slower degradation as compared to softwoods, the hardwood mulch still requires a maintenance program for removal and replacement on a determined frequency.

If mulch in a stormwater BMP has been designed as a site measure for metals removal from stormwater, the maintenance program should be robust and specific in order to fully utilize the expected adsorption performance of the mulch. If mulch layers are not regularly removed and replaced according to the maintenance schedule, they will inevitably degrade, potentially releasing much of the adsorbed pollutants that were intended to be removed. Additionally, the degraded mulch will not provide a matrix to adsorb target pollutants from future intercepted runoff. The performance declines and the pollutants initially captured may then be conveyed downstream or infiltrated at a greater rate than expected. Abandoned or lightly staffed sites and facilities may not benefit from BMPs dependent on mulch performance if the maintenance program is unlikely to be well-managed.

Next Steps

These four types of stormwater management BMPs cover design considerations pertaining to the fundamentals of infiltration, ponding, surface flow, and mulching. TRC’s expertise in these areas, our diligence in analyzing existing site conditions and care in selecting project design teams have helped our clients minimize stormwater management risks. To learn more about stormwater management regulations or designs, view our related services or contact TRC today.