Stormwater Management: When Is Green Not So Green?

Current national stormwater policy may have adverse effects on public and private water supplies. Shallow groundwater, which is increasingly being relied on for drinking water, irrigation, stream baseflow, and drought relief, is now becoming a sink for unwanted stormwater contaminants to avoid direct discharge to surface water. This policy of infiltration without properly considering implications for groundwater quality should be improved so that society's contaminants are not transferred from one water resource to another just to avoid paying the full cost of today's stormwater management. Stormwater regulatory programs and green infrastructure practices focus first on reducing pollutant loads to surface water, with minimal consideration of pollutant load diverted to groundwater. Best management practices, which provide direct recharge, such as porous pavement, retention ponds, shallow injection wells, as well as agricultural and roadway drains, are commonly used. Frequently the only design criterion for stormwater infiltration is the infiltration rate. Infiltrated stormwater can carry pollutants (nitrogen; pesticides; metals, oil, and grease from road surfaces and gas stations; hazardous waste spills; and salts used in road deicing) as well as cause hydraulic problems (mounding, slope stability, and subsurface flooding of infrastructure). Governmental and professional organizations, including the National Research Council (https://doi.org/10.17226/12465), USEPA Underground Injection Control Program, and Water Environment & Reuse Foundation (www.werf.org), have examined the groundwater impact issue, but issued often vague, general cautions about the risks. For instance, the guidance for the Underground Injection Control program only notes that infiltration through stormwater drainage wells has the potential to adversely impact groundwater supplies. State and local stormwater infiltration guidance typically focuses on avoiding "hotspots" already contaminated so as not to move contaminated groundwater in unanticipated directions. Most guidance documents recommend fixed-distance setbacks for infiltration sites instead of empirically determined or engineered structures to preclude adverse subsurface effects. While these structures may initially function as designed, without guidance and implementation of routine maintenance, they will not continue to do so. Guidance and regulation rarely include mention of groundwater monitoring. We understand that USEPA is starting to identify some key concerns regarding groundwater impacts. These include recognizing aquifer complexity and long-term groundwater monitoring needs near stormwater facilities. Finally, using shallow groundwater for the disposal of todays' stormwater problems may only be delaying a problem rather than solving it. Hydraulic loading by managed stormwater infiltration can overload natural treatment processes. Impacted groundwater can ultimately discharge to surface waters resulting in long-term degradation, especially under low flow conditions. This concept was recently addressed in a U.S. Ninth Circuit Court of Appeals decision (February 1, 2018, Hawaii Wildlife Fund v. County of Maui). The panel found that the disposal wells were "point sources" that discharged "pollutants" into groundwater, that eventually entered surface water and therefore these wells fell under the purview the National Pollutant Discharge Elimination System. On February 20, 2018, the U.S. Environmental Protection Agency issued a request for comment regarding the adequacy of its regulatory programs under the Clean Water and Safe Drinking Water Acts addressing stormwater discharges to groundwater "in direct hydrologic connection to surface water" (83 FR 7126 comments due May 21, 2018). With the complexity of the subsurface, a single approach to managing stormwater infiltration and protecting groundwater quality will not be appropriate. Most guidance documents do note that additional research into groundwater contamination is warranted. Scientific research is needed to identify less risky stormwater infiltration practices, quantify impacts on groundwater quality and quantity, develop appropriate monitoring practices, improve pollutant removal prior to infiltration, and discern sound hydrogeologic and engineering design practices in the siting and design of stormwater facilities. Research is also needed to understand the future effects and costs of stormwater disposal practices and to develop a more advantageous and desirable policy for all water users.