A tiered complexity conceptual framework for treating water soluble, hydrophilic contaminants in green stormwater infrastructure

Introduction and motivation

Green stormwater infrastructure (GSI) as a nature-based solution for urban stormwater has historically been designed to mitigate flooding, but is increasingly implemented to improve water quality,^1,2 enhance human environments,³ and augment water supplies.⁴ Dissolved, hydrophilic contaminants are a growing concern in urban stormwater,^5,6 presenting a wicked challenge that can yield deleterious impacts to water quality and limit potential beneficial use. Current GSI systems maintain relatively short hydraulic retention times to maximize runoff mitigation. Sandy media amendments are highly effective for capture of particles or particle-associated contaminants but do not adequately treat water soluble, hydrophilic contaminants. There is growing realization that many contaminants in stormwater are dissolved and not particle associated.^5–7 Examples include trace organic contaminants, dissolved nutrients, dissolved metals, and deicing salts. This challenge raises the question: How can we improve the quality of the most stormwater using GSI in a world of limited resources for implementation and maintenance?

A tiered conceptual framework to address dissolved, hydrophilic contaminants

Decisions to improve GSI are often haphazard, focused on managing stormwater quantity (i.e., peak flows, infiltration volumes) and removing particles/particle-associated compounds. There is a critical need for decision support to focus enhanced GSI research and implementation in the context of dissolved, hydrophilic contaminants and propose or prescribe treatment interventions. The challenges related to persistent, mobile, and toxic (PMT) substances in stormwater and their low removal in GSI have been described in a prior ESWRT review.⁵ We recognize that stormwater quality regulations for GSI systems are often lacking, particularly regarding dissolved, hydrophilic contaminants. Fit-for-purpose treatment goals and targets need to be defined to comply with existing regulations (e.g., environmental quality) or to follow precautionary principles in collaboration with local communities/stakeholders. To minimize risks associated with dissolved, hydrophilic chemicals in stormwater, we propose a tiered complexity conceptual framework ranging from the simplest, least energy/resource intensive GSI intervention up to the most resource-intensive approaches to maximize total water quality improvements (Fig. 1):

Fig. 1 Tiered conceptual complexity framework to address interventions for dissolved, hydrophilic contaminant removal from urban stormwater in green stormwater infrastructure (GSI). Increasingly complex interventions can be applied to achieve reductions in environmental and human health risks associated with stormwater contaminants, thereby maximizing resource efficiency and improving chemical water quality.

Tier 1: [simplest/least resource intensive] Improved Hydraulics

GSI practices must be designed, implemented, and operated to ensure proper distribution and retention of incoming stormwater to capture contaminants (including dissolved, hydrophilic and particle-associated). Sometimes, soil surfaces higher than the inlet and/or outlet structure or a lack of maintenance, prevents stormwater from attaining full surface distribution, limiting opportunities for contaminant sorption, transformation, or plant uptake. Many GSI practices infiltrate faster than recommended by local stormwater GSI design manuals;^8,9 therefore, significant opportunities exist to increase water retention time and use the full storage volume without the need to change existing design criteria. For example, tracer tests demonstrated a hydraulic retention time <2 h, which limited removal of benzotriazole.¹⁰ Decreased hydraulic conductivity of the media would enhance sorption, plant uptake, and transformation of benzotriazole.¹¹ Early efforts on flow control employed up-turned pipes connected to underdrains to maintain saturated anoxic conditions for denitrification (anoxic conditions can, however, limit degradation of many organic chemicals). Recently, automated real-time flow controls enabled by internet-connected sensors have been successfully used to control flows in GSI.¹² If combined with redox sensors, this approach can optimize hydraulics while tuning redox conditions, thereby generating opportunities to increase sorption and transformation for dissolved, hydrophilic contaminants that are not highly persistent but would benefit from increased hydraulic retention time.

Tier 2 [medium complexity/ intensity] Improved Media

Conventional infiltration-based GSI practices contain media consisting primarily of sand and compost to maximize infiltration and support plant growth, which unfortunately poorly removes dissolved, hydrophilic contaminants.⁷ Increasing compost content can improve sorption but can generate undesirable nutrient leaching. Iron oxides added to geomedia have proven effective at capturing dissolved phosphorus.¹³ Efforts to incorporate highly sorptive black carbon materials (i.e., activated carbon or biochar) as a geomedia component are effective at capturing dissolved, hydrophilic trace organic contaminants while maintaining hydraulic conductivity and minimal leaching.^14–18 Sorption capacity of black carbon can be regenerated in situ through biological processes (i.e., microbial biofilms,¹⁹ fungal degradation,^20,21 plant uptake/transformation [an ESWRT review covers plant processes in bioretention²²]) to sustain removal of captured non-persistent contaminants. Bioaugmentation of GSI ²³ and development of novel geomedia containing encapsulated organisms and sorptive materials to retain biodegrading microbes proximal to sorbed organic contaminants^24–26 can potentially renew sorption capacity. Analogous to how activated sludge revolutionized wastewater treatment by decoupling solids residence time from hydraulic residence time, biologically active sportive media enables capture of many dissolved, hydrophilic organic contaminants during infiltration events with subsequent biodegradation during antecedent dry periods to sustain removal. Some types of metal oxides-containing geomedia amendments are also capable of transforming organic contaminants in situ;^27,28 however, regeneration remains a challenge.^29,30 Improved geomedia can support dissolved, hydrophilic contaminant removal at relatively low costs, via pre- or post-construction amendments.

Tier 3 [highly complex/specialized “hot spot”] Advanced Treatment Options

Numerous dissolved, hydrophilic organic contaminants in stormwater can be classified as (very) persistent and (very) mobile with potential toxicity.⁵ These intrinsic properties yield minimal removal in conventional GSI due to a lack of sorption or biodegradation, even with improved media. Because persistent and mobile substances can spread throughout the water cycle and enter drinking water sources, advanced stormwater treatment technologies are gaining increased interest. Oxidation processes, involving chemical oxidants (e.g., hydrogen peroxide [H₂O₂] or persulfate) and/or UV light are particularly promising to abate persistent and mobile organic contaminants. For instance, an electrochemical advanced oxidation process has been proposed, where H₂O₂ is produced on site and converted to hydroxyl radicals via UV lamp.^31,32 Solar-driven advanced oxidation processes have been studied for herbicide removal from stormwater.³³ Persulfate can be activated with biochar for the removal of a variety of stormwater contaminants,³⁴ allowing combined adsorption and oxidation. Although various oxidation processes are used in drinking water and wastewater treatment, applications for stormwater are still being explored in laboratory studies. Pilot- and full-scale investigations are needed to address practical challenges; e.g., related to oxidant supply or by-product formation. Ion exchange resins for per- and polyfluoroalkyl substances (PFAS) removal have been investigated, with the option to regenerate the resins and degrade PFAS (e.g., UV/sulfite),³⁵ but stormwater applications are lacking. Advanced stormwater treatment involves the highest complexity (i.e., equipment, materials, operation, and maintenance); therefore, we propose advanced stormwater treatment for: (i) contamination hotspots [e.g., highway runoff, industrial stormwater], (ii) neighborhood-scale GSI systems collecting larger water volumes, and (iii) the protection of drinking water sources impacted by or augmented by urban stormwater.

Conclusions and outlook

Green stormwater infrastructure (GSI) is a critical tool for improving stormwater quality, including the wicked challenge of dissolved, hydrophilic contaminants poorly removed in conventional systems. Herein, we propose a tiered conceptual framework of progressively more complex, costly, and resource-intensive interventions to capture and/or remove/degrade dissolved, hydrophilic contaminants that can pose risks to ecosystems and drinking water sources. Our overall goal is to maximize water quality improvement and focus intervention implementation efforts for GSI technologies in a world with limited resources. Research to advance all three Tiers should be expanded, and more data collected to allow effective chemical mixture risk assessment in urban stormwater and GSI. We close with the continued appeal that contaminant source control is the most effective and least-costly approach to lowering watershed loads. Indeed, some highly soluble contaminants, such as deicing salts or trifluoroacetate (TFA), cannot practically be removed in GSI and source control is the only realistic approach.

Conflicts of interest

The authors declare no competing interests.

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