Wildfire impacts on surface water quality parameters: Cause of data variability and reporting needs

Surface runoff mobilizes the burned residues and ashes produced during wildfires and deposits them in surface waters, thereby deteriorating water quality. A lack of a consistent reporting protocol precludes a quantitative understanding of how and to what extent wildfire may affect the water quality of surface waters. This study aims to analyze reported pre- and post-fire water quality data to inform the data reporting and highlight research opportunities. A comparison of the pre-and post-fire water quality data from 44 studies reveals that wildfire could increase the concentration of many pollutants by two orders of magnitude. However, the concentration increase is sensitive to when the sample was taken after the wildfire, the wildfire burned area, discharge rate in the surface water bodies where samples were collected, and pollutant type. Increases in burned areas disproportionally increased total suspended solids (TSS) concentration, indicating TSS concentration is dependent on the source area. Increases in surface water flow up to 10 m³ s^-1 increased TSS concentration but any further increase in flow rate decreased TSS concentration, potentially due to dilution. Nutrients and suspended solids concentrations increase within a year after the wildfire, whereas peaks for heavy metals occur after 1-2 years of wildfire, indicating a delay in the leaching of heavy metals compared to nutrients from wildfire-affected areas. The concentration of polycyclic aromatic hydrocarbons (PAHs) was greatest within a year post-fire but did not exceed the surface water quality limits. The analysis also revealed inconsistency in the existing sampling protocols and provides a guideline for a modified protocol along with highlighting new research opportunities. Overall, this study underlines the need for consistent reporting of post-fire water quality data along with environmental factors that could affect the data so that the post-fire water quality can be assessed or compared between studies.

Can water composition and weather factors predict fecal indicator bacteria removal in retention ponds in variable weather conditions?

Retention ponds provide benefits including flood control, groundwater recharge, and water quality improvement, but changes in weather conditions could limit the effectiveness in improving microbial water quality metrics. The concentration of fecal indicator bacteria (FIB), which is used as regulatory standards to assess microbial water quality in retention ponds, could vary widely based on many factors including local weather and influent water chemistry and composition. In this critical review, we analyzed 7421 data collected from 19 retention ponds across North America listed in the International Stormwater BMP Database to examine if variable FIB removal in the field conditions can be predicted based on changes in these weather and water composition factors. Our analysis confirms that FIB removal in retention ponds is sensitive to weather conditions or seasons, but temperature and precipitation data may not describe the variable FIB removal. These weather conditions affect suspended solid and nutrient concentrations, which in turn could affect FIB concentration in the ponds. Removal of total suspended solids and total P only explained 5% and 12% of FIB removal data, respectively, and TN removal had no correlation with FIB removal. These results indicate that regression-based modeling with a single parameter as input has limited use to predict FIB removal due to the interactive nature of their effects on FIB removal. In contrast, machine learning algorithms such as the random forest method were able to predict 65% of the data. The overall analysis indicates that the machine learning model could play a critical role in predicting microbial water quality of surface waters under complex conditions where the variation of both water composition and weather conditions could deem regression-based modeling less effective.

Iron amendments minimize the first-flush release of pathogens from stormwater biofilters

First flush or the first pore volume of effluent eluted from biofilters at the start of rainfall contributes to most pollution downstream because it typically contains a high concentration of bacterial pathogens. Thus, it is critical to evaluate designs that could minimize the release of bacteria during a period of high risk. In this study, we test the hypothesis of whether an addition of iron-based media to biofilter could limit the leaching of Escherichia coli (E. coli), a pathogen indicator, during the first flush. We applied E. coli-contaminated stormwater intermittently in columns packed with a mixture of sand and compost (70:30 by volume, respectively) and iron filings at three concentrations: 0% (control), 3%, and 10% by weight. Columns packed with a mixture of sand and iron (3% or 10%) without compost were used to examine the maximum capacity of iron to remove E. coli. In columns with iron, particularly 10% by weight, the leaching of E. coli during the first flush was 32% lower than the leaching from compost columns, indicating that the addition of iron amendments could decrease first-flush leaching of E. coli. We attribute this result to the ability of iron to increase adsorption and decrease growth during antecedent drying periods. Although the addition of iron filings increased E. coli removal, the presence of compost decreased the adsorption capacity: exposure of 1 g of iron filings to 1 mg of DOC reduces E. coli removal by 8%. The result was attributed to the alteration of the surface charge of iron and blocking of adsorption sites shared by E. coli and DOC. Collectively, these results indicate that the addition of sufficient amounts of iron media could decrease pathogen leaching in the first flush effluent and increase the overall biofilter performance and protect downstream water quality.

Nitrate removal uncertainty in stormwater control measures: Is the design or climate a culprit?

Eutrophication is caused by excess nitrate and other nutrient exported via stormwater runoff to surface waters, which is projected to increase as a result of climate change. Despite recent increases in the implementation of stormwater control measures (SCM), nutrient export has not abated, indicating poor or inconsistent removal capacities of SCM for nitrate. However, the cause of the variability is unclear. We show that both design and local climate can explain nitrate removal variability by critically analyzing data reported on the international BMP database for nitrate removal by four common types of SCM: bioretention cells, grass swales, media filters, and retention ponds. The relative importance of climate or design on nitrate removal depends on the SCM type. Nitrate removal in grass swales and bioretention systems is more sensitive to local climate than design specifications, whereas nitrate removal in the retention ponds is less sensitive to climate and more sensitive to design features such as vegetation and pond volume. Media filters without amendment have the least capacity compared to other SCM types surveyed, and their removal capacity was independent of the local climate. Adding amendments made up of carbon biomass, iron-based media, or a mixture of these amendments can significantly improve nitrate removal. The type of carbon biomass is also a factor since biochar does not appear to affect nitrate removal. This analysis can help inform the selection of SCM and modification of their design based on local and projected climate to maximize nitrate removal and minimize eutrophication.

Conventional and amended bioretention soil media for targeted pollutant treatment: A critical review to guide the state of the practice

Bioretention systems are widely used green infrastructure elements that utilize engineered bioretention soil media (BSM) for stormwater capture and treatment. Conventional bioretention soil media, which typically consists of sand, sandy loam, loamy sand or topsoil amended with compost, has limited capacity to remove and may leach some stormwater pollutants. Alternative engineered amendments, both organic and inorganic, have been tested to supplement BSM. Yet, municipalities and regulatory agencies have been slow to adopt these alternative amendments into their design specifications, partly because of a lack of clear guidance on how to select the right amendment to treat a target stormwater contaminant under highly variable climatic conditions. This article aims to provide that guidance by: (1) summarizing the current design BSM specifications adopted by jurisdictions worldwide, (2) comparing the performance of conventional and amended BSM, (3) highlighting advantages and limitations of BSM amendments, and (4) identifying challenges for implementing amendments in field conditions. The analysis not only informs the research community of the barriers faced by stormwater managers in implementing BSM amendments but also provides guidelines for their adoption by interested agencies to comply with existing regulations and meet design needs. This feedback loop could catalyze further innovation in the development of sustainable stormwater treatment technologies.

Size-dependent biochar breaking under compaction: Implications on clogging and pathogen removal in biofilters

Breaking of biochar during compaction of amended soil in roadside biofilters or landfill cover can affect infiltration and pollutant removal capacity. It is unknown how the initial biochar size affects the biochar breaking, clogging potential, and contaminant removal capacity of the biochar-amended soil. We compacted a mixture of coarse sand and biochar with sizes smaller than, similar to, or larger than the sand in columns and applied stormwater contaminated with E. coli. Packing columns with biochar pre-coated with a dye and analyzing the dye concentration in the broken biochar particles eluted from the columns, we proved that biochar predominantly breaks under compaction by disintegration or splitting, not by abrasion. Increases in biochar size decrease the likelihood of biochar breaking. We attribute this result to the effective dissipation of compaction energy through a greater number of contact points between a large biochar particle and the adjacent particles. Most of the broken biochar particles are deposited in the pore spaces of the background geomedia, resulting in an exponential decrease in hydraulic conductivity of amended sand with an increase in suspended sediment loading. The clogging rate was higher in the columns with small biochar. The columns with small biochar also exhibited high E. coli removal capacity, partly because of an increase in bacterial straining at reduced pore size after compaction. These results are useful in selecting appropriate biochar size for its application in soils and roadside biofilters for stormwater treatment.

Compaction conditions affect the capacity of biochar-amended sand filters to treat road runoff

Amending roadside soil with adsorbents such as biochar can help remove pollutants from road runoff. To maintain soil stability, the roadside soil requires compaction. However, it is unknown how compaction conditions affect the capacity of biochar-augmented roadside biofilters to infiltrate stormwater and remove pollutants. This work examines the effect of compaction conditions on the release of biochar particles disintegrated during compaction, and the change in their capacity to infiltrate stormwater and remove E. coli. The net loss of biochar particles by mobilization with stormwater was insignificant compared to the biochar remained in the filters. The initial release of biochar particles in wet-compacted biochar columns was greater than that in dry-compacted biochar. The results revealed that compaction can affect the release of biochar particles in a series of three-step processes: generation of particles by disintegration of large biochar under compaction, diffusion of particles deposited near grain walls to bulk pore water, and transport and retention of particles in constricted pore paths based on pore water connectivity. Under similar conditions, compost columns released more particles than biochar columns, suggesting biochar is more stable than compost under compaction. E. coli removal in wet-compacted columns was greater than removal in dry-compacted columns, owing to greater pore path connectivity in wet-compacted columns. These results indicate that addition of moisture during compaction can increase contaminant removal, initial particle release, and infiltration capacity of biochar-augmented sand filters for road runoff treatment. The results would help develop design guidelines for roadside stormwater treatment systems that require compaction of filter media.

Stormwater green infrastructures retain high concentrations of TiO2 engineered (nano)-particles

Stormwater conveys natural and engineered (nano)-particles, like any other pollutants, from urban areas to water resources. Thus, the use of stormwater green infrastructures (SGI), which infiltrate and treat stormwater, can potentially limit the spread of engineered (nano)-particles in the environment. However, the concentration of engineered (nano)-particles in soil or biofilter media used in SGI has not been measured due to difficulties in distinguishing natural vs. engineered (nano)-particles. This study reports, for the first time, the concentration and size distribution of TiO₂ engineered (nano)-particles in soils collected from SGI. The concentrations of TiO₂ engineered (nano)-particles were determined by mass balance calculations based on shifts in elemental concentration ratios, i.e., Ti to Nb, Ti to Ta, and Ti to Al in SGI soils relative to natural background elemental ratios. The concentrations of TiO₂ engineered (nano)-particles in SGI soils varied between 550 ± 13 and 1800 ± 200 mg kg^-1. A small fraction of TiO₂ engineered (nano)-particles could be extracted by ultrapure water (UPW) and Na₄P₂O₇; however, the concentration of TiO₂ engineered (nano)-particles was higher in the Na₄P₂O₇-extracted suspensions than in UPW-extracted suspensions. The concentration of TiO₂ in the nanosize range increased with the increase in extractant (Na₄P₂O₇) volume to soil mass ratio due to the increased disaggregation of soil heteroaggregates. The size distribution of TiO₂ engineered (nano)-particles in the < 450 nm Na₄P₂O₇-extracted suspension from one of the SGI soils was determined by asymmetrical flow-field flow fractionation coupled to inductively coupled plasma-mass spectrometer, and was found to vary in the range of 25-200 nm with a modal size of 50 nm. These results demonstrated that the increase in the Ti to natural tracers (e.g., Nb, Ta, and Al) elemental ratios in the SGI soil relative to bulk soil can be used to estimate the concentration of TiO₂ engineered (nano)-particles in SGI.

Microbial quality of surface water and subsurface soil after wildfire

Runoff from wildfire affected areas typically carries high concentrations of fine burned residues or eroded sediment and deposits them in surface water bodies or on subsurface soils. Although the role of wildfire residues in increasing the concentration of chemical contaminants in both environments is known, whether and to what degree wildfire residues may affect microbial contaminants is poorly understood. To examine the effect of wildfire residues on growth and die-off of Escherichia coli (E. coli) -a pathogen indicator, we mixed stormwater with E. coli and suspended particles from the pre- and post-wildfire area in batch reactors and monitored E. coli concentration. E. coli grew initially in the presence of all particles, but the relative E. coli concentration was 10 times lower in the presence of wildfire residues than in natural soil from unaffected areas. Wildfire residues also decreased the persistence of E. coli during a 15-day incubation period. These results indicate that the growth or persistence of E. coli in surface water in the presence of wildfire residues was less than that in the presence of unburned soil particles, potentially due to depletion of nutrient concentration and/or loss of viability of bacteria in the presence of wildfire residues. To examine the transport potential of wildfire residues and their ability to facilitate the transport of E. coli in the subsurface system, suspensions containing wildfire residues and/or E. coli were injected through unsaturated sand columns-a model subsurface system. Transport of wildfire residues in sand columns increased with decreases in the depth and increases in the concentration of particles, but increased transport of wildfire residues did not result in the increased transport of E. coli, suggesting wildfire residues do not facilitate the transport of E. coli. Overall, the results indicate that wildfire residues may not increase the risk of the microbial contamination of surface water or groundwater via subsurface infiltration.

Biochar increases nitrate removal capacity of woodchip biofilters during high-intensity rainfall

Stormwater biofilters have been increasingly used to mitigate the impact of climate change on the export of contaminants including nitrate to water bodies. Yet, their performance is rarely tested under high-intensity rainfall events, which are predicted to occur more frequently under climate change scenarios. We examined the potential of biochar to improve the resilience of woodchip biofilters under simulated high-intensity rainfall events and linked denitrification to biochar-mediated changes in hydrological (physical), chemical, and biological properties of woodchip biofilters. Results showed that nitrate removal capacity of woodchip biofilters decreased with increases in rainfall intensity or duration and decreases in antecedent drying time. However, adding biochar to woodchips significantly decreased the exhaustion rate of woodchips, only when the hydraulic residence time (HRT) was less than 5 h. At longer HRT (>5 h), the benefits of biochar became less apparent. We attributed the improved denitrification during high nitrate loading to biochar's ability to decrease dissolved oxygen in pore water and increase water holding capacity and retention of dissolved organic carbon and nitrate-all of which could increase nitrate utilization. Biochar increased the net microbial biomass but did not affect the relative abundance of denitrifying genes, which indicates that a shift in microbial biomass could not fully explain the observed increase in nitrate removal in biochar-augmented woodchip biofilters. Overall, the results showed that biochar could increase the resiliency of woodchip biofilters for denitrification in high-intensity rainfall events, a worst-case scenario, thereby mitigating the water quality degradation during climate change.

Plenty of room for carbon on the ground: Potential applications of biochar for stormwater treatment

Low impact development (LID) systems are increasingly used to manage stormwater, but they have limited capacity to treat stormwater-a resource to supplement existing water supply in water-stressed urban areas. To enhance their pollutant removal capacity, infiltration-based LID systems can be augmented with natural or engineered geomedia that meet the following criteria: they should be economical, readily available, and have capacity to remove a wide range of stormwater pollutants in conditions expected during intermittent infiltration of stormwater. Biochar, a carbonaceous porous co-product of waste biomass pyrolysis/gasification, meets all these criteria. Biochar can adsorb pollutants, improve water-retention capacity of soil, retain and slowly release nutrients for plant uptake, and help sustain microbiota in soil and plants atop; all these attributes could help improve removal of contaminants in stormwater treatment systems. This article discusses contaminant removal mechanisms by biochar, summarizes specific biochar properties that enhance targeted contaminants removal from stormwater, and identifies challenges and opportunities to retrofit biochar in LID to optimize stormwater treatment.