Determination of sediment sources following a major wildfire and evaluation of the use of color properties and polycyclic aromatic hydrocarbons (PAHs) as tracers

Purpose

This research aimed to determine if a severe wildfire caused changes in the source of sediment being delivered to downstream aquatic systems and evaluate the use of polycyclic aromatic hydrocarbons (PAHs) and color properties as tracers.

Methods

Sediment samples were collected from 2018 to 2021 in three tributaries impacted by the 2018 Shovel Lake wildfire and from two sites on the mainstem of the Nechako River, British Columbia. Source samples were collected from burned and unburned soils as well as from channel banks and road-deposited sediment. Samples were analyzed for color properties and for the 16 US Environmental Protection Agency priority PAHs. After statistical tests to determine the conservatism and ability to discriminate between sources by the tracers, the MixSIAR unmixing model was used, and its outputs were tested using virtual mixtures.

Result

In the tributaries, burned topsoil was an important contributor to sediment (up to 50%). The mainstem Nechako River was not influenced as significantly by the fires as the greatest contributor was banks (up to 89%). The color properties provided more realistic results than those based on PAHs.

Conclusion

In smaller watersheds, the wildfire had a noticeable impact on sediment sources, though the impacts of the fire seemed to be diluted in the distal mainstem Nechako River. Color tracers behaved conservatively and discriminated between contrasting sources. Due to their low cost and reliability, they should be considered more widely. While PAHs did not work in this study, there are reasons to believe they could be a useful tracer, but more needs to be understood about their behavior and degradation over time.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11368-023-03565-0.

Identifying spatial influence of urban elements on road-deposited sediment and the associated phosphorus by coupling Geodetector and Bayesian Networks

Elevated particles and phosphorus washed from road-deposited sediment (RDS) are noteworthy causes of eutrophication in urban water bodies. Identifying how urban elements (e.g., dwellings, roads) spatially influence RDS and the associated phosphorus can help pinpoint the primary management areas for RDS pollution and therefore effectively mitigate this problem. This study investigated spatial influence of urban elements on RDS build-up load and phosphorus load in Hanyang district of Wuhan city in central China. Bayesian Networks (BNs), combined with geographical detector (Geodetector) and correlation analysis, were applied to quantify spatial association between kernel density of urban elements, RDS build-up load and phosphorus load in RDS. Results showed that (1) areas with higher density of factories related elements usually had elevated level of RDS build-up load, aluminum-bound phosphorus (Al-P), occluded phosphorus (Oc-P), organophosphorus (Or-P). Higher load of RDS associated iron-bound phosphorus (Fe-P) and apatite phosphorus (Ca-P) usually occurred where dwellings, catering, and entertainment related elements were concentrated. (2) Urban elements mainly showed positive correlation with RDS build-up load, Fe-P, Ca-P, De-P (detrital apatite phosphorus), while they chiefly showed negative correlation with Ex-P (exchangeable phosphorus), Al-P, Oc-P, and Or-P. Bus stations, dwellings, and factories related elements had relatively strong determinant power over spatial stratified heterogeneity of RDS and RDS-associated phosphorus. (3) Geodetector and correlation analysis could boost factors filtering and construction of network structures in the process of developing BNs models. The developed BNs resulted in sound prediction of <150 μm RDS build-up load and phosphorus load, given that the prediction accuracy of models ranged from 0.532 to 0.657. These findings demonstrate that urban elements are useful spatial predictors of RDS pollution, and coupling Geodetector and BNs is promising in RDS pollution prediction and supporting urban nonpoint source pollution management.

Road-deposited sediments mediating the transfer of anthropogenic organic matter to stormwater runoff

It has been regarded that road-deposited sediment (RDS) is one of the important sinks of anthropogenic pollutants as well as the major source of pollutants in stormwater runoff. However, the role of RDS, as a mediator of pollutants to the stormwater runoff, has not yet been investigated so far. Therefore, in this study, the leaching of dissolved pollutants, especially dissolved organic matter (DOM) from RDS, in synthetic precipitation was investigated. A significant amount of metals, nutrients, dissolved compounds, and DOM was leached. The leaching of DOM during 10 sequential leachings was 1811.3 and 2301.7 mg C/kg for larger (63 μm-2 mm) and smaller (< 63 μm) RDS, respectively. The results of UV/Vis spectroscopy, fluorescence spectroscopy, and size exclusion chromatography showed that the leached DOM was of anthropogenic/abiotic origins with lower molecular weight and humification degree. It is ubiquitous in stormwater runoff and industrial discharges and differs from natural organic matter. The results strongly suggest that RDS is an important mediator transferring anthropogenic pollutants to stormwater runoff. In addition, the removal of RDS, such as sweeping, would significantly reduce the pollutants input to the runoff.

Impact of green infrastructure on the mitigation of road-deposited sediment induced stormwater pollution

As a vital stormwater pollution source, the pollutants associated with road-deposited sediment (RDS) have become a growing concern in urban water management. Green infrastructure has exhibited great potential in stormwater pollution mitigation, but is not comprehensively understood yet due to the influences of complex RDS-associated pollutant migration processes (i.e., build-up, wash-off, and discharge). In this study, a city-scale hydraulic and water quality model was used to analyze the migration and removal processes of four RDS-associated pollutants (total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP)) under different hydrological patterns, land-cover types, and green infrastructure installation locations. Results show that the antecedent dry-weather period was the main factor influencing RDS build-up, while the precipitation pattern strongly impacted RDS wash-off, discharge, and removal. The downstream-installed green infrastructures reduced the RDS-induced stormwater pollution by up to 68% and relieved the pollution-mitigation pressure of the studied drainage networks by almost 50%. The TSS and COD removal rates were higher (62.22-68.09%) near green space, while those of TN and TP were higher around buildings and roads (40.00-62.50%). Sensitivity analysis indicated that seven parameters regarding the surface layer characteristics and soil texture class strongly impacted the pollution-mitigation performance among the 31 technical parameters of green infrastructure. The results of this study would assist urban water management by optimizing green infrastructure for stormwater pollution mitigation.

Impacts of key factors on heavy metal accumulation in urban road-deposited sediments (RDS): Implications for RDS management

Understanding of the impacts of key influential factors on RDS related heavy metal (HM) contamination is crucial for developing effective RDS management strategies to support the stormwater pollution mitigation. In this paper, three factors (i.e. traffic condition, slope of road and antecedent dry period) were considered to investigate their influences on the accumulation of RDS and adsorbed HMs. Positive correlations between truck and Zn/Ni content in RDS, car and Cu/Pb content in fine RDS, as well as bus and Cu content in coarse RDS were observed. Relative to sloping stretches, RDS from level stretches generally presented finer size distributions and aggravated HM contamination in fine fractions. Moreover, the fine RDS and adsorbed HMs increased significantly with the accumulation of RDS. Based on these findings, optimized RDS management strategies were proposed to enhance the removal of washable RDS in pollution hotspots. For example, tandem sweeping technologies, performing high efficiency in capturing fine particles, are suggested to be employed during the steady period of RDS accumulation, in order to reduce fine RDS effectively. The outcomes of this study provide useful reference for the source control of stormwater runoff pollution.

Characterization of the contribution of road deposited sediments to the contamination of the close marine environment with trace metals: Case of the port city of Busan (South Korea)

We examined the concentrations of 12 trace metals in road-deposited sediments (RDS) and their contributions to the pollution of harbor/marine sediments in the port city of Busan, South Korea. The concentrations of Cr, Cu, Zn, Cd, Sn, Sb, and Pb in RDS affected by industrial and traffic activities were 6.7-25.7 times higher than those in marine sediments. The enrichment factors indicate that RDS are extremely polluted with Sb and moderately to strongly polluted with Cr, Cu, As, Sn, Pb, Zn, and Cd. The mean modified pollution index distinguished between unpolluted marine sediments, moderately to heavily polluted harbor sediments, and severely polluted RDS. Furthermore, harbor/marine sediments close to shipyards and the mouths of streams and rivers were found to be moderately polluted with Cu, Zn, Cd, Sb, and Pb. RDS containing trace metals accumulate on impervious surfaces and flow into the marine environment via untreated stormwater runoff.

Size distributions and heavy metal pollution of urban road-deposited sediments (RDS) related to traffic types

Little is known about the particles attached on the surfaces of coarse (> 150 μm) road-deposited sediments (RDS), which are potential contributors of stormwater pollution. Therefore, the size distributions and heavy metal (including Cu, Pb, Zn, Cr, and Ni) pollution of RDS were evaluated taking these attached particles into consideration. Moreover, the relationships of RDS contamination with traffic types were further discussed. The results showed that < 22, 22-38.5, 38.5-150, > 150 μm RDS and attached particles accounted for 2.1 ± 1.8%, 8.5 ± 5.7%, 50.7 ± 6.8%, 38.6 ± 8.3%, and 2.0 ± 1.2% of total RDS mass, respectively. The size distributions of attached particles were comparable to that of representative stormwater particles, suggesting their great potential to contribute stormwater particles by desorption. The Zn pollution of attached particles was remarkable, which was significantly higher than that of both fine (< 150 μm) and coarse (> 150 μm) RDS. The Cu and Pb pollution were comparable to that of fine RDS, which were significantly higher than that of coarse RDS. Conversely, the Cr and Ni pollution were comparable to that of coarse RDS, which were significantly lower than that of fine RDS. Traffic types were found to be closely related with RDS contamination, and thus are indicative of specific RDS pollution. For example, electrombile in old residential area is indicative of abnormal Pb pollution due to past emissions of leaded gasoline; lorry is indicative of remarkable Zn pollution of > 22 μm RDS due to abrasion of tires and metallic substances. Accordingly, traffic type is accessible to identify the typical pollutants in RDS to promote effective RDS management.

Tropical climate effect on the toxic heavy metal pollutant course of road-deposited sediments

In modern society, the intense vehicle traffic and the lack of effective mitigating strategies may adversely impact freshwater systems. Road-deposited sediments (RDS) accumulate a variety of toxic substances which are transported into nature during hydrologic events, mainly affecting water bodies through stormwater runoff. The aim of this study was to evaluate the RDS metal enrichment ratio between the end of wet season and the middle of the dry season for Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn in samples from Natal, Brazil. Twenty RDS, drainage system and river sediment samples were collected in the wet and dry seasons using a stainless-steel pan, brush and spatula. In the laboratory, the samples were submitted to acid digestion and heavy metal concentrations were measured by atomic absorption spectrometry (AAS). A consistent RDS enrichment by heavy metals in dry season samples was followed by an increase in the finest particle size fraction (D < 63 μm). Maximum concentrations were 5, ND, 108, 23810, 83, ND, 77 and 150 mg kg^-1 for Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn, respectively. The RDS enrichment ratio was Cr(1.3 × ), Cu(2.6 × ), Fe(3.3 × ), Mn(1.5 × ), Pb(1.5 × ) and Zn(2.1 × ). The Geo-accumulation Index values showed that RDS were moderately polluted for Cu and slighted polluted for Zn and Pb. Principal Component Analysis (PCA) showed that the accumulation of toxic heavy metals decreased according to water flow.

Land-use-dependent spatial variation and exposure risk of heavy metals in road-deposited sediment in Villavicencio, Colombia

Urbanization-induced land-use changes cause several environmental problems, especially in developing countries due to a lack of sufficient urban planning. This study was performed in a medium-size city of Villavicencio, Colombia. Copper, lead, nickel, zinc, chromium, manganese, and cadmium in road-deposited sediment (RDS) from different land uses were determined. Multiple geo- and statistical approaches of geographic information system mapping, Pearson correlation, Kruskal-Wallis H, hierarchical cluster analysis (HCA), and principal component analyze (PCA) were employed to assess the influence of land use on the metals' spatial distribution. The enrichment of given metals in RDS was evaluated by geo-accumulation (I_geo) and pollution load (PLI) indexes. The exposure human health risk was assessed by hazard index (HI). Results show that the average contents of the given metals decreased in the order of commercial > residential > highway > government institutions and public parks areas. Commercial areas thereafter always have the highest metals enrichment (I_geo) and pollution level (PLI). HI assessment indicates that child has a higher health risk than adult due to the exposure to metals in RDS. HCA analysis reveals that surface roughness had a more direct influence than land-use type on metals' distribution. Kruskal-Wallis H test further suggests land-use type had a significant influence on certain metals' spatial variation. Two potential (group) sources of geochemical and vehicular sources, along with leaded petrol and paintings, were inferred to be the main contributors to metals in RDS by PCA analysis.

Evaluation of the methods for quantifying particle wash-off loadings in urban impervious surfaces at small scales

Quantifying road-deposited sediment (RDS) wash-off is critical in estimating pollutant load and for supporting the designs of urban storm water low-impact development (LID) best management practices (BMPs). The objective of this study was to identify a simple and effective small-scale wash-off load estimation and prediction method suitable for BMP engineering. The reliabilities, complexities, and applicabilities of four prediction methods (i.e., the event mean concentration (EMC) equation method, the exponential equation method, and the RDS mass difference before and after rainfall event method, and the RDS index model) were tested for three sites (i.e., a newly built area, and built-up area, and conservation area) during five natural rainfall events. Taking the calculations by the EMC method as standard results revealed the following. (1) The degree of agreement of the methods with the standard results in descending order was exponential equation method > RDS index model > RDS mass difference. (2) The degree of complexity of the studied methods in descending order was exponential equation method > the EMC equation method > the RDS index model > the RDS mass difference. (3) When considering the applicability of the models in terms of LID-BMP engineering, the RDS index model could be considered optimal for supporting the design of urban storm water BMPs.

A simplified method for determining potential heavy metal loads washed-off by stormwater runoff from road-deposited sediments

A simplified method is proposed for determining the potential load of heavy metals (HMs) derived from the wash-off caused by surface runoff on road-deposited sediment (RDS). The method consists of three phases: (i) characterization of RDS load wash-off, (ii) assessment of HM load in dry weather, and (iii) application of a wash-off equation. Two processes were included in the wash-off equation: HM transport (solid fraction) and HM leaching (dissolved fraction). The average wash-off of HMs ranges from 16.6 to 46.3%, relative to the total mass of HMs associated with dry-weather RDS (Pb, Zn, Cu, Cr, Ni, Cd, Fe, Mn, Co, and Ba). Cd, Mn, and Zn presented the highest wash-off in the areas studied. The size fraction below 250μm contributed an average of 86.7% of potential HM load washed-off from RDS. Based on the phenomena included in the wash-off equation, it was observed the following order of precedence: transport of RDS<250μm, leaching of RDS<250μm, and leaching of RDS≥250μm. Solid and dissolved fractions contributed 70.7 and 29.3% of the potential HM load washed-off by runoff from RDS, respectively. The proposed method serves as a management tool for road HM pollution during rain.

The influence of land use on source apportionment and risk assessment of polycyclic aromatic hydrocarbons in road-deposited sediment

The pollution load of urban runoff is boosted due to the washing away of road-deposited sediment (RDS). Therefore, a source-oriented mitigation strategy is essential to integrated stormwater management. This study showcases the influence of land use dependent source apportionment and risk assessment of polycyclic aromatic hydrocarbons (PAHs) in RDS. Samples were collected from areas of different land uses, including commercial city centre, highway, residential rural and campus areas. According to the positive matrix factorisation (PMF) receptor model, different primary sources were identified at different land use areas. Generally, potential sources of gasoline- and diesel-powered engine emissions and other pyrogenic sources of biomass, coal, and wood combustions were identified as main sources of PAH content in RDS. The source specific risks posed by PAHs at different land uses were further estimated by the incremental lifetime cancer risk (ILCR). This shows that the mean ILCRs of the total cancer risk for children and adults at the given land uses were lower than the baseline value of an acceptable risk. However, the potential exposure risk to RDS adsorbed PAHs for children was considerably higher than that for adults. Vehicular emissions and wood combustion were the major contributors to the cancer risk with average contributions of 57 and 29%, respectively.

Influences of land use and antecedent dry-weather period on pollution level and ecological risk of heavy metals in road-deposited sediment

Road-deposited sediment and its adsorbed pollutants have been regarded as significant sources of urban diffuse pollution. In this study, the solid-phase concentrations (mg/g), surface load (mg/m²) and chemical fractionation of zinc (Zn), copper (Cu) and cadmium (Cd) were determined. Geo-accumulation (I_geo) and ecological risk (RI) indexes were employed for metal risk assessment. Results show that the highest solid-phase concentrations of Zn and Cu were usually found at an industrial area. However, Cd had the highest solid-phase concentrations at a rural area, followed by a commercial area. The surface loads of Zn and Cu decreased along the city centre to city border gradient. However, Cd was distributed irregularly. In terms of chemical fractionation, the predominant components of Zn and Cd were identified in the unstable exchangeable fractions, indicating high potential ecological risks to the aquatic environments. Cu posed a comparably low risk due to the high proportions of the stable components of residual and oxidisable fractions. According to a two-dimensional hierarchical cluster analysis, Zn and Cu surface loads were dominantly influenced by the antecedent dry-weather period; Cd contents were strongly land-use type dependent. In addition, the enrichment capability was ranked as Zn > Cu > Cd determined by I_geo index. The sampling site dependent potential ecological risk was determined as rural area (R) > commercial city centre (W) > federal highway (B) > industrial area (I) > main road (S) > secondary road (A) by the RI index.

Characterization of heavy metal desorption from road-deposited sediment under acid rain scenarios

Road-deposited sediments (RDS) on urban impervious surfaces are important carriers of heavy metals. Dissolved heavy metals that come from RDS influenced by acid rain, are more harmful to urban receiving water than particulate parts. RDS and its associated heavy metals were investigated at typical functional areas, including industrial, commercial and residential sites, in Guangdong, Southern China, which was an acid rain sensitive area. Total and dissolved heavy metals in five particle size fractions were analyzed using a shaking method under acid rain scenarios. Investigated heavy metals showed no difference in the proportion of dissolved fraction in the solution under different acid rain pHs above 3.0, regardless of land use. Dissolved loading of heavy metals related to organic carbon content were different in runoff from main traffic roads of three land use types. Coarse particles (>150μm) that could be efficiently removed by conventional street sweepers, accounted for 55.1%-47.1% of the total dissolved metal loading in runoff with pH3.0-5.6. The obtained findings provided a significant scientific basis to understand heavy metal release and influence of RDS grain-size distribution and land use in dissolved heavy metal pollution affected by acid rain.

The influences of dissolved organic matter and surfactant on the desorption of Cu and Zn from road-deposited sediment

This study showcases the desorption behaviours of copper (Cu) and zinc (Zn) in road-deposited sediment (RDS). Batch tests were conducted to investigate the influences of rainwater, major wastewater constituents of dissolved organic matter (DOM) and surfactant on the metals leaching from RDS. Results show that the rainwater solutions considerably enhanced the total amounts of Cu (319 ± 46% of the total leaching amount by blank solutions) and Zn (617 ± 130%) released from RDS compared with blank solutions. DOM enhanced the leaching of Cu from RDS at a neutral pH. By contrast, DOM had an adverse effect on the mobilization of Zn. In the absence of DOM, a higher concentration of sodium dodecyl sulfonate (SDS) slightly increased the release of Cu from RDS than a lower concentration of SDS. However, the existence of SDS suppressed the release of Zn from RDS.

The build-up dynamic and chemical fractionation of Cu, Zn and Cd in road-deposited sediment

This study investigates the build-up dynamics of heavy metals on impervious urban surfaces with different antecedent dry-weather periods (ADPs) and land-use types. Solid-phase concentration (mg/kg), surface load (mg/m(2)), and chemical fractionation of Zn, Cu and Cd in bulk- and size-fractionated road-deposited sediment were determined. The inherent correlations among particle size distribution, ADP, land use, and chemical fractionation were analysed by hierarchical cluster analysis. Results show a clear build-up phenomenon of Cu and Zn at a city commercial centre and a highway area. Cd had complex build-up patterns. With regard to chemical fractionation, Zn and Cd could pose higher risks than Cu to aquatic biota after a longer ADP. Special attention should be paid to the significant risk in a rural area because of the high proportion of exchangeable chemical fractionation in terms of the unstable Cd component. Hierarchical cluster analysis indicates that ADP had a strong influence on build-up processes of sediments and associated metals. The metal contents were very dependent on the particle size distribution. However, the chemical fractionation of metals was dependent to a lesser extent on the land-use type.

Index models to evaluate the potential metal pollution contribution from washoff of road-deposited sediment

The proper evaluation of pollutant strength and loads associated with road-deposited sediment (RDS) is crucial for controlling diffuse pollution in urban areas. A new index model, which combines source and transport factors and is called the RDS index, was developed using RDS characteristics (e.g., the amount, grain size, mobility, and metal concentrations) and used in a case study in the Beijing region. The observed and weighted RDS characteristics along an urban-rural gradient, which included central urban (UCA), urban village (UVA), central suburban county (CSA), rural town (RTA), and rural village (RVA) areas, were used to calculate the RDS index for the pollutant load (RDSindex,load) and the pollutant strength (RDSindex,strength). Our results demonstrated that the RDSindex,load and RDSindex,strength values both changed significantly along the urban-rural gradient. RDSindex,strength increased along the urban-rural gradient and the RDSindex,load value along the main roads decreased in the order RVA > UCA > CSA > RTA. The method offers a new way of assessing metal pollution in RDS and provides an important scientific basis for controlling pollution caused by RDS washoff.

Polycyclic aromatic hydrocarbons in road-deposited sediments, water sediments, and soils in Sydney, Australia: Comparisons of concentration distribution, sources and potential toxicity

Sixteen polycyclic aromatic hydrocarbons (PAHs) considered as priority environmental pollutants were analysed in surface natural soils (NS), road-deposited sediments (RDS), and water sediments (WS) at Kogarah in Sydney, Australia. Comparisons were made of their concentration distributions, likely sources and potential toxicities. The concentrations (mg/kg) in NS, RDS, and WS ranged from 0.40 to 7.49 (mean 2.80), 1.65 to 4.00 (mean 2.91), and 0.49 to 5.19 (mean 1.76), respectively. PAHs were dominated by relatively high molecular weight compounds with more than three fused benzene rings, indicating that high temperature combustion processes were their predominant sources. The proportions of high molecular weight PAHs with five or six fused benzene rings were higher in NS than in RDS, whereas the low molecular weight PAHs were higher in RDS. Concentrations of all PAHs compounds were observed to be the lowest in WS. The concentrations of most of the high molecular weight PAHs significantly correlated with each other in RDS and WS. All PAHs (except naphthalene) were significantly correlated in NS suggesting a common PAH source. Ratios for individual diagnostic PAHs demonstrated that the primary source of PAHs in WS and NS was of pyrogenic origin (combustion of petroleum (vehicle exhaust), grass, and wood) while in RDS it was petrogenic (i.e. unburned or leaked fuel and oil, road asphalt, and tyre particles) as well as pyrogenic. The potential toxicities of PAHs calculated using a toxicity equivalent quotient (TEQ) were all low but higher for NS compared to WS and RDS.