Contribution of trace metals from atmospheric deposition to stormwater runoff in a small impervious urban catchment

A risk-based approach to prioritise catchments for diffuse metal pollution management

Catchment management integrates a wide range of water management aspects taking into account the interaction between land-use and water quality. While most sources of pollution can be identified, monitoring and tackling diffuse pollution are difficult due to the abundance of its sources and pathways. As a result, the application of risk assessment of diffuse pollution at the catchment level has been limited. This study aimed to develop a risk-based approach to rank and prioritise catchments based on the need for managing diffuse metal pollution, using a multi-criteria evaluation framework. Source-pathway-receptor linkages were the basis for the selection of criteria and associated indicators. The methodology was applied to eight catchments located in the Humber River Basin District (RBD) that were comparatively evaluated, and difficulties in selecting weightings for the criteria used were discussed. Results showed that Don and Rother, Aire and Calder and Trent were the three catchments with the highest risk of diffuse metal pollution in this RBD. The findings demonstrate the potential of the proposed risk-based approach to inform more detailed investigations to follow and prioritise the need for investments on relevant programmes to prevent diffuse metal pollution in catchments where these are needed most.

Stormwater run-off from an industrial log yard: characterization, contaminant correlation and first-flush phenomenon

The stormwater run-off generated in an industrial log yard during eight run-off events was studied with the main focus on the transport of toxic metals. Associations between water quality constituents and potential surrogates were evaluated by correlation analysis. The first-flush phenomenon was verified by normalized M(V) curves. The results have shown that, whereas some metals such as Zn, Ba, Cd, As and Fe were always detected in these waters, others (Cr, Pb, Cu, Ni, V, Co) were not. Large variations in the water constituents' concentrations were observed, with Fe, Pb and V being the most variable ones. Concentrations of Zn and Cu in the run-off waters exceeded the values established by the Swedish environmental authorities in 100% and 97% of samples, respectively. The correlation analyses indicated TSS as a potential surrogate of Pb, V, Co, Ni, As, Ba, Cr and COD (0.949 > R > 0.808), making it reasonable to state that a treatment system with focus on TSS removal would also reduce toxic metals from these waters. The first-flush phenomenon was evident for most of the constituents. Significant differences (p < 0.05) in the first-flush magnitude of different run-off events were observed confirming that hydro-meteorological variables such as dry period, precipitation duration and average intensity play important roles. Metal loads originating from the log yard were mainly composed ofZn, Cu and Ba. Knowledge of the physicochemical characteristics, discharge dynamics and the storm variables involved in the process is a crucial step for the proposal and implementation of a stormwater management programme.

Atmospheric deposition and storm induced runoff of heavy metals from different impermeable urban surfaces

Contaminants deposited on impermeable surfaces migrate to stormwater following rainfall events, but accurately quantifying their spatial and temporal yields useful for mitigation purposes is challenging. To overcome limitations in current sampling methods, a system was developed for rapid quantification of contaminant build-up and wash-off dynamics from different impervious surfaces. Thin boards constructed of concrete and two types of asphalt were deployed at different locations of a large carpark to capture spatially distributed contaminants from dry atmospheric deposition over specified periods of time. Following experimental exposure time, the boards were then placed under a rainfall simulator in the laboratory to generate contaminant runoff under controlled conditions. Single parameter effects including surface roughness and material composition, number of antecedent dry days, rain intensity, and water quality on contaminant build-up and wash-off yields could be investigated. The method was applied to quantify spatial differences in deposition rates of contaminants (TSS, zinc, copper and lead) at two locations varying in their distance to vehicle traffic. Results showed that boards exposed at an unused part of the carpark >50 m from vehicular traffic captured similar amounts of contaminants compared with boards that were exposed directly adjacent to the access route, indicating substantial atmospheric contaminant transport. Furthermore, differences in contaminant accumulation as a function of surface composition were observed. Runoff from asphalt boards yielded higher zinc loads compared with concrete surfaces, whereas runoff from concrete surfaces resulted in higher TSS concentrations attributed to its smoother surfaces. The application of this method enables relationships between individual contaminant behaviour and specific catchment characteristics to be investigated and provides a technique to derive site-specific build-up and wash-off functions required for modelling contaminant loads from impermeable surfaces.

Evaluation of surface runoff and road dust as sources of nitrogen using nitrate isotopic composition

Stable nitrogen and oxygen isotope ratios of nitrate (δ(15)N-NO(3) and δ(18)O-NO(3)) have recently been used to identify nitrogen sources in water environments. However, there have been no investigations designed to determine nitrate isotopes in non-point sources in urban areas for evaluating the impact of surface deposits on nitrogen in surface runoff. In this study, we collected rainwater, surface runoff and surface deposits (road dust, roof dust and soil) to evaluate the nitrogen sources in surface runoff using nitrate isotopes. There were no large differences in δ(15)N-NO(3) among rainwater (-0.3‰ to 1.5‰), surface runoff (-2.7‰ to 0.4‰), leachates from road dust (-5.8‰ to 6.2‰) and soil (-11.5‰ to 0.6‰). In contrast, the δ(18)O-NO(3) in surface runoff (28.5-47.9‰) was lower than that in rainwater (62.7-78.6‰), and higher than that in leachates from road dust (6.1-27.6‰) and soil (-1.1‰ to 6.6‰). δ(18)O-NO(3) is a useful indicator for evaluating the NO(3)-N sources in surface runoff. Using this indicator, NO(3)-N from road dust was estimated to account for more than half of the NO(3)-N in surface runoff. This is consistent with a result based on a comparison of their loads per unit surface between rainwater and surface runoff, which also showed that most of the nitrogen in surface runoff was derived from surface deposits.

Characteristics and environmental mobility of trace elements in urban runoff

The spatial and temporal distribution of various trace elements in water and suspended solids in urban runoff from residential and industrial sites was studied. 240 sequential urban runoff samples from 39 rain/storm events were collected, processed and analyzed for 13 elements (12 metals - Al, Co, Cd, Cr, Cu, Fe, Mn, Pb, Ni, Ti, V, and Zn, and one metalloid--As). The experimental protocol used in this study was validated using standard reference material (NIST 1648, urban particulate matter) and certified rain water samples. Good agreement was obtained between the certified and measured values. Al, Fe and Zn were found to be abundant in both residential and industrial runoffs. Some of the metals demonstrated first flush phenomena. Investigation of dissolved fraction, environmentally mobile fraction and total concentration for the 13 elements revealed that trace elements in industrial runoff were highly enriched as compared to those in the residential runoff. The environmentally mobile fraction was quite significant for most of the trace elements. Statistical correlations among the metals were studied, and principal component analysis (PCA) was used for identification of the major sources of the metals/metalloid in both residential and industrial runoffs. Crustal leaching, paints from building walls, and atmospheric deposition were found to be the main sources of metals/metalloid in runoff from the residential site while emissions from petrochemical and semiconductor industries, metal corrosion and vehicular emissions were the main sources of metals/metalloid in runoff from the industrial site. These results are presented and discussed in this paper.

Environmental applications of ICP-MS for simultaneous determination of trace elements and statistical data analysis

Water samples collected from two lakes separated from each other by the cascade of four small lowland reservoirs were subjected to analyses in order to compare contamination levels in these ecosystems. Inductively coupled plasma mass spectrometry was used to determine eight elements: Al, As, Cd, Cr, Li, Mn, Sb, and Zn. Statistical methods were applied in order to assess the degree of self-purification of water, during its flow from Swarzedzkie Lake to Malta Lake. Obtained distinctly, lower concentrations of all studied elements in Malta Lake than in Swarzedzkie Lake indicated the role of self-purification processes, which occurred in the distance between these lakes.

Characterisation of atmospheric deposition as a source of contaminants in urban rainwater tanks

To characterise atmospheric input of chemical contaminants to urban rainwater tanks, bulk deposition (wet+dry deposition) was collected at sixteen sites in Brisbane, Queensland, Australia on a monthly basis during April 2007-March 2008 (N=175). Water from rainwater tanks (22 sites, 26 tanks) was also sampled concurrently. The deposition/tank water was analysed for metals, soluble anions and selected samples were additionally analysed for PAHs, pesticides, phenols, organic & inorganic carbon. Flux (mg/m(2)/d) of total solids mass was found to correlate with average daily rainfall (R(2)=0.49) indicating the dominance of the wet deposition contribution to total solids mass. On average 97% of the total mass of analysed components was accounted for by Cl(-) (25.0%), Na (22.6%), organic carbon (20.5%), NO(3)(-) (10.5%), SO(4)(2-) (9.8%), inorganic carbon (5.7%), PO(4)(3-) (1.6%) and NO(2)(-) (1.5%). For other minor elements the average flux from highest to lowest was in the order of Fe>Al>Zn>Mn>Sr>Pb>Ba>Cu>Se. There was a significant effect of location on flux of K, Sb, Sn, Li, Mn, Fe, Cu, Zn, Ba, Pb and SO(4)(2-) but not other metals or anions. Overall the water quality resulting from the deposition (wet+dry) was good but 10.3%, 1.7% and 17.7% of samples had concentrations of Pb, Cd and Fe respectively greater than the Australian Drinking Water Guidelines (ADWG). This generally occurred in the drier months. In comparison 14.2% and 6.1% of tank samples had total Pb and Zn concentrations exceeding the guidelines. The cumulative mean concentration of lead in deposition was on average only 1/4 of that in tank water over the year at a site with high concentrations of Pb in tank water. This is an indication that deposition from the atmosphere is not the major contributor to high lead concentrations in urban rainwater tanks in a city with reasonable air quality, though it is still a significant portion.

Partitioning and granulometric distribution of metal leachate from urban traffic dry deposition particulate matter subject to acidic rainfall and runoff retention

Vehicular transportation coupled with urban hydrology is a significant source as well as vector of particulate matter (PM) and particulate-bound metal inventories in urban systems. This study examines the granulometric distribution of metals from dry deposition PM generated from 17 dryfall periods and equilibrium metal partitioning with runoff PM distribution from eight rainfall-runoff events at an urban inter-state watershed in Baton Rouge, LA. Dry deposition PM is a coarse non-uniform gradation with a d(50 m)=304 microm and a peak surface area at 106 microm. Results indicate acid rain is not a significant metal contributor to runoff but is capable of leaching metals from PM to runoff. Retained runoff partitioning resulted in particulate-bound predominance for As, Cd, Cr, Cu, Pb, and Zn while Ca and Mg remained predominately dissolved. The finer PM fraction (<75 microm) generates the highest metal concentrations and the highest metal mass is associated with the coarser fraction (>75 microm). This coarse fraction is also the most labile when exposed to acidic rainfall; generating up to 90% of the total metal mass leached from the entire PM gradation. Comparing dry deposition and runoff PM of equal mass and size gradation, retained runoff PM is enriched with metals (except Pb). Results indicate the labile coarse fraction of dry deposition PM can be a significant source of metal leaching while runoff PM (mobilized dry deposition PM) stored in a BMP can be metal-enriched with the potential for re-leaching or scour.

A load model based on antecedent dry periods for pollutants in stormwater

A load model for stormwater constituents is proposed that describes mass retained on the drainage area after previous storms, as a function of a weighted sum of previous antecedent dry periods. It was used to compute the event load for 14 constituents--zinc, copper, cadmium, nickel, chromium, lead, mercury, silver, total suspended solids (TSS), E. coli, fecal coliform, total soluble phosphorus, total phosphorus, and 5-day biochemical oxygen demand. The results show acceptable fits for most constituents based on over 400 rain events (2000 to 2004) in Milwaukee, Wisconsin. Considering the retained pollutant mass after one previous storm rather than assuming the area to be clean after the previous storm gives more accurate results for all constituents, except TSS and silver. The model can be used for mixed land-use areas. The ratios of deposition fluxes of zinc and individual metals were determined at a major parking lot producing a profile characteristic of automobile emissions.

Predicting estuarine sediment metal concentrations and inferred ecological conditions: an information theoretic approach

Empirically derived relationships associating sediment metal concentrations with degraded ecological conditions provide important information to assess estuarine condition. Resources limit the number, magnitude, and frequency of monitoring activities to acquire these data. Models that use available information and simple statistical relationships to predict sediment metal concentrations could provide an important tool for environmental assessment. We developed 45 predictive models for the total concentrations of copper, lead, mercury, and cadmium in estuarine sediments along the Southern New England and Mid-Atlantic regions of the United States. Using information theoretic model-averaging approaches, we found total developed land and percent silt/clay of estuarine sediment were the most important variables for predicting the presence of all four metals. Estuary area, river flow, tidal range, and total agricultural land varied in their importance. The model-averaged predictions explained 78.4, 70.5, 56.4, and 50.3% of the variation for copper, lead, mercury, and cadmium, respectively. Overall prediction accuracies of selected sediment benchmark values (i.e., effects ranges) were 83.9, 84.8, 78.6, and 92.0% for copper, lead, mercury, and cadmium, respectively. Our results further support the generally accepted conclusion that sediment metal concentrations are best described by the physical characteristics of the estuarine sediment and the total amount of urban land in the contributing watershed. We demonstrated that broad-scale predictive models built from existing monitoring data with information theoretic model-averaging approaches provide valuable predictions of estuarine sediment metal concentrations and show promise for future environmental modeling efforts in other regions.