The environmental fate of traffic-derived metals in a section of Wet Tropics World Heritage Area (WTWHA), Far North Queensland (FNQ)

Pratt, Chris (2006) The environmental fate of traffic-derived metals in a section of Wet Tropics World Heritage Area (WTWHA), Far North Queensland (FNQ). PhD thesis, James Cook University.

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The major aim of this research was to resolve the following question: What are the key processes affecting the concentrations, mobility and bioavailability of traffic-derived metals (Cd, Cu, Pb, Ni, Zn, Pd and Pt) in roadside environments in a section of the wet-dry tropics in northern Australia? Specific areas investigated included the Kuranda Range Road, northwest of the city of Cairns; the Captain Cook Highway at the base of the Kuranda Range Road; and adjoining streams and grassed fields. The Kuranda Range Road traverses World Heritage-listed rainforest and the Queensland Department of Main Roads plans to upgrade the road from two lanes to four. Materials analysed in the study comprised bedrock, road sediments, road runoff waters, stream sediments, roadside topsoils, and grasses. Additionally, background stream sediment, stream water, topsoil and grass samples were collected away from roads. Geochemical analyses of the road sediments from the Kuranda Range Road revealed variable total metal concentrations (median values: 0.19 mg/kg Cd, 41.7 mg/kg Cu, 53.3 mg/kg Pb, 38.8 mg/kg Ni, 852 mg/kg Zn, 0.035 mg/kg Pd, 0.086 mg/kg Pt). Moreover, the studied road sediments exhibited metal enrichment (Ni excepted) relative to background stream sediments (maximum enrichment factors: Cd 1.8x, Cu 1.5x, Pb 6.8x, Zn 17.3x, Pd 49.5x, Pt 82x). Partial (citrate dithionite) and sequential (as per the method of Tessier et al. 1979) extractions were performed on the road sediments to examine their metal host sites. The results demonstrated that approximately 35 % to 95 % of the sediments’ metal content was accommodated by acid (HF-HNO3-HClO4)-insoluble fractions, likely residual silicates. However, significant (p<0.01) positive correlations between the Corg and Cd, Cu, Pb and Zn concentrations in the road sediments pointed to metal hosting by an organic source, most likely tyre rubber shreds. The extraction techniques revealed that metals associated with tyre rubber are not readily removed by extraction reagents. Hence, other methods, including correlation analyses between metal concentrations and Al, Mn, Fe and Corg values, are necessary to accurately interpret metal hosting within road sediments. Chemical analyses were performed to evaluate the mobility of Cd, Cu, Pb, Ni and Zn within road runoff waters on the Kuranda Range Road. Maximum Cu, Pb and Zn levels in filtered (<0.45 μm) road runoff waters taken in November 2004 (after a prolonged absence of rainfall) were 8x, 6x and 12x greater than their respective highest values in samples acquired in February 2003 and January 2004 (following heavy rainfall). Such temporal metal distribution data for road runoff waters suggest that large volumes of rainfall in wet-dry tropical regions are capable of mobilising high levels of metals from road surfaces during the initial flushing event (i.e. the ‘first flush’). Furthermore, laboratory leaching and ponding experiments conducted on road sediments indicated that a small proportion of the total heavy metal content (<10 %) of road sediments is readily dissolved in distilled water. In the leach tests, aqueous Cd, Cu, Ni and Zn concentrations showed a pronounced ‘first flush’ effect (i.e. metal values were much higher in the first few samples than in the remaining leachates). To explore the dispersion of metals from road surfaces, stream sediments from Avondale Creek (intersecting the Kuranda Range Road) were analysed for their total metal contents and Pb isotopic ratios (208Pb/206Pb, 207Pb/206Pb, 208Pb/204Pb and 206Pb/204Pb). The results revealed: a) elevated total Pb (29.6 mg/kg) and Pt (0.025 mg/kg) concentrations in the sediments collected downstream of the road compared to sediments upstream of the road (Pb = 7.3 mg/kg, Pt = 0.006 mg/kg); and b) non-radiogenic Pb isotopic signatures (characteristic of Broken Hill Pb used in petrol) in sediment samples downstream of roads relative to background stream sediments. The results likely reflect contamination of the catchment by road sources. The verification of metal contamination within Avondale Creek triggered an investigation into the bioavailability of traffic-derived metals. This involved an assessment of the uptake of soil-hosted metals by a grass species (Melinis repens), growing adjacent to the Kuranda Range Road. Median total metal concentrations in topsoils collected adjacent to the road were much higher than median total metal values in topsoils taken 5 metres from the road edge. In the M. repens grass specimens, Cu, Pb, Ni and especially Zn concentrations were elevated in roots acquired from immediately adjacent to the Kuranda Range Road. M. repens clearly has the ability to incorporate high concentrations of trace elements when growing on contaminated roadside soils, particularly Zn and to a lesser degree Cu, Pb and Ni. Additionally, extractions using a DTPA-CaCl2-TEA-HCl (DTPA) solution revealed a significant positive correlation (p<0.01) between soil-DTPA and root Zn levels in the roadside M. repens samples. This indicates that the DTPA reagent is a rudimentary indicator of Zn to the roots of this grass species. Metal concentrations in M. repens samples grown in road sediments as part of a greenhouse experiment, were similar to the values exhibited by the field specimens. Moreover, the metal levels extracted from the road sediments by an EDTA-NH4HCO3 solution were commensurate with DTPA-extractable values, indicating that both of these solutions target similar metal fractions in road sediments. The final research phase examined remediation measures for road runoff waters on the planned Kuranda Range Road Upgrade. A treatment selection process identified dissolved metals as the most significant category of pollutants because of their high lability and potential toxicity. Site constraints, including the close proximity of the road to sensitive water catchments, indicated that at-source pollutant attenuation will be the most effective remediation option for the road upgrade. Thus, existing at-source primary treatment measures (e.g. trash racks); secondary technologies (including sand filters); and tertiary structures (such as biofilters) were identified as the most suitable treatment options for the Kuranda Range Road Upgrade. Few tertiary treatment devices exist for road runoff waters (the StormFilter is an exception). Hence, this research explored the capacity of commonly-available materials, including mushroom compost and bentonite, to remove dissolved metals from road sediment leachates. In laboratory experiments conducted in this project, mushroom compost and bentonite displayed strong capacities to reduce dissolved heavy metal concentrations in road sediment leachates (Pb and Zn removal over 80 %). Both materials were very fast-acting (<5 minutes) in achieving metal attenuation. It is envisaged that these adsorptive materials have the potential to be included into structures (such as sand filter beds) that can achieve tertiary treatment of road runoff waters on the upgraded Kuranda Range Road. Overall, this research demonstrated that annual wet-dry climate cycles control the concentrations, mobility and bioavailability of traffic-derived metals in roadside corridors in the tropics. Metals accumulate in roadside sediments and soils during the prolonged ‘dry season’ from April to October, and are mobilised by road runoff waters over the ‘wet season’ (November to March). Mobile metals are bioavailable to organisms living adjacent to roads. Consequently, remediation strategies that can reduce the dispersal of these contaminants into natural environments are important in road design and maintenance in the tropics. The use of adsorptive materials such as bentonite in sand filter beds is presented as one such remediation option.

Item ID: 1171
Item Type: Thesis (PhD)
Keywords: Geochemical analyses, Total metal concentrations, Road sediments, Stream sediments, Metal host sites, Metal mobility within road runoff waters, Dispersion of metals from road surfaces, DTPA reagent, Bioavailability, Uptake of soil-hosted metals by a grass species, Melinis repens, Remediation measures, Kuranda Range Road Upgrade, At-source pollutant attenuation, Mushroom compost, Bentonite, Tertiary treatment, Annual wet-dry climate cycles, Use of adsorptive materials, Kuranda Range Road, Cairns, Captain Cook Highway, World Heritage-listed rainforest
Date Deposited: 26 Oct 2006
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management @ 0%
04 EARTH SCIENCES > 0406 Physical Geography and Environmental Geoscience > 040608 Surfacewater Hydrology @ 0%
05 ENVIRONMENTAL SCIENCES > 0503 Soil Sciences > 050304 Soil Chemistry (excl Carbon Sequestration Science) @ 0%
07 AGRICULTURAL AND VETERINARY SCIENCES > 0799 Other Agricultural and Veterinary Sciences > 079901 Agricultural Hydrology (Drainage, Flooding, Irrigation, Quality, etc) @ 0%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050204 Environmental Impact Assessment @ 0%
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