TITLE: PARTICLE SIZE DISTRIBUTION OF URBAN RUNOFF - AUSTRALIA VS INTERNATIONAL: TEN YEARS ON

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1 TITLE: PARTICLE SIZE DISTRIBUTION OF URBAN RUNOFF - AUSTRALIA VS INTERNATIONAL: TEN YEARS ON Authors: Dr Darren Drapper, Drapper Environmental Consultants Contact Details: P I M I E darren@drapperconsultants.commailto:darren@drapperconsultants.com Peer Reviewed Paper PRESENTER BIOGRAPHY Darren is an Environmental Engineer with 18 years experience in the stormwater field. He has undergraduate and post-graduate degrees in Environmental Engineering and qualifications in WHS and an MBA. Darren s experience includes government, research organisations, consulting and manufacturing. He has a researched, designed, constructed and monitored WSUD stormwater treatment measures across Australia and internationally. ABSTRACT Research into particle size distributions in urban runoff was initiated with the work for the United States National Urban Runoff Program (NURP) in the early 1970s. Several studies extended this work through evaluating the effectiveness of street-sweeping on removing sediment from road surfaces and thereby reducing pollutant loads. An Australian literature review undertaken in the late 1990s collated and compared the US data with a limited range of results from Australian studies. This document has been widely referenced even into the current decade, with the conclusion drawn that Australian stormwater runoff particle size distributions are significantly smaller than those observed overseas. A literature review of recent research and a detailed evaluation of the seminal research has revealed a contrasting conclusion. It would appear that the sampling method and the analytical process explains much of the variability originally observed. Further, research on the Particle Size Distribution (PSD) of Australian road runoff confirms a range that incorporates those PSDs observed internationally. Recent studies by Trinity College, Dublin; the United State Geological Survey and Ontario Ministry of the Environment demonstrate that the particle size distribution of urban runoff, are not statistically significantly different to those PSDs observed in Australian research. This paper discusses the historical research into urban runoff particle size distributions, evaluates a larger dataset of Australian PSD research, and compares this research with recent international studies utilising similar sampling and analytical methods. It proposes an alternate conclusion that Australian urban runoff PSDs are not significantly different to those observed overseas. INTRODUCTION Since the early 1970s it has been recognised that the sediment load of urban runoff carries significant amounts of pollutants (Sartor and Boyd, 1972). Originally, it was thought that the major area of concern was the ability of sediment to smother aquatic ecosystems. However, as further research was performed, the pollutants attached to the sediment have been identified. The studies observed that particularly the sediment <246 microns contained more than 90% of phosphates, 50% of Kjeldahl nitrogen, and 50% of heavy metals (op. cit, 1972). Research conducted by Vaze and Chiew (2004) nearly 30 years later confirmed that almost 50% of heavy metals are associated with particles <200 microns, and 75% associated with particles <500 microns. The particle size distributions (PSD) from this research into urban runoff and street surface pollutants have been the basis of many guidelines internationally. It guided management measures to target specific pollutants and particulate fractions. Over time, this research has been compared with Australian studies and used to highlight the difference between particle sizes observed here and the United States (US). A closer inspection of the methodology and analytical methods reveals a different perspective.

2 UNITED STATES HISTORICAL DATA The research originally summarised by Sartor and Boyd (1972) was focussed on sediments from street surfaces. The sediments were thought to be effectively managed by street-sweeping practices. Experiments conducted for the research included two separate methodologies. The first involved washing the street surface using a rainfall simulator, and vacuum collection of the entire water sample as it passed along the kerb. The second methodology collected sediment using a small industrial vacuum sweeper. Hand sweeping with a stiff bristle broom and washing with a high pressure hose was also conducted on selected monitoring sites as the research progressed. The rainfall simulator washed the surface at a constant rate of 12.5mm/hr for a 1 hour period - less than an equivalent 3 month event in Brisbane, but similar to a 3 month Average Recurrence Interval (ARI) event in Sydney. Particle Size analysis of the captured sediment sample was then conducted using dry sieving, wet sieving and sedimentation pipette analysis. Laser particle sizing was not widely used at the time. The PSD analyses showed a range of median diameters (d50) between 300 microns and 1,050 microns. The average PSD data is presented in Figure 1. Ellis and Revitt (1981) conducted research on four road sites, collecting sediment samples from the gutters after several weeks of dry weather. The samples were oven-dried, riffled and the PSD was determined by dry and wet sieving and standard pipette analysis in accordance with BS1377. They found that the sediments exhibited log normal size distribution curves with d50 of approximately 150 microns. PSD information on the four monitored sites is presented in Figure 1. Rainfall runoff samples were also collected from a 15m sampling trough along the edge of the I-75 urban highway in Cincinnati, Ohio in 1995 (Sansalone and Buchberger, 1997). Collected samples were oven-dried and then the PSD determined through mechanical sieve analysis from 9,500 microns to 25 microns. Five rainfall events were captured in the spring to autumn of that year. The research observed a d50 of between 500 microns and 600 microns. Figure 1 presents the collated data graphically.

3 Figure 1. Historical US data HISTORICAL AUSTRALIAN RESEARCH Research into the PSD of urban runoff in Australia was relatively limited until recently. The PhD research completed by Ball and Abustan (1995) also investigated road sediment particle sizes by collecting end of pipe water samples. These samples were analysed using a Laser Mastersizer/E Particle Analyser. This analytical method constrains the PSD between 1.2 microns to 600 microns, contrary to the historical data presented in the United States. The d50 identified in the research was separated according to the rising limb, peak and falling limb of the hydrograph. The PSDs also showed log-normal distributions with a d50 of 49 and 73 microns in the rising limb, 52 and 73 microns in the peak, and 39 to 70 microns in the falling limb of the hydrograph. Particles less than 100 microns were found to represent between 70% and 92% of the total sediment mass in the collected samples. The Co-operative Research Centre for Catchment Hydrology (CRCCH) undertook research into the PSD of road runoff, and prepared the Water Sensitive Road Design document including limited Australian data (Wong et. al. 2000, Lloyd and Wong, 1999). This summarised the Ball and Abustan (1995) research, studies by Lloyd and preliminary data by Drapper (1999). The CRCCH document modified the US data by cropping particle sizes greater than 500 microns from the comparison. Figure 2 is an adaptation of the original graph presented in the Water Sensitive Road Design document. The CRCCH document concluded that even with the cropped international data, the Australian data showed 65% of the sediment sample was finer than 100 microns, compared with the international data showing less than 25% finer than 100 microns. It is the author s experience that this conclusion led many stormwater practitioners to believe that Australian soils have much finer PSDs than those internationally and international field tests on stormwater Best Management Practices (BMPs) were not, therefore, applicable in Australia. In fact, recent guidelines prepared for Auckland Council (Wong et al. 2011) and Gold Coast City Council (DesignFlow, 2014) reinforce this perception by applying a conversion factor between 50% and 75% on international performance due primarily to the assumption that PSDs are different. And the MUSIC software help file indicates that Melbourne and Brisbane catchments are characterised by finer particle size distributions with a graph indicating a d50 of ~70 microns.

4 Figure 2. Comparison of particle size distribution research from Australia and overseas, adapted from Lloyd and Wong (2000) As the CRCCH document was being published, doctoral research was also being undertaken in southeast Queensland looking into the characteristics of road runoff (Drapper, 2001). This research collected 214 full event grab samples from a range of road surfaces from rural highways to dual lane motorways. PSD analysis was undertaken on the samples using a Malvern Mastersizer Laser Particle Sizer. When compared with the highest (Sansalone & Buchberger, 1997) and lowest (Ellis & Revitt, 1981) PSD from the historical data out of the United States, it can be seen that the two datasets overlap. Taking into account the different sample collection methods and the different analytical procedures to determine the PSDs, it is noted that the Australian PSDs are no longer as visually separate as was originally presented.

5 Figure 3. SEQ Particle Size Distributions and International Data Comparison This Australian data has been collated from research completed approximately a decade ago. So to ensure it remains relevant and applicable currently, a further investigation of current international research was warranted. LATEST INTERNATIONAL RESEARCH The Ontario Ministry of the Environment conducted research in 2012, sampling untreated stormwater runoff from six impervious carpark and road catchments over six rainfall events. Samples were collected directly from the paved surface and from the stormwater outlet as Grab samples and via automated samplers. Analysis of the PSD was determined by both field evaluation and Laser particle sizing. Median particle sizes of the samples (d50) were observed from 4.2 microns to 31.1 microns. Fifty percent (50%) of the particles were observed to be finer than 13.7 microns, with 90% of particles less than 55 microns (Goncalves and Van Seters, 2012). The PSD data for the Mixed Residential catchment is presented in Figure 5, identified as Toronto Mixed Res. Of interest, the Toronto research compared the PSD from grab samples against those collected by an autosampler and found reasonable correlation, except for one site where the collection point was relocated from the pipe invert to higher in the flow profile. Stormwater runoff was also collected from an urban area of Kimmage, Dublin. Flow-weighted water samples were collected with ISCO6712 auto-samplers. PSD analysis was completed using a Malvern Mastersizer Laser diffraction method. The results of the Laser diffraction demonstrated a median particle diameter of approximately 38 microns. Comparison was also undertaken of the sediment captured in the gully pits of the Kimmage study area, locations shown in Figure 4. PSD analysis was completed using sieve analysis and observed a d50 of ~1,000 microns for the gully pit sediments. The average PSD of the autosampler-collected samples is presented in Figure 5 identified as Dublin (Morgan, 2014).

6 Figure 4. Aerial image of Kimmage, Dublin monitoring area (Morgan, 2014) The United States Geological Survey (USGS) characterised the PSD in runoff from six types of urban catchment areas including parking lots, small streets, collector streets, arterial streets, rooftop and mixed use (Selbig and Bannerman, 2011). Flow-weighted runoff samples were collected using autosamplers. PSD analysis was undertaken on the collected sample via a wet sieve analysis down to 32 microns and a Coulter counter for particles less than 32 microns. PSD analysis was conducted on between 20 and 94 events depending on the catchment type. On an arterial street (n=48), the average d50 was observed to be ~100 microns. On the mixed use catchment (n=20), the average d50 was observed to be ~80 microns. The PSDs for the arterial street and mixed use catchments are presented in Figure 5. As was seen in Figure 3, the original band of international data presented in 1999 appears to have presented a skewed interpretation by comparing different collection methods, and different particle sizing techniques. Using the CRCCH corrected data (by removing the portion above 500 microns) from Figure 3 and comparing with a larger Australian dataset indicated that the international data range was still relatively high, but overlapped. Figure 5 updates this historical data with the latest international research.

7 Figure 5. Comparison of Australian and International PSD data for stormwater runoff Figure 6. Boxplots of Australian and International d50 (microns) data for stormwater runoff

8 Figure 7. Scattergrams of Australian and International d50 (microns) data for stormwater runoff Statistical comparison of the d50 for the International research across all sites (n=261), against the Australian data set (n=216) indicates that they are not significantly different at a 95% confidence interval. Box plots and scattergrams of the two datasets are presented in Figures 6 and 7. This challenges the previous view that the PSD in Australian stormwater runoff is much finer than that observed internationally. It also challenges the perception that international results for TSS removal of field monitored BMPs must be factored down due to different PSDs. CONCLUSIONS Research into the particle size distribution of sediments in stormwater runoff was originally reported in US publications from the early 1970s. This work was reviewed and compared with a limited Australian dataset as part of the CRCCH urban stormwater projects. The CRCCH publications compared international results against Australian research however, omitted to consider the different collection techniques and analytical methods. The conclusion intimated from the comparison was that the PSDs of Australian stormwater runoff were significantly finer than that reported overseas. That conclusion has been perpetuated in recent Australian guidelines by discounting international field test data on the performance of stormwater BMPs because the runoff PSDs were considered significantly different. New research conducted in Australia and internationally reveals that the PSDs are not statistically, significantly different when considering similar collection methods and similar analytical techniques. Therefore, it calls into question the practice of discounting international field performance data for stormwater BMPs on the grounds of PSD and the author recommends a revision of current guidelines formed on this assumption. REFERENCES Ball, J.E. and Abustan, I. (1995) An Investigation of Particle Size Distribution during Storm Events from an Urban Catchment, The Second International Symposium on Urban Stormwater Management, Melbourne, Australia, July, DesignFlow (2014) Draft Independent review of proprietary devices, for Gold Coast City Council, unpublished.

9 Drapper, D.W. (2001) Pilot Study of Pollutants in Road Runoff and Evaluation of Best Management Practices for South-east Queensland, PhD Thesis, Griffith University. Ellis, J.B. and Revitt, D.M. (1982) Incidence of Heavy Metals in Street Surface Sediments: Solubility and Grain Size Studies, Water, Air and Soil Pollution 17, , D. Reidel Publishing Co, USA Goncalves, C. and Van Seters, T. (2012) Characterization of Particle Size Distributions of Runoff from High Impervious Urban Catchments in the Greater Toronto Area, Final Report, Toronto and Region Conservation Authority. Lloyd, S.D. and Wong, T.H.F. (1999) Particulates, associated pollutants and urban stormwater treatment, Proceedings of the 8 th International Conference on Urban Storm Drainage, 30 th August 3 rd September, 1999, Sydney. Morgan, D. (2014) Characterisation of sediment and associated pollutants in urban runoff, PhD Thesis, Trinity College, Dublin Sansalone, J.J. and Buchberger, S.G. (1997) Characterization of Solid and Metal Element Distributions in Urban Highway Stormwater, Water Science and Technology, Vol. 36, No. 8-9, , Elsevier Science Ltd. Sartor, J.D and Boyd, G.B. (1972) Water Pollution Aspects of Street Surface Contaminants, USEPA Document EPA-R , November Selbig, W.R. Urban Water Journal (2013): Characterising the distribution of particles in urban stormwater: advancements through sampling technology, Urban Water Journal Selbig, W.R. and Bannerman, R.T. (2011) Characterising the Size Distribution of Particles in Urban Stormwater by Use of Fixed-Point Sample-Collection Methods, US Geological Survey Open-file Report , 14p Selbig, W.R. and Fienen, M.N. (2012) Regression Modeling of Particle Size Distributions in Urban Storm Water: Advancements through Improved Sample Collection Methods, Journal Of Environmental Engineering, Vol. 138, No. 12, December 2012, ASCE. Vaze, J. and Chiew, F.H.S. (2004) Nutrient Loads Associated with Different Sediment Sizes in Urban Stormwater and Surface Pollutants, Journal of Environmental Engineering, Vol. 130, No. 4, April1, 2004, ASCE. Wong, G., Ansen, J-A., and Fassman, E. (2012) GD03 Proprietary Devices Evaluation Protocol (PDEP) for Stormwater Quality Treatment Devices. Prepared by Auckland Council. Wong, T.H.F, Breen, P. and Lloyd, S. (2000) Water Sensitive Road Design, Technical Report 00/1, August 2000, CRC for Catchment Hydrology.