SWEPT UNDER THE CARPET? ROAD DERIVED SEDIMENTS: FRIEND OR FOE?

Peter Mortimer Technical Services Manager, Downer NZ Ltd, Dunedin, New Zealand

Kirsty Marlow Divisional Manager, Downer NZ Ltd, Queenstown, New Zealand

Abstract

It is common practice to dispose in landfill the material removed from road surfaces and transported into sumps as part of rain runoff. The material contains contaminants including heavy metals, especially Lead, Copper and Zinc, which has detrimental effects to the receiving environments from landfill leachate. However, a landfill tends to be acidic, and this environment encourages the heavy metals (especially Zinc) to go into solution forming a neutral salt, and leach into the surrounds. Better, if possible to encapsulate the heavy metals elsewhere in a useful product and avoid adding to landfills. Downer undertook research and commissioned laboratory testing as part of a project development for Queenstown Lakes District Council in order to investigate this possibility. This paper will describe the development of a sampling method, and results of testing compared with industry solid waste management limits. Uses to date in the Queenstown Lakes District are discussed, as well as the potential for further use as an economic commodity.

Key Words

Road Sump Waste, Street sweepings, Road Derived Sediments, Hazardous waste, Landfill, recycling

Introduction

Road maintenance contracts with an urban component commonly have a requirement to remove the build up of detritus from the road surface and kerb and channel and the sediments and deposits within surface drainage sumps. It is usually the contractors responsibility to dispose of this material in a responsible manner. The cost of this disposal (and thus the price allowed for in tenders) is directly related to the disposal method which is obviously dependant on the definition of the materials being disposed.

Within the context of the Queenstown Lakes District Council (QLDC) road maintenance contract, these road sweepings were not specifically defined as a substance requiring special treatment and information about the degree of toxic contamination was not provided in the contract documents. Standard practice of the time was to remove the refuse from the sweepings and take the material to landfill by the truckload as cover material or clean fill. This worked well up until the point that the landfill operator of the time refused to take the street sweepings on the ground that they had test results to show that the degree of contamination breached the limits set by the Ministry for Environment for their class B landfill rating. The cost for disposal rose from under $10/tonne to over $200/tonne. So now what were we to do? Time was of the essence as the drainage maintenance budget was being rapidly consumed by the landfill provider. The nearest class A landfill was Dunedin and to take the material to Dunedin was equally cost prohibitive and just didn‟t make sense. After all, this isn‟t nuclear waste, right?

About the QLDC Road Maintenance Contract

Downer has undertaken road maintenance on behalf of the Queenstown Lakes District Council since 2004.

The roading network is approximately 817 km in length, 470km of which is sealed road. There is a reasonably diverse road user profile, with a high percentage of tourist traffic, but a relatively low number of heavy vehicles as farming activity is limited, and there is currently no forestry activity. The district has a low rate payer base relative to road user numbers.

The Queenstown Lakes District roading network is diverse in nature

Whilst the district contains many historic and relatively low volume roads, for instance Macetown Road, Tobins Track, and the Skippers Canyon Road, it is also home to built-up residential and commercial centres, particularly in the Queenstown and Wanaka townships.

To give an indication of the scale of the RDS generating activity, in the 2011-2012 financial year, 1900km of road sweeping was undertaken, and 1334 sumps cleaned. Annual quantities of RDS collected are variable and at least partially dependent on the severity of the preceding winter and therefore how much ice grit has been spread. Network wide, this would total anywhere between 1000 and 2000 tonnes.

Ice grit and sealing chip make up the majority of RDS recovered.

The emerging problem

When this problem was realised, it was accepted by QLDC that the reclassification of the RDS from general material that has historically been treated as clean fill, to a hazardous, contaminated material, was outside of the scope of the current contract. Therefore any additional cost associated with either interim or long term solutions would be borne by QLDC. An agreed interim solution was put in place which was:

Decant any liquid component in to the foul water system Continue to dispose of the resulting dry solids to landfill, with the landfill operator treating them prior to disposal within the landfill

As indicated, the cost of treatment and disposal of the predominant solid portion presented QLDC with an unsustainable activity. Therefore the Downer and QLDC team commenced a project to find out as much as possible about Road Derived Sediments (RDS) so that we could avoid any breach in resource consents and environmental certification, whilst providing QLDC with a practical and cost effective solution to the management of RDS. We developed a sampling methodology and laboratory testing regime designed to learn more about the type of contaminants and their variability. We also completed testing on samples treated using an alternative method identified through a literature search.

Landfill legislation As road maintenance contractors, landfill management and legislation is not our specialty, however we needed to understand more to know how to provide a solution to this problem. Landfills within New Zealand require resource consents to operate under the Resource Management Act 1991 (RMA). The basis for the development of resource consents is provided in a set of “guidelines” published by the Ministry for the Environment (MfE) particularly Module 2: Hazardous Waste Guidelines – Landfill Waste Acceptance Criteria and Landfill Classification. Figure 1 provides a simple workflow describing the acceptance criteria for waste at any landfill.

Figure 1, Waste acceptance decision process Source; MfE Module 2: Hazardous Waste Guidelines, landfill waste acceptance criteria.

Using this workflow and assuming from the start that RDS is not a “Prohibited” substance then this should be relatively straight forward. We get down to question four where the user is asked if the waste is “asterisked on the NZ Waste List?” At this point after doing a search for the NZ Waste list we identify code “20 03 03 Street Cleaning Residues” under the sub category of “Other Municipal waste” as being the closest description to our waste. It does not have an asterisk against it therefore deemed acceptable for disposal? There is absolutely no requirement to complete screening testing to prove otherwise and nor does the guideline state that and “special treatment” is required.

However, the QLDC landfill operator had completed screening testing and deemed the wasted “prohibited”. In the interests of gaining more knowledge we undertook quite extensive Toxicity Characteristic Leaching Procedure (TCLP) and Particle Size Distribution testing to determine the extent of our (QLDC and Downer) problem.

Development of testing regime

After completion of a literature review the Land Transport New Zealand (Now New Zealand Transport Agency) research paper 345 “Contaminant Characterisation and toxicity of road sweepings and catchpit sediments: Towards more sustainable reuse options” (Depree, C) was identified as being cornerstone research for the New Zealand context. The main premise of this work suggests Road Derived Sediments (RDS) are characterised as a prohibited substance. They sampled RDS from various locations within Auckland, Hamilton and Christchurch. The findings of this research showed there was potential reduction in the leachability of heavy metals through stabilisation with compost and also perhaps a reduction by allowing the materials to “cook”, meaning leaving them to weather or cure in the elements prior to disposal. Some aspect of “Passive bioremediation” may also be at works here. With these results in mind we agreed with QLDC to obtain samples from several different locations within Queenstown and Wanaka which were representative of what we considered high to low stress loading areas. We would assess the individual initial toxicity level using the TCLP method. We decided to test the affect of “cooking” the sample and also mixing the sample with locally produced compost as a means of stabilising the heavy metals. We used Hills Laboratory in Hamilton to advise on testing methodology and undertake the tests. The goal of the testing was to firstly identify if we are in fact dealing with a material with toxicity that does breach the class B landfill guidelines and if so by how much. Additionally we wanted to understand how much in-organic material we were collecting as well as particle size distribution of this material fraction to be able to determine if the in organic component could be reused or recycled. Testing for heavy metals (Lead, Copper and Zinc) was undertaken as these are the main heavy metal contaminants present and of concern to the land fill operator. This is backed up by the findings within research report 345.

Sampling methodology

Based on the agreed testing regime, above, samples were taken from each the following locations, in both the Queenstown and Wanaka areas:

Mudtank in “high stress” area such as at the bottom of a hill Mudtank in CBD Mudtank in other urban area Roadside sweepings (collected from kerb and channel or road surface)

Typical “high stress” mudtank location

A further sample was generated by combining RDS from each of the four locations above to simulate the actual process of combining sweeper truck loads at a handling facility to cart the waste in larger quantities to landfill. Again one for the Queenstown area and one for the Wanaka area.

Each of the resulting ten samples was then split in two, one to be tested immediately following collection/combination, the other to be tested after a four to six week period of “cooking”. This made a total of twenty samples to be tested.

Results of laboratory testing As a result of the lab testing and subsequent interpretation of the results, the following conclusions were drawn:

Of the twenty three samples tested, four individual samples from high stress sumps, CBD sumps and road sweepings and all from the Queenstown area, had PH results/reactivity that on paper could cause harm to plant or animal life, but from other urban areas and in combination they did not. Therefore it was recommended that the RDS is always stockpiled, stored and used as a combination of the materials collected.

No individual or combined sample breached the recommended limits for overall contaminant concentrate, meaning it could be used with no harm to plant or animal life in that respect.

1.600 Leachability Limit TCLP (Acidic)

1.400 Class B Landfill Threshold

1.200

1.000

0.800 mg/kg 0.600

0.400

0.200

0.000 Lead Copper Zinc

Figure 2, Graph of Average, 15 th percentile and 85th percentile Contaminant Concentration against Class B landfill limits.

Of the twenty three samples tested, five individual samples (from high stress, CBD but mainly from roadside sweepings) would tend to leach zinc if encapsulated in an acidic environment such as a landfill (TCLP), however from other urban areas and in combination they would not. Therefore as long as the materials are combined once collected, they could be disposed of to landfill without the need for further treatment.

No leachability limits (TCLP) were breached for Lead and Copper, when exposed to the simulated acidic landfill conditions.

350.0 Average Contaminant 300.0 Concentration Class B Landfill Threshold 250.0

200.0

mg/ml 150.0

100.0

50.0

0.0 Lead Copper Zinc

Figure 3, Graph of Average, 15th percentile and 85th percentile of leachability TCLP against class B landfill threshold.

No leachability limits were breached for any heavy metals, when exposed to simulated rainfall conditions (SPLP). Therefore the RDS can be used in an outside environment without any detrimental effect.

1.200 Leachability Limits 'Contaminant Results SPLP 1.000 (Freshwate+'Contaminant Results)

0.800 Class B Landfill threshold

0.600 g/ml

0.400

0.200

0.000 Lead Copper Zinc

Figure 4, Graph of Average, 15th percentile and 85th percentile of leachability SPLP against class B landfill threshold.

Current management practice and future potential for use

Currently, the liquid portion continues to be decanted to the foul water system – this predominantly occurs during targeted annual mudtank cleaning. The solid portion, much of which is already dry when collected, is stockpiled at agreed locations around the network, and always comprises a combination of RDS from various locations. Visual inspection identifies trash and leaf litter components, which are isolated and are disposed of to landfill. The remaining material is mixed periodically and before use, and samples are tested annually to ensure that the collected material is still behaving as the initial testing indicated it would.

Typical stockpile

Whilst at the time of the project, preliminary work was undertaken to investigate multiple uses for the material, changes in personnel meant that momentum was lost. Therefore, uses to date have been limited to the construction of unsealed footpaths.

RDS in use for construction of unsealed footpaths in Arrowtown

However, potential still exists for this material to be used much more extensively. Possibilities include:

Pipe bedding material Combine with other aggregates for use in maintenance metalling and modified Otta seal construction Binding agent for unsealed roads with high coarse fraction Backfill for kerb and channel renewal reinstatement Footpath pavement aggregate Screen out and re-use the ice-grit portion Soil conditioner Fill material in bunds and berms

Downer plans to undertake further investigations to determine the economic viability of each potential use.

Treatment Test results In research report 345 it discusses a form of treatment to mix compost to bind and reduce the bioavailability of the heavy metals. (Depree, C. 2008). This treatment method has a lot of potential to reduce disposal costs and also provide a material for reuse by council on roadside plots and gardens. As such we felt this was a very promising treatment method should contaminant results require us to stabilise the materials. The compost was a locally produced mix produced from shredded green waste at the Wanaka landfill. It was mixed at 10%, 20% and 50% concentrations. The results below show some interesting incites. Firstly we see a 10% mix dramatically reduced the contaminant concentration within the mix. Secondly that higher percentages of compost had less affect on the contamination level. Most interestingly of all was that the 100% compost mix when tested showed the second highest contamination concentration of all. This is a good example of how by mixing a small percentage of compost can provide a much greater amount of useable growth medium. In this case the growth medium will have less contamination than the original compost itself.

120 Contaminant Concentration 10% Compost mix 100 50% compost Mix 100% Compost 80

60 mg/kg

40

20

0 Lead Copper Zinc

Figure 5, Contaminant Concentration of Wanaka sump waste with various compost mixes.

As described in the section above. Current practice is to stockpile recovered RDS at various locations within the network. To confirm the contamination concentrations are under acceptable limits we sampled from five locations. Below is a summary of the test results for freshwater leachability being a reflection of the potential to leach whilst under natural rainfall. As we can see the average and 85th percentile value are small indeed. The class B landfill limits of 0.5 for Lead and Copper and 1.0 for Zinc has not been shown as this made the chart difficult to read due to the significant difference in figures.

0.0350

Leachability Limit SPLP 0.0300 (freshwater)

0.0250

0.0200 g/ml 0.0150

0.0100

0.0050

0.0000 Lead Copper Zinc

Figure 6. Average, 15th percentile and 85th percentile leachability limit SPLP (freshwater) of stockpiles

A Council Wide Problem The collection and disposal of RDS is generally considered a contractors issue. The maintenance of water channels and sumps is indeed funded by NZTA however we suggest this is a community issue and as such a whole of council issue. We all drive vehicles and so it is collectively our problem. Through road maintenance contracts the contractor is responsible for the cost of disposal however this cost is established on the basis of how the contractor can dispose the material. Council „revenue‟ comes from a general rate take and from NZTA subsidy which comes from fuel exercise duty thus the whole community pays for the disposal of this material in that regard. These rates can potentially reduce with some inter council department co-operation and agreement. As eluded to above, there is a number of potential reuses for the processed RDS which is being done in other cities world wide. For instance with the co-operation of a Parks and Reserves department the RDS can be combined with mulch or compost and be used as a soil conditioner or growth medium for flower beds. This can be extended to being and acceptable produce for road side landscape areas. The processed RDS could be used as a pipe bedding material if it was allowed through Water and Wast services contracts or better could be made available for contractors to use “free” within the tender documents.

Conclusion

So, Road Derived Sediments –are they truly a friend or still a foe? It is our conclusion that based on the evidence of test results collected for the Queenstown Lakes District that RDS are a friend. Further they pose no harm to the environment when stored in a non acidic environment and can be safely disposed of at a class B landfill without treatment. However there is much potential to recycle and reuse this resource with some acceptance and co-operation from Local Authority departments and save filling landfills with usefull materials. Because of the NEC3 Cost reimbursable model used in Queenstown, any saving made as a result of reuse is either used to fund more work on the transport network or banked as a saving to council and therefore the rate payers of QLDC.

References Depree C, 2008, „Land Transport New Zealand Research report 345, Contaminant characterisation and toxicity of road sweepings and catchpit sediments: Towards more sustainable reuse options‟.

„New Zealand Waste List”, Ministry for the Environment. From internet (http://www.mfe.govt.nz/issues/waste/content.php?id=25&code=20)

„Module 2: Hazardous Waste Guidelines, Landfill waste acceptance criteria and landfill classification‟, Ministry for the Environment, published may 2004. From internet.

Author Biographies

Peter Mortimer is Divisional Manager of Technical Services for Downer in the Otago and Southland areas, based in Dunedin. In this role he is responsible for a team performing technical service functions for both Intelligent Transportation System and road engineering disciplines. He has worked in the consultancy, road controlling authority and contracting sectors over the past 20 years, the last 11 of which have been spent with Downer. Peter and his team provide valuable technical and asset management input to numerous projects include thing Coastal Otago Hybrid road maintenance contract and the ongoing Caversham four-laning.

Kirsty Marlow is Divisional Manager responsible for all Downer activities in the Queenstown Lakes and Central Otago districts. Hailing from the UK, she is a Chartered Professional Engineer with over 20 years contracting experience, and moved to New Zealand nine years ago. Since then she has been predominantly involved in the management of Roading Maintenance activities in both the Local Authority and State Highway arena, and contributes to the strategic direction and ongoing improvement of Downer‟s road maintenance approach nationally.