C& D landfill Burrell Demolition Ltd : Response to Section 92 Requests : Leachate Management

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1 Opus International Consultants Ltd Whakatane Office Concordia House, Pyne Street PO Box 800, Whakatane 3158 New Zealand 4th October 2013 t: f: w: Christine Morgan C&D Landfill Ltd 2 Landfill Road Happy Valley Wellington Dear Christine C& D landfill Burrell Demolition Ltd : Response to Section 92 Requests : Leachate Management We have reviewed leachate management for the proposed western extension of the C &D landfill and advise as follows: 1. Water Quality Monitoring to Date Water quality is monitored at 4 sample locations as shown on the attached figure Water Quality Sample Locations : -Site 1 - upstream of current south C & D fill - Site 2 - Discharge from culvert outfall from western gully - Site 3 Careys Gully stream downstream of western gully - Site 4 Seep area above landfill access road None of these sites are a true representation of the leachate or drainage water sourced solely from the C & D fill, but do give an indication of the nature of leachate to be expected from the extension fill. Monitoring results are appended. Site 1 is upstream of any landfill influence and is characterised by high water quality with low levels of dissolved salts (conductivity ms/m) and very low levels of dissolved metals. Site 2 at the downstream toe is understood to be sampling a mixture of clean water from the upstream western gully, drainage from the historic Wellington City closed landfill and the C & D southern fills. This sample site shows some landfill influence with higher alkalinity and conductivity (42-49 ms/m) and higher metal levels. Iron and manganese in particular are considerably higher than the upstream sample (7.56 g/m 3 and g/m 3 respectively on 4 March 2013). Notably ammoniacal nitrogen levels are still very

2 low. This site does not therefore explicitly establish the character of leachate from the C & D fill. Site 3 picks up flow from both branches of the Carey Gully stream and so also includes potential influences from the Wellington City Council landfills. Site 4 being the seep has higher alkalinity and conductivity again (180 g/m 3 and 68 ms/m respectively). Iron and manganese are in a similar range to Site 2. The seep has slightly elevated ammoniacal nitrogen at g/m 3. This is indicative of leachate (albeit quite dilute) from a fill containing organic material. However it is not clear from the location whether this site is solely sourced from C & D or may include some old municipal mixed refuse. 2. Expected Character of Construction & Demolition site leachate Leachates from construction and demolition type landfills are variable reflecting the particular waste stream entering the site. In particular the proportions of organic matter (as wood and wood products, vegetation, cardboard and paperboard) can vary widely from site to site. This makes prediction of a leachate composition on the basis of published data from other sites uncertain. However the following generalisations can be made 1 : If levels of organic matter are low, say less than 5% by weight then nitrogen levels (as ammoniacal nitrogen or nitrate) will be low relative to municipal waste fills Levels of dissolved salts from the major anions and cations will be elevated, showing up as higher conductivity and TDS (total dissolved solids) A major contributor to TDS is often sulphate and calcium sourced from dissolution of plasterboard 2. Sulphate levels in excess of 1,000 g/m3 are not uncommon for construction and demolition landfills with large inputs of plasterboard. If sulphate reducing bacteria are present sulphate will be converted to H 2S (hydrogen sulphide) Dissolved iron and manganese levels are commonly elevated being mobilised from residual metal items and from incoming soil. Unless the site is taking soil from contaminated sites then hydrocarbon levels would be very low A range of heavy metals may be present including commonly zinc, copper, lead and mercury. These will be sourced from miscellaneous items in the waste mix, lead paint residue, small electrical items etc but are generally found at low levels. Boron may be elevated if substantial quantities of boric treated framing timber are present. Page 2

3 Referring to the proposed waste acceptance criteria in the C & D Landfill Management Plan Sections and Acceptable and Unacceptable Materials, the following points are relevant: While the site predominantly accepts inert materials such as soil from site excavations and demolition rubble ie brick and concrete, a range of other materials are listed in the site acceptance criteria, including - Sheet roofing material - Framing timber which may be treated - Steels in form of rebar and tendon - Galvanised steel - Plasterboard - Plastics eg as electrical fittings and packaging - Floor coverings adhering to concrete and timber - Asphalt - Electrical wiring as part of mixed demolition loads - Cardboard packaging While most of the materials disposed are essentially inert in the landfill, a number of items in the above list do have potential to release contaminants to leachate. Importantly, the waste acceptance criteria limit the organic content as woodwaste, vegetation and all other packaging, carpet etc to a maximum of 5% by weight. C&D advise that in practice actual wood waste content of the loads would be considerably less, but they do wish to maintain the 5% limit to allow some contingency for mixed loads such as from site clearing or slip material. Waste is sorted as received to maximise the recovery of useful materials and in particular metal items. However it is inevitable that some items will not be recoverable being too small or mixed with general rubble. Overall therefore the waste stream can be expected to generate a leachate with elevated levels of major anions/cations, but low levels of nutrients, heavy metals and organic contaminants. While there are no specific measurements of leachate from the existing fill, the water quality observations noted above generally support this conclusion. Should the proportion of organic materials rise above 5% then it would be expected that levels of ammoniacal nitrogen in the leachate will start to rise. 3. Leachate quantity The quantity of leachate generated from the proposed fill will be a function of: - Area of the fill surface ie rainwater catchment - Extent of temporary and permanent capping on the fill which is able to shed water to stormwater (as opposed to allowing it to percolate into the fill) - Inflow of surface water from upslope catchments Page 3

4 - Inflow from groundwater sources The direct catchment area is 3.68 ha for Stage 1W. The average annual rainfall at the site is estimated to be 1450mm approx., being some 1.17 x the kelburn gauge rainfall, reflecting the proximity of the site to the south coast (NZET 4 1/10/2013 Section 3.1), ranging from 700mm in a very dry year to 2100 mm in a peak wet year. The peak monthly rainfalls are from May to August being typically mm /month. The relatively small fill platform and large vertical lift will limit the practicality of placing intermediate capping. For leachate assessment purposes a conservative (ie high) position has been taken of assuming all the Stage 1w area of 3.68 ha is open to incident rainfall. The nature of the C & D fill material being rubble and open graded means it will tend to be permeable to incident rainfall with little runoff except in heavy rains and on compacted access roads. Inflow from the upper catchment slopes is to be managed by contour stormwater drains. These will intercept the water and shed it clear as stormwater with minimal contribution to water percolating into the fill (NZET 4 Section 4.4 discusses the contour interceptor drains). Inflow from groundwater sources is expected from the infilled colluvium gullies that feed into the main western gully. These gully features will be traversed by the contour drains. It is proposed to excavate into, or more likely tap into these with horizontal well points, to intercept water draining down the interface between the colluvium and the greywacke. This will minimise the volume of groundwater reaching the gully floor. However realistically it is not expected to be feasible to intercept all the flow in these gullies and so no reliance on interception has been made in assessing groundwater contributions. On the basis of the above assumptions the following conservative assessment of the groundwater/leachate quantity for Stage 1W is approximately 250 m 3 per day (3.0 l/s approx.) as an annual average, derived as: - 30% of incident annual rainfall on the 3.68 ha of landfill area below the contour stormwater drains making some 44 m 3 /day. - A net 300mm of annual rainfall being the groundwater component from the colluvial gullies of the upslope catchments (25 ha) making some 205 m 3 /day. The actual evapotranspiration at Kelburn is 852 mm/yr leaving typically 596 mm for surface runoff and groundwater contribution. Typically around 20% of precipitation in a well vegetated catchment with rainfall of around mm shows as stream flow 3. This leaves 300mm for the subsurface flow component. Note this is probably on the high side, with more typically 10% of incident rainfall contributing to groundwater flows. The water flow from the fill will therefore be a mixture of leachate from the fill and groundwater and thus be diluted approx. 4:1. While flows will vary with season and rainfall, the fill mass and groundwater contribution will attenuate the peak flows from individual rainfall events. For the purpose of sizing outfall pumps and pipework etc a peak of 4x the base flow is assumed, but would need to be confirmed by final design. Page 4

5 4. Proposed Leachate Management Strategy Recognising that there are uncertainties over both the quantity and contaminant levels of leachate which may be generated by the proposed C & D fill, a tiered management approach is proposed as follows: (i) (ii) (iii) (iv) (v) (vi) (vii) Prior to filling commencing, construct a groundwater/leachate collection system into the gully base (Figures 1-3). This will consist of a toe dam at the downstream end of each stage to pick up groundwater/leachate flow off the greywacke basement and direct it into a collection pipe. The gully floor is steep at a 1:5 longitudinal gradient. The alluvial fill depth is variable, from 0- approx 2.0 m. A site at the toe will be selected with moderate width and minimal alluvial depth at which to construct the toe detail. A toe bund of low permeability material such as compacted weathered greywacke or waste concrete will be constructed onto the base rock to intercept the downslope leachate/groundwater flow (Figure 2) Running up the base of the main gully will be a highly permeable rubble filled trench. This will pick up water infiltrating down through the main fill, plus any side inflows from the paleogullies (specific side taps included as needed) The combined groundwater/leachate flow will be directed into a lined retention pond at the head of Stage 2 including a flow measurement and water quality monitoring point. From the retention pond direct flow through a constructed wetland prior to discharge to the Western gully stream, subject to compliance with discharge conditions after reasonable mixing. Minimise contribution of upslope catchment to water entering the fill using contour drains as per NZET 4 Cap and grass completed fill areas as soon as feasible, including using intermediate capping, so more surface water can be shed to the contour drains and clear of the fill at an early point Provide contingency system for discharge of groundwater/leachate to sewer as trade waste should a direct discharge not be in compliance. This will be subject to detailed design. Two options arise, being either to pump the flow away via the South C & D fill, or (preferred) to construct a new contour access bench and pipeline down to the existing culvert. Note that this bench will be required in future as part of the Stage 2 stormwater control. In either case the flows are predicted to be quite manageable even allowing for peaking in wet weather. Page 5

6 5. Lining of the Fill It is not proposed to line the gully base and sides beneath the proposed fill, apart from the localised treatment at the toe stage to intercept groundwater/leachate flow. The rationale for this is: (i) (ii) (iii) The hydrogeological assessment (GeoScience October ) identifies that the gully base is the primary end point of groundwater flow from the rock mass, primarily from the colluvium filled gullies. Ie any flow component of groundwater out into the surrounding rock mass will be (a) very small in quantity and (b) reliant upon travel through weathering features such as shear zones and fissures and be relatively slow. Provision of a high permeability flow path with a rubble filled trench in the valley floor will engineer a preferential flow path that is orders of magnitude higher in transmissivity than the rock mass. Therefore flow from the fill into the surrounding country will be minor provided the base of the fill is well drained. The leachate is predicted to have low levels of contaminants and to be at or close to a level allowing direct discharge after wetland treatment 6. Monitoring The following monitoring is proposed for the groundwater/leachate discharge: (i) (ii) (iii) (iv) Weekly recording of flow at a v-notch weir on the discharge (if to stream) or by flow meter on pump (if pumped) Weekly conductivity of discharge Quarterly key indicator parameters of EC, ph, Ammoniacal nitrogen, nitrate nitrogen, DRP, Chloride, Potassium, Iron, Sulphate, Boron Annual full suite of above plus metals (as dissolved) Arsenic, Cadmium, Copper, Chrome, Lead, Manganese, Nickel, Zinc, also BOD 5, TPH and organics (semivoc) 7. Future Staging Stage 1W is discrete and quite separate from any past activity on site and can be effectively managed as described. Data collected from Stage 1W will be quite specific to the effects of the C &D fill and will be used to inform leachate management of future stages. At this stage the following is envisaged: (i) Stage 1N upstream of the land bridge This will be filled with selected inert soil and rubble with minimal leachate generating potential until the level of the land bridge runs out into the gully floor. At that point future flows of stormwater and leachate can be directed across the fill. In addition more Page 6

7 area will be available on the completed fill for construction of (lined) treatment ponds for leachate from the upslope Stages 1W, 2W and 3W (NZET Section 4.2). (ii) Stage 2 With Stage 1N complete as above the toe detail from Stage 1W can then be replicated for Stage 2. For this stage a large area is available on Stage 1 N for further leachate treatment and simple gravity flow is available for discharge. (iii) Stage 3 Stage 3 is an overlay of Stages 1W and 2 and can rely upon the infrastructure for Stage Landfill Gas The C & D fill will be a minor generator of landfill gas. Contributors to landfill gas would be the small organic content. Wood as stumps and framing timber would be very slow to degrade. Only finer material such as vegetation, packaging, textiles (carpets) etc would contribute to any degree. However with an overall organic content limited to 5% inclusive of wood and stumps, the remaining fraction is small. Plasterboard may contribute to H 2S generation through action of sulphate reducing bacteria. Receptors for landfill gas will be limited to site staff on the fill itself. Their exposure would be trivial compared to personnel working on a normal refuse landfill. The nearest public receptors would be walkers on the ridge top tracks some 100m horizontally and 200m vertically above in a very exposed location. Or the recycling area some 300m down the gully on the other side of the main landfill access road. On this basis we conclude the landfill gas generation to be of less than minor impact and that no further specific analysis is warranted. References 1 : Melendez, B. A. (1996). A study of leachate generated from construction and demolition waste landfills. MS thesis, Dept. of Envir. Engrg. Sci., University of Florida, Gainesville, Fla 2: Townsend, Jang & Thurn, 1999 Simulation of Construction & Demolition Waste leachate, J. Environ Eng 125(11) 3: Fahey & Rowe 1992, in The Physical Environment A New Zealand Perspective Ed Sturman & Spronken-Smith, OUP : NZET 2013: C& D Landfill Burrell Demolition Ltd Response to S92 requests Stormwater 5: Geoscience 2013, C&D Landfill Expansion Geotechnical Assessment Page 7

8 Attachments Figures Water Quality Sample Locations Figure C 4 : GeoScience Stage 1W Plan Figure 1 : Landfill Toe Detail and cross sections Water Quality Data Yours faithfully Peter Askey Principal Environmental Engineer Page 8