Ash Disposal - Transport and Technical Services (2) - Submission - 12 June 2012

Planning, Environment & Design

States of Jersey Transport and Technical Services

Quality Management

Contents  Executive Summary  i

1. Introduction  1

1. Background  1
2. Scope of Report  2
3. Limitations  2
4. Disclaimer  2

2. Hydrogeological Conceptual Model  3

1. Introduction  3
2. Sources  3 Municipal Solid Waste Incinerator Ash Residue 6
3. Pathways  8
4. Receptors  19

3. Baseline Data Collection  25
   1. Introduction  25
   2. Water Quality Determinands  25
   3. Sampling Methodology  26

5. Results  30

1. Introduction  30
2. In-Situ Water Quality  30
3. Heavy Metals  33
4. Nutrients  38
5. Phenols  43
6. Total Petroleum Hydrocarbons  43
7. Polyaromatic Hydrocarbons (PAHs)  43

5. Summary  47
   1. General  47
   2. Receptor Baseline Water Quality  49
   3. Pathway Baseline Water Quality  49
   4. Ash Cell Baseline Water Quality  50
   5. Conclusions  51

7. References  52

Figures

Figure 1  Location plan.

Figure 2  La Collette Headland overview.

Figure 3  Below water fill material.

Figure 4  Discharge consents and soakaways.

Figure 5  Fuel storage and transport.

Figure 6  Groundwater level ranges & lag times (spring & neap

tides).

Figure 7  Spring tide groundwater levels (m ACD) - Feb & June

2011.

Figure 8  Baseline monitoring locations.

Figure 9  Salinity - in-situ readings (average).

Figure 10  Electrical Conductivity (µS/cm) - in-situ readings

(average).

Figure 11  pH & Redox - in-situ readings (average). Figure 12  Zn & Cu concentrations (average). Figure 13  Fe concentrations (average).

Figure 14  Ammonium & Nitrate concentrations (average). Figure 15  Total TPH (µg/l) concentrations (average).

Figure 16  PAH - Anthracene & Benzo(a)pyrene concentrations

(average).

Figure 17

Plates  Plate 2.1 Plate 2.2

Tables  Table 2.1

Table 2.2

Table 2.3 Table 2.4 Table 2.5 Table 2.6 Table 2.7 Table 2.8

Table 2.9 Table 2.10 Table 2.11 Table 3.1

Table 3.2 Table 4.1 Table 4.2 Table 4.3

Table 4.4 Table 4.5

PAH sum (µg/l) concentrations (average).

Initial Source – Pathway – Receptor Model.  8 Variation in hydraulic gradient during the tidal cycle

(schematic)  16

Key potential contaminant sources within the wider

La Collette development area that may influence baseline water quality.  4

Key potential contaminant sources within the La Collette Waste Management Facility area that may influence baseline water quality.  5

Summary of leachate analysis results (WRC 2003

and 2007)  6 Summary of actual versus predicted leachate

extraction volumes (maximum run-off)  10 Actual versus predicted extraction volume

differences compared to total cell volume.  10 Summary of groundwater level monitoring results

(2010 – 2011).  12 Estimated duration of groundwater exit and entry

into WMF.  16

Phase 1 versus Phase 2 groundwater level

monitoring (13 Feb – 16 March 2011)  17 Potential receptors.  19 Sensitivity rankings for specific ecological receptors  21 La Collette water quality standards.  23

In-situ water quality parameters during 20 minute purging.  27

Water quality results for BH3d (4 – 5 April 2011)  28 pH and redox statistics for southern ash cells  32 Average dissolved heavy metal concentrations  34

Number of monitoring points where average heavy metal concentrations exceeded EQS  35

Average nutrient concentrations.  39

Anthracene and Benzo(a)pyrene concentrations in groundwater.  44

Appendices

Appendix A  Figures

Appendix B  Potential Contaminant Sources Associated with

La Collette Headland

Appendix C  In Situ Water Quality Results

Appendix D  Laboratory Analytical Results

Appendix E  Extracted Data & Statistics

Appendix F  Groundwater Level Hydrographs Appendix G  Ecological Water Quality Literature Review

Executive Summary

This report presents a review of the monitoring results from a six month baseline water quality monitoring survey of the La Collette Waste Management Facility (WMF), in conjunction with knowledge gained from parallel studies and groundwater level monitoring to provide an updated hydrogeological conceptual model. This document is a stand-alone report that will be used to inform any future operational water quality monitoring strategy or impact assessments for future development proposals.

The La Collette Waste Management Facility (WMF) currently operates under a deemed waste management licence. Since 1995 the WMF has received a mixed waste stream with inert materials deposited behind the rock armour wall below the mean High Water Spring Tide level (11 m above ACD[1]), and incinerator ash residue (predominantly bottom ash) placed in 4 – 5 m deep lined cells above the spring line. Ancillary activities at the site include green composting, aggregate recycling and the temporary storage of bio-remediated soils and street sweepings on top of lined cells. Historic and other current activities beyond the site that may influence water quality within the wider water environment (including the Ramsar designated area to the south and east of the WMF) include: a remediated diesel spill in the northern corner of the southern working area of the site; historic temporary bulky waste storage within unlined pits; and discharges from the Energy from Waste / JEC Plant outfall.

To gather scientific evidence on baseline water quality, and to ascertain if there are any significant impacts of WMF operations on water quality within the within the wider water environment, monthly sampling at 42 locations (18 ash cells, 7 boreholes, 5 springs issuing from inert waste material underlying the ash cells, and 12 low tide seawater locations within the Ramsar area beyond the sea wall) were undertaken between March and August 2011. Laboratory analysis for each sample for up to 75 chemicals (including heavy metals, nutrients, and organic hydrocarbons), chosen as representative of potential contaminants that may by associated with historic or current activities at the WMF and surrounding area. This is considered to represent an effective spring / summer dataset, sufficient to rationalise future monitoring requirements and to form a baseline for comparison to inform any subsequent impact assessments.

Water quality standards are used as guidelines to assess water quality, but this benchmark is not regarded as critical to the water quality required to protect sensitive receptors, rather as an indication where raised levels exist as a result of background, legacy or possibly current operations.

No biological water quality or biota sampling was undertaken as part of this baseline monitoring survey as this would form part of wider environmental monitoring and would not be a direct assessment of the sites contribution to water quality.

GROUNDWATER MOVEMENTS

With respect to the WMF, groundwater ingress to and egress from the site during the tidal cycle is both via the Phase 2 rock armour wall / inert waste, and via natural channels filled with beach deposit material within the underyling diorite / granodiorite bedrock. Tidal variation within groundwater levels, combined with the lag time between peak tidal levels and peak groundwater levels, suggest a difference in groundwater behaviour in the west of the site to that in the east of the site.

In the west little tidal variation (0.25 m) is observed with lag times up to 6 hours, suggesting limited interaction between the sea and groundwater, plus groundwater levels are lower than those to the east suggesting a groundwater low area. In contrast, in the east groundwater movements appear more dynamic with greater tidal variation (1 – 3 m) and shorter lag times (1½ - 3 hours) suggesting a greater degree of interaction between the sea and groundwater in this area.

Basal elevations of lined ash cells is approximately 1 m (or more) above the highest groundwater level recorded. A basic water balance assessment of leachate volumes within the ash cells suggests hydraulic containment (over three levels of cells) is performing within climatic expectations, suggesting that liners are performing to engineering expectations without a loss of leachate to underlying inert waste / groundwater. Comparison of groundwater quality to ash cell quality further supports this.

RECEPTOR WATER QUALITY

Groundwater within the inert waste of the WMF does not have an amenity or water supply use. As such it is not considered to be a key receptor, with monitoring focused on its ability to act as a pathway for chemicals to the wider water environment.

The key receptor of interest is the coastal waters of the adjacent South East Coast of Jersey Ramsar. A brief literature review indicates that generally key Ramsar species are broadly tolerant to significant changes in water quality, with the exception of:

• Common Eelgrass – sensitive to nutrient changes, principally nitrates and phosphates.
• Maerl & Sweet Cup Coral – sensitive to changes in sediment concentrations.
• Native Oyster – sensitive to changes in some heavy metals during the larval stage (principally mercury, copper, cadmium and zinc), although the adult population are generally tolerant of high levels of copper and zinc.

The evidence from publically available information would suggest that large populations of the above species are not found adjacent to the La Collette headland. However they will still remain a consideration within any future impact assessments.

Currently accepted water quality standards tend to be more conservative / onerous than the limited published research on tolerable water quality concentrations for key indigenous species would suggest. As such the standards have been used for guidance only, with comparison made to a local natural baseline sea water monitoring point for context. Being located at the furthest distance from waste activities and other activities associated with the La Collette Headland, SW1 was considered to represent a local natural baseline monitoring point to compare to.

Beyond the rock armour wall average concentrations of organic hydrocarbons (phenols, BTEX[1], and Polyaromatic Hydrocarbons), nitrates, phosphates and the majority of heavy metals did not exceed water quality standards. Average copper concentrations marginally exceeded the water quality standard at two isolated seawater monitoring points (of the 12 locations sampled), although the reason for this marginal local effect is unknown it is not regarded as a matter for concern for the sensitive receptors.

Widespread receptor exceedence of water quality standards in respect of iron and ammonium is noted, including at SW1. This may be potentially influenced by the natural granodiorite geology (in the case of iron) or a wider water environment impact from surrounding anthropogenic activities (in the case of ammonium), in combination with a small diffuse contribution from the WMF. However this cannot be confirmed, or the precise contribution from the WMF to the wider water environment, in the absence of wider water environment monitoring (which is beyond the remit of Transport and Technical Services).

PATHWAY WATER QUALITY

Springs issuing from the inert waste, representing a more generic water quality from the inert waste body, within the site reflect a similar water quality to that of seawater beyond the sea wall (i.e. the majority of contaminants are present at concentrations below water quality standards).

Groundwater quality further inland within the WMF site (recorded in boreholes sampling a restricted local sampling area) generally reflects a baseline chemical water quality signature associated with historic activities and inert waste disposal, although water quality within the waste springs and seawater beyond does not suggest that such historic influences are having a significant impact on Ramsar water quality.

The near neutral – slightly alkaline pH environment, combined with generally reducing conditions (only marginally oxidising once water reaches the open sea beyond the sea wall) are not conducive to significant degradation chemical reactions. As such, reduction in contaminant concentrations observed between groundwater and the sea is predominantly due to dilution associated with the twice daily flushing of sea water through the site. Within open sea, a short distance beyond the sea wall, conditions would be expected to become oxidising promoting oxidation and further reduction in recorded concentrations.

ASH CELL WATER QUALITY

Overall ash cell leachate concentrations were observed to be at the low end of the range of concentrations reported within WRCs 2003 / 2007 leaching studies. Heavy metal concentrations were found to be similar in scale to that which may be associated with an urban run-off.

Water quality in individual cells is highly variable, with no obvious trends (from the review of monitoring data collected to date) associated with the age of the ash. Variation in leachate composition may be more reflective of variations in the composition of the waste initially burnt masking any obvious trends in ash degradation over time.

Widespread exceedence of water quality standards for arsenic, copper and nickel was noted, with only occasional cells recording exceedences for chromium, manganese and iron. Ammonium concentrations varied over several orders of magnitudes. Anthracene and benzo(a)pyrene levels marginally exceeded the water quality standard, however concentrations of other organic contaminants (phenol, naphthalene and BTEX) were generally below water quality standards.

CONCLUSIONS

The six month baseline  water  quality monitoring survey,  plus groundwater  level monitoring, has considerably improved our understanding of the hydrogeological and water quality regime of the La Collette WMF (and immediate surrounding area).

Water quality in the sea surrounding the site was found to be generally of a quality that would not be of concern for the sensitive receptors that in the area. The evidence examined would suggest that the ash cells are effective in containment of the low level contaminants in the cell water.

Inert  waste protocols  implemented by TTS at the  WMF over  the past  15+  years, together  with implementation of appropriate mitigation measures (e.g. lining and capping of ash cells), have reduced the risk of a significant introduction of contaminants into underlying groundwater. In combination with the dilution effect of a site that is flushed through twice daily, concentrations recorded in sea water are within acceptable limits.

It is acknowledged that this current monitoring dataset represents a spring / summer baseline, and as such will be supplemented with additional data during operational monitoring (proposed to be undertaken quarterly and focused on key monitoring locations / determinands). In combination with any wider water environment monitoring undertaken by parties other than TTS, such information will, in the future, enable further improvements to the understanding of La Collettes contribution to the water quality the wider water environment (including the Ramsar designated area to the south and east of the WMF).

1. [1]Introduction

1. BACKGROUND

1. The La Collette Waste Management Facility (hereafter referred to as the WMF, the boundary of which is shown in Figure 1) currently operates under a deemed waste management licence. Since 1995 theWMF has received a mixed waste stream3 with inert materials deposited behind the sea wall up to a level between the Mean High Water Spring (MHWS) level (approximately 10.96 m ACD[2]) and the top of the sea wall (14 – 15 m ACD), and ash placed in lined cells above the spring line.
2. Since 2003 an aggregate recycling facility has been incorporated into the inert process, removing suitable granular material as secondary aggregate product for construction re-use and some metals inclusions for scrap recovery from the waste stream prior to disposal. Other activities on-site include green waste composting, the temporary processing of bio- remediated soils and storage of street sweepings on top of lined cells.
3. Although not obliged to have a water quality monitoring strategy in place under current permissions and licences, given the proximity of the WMF to the internationally designated South East Jersey Coast Ramsar area, the States of Jersey Environment Protection Department wanted to see a robust strategy for water quality monitoring brought into place.
4. To gather scientific evidence in support of planning / waste management licence applications at the WMF, and in support of the current deemed waste management licence, TTS commissioned Capita Symonds Ltd (Capita Symonds) in June 2010 to develop a baseline water quality monitoring strategyto ascertain if there is any significant impact of WMF operations on receptor water quality. The early results of such baseline monitoring could inform refining the monitoring programme and network, such that an appropriate water quality monitoring strategy to be implemented during site operations can be developed.
5. Monthly baseline water quality monitoring was undertaken between March and August 2011. In addition other parallel pieces of work have been undertaken to inform our understanding of the hydrogeological system and water quality within the WMF and surrounding environment. Referred to throughout this document, they principally comprise:

• La Collette Water Quality Monitoring Strategy. Stage 1: Contextual Factual Report (Sept 2010).
• La Collette Waste Management Facility and Surrounding Area. Water Quality: Source Analysis Review (May 2011).
• La Collette Waste Management Facility. Ash Cells: Hydrogeological Conceptualisation (November 2011).

2. SCOPE OF REPORT

1. The purpose of this report is to review the monitoring results from the 6 month baseline water quality monitoring period, together with knowledge gained from parallel studies and groundwater level monitoring to present an updated hydrogeological conceptual model (to that last documented in Sept 2010).
2. This document is a stand-alone report that will be used to inform any subsequent development of a water quality monitoring strategy during operation, Environmental Impact Assessment for future planning applications, or Hydrogeological Risk Assessment in support of Waste Management Licence applications.
3. Section 2 presents an updated Source-Pathway-Receptor hydrogeological conceptual model, drawing upon the results of the aforementioned parallel studies. Section 3 factually reports the extent of the monitoring undertaken during the baseline period (in terms of approach, methodology, determinand suite and sample locations).
4. Water quality monitoring results are summarised in Section 4 with full data sets presented in Appendices C – E. An over-arching summary is presented in Section 5.

3. LIMITATIONS

1. The purpose of the baseline period of monitoring has been to characterise existing water quality at the WMF and immediately beyond the sea wall, such that the data maybe used as baseline for judgement of future development proposals interms of hydrogeological risk assessments, and against which future monitoring data can be compared.
2. As such the focus for the water quality monitoring is the WMF and its potential for ongoing diffuse discharge to the wider water environment (including the Ramsar designated area to the south and east of the WMF), rather than on establishing water quality of the wider marine environment. No biological water quality or biota sampling was undertaken as part of this baseline monitoring survey as this would form part of wider environmental monitoring and would not be a direct assessment of the sites contribution.
3. Although the gathering of baseline monitoring data has helped to further develop our conceptual knowledge of the site, the baseline monitoring is not intended to fully explain the complexities of the hydraulic regime at the site nor reach definitive conclusions as to the influence of historic activities on the water quality regime.
4. No numerical modelling of water levels, or calculation of groundwater / surface water mass fluxes has been undertaken as part of this work.

4. DISCLAIMER

1.4.1  This report is for the use of States of Jersey Transport and Technical Services Department only and should not be relied upon by other parties unless advised by Capita Symonds Ltd in writing.

Conceptual Model

2. Hydrogeological Conceptual Model

1. INTRODUCTION

1. An initial source-pathway-receptor (SPR) conceptual model was outlined in Capita Symonds

Stage 1 Report (Sept 2010). The aforementioned report highlighted several areas of conceptualisation where knowledge (at that time) was limited, including:

• An improved understanding of baseline influences of other potential sources in the wider water environment;
• Evidence for the integrity of the ash cells;
• An improved understanding of the nature of fill material below water within the La Collette WMF, and its influence on water quality;
• The degree of hydraulic connection across the site; and
• Water quality in the receptor (beyond the sea wall).

2. To address these knowledge gaps, further work[1] (including some periods of groundwater level recording and 6 months of baseline water quality monitoring – see Section 3 for detail) has been undertaken.
3. This section presents an updated hydrogeological conceptual model for the WMF and surrounding area, drawing upon previous work and additional information (excluding consideration of water quality results that are discussed in Section 4) gathered since the

aforementioned report was written.

2. SOURCES

1. Built out from the 1960s to the present day, the La Collette development area currently

encompasses a number of different zones (Figure 2):

• Pre-1965 reclamation (encompassing Jersey Electricity Corporation (JEC) power plant).
• 1970/1980s Phase 1 reclamation (encompassing the abattoir, current Esso / Rubis fuel consortium and various light industry).
• 1990s to date Phase 2 reclamation (encompassing the Connex bus depot, the Energy from Waste facility and activities (both past and present) at the La Collette Waste Management Facility itself).

2. Key potential sources that may influence water quality within La Collette or the surrounding area are summarised in Tables 2.1 (wider area) and 2.2(WMF area), with expanded notes contained in Appendix B. Formore detail the reader is referred to the main Source Analysis Review report (May 2011). Further discussion of potential leachate associated with the ash source is outlined from paragraph 2.2.3 onwards.

Table 2.1:  Key potential contaminant sources within the wider La Collette development area that may influence baseline water quality.

(derived from Capita Symonds Source Analysis Review, May 2011).

Table 2.2:  Key potential contaminant sources within the La Collette Waste Management Facility area that may influence baseline water quality

(derived from Capita Symonds Source Analysis Review, May 2011).

MUNICIPAL SOLID WASTE INCINERATOR ASH RESIDUE

3. Of the 5-8% of fill received previously from the Bellozane municipal solid waste incinerator (MSWI) residues / ash received at the WMF, it is estimated 21% comprises bottom ash, 4% fly ash and 1% metal inclusions. In the absence of detailed information on waste materials by cell, it is conservatively assumed that all cells historically received a mixture of bottom ash, fly ash, scrap inclusions material, with some occasional putrescible material and small quantities of contaminated soils historically.
4. The La Collette Energy fromWaste(EfW) facility has a new process producing grate ash with ferrous metal separation and flue gas treatment residues. The plant which came on- line in 2011, will be fully operational in 2012. As such the receipt of wastes from Bellozane ceased in 2010 and has been replaced with intake of MSWI residues from La Collette (EfW). It is estimated that for each tonne of MSW burned within the new facility 25% will become bottom ash residue, 2% non-hazardous ferrous metals, and 4% Air Pollution Control (APC) residue (Capita Symonds EfW Residue High Level Review, Sept 2010). For the purpose of review of baseline water quality monitoring data however, the main area of focus is on potential leachate concentrations associated with previous ash from the Bellozane incinerator. This did not include APC residue since there were no such control measures in place at the historic plant.
5. Capita Symonds Stage 1: Contextual Factual Report (Sept 2010) provides a review of desk study information available on the potential composition of leachate from Incinerator Bottom Ash (IBA) and Fly Ash. Key points are summarised below for context to this report.

Bottom Ash

6. Bottom ash particles range in size from a fine gravel to a fine sand with verylow percentages of silt-clay sized particles. WRCs 2003 and 2007 ash characterisation studies examined the leaching potential of ash products generated by the Bellozane MSWI (receiving a mixture of municipal solid waste, shredded domestic and wood waste plus electrical goods). Table 2.3 summarises the range of inorganic and organic results from the two studies.

Table 2.3:  Summary of Leachate Analysis results (WRC 2003 and 2007)

7. It is noted that the above studies did not report on potential iron concentrations within leachate samples derived from the Bellozane bottom ash in 2003 and 2007. Values quoted from a german paper (Pfrang-Stotz & Reichelt, 2000) show a range in iron concentration of 48 – 75 ug/l.
8. With respect to potential non-metal concentrations within the IBA leachate, the Environment Agency[1] note a range of concentrations for Ammoniacal Nitrogen (100 – 1,000 mg/l), Chloride (1,000 – 10,000 mg/l), and Sulphate (1,000 – 10,000 mg/l). They also note that a lysimeter study undertaken within IBA yielded Chloride concentrations of approximately 10,000 mg/l and Sulphate concentration of 550 mg/l.
9. It can be seen that leachate composition may potentially be quite varied, and tends to fail environmental water quality standards, as such disposal to lined pits is considered appropriate.

Fly Ash

10. Background research indicates that only 2% of fly ash component is soluble, with leachate tending to be characterised by a non-reducing and alkaline environment. Arsenic (As) is usually present in its anionic form, with solubility tending to be suppressed by the presence of calcium. However pH may decrease over time as calcium oxide is used up, leading to an increased risk of heavy metal dissolution. Surface iron oxide coatings, combined with alkaline pH, have a dominant influence on the sorption of trace heavy metals. In the case of the Bellozane ash, results from the WRC characterisation studies would suggest initial leachate pHs of 10.5 – 12.5.
11. Fly ash exceedences in general tend to be with respect to: calcium (Ca); magnesium (Mg); sodium (Na); potassium (K); sulphate (SO4); chloride (Cl); fluoride (F); nitrate (NO3), selenium (Se); and boron (B). Exceedences tend to reach peak highs rather than sustained concentrations over a greater period of time. B tends to become quickly sorped to clays and iron oxides. Dioxins and furans are predominantly concentrated in the fly ash (0.2-10 ng/kg compared to 0.001-0.01 ng/kg in the bottom ash of the Bellozane waste residue, WRC 2007 report).
12. With respect to Ammonia in fly ash leachate, Total Kjeldahl Nitrogen[2] concentrations are noted in Environment Agency research (Robinson et al, Sept 2004) tobe approximately 25 mg/l. Only trace concentrations of PAHs and BTEX were noted, with oil concentrations less than 1 mg/l.

3. PATHWAYS

1. As highlighted in Capita Symonds Stage 1 Report (Sept 2010), there are several potential hydrogeological pathways (both natural and anthropogenic in nature) via which contamination from sources identified in Section 2.2 may reach the surrounding surface water environment to the south and east of La Collette to a greater or lesser degree. Illustrated in Plate 2.1 below, these include:

• If liner integrity were compromised leakage from the ash cells to underlying inert waste material;
• Infiltration of rainfall, and run-off from site activities, to ground (inert waste);
• Groundwater flow via tidal ingress in and out of inert waste to adjoining lagoons within the WMF or discharging as springs / streams through rock armour walls (Phases 1 and 2); and
• Sea water movement directly through rock armour wall to Western and Eastern Lagoons within WMF.

Plate 2.1:  Initial Source – Pathway – Receptor Model (taken from Capita Symonds Stage 1: Contextual

Factual Report, Sept 2010).

Rainfall

Seawall Embankment (14 – 15 m ACD)

Mean High Water  High Admiralty Tide (12 m ACD) Spring (11 m ACD)

Perched Water  Intertidal Flow  Low Admiralty Tide (0.5 m ACD)

Sea bed deposits in channel – possible pathway?

Possible pathway through bedrock?

ASH CELL LINER LEAKAGE POTENTIAL

2. The ash cells at the site have been (and continue to be) constructed above the Mean High Water Spring line level of approximately 11m ACD, and are designed to operate on a hydraulic containment basis, whereby leachate in the ash waste is prevented from percolating into the ground beneath by a low permeability liner. Disposal commenced in the northern most part of the WMF from 1995, gradually moving south and south west over time.
3. Ash cells are 4 -5 m deep, lined, with basal drainage layers for leachate collection and a rainshed capping installed after cell filling has been completed. Access pipes are located in cell corners to facilitate pumping out of infiltration leachate when evaporation is exceeded by rainfall rates in uncapped cells before final capping. 2 - 3 layers of ash cells arranged on top of each other have been created over time.

4. The infiltration of rainfall into the cells (and hence leachate generation) after closure is also designed to be minimised either by the emplacement of a low permeability cap, or succeeded with further cells with caps at a higher level. In total there are up to three levels of ash pits (low, medium and high) within the WMF. Monitored for leachate levels, the pits have maintained veryminimal or no accumulation of water in the completed pits and only occasional extraction pumping has been required.
5. In 2010/11 there has been an increasing demand for areas on the site to manage potentially contaminated or lightly contaminated waste material to allow bioremediation to occur, or to ensure this material does not go in to the general fill. Areas above the ash pits have been used as they provide a lined capture point for any potential contaminants.
6. This activity has required a number of cells to be left uncapped for longer periods, providing a pathway for rainwater to infiltrate and generate leachate. To maintain a leachate level that does not over-top the pit walls, leachate is routinely abstracted from a number of ash cells (via the access pipes / dip tubes) and removed from site by tanker to the Bellozane Sewerage Treatment Works.
7. To improve our understanding of the degree of ash cell liner integrity, and hence the potential for leakage of leachate to underlying inert waste material, Capita Symonds undertook a broad review[1]of the ash cells (in terms of leachate volumes, levels and quality). This included comparison of physical levels and estimated volumes of leachate to tanker records, to broadly ascertain whether leachate volumes were as expected within the cells (given the information available on their design, site conditions and local weather patterns).
8. Working upon the assumption that leachate is pumped to maintain a head level (i.e. there is no change in storage), and that there is no ingress from groundwater (given that the pits lie above groundwater) the volume of "water in" to the system should be equal to the volume of "water out" if there is no leakage from the cell. A basic, simple water balance spreadsheet assessment was undertaken for existing ash cells from which leachate was extracted in 2010/11 to predict theoretical abstraction volumes required to maintain a steady leachate head (i.e. to prevent the depth of leachate increasing), if the assumed climatic catchment and cells hydraulic containment were functioning as expected. The spreadsheet model was run for each pumped cell for an autumn-winter scenario and a winter-spring scenario (with zero or maximum run-off taken into account).
9. Whilst the spreadsheet model was not calibrated with site-specific evaporative / interception data, these theoretical leachate extraction volumes were compared to records of actual leachate extraction to ascertain whether the ash cells, broadly speaking, had predicted leachate volumes within them. If the volume actually abstracted was less than predicted, this may potentially indicate some undetermined contribution or loss to the cells, or storage of leachate in perched water bodies not accessible to the dip tubes. Alternatively if the extracted volume was found to be greater than predicted this may potentially indicate an input derived from more than just rainfall.