Paper Published in the Journal of the Institution of Water & Environmental Management, 1991, Vol. 5., No. 6 December.
Protecting Water Resources from the Effects of Landfill Sites: Foxhall Landfill Site
Site Map
Site Map
By C. Palmer, CEng, MICE, MinstWM (Fellow), and P. J. Young, BSc, CChem, MRSC, MinstWM (Member)
ABSTRACT
This paper describes the work carried out to protect the ground and surface waters close to the Suffolk County Council Foxhall Landfill Sites. Monitoring of groundwater showed that a deterioration was occurring, likely to impact adversely on water quality in the adjacent stream. The National Rivers Authority expressed their concern, and measures have been introduced to eliminate this pollution risk. Contaminated groundwaters flowing towards a local watercourse were intercepted by a "Trammel" type geotextile cut-off drain and flow to a central collection sump. Automatic monitoring of contaminant levels at this point controls the treatment and disposal options. Where water quality is unacceptable for discharge to the local watercourse, the intercepted groundwaters are treated by irrigation onto the capped surface of a restored landfill. If quality remains unacceptable for discharge, further irrigation treatment or discharge to a percolation area utilizing aquifer attenuation properties is possible.
Key words: Capping; discharge; disposal; groundwater; irrigation; landfill; remedial measures; water resources.
INTRODUCTION
Since 1974, when the County Council became responsible for waste disposal, all of Suffolk’s household and most of its dry industrial wastes have been disposed of to landfill. At the time of reorganisation all operational public sector landfill sites transferred from the districts to the county council. While the majority of these sites had very little remaining capacity, two or three were of a significant size and continued to play a major role in the disposal of Suffolk’s waste, including the Foxhall Landfill Site.
Over the years the County Council rationalised the number of operational landfill sites, concentrating its resources in a smaller number of larger sites. Suffolk County Council currently operates five landfill sites, all of which consist of mineral workings for sand and gravel or chalk.
Since 1974 the quantity of household waste handled by Suffolk County Council has steadily increased. Currently some 300,000 tonnes per annum are disposed of. During the same time, the annual quantity of commercial and industrial waste handled at Council Council sites increased to a peak of about 210,000 tonnes per annum in 1978/88. The levels of industrial wastes being received have been deliberately curtailed during recent years.
The waste materials within a landfill site will decompose over a period of many years, the rate of decomposition depending principally upon the moisture content, temperature and constituents of the waste. The scale of gas production and also leachate generation has only been recognised comparatively recently.
Over recent years the whole concept of disposing of waste into the ground has become more scientifically based. In 1971 the Department of the Environment’s (DoE) advice contained in the Sumner Report (1) amounted to only one chapter on the subject of protecting water resources, and its general conclusion was that potential disposal sites having "subsoils such as sand and gravel would act as an effective filter for bacterial and organic contaminants. For contaminants such as chloride and sulphate reliance would mainly have to be placed on dilution but generally it is considered this will be sufficient"
Following a period of intense government sponsored research work in the mid-1970s, the DoE published a far more scientific report in what has become known as the Brown Book(2). The findings from the programme were relevant to a wide range of situations likely to be encountered in the landfill disposal of wastes. The report drew attention to the advantage of an unsaturated zone beneath a landfill to provide attenuation opportunities for leachate by chemical and biochemical processes. The report concluded that very few documented cases of significant groundwater contamination due to landfill had occurred, indicating that landfill using dilute and attenuation philosophies was acceptable.
Over the period covered by this general advice from the DoE, all the landfills developed in Suffolk were therefore based upon the dilute and attenuate philosophy using the underlying sub-soils (raised above the water table where necessary) to provide an attenuating medium. In particular three sites on the eastern side of Suffolk (Foxhall, Kesgrave and Wangford) were all developed in former sand and gravel workings.
Planning permissions and subsequently site licences were granted following consultations with the water authority. The sites were operated in accordance with planning and site licence conditions. All these sites were equipped with groundwater boreholes and routine sampling was undertaken, the results of which were copied to the water authority and considered by officers of the waste disposal authority.
Over the past three years it is, however, evident that these three sites (Foxhall, Kesgrave and Wangford), on which landfilling commenced in 1963, 1970, and 1980 respectively, are all giving rise to contamination of groundwater which is threatening the quality of local surface water streams or rivers.
This paper sets out the investigations carried out jointly by Suffolk County Council and their consultants (Aspinwall) on one of those sites (Foxhall) and the measures proposed and now being installed to combat the problem.
HISTORY AND SETTING OF FOXHALL LANDFILLS
Aspinwall and Company were first requested to advise Suffolk County Council concerning the monitoring and control of the impact of leachate from the Foxhall landfill sites in early 1988. Work has progressed steadily from assessing the appropriate design for future phases of the landfill, Foxhall III and IV, to assessing the impact of leachate released from the existing Foxhall I and Foxhall II sites, through to the recommendations for design of remedial works to protect local surface waters. Work in early 1988 built on the extensive monitoring record which had been established by Suffolk County Council over the many years of previous landfilling activity. It was primarily aimed at producing an assessment of the existing landfills, and on the basis of this impact recommended an improved approach for the small extension called Foxhall III which was currently under design.
The Foxhall landfills are situated 1 km west of the A12 trunk road, which forms the eastern bypass to Ipswich, about 7 km east of the centre of Ipswich (Fig. 1). The landfills are located in pits which were formerly worked for sand and gravel. No sub-water table working took place and a minimum of 2 m unsaturated zone was left at the completion of mineral extraction.
The geology of the sites is a sequence of glacial sands and gravel, overlying Red Crag, which in turn rests upon London Clay. The surface of the London Clay has been affected locally by superficial glacial disturbances and/or tectonic movements and downwarping, such that borehole logs have shown a mound or ridge within the clay surface. This reaches a maximum height below Foxhall II, and in the centre of the landfill waste rests directly on the clay. The Red Crag which overlies the London Clay consists of a lower part containing shells and an upper part without shells, which is very similar to the overlying sands and gravels of glacial age. Typical depths of the sequence are shown below:
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The lower portion of the sand and gravel sequence in the Red Crag is saturated and groundwater flows in this deposit towards the Mill River, which is the principal surface watercourse to the south of the site. The Mill River flows over London Clay with associated alluvium deposits in the valley.
Foxhall I landfill was filled from 1963 to 1982, during which period it had 800,000 m3 of waste deposited. Landfilling then transferred in 1982 to Foxhall II, which was completed towards the end of 1988. Foxhall III provided less than 18 months capacity for the period around 1989, and since 1990 landfilling has continued in Foxhall IV, which forms the western part of a triangular area still undergoing mineral extraction.
ASSESSMENT OF IMPACT ON LOCAL WATER RESOURCES
The first task in early 1988 was to predict the time, quantity and quality of leachate from both Foxhall I and Foxhall II in order to assess the long-term impact of these sites. A relatively sophisticated desk study and calculation of leachate production was carried out taking advantage of the detailed knowledge which Suffolk County Council had regarding the input quantities and phased filling of both sites, together with the extensive geological and hydrogeological information from ten previous site investigations.
In the early part of 1988 fifteen boreholes were available for the monitoring of groundwater quality in the vicinity of the Foxhall landfills, and a further five boreholes and eight pressure vacuum lysimeters were installed during the year. This monitoring dated back to July 1979 and is summarised in Table 1.
The monitoring of these boreholes had already alerted Suffolk County Council to the gradual deterioration in water quality beneath the oldest landfill, Foxhall I. The highest concentration of chloride was approximately 1000 mg/l and of ammonium ions was nearly 600 mg/l in borehole 12, although these are peak concentrations and the boreholes around Foxhall I typically recorded 100-200 mg/l chloride and generally less than 10 mg/l ammonium ions up to the beginning of 1988.
A risk assessment was therefore performed to predict further concentrations from this landfill. the approach was involved the calculation of dilution of conservative contaminants such as chloride. It is based on detailed water balance to define when the absorptive capacity of the refuse in Foxhall I became used up. Due to the progressive filling of the site it was necessary to consider how leachate production would begin to the north and move steadily southwards until the whole site became leachate generating. The detailed information available concerning the operation of the landfill allowed a reasonably accurate picture to be drawn of how leachate generation was likely to be distributed within the site.
Using limited field data available on the cation exchange capacity (CEC) of the underlying sand strata, it was possible to calculate what the attenuation of ammonium ions would be at an efficiency of removal of 37% in the unsaturated zone (3) and of a minimum 10% in the saturated zone, which is based on previous experience of the consultants. The calculation used for ammonium ion was based on a worst case situation; including the earliest possible date of leachate generation, a lower end of observed efficiency of ammonium removal, and the highest permeability of the range observed for the underlying strata. However, the possible range for each of these input figures is relatively small, and the results were considered to predict likely behaviour beneath the landfill within a few years of the observed deterioration in water quality taking place. A summary of the ammonium ion dilution and attenuation calculation for the three Foxhall landfills is given in Table II.
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The results showed that in worst case scenario, significant groundwater contamination by ammonium ions would reach the spring line which discharges into the Mill River by 1990. with respect to the impact of the existing landfills the following key recommendations were made:
- The levels and quality of leachate and groundwater within and around the landfills should be carefully monitored. The information will help to identify remedial action and site specific predictions of attenuation performance and leachate generation throughout the Foxhall landfill complex.
- The individual impact of Foxhall II should be limited by incorporating as single fuller’s earth attenuating layer into the base of the quarry using powdered fuller’s earth at the rate of 20 kg/m3. The existing bentonite slurry should be mixed with and applied to the lower walls of the quarry , as a low permeability bund. Phased filling and daily cells should be used to minimise leachate generation.
- Monitoring of the unsaturated zone beneath Foxhall III is recommended by the use of pressure vacuum lysimeters. This will enable predictions to be made about future landfill designs such as Foxhall IV and to define further measures required to limit the impact of this phase, should these be necessary.
- It is recommended that three boreholes are drilled through the waste in each of Foxhall I and II. The boreholes through the waste will be used to calibrate the desk study predictions and enable a remedial works programme to be timed to give best effect.
SELECTION OF CONTINGENCY MEASURES
All of the above recommendations were acted upon by Suffolk County Council and the consultants in the shortest practicable timescale. However, due to a number of difficulties, such as the need to gain access to land outside Suffolk County Council’s ownership, the calibration of the initial impact assessments of Foxhall I and Foxhall II took a period of about one year.
The drilling of boreholes through Foxhall I and Foxhall II landfill showed that the moisture content in both landfills was not as uniform as the desk study predictions would suggest. Perched leachate exists within the waste whilst other horizons continue to absorb the moisture as it moves down through the waste. However, the field investigations showed that some leachate was already reaching the saturated zone by fissure flow between the perched water tables within the landfill.
The calibration work suggested that the dates for onset of leachate generation, and for initial contamination of the spring line near the Mill River from Foxhall I, were approximately correct, although the quantities of leachate arriving would not lead to the predicted impact for a longer period of time. From Foxhall II contamination could reach the spring line by 1992, due to the initial release of small volumes of leachate in advance of the main body of waste reaching its field capacity. However, the prediction of large-scale leachate production leading to the derogation of water quality stated in Table III would not take place until around the turn of the century.
In view of the sensitivity of the Mill River, which is a Class 1A watercourse, the initial pollution predicted from Foxhall I and from Foxhall II in 1990 and 1992 respectively required contingency measures to be in place by those dates. Detailed recommendations were made therefore to continue monitoring of groundwater using the existing borehole network, to instigate Suffolk County Council’s own monitoring of local surface water quality, together with a detailed investigation along the spring line to the north of the Mill River, where polluted groundwater from Foxhall I was predicted first to emerge.
A detailed assessment of the impact of capping the landfills was made; the value of capping Foxhall II was found to be much greater than for Foxhall I due to the later filling of this site and the large absorptive capacity remaining in the refuse. Early capping could delay the onset of leachate production from the main body of the site by a considerable period of time, according to the efficiency of the cap adopted.
The contingency measures investigated for the site and the initial cost estimates (present worth) for their implementation are given in Table III. Before arriving at the costs given in Table III an assessment of five different designs of groundwater cut-off had been made. The alternative interception methods using bentonite/cement slurry cut-offs, concrete diaphragm walls, sheet piling, or a granular drainage system were rejected on the basis of their high cost. The cost of concrete diaphragm walls was almost an order of magnitude larger than for the adopted "Trammel" type drain.
Option 1 (Table III) is the preferred option and is based on spray irrigation over the capped and completed landfills, together with discharge to the Mill River, or via a percolation plot depending on the quality of the run-off from the spray irrigation areas. The percolation plot would provide stand-by treatment capacity by controlled application through the unsaturated zone, where absorption and attenuation will take place, as has been observed beneath the existing landfills. The long term expenditure was calculated on the basis of needing to intercept contaminated groundwater from Foxhall I and Foxhall II, which requires a total irrigation area equal to the two completed landfills. Percolation will require a plot close to half the irrigation area to ensure that contamination is retained within the unsaturated zone. At this stage the irrigation system is only designed to use Foxhall II, as the groundwater interception drain is designed to intercept water from Foxhall landfills I and III, and only part of II.
Option 2 is a variation of the first, such that if percolation cannot be achieved adjacent to the existing landfills, a remote plot situated further afield would be sought. However, this option does not include for irrigation treatment and was considered unlikely to be acceptable as it would rely on percolation treatment through the unsaturated zone to an unacceptable degree.
Option 3 considered alternative disposal arrangements to the use of the percolation plot, using pipeline discharge of the runoff water to various locations shown in Table III. All these options would require considerable environmental impact work to assess the implications of discharging the variable quality runoff from the irrigation plot to sensitive surface water locations.
Option 4 considered direct discharge of the collected contaminated groundwater to the four locations listed in Table III. All these options would require a formal environmental impact assessment as the implications of discharging the untreated water would need to be carefully assessed.
Option 5 considered discharge of the collected contaminated groundwater by pipeline to the sewerage system. This would involve the installation of a new pipeline from Foxhall II to either the Cliff Quay sewage-treatment works or Dobbs Lane pumping station, which would not be available at the initial stage of the site works.
The latter option of a shorter pipeline to the Dobbs Lane pumping station therefore required an initial period of irrigation. Although favoured by the National Rivers Authority (NRA), the costs were calculated to be 2-3 times as much as adopting the preferred route of irrigation and percolation.
Under these circumstances the preferred option of irrigation combined with percolation was selected for the contingency measures for dealing with contaminated groundwater at the site.
Detailed design work and contractor selection therefore proceeded for the two main elements of the contingency measures: firstly the capping with a synthetic membrane of Foxhall II, and secondly the construction of the initial phase of the groundwater interception and treatment scheme.
The technical and design work being carried out by the consultants was at this time matched by Suffolk County Council, with a significant increase in monitoring activity and preparation of reports to enable the raising of the necessary capital finance for the substantial sums needed to complete the contingency measures. The tight timetable set to prevent contamination reaching the Mill River at potentially damaging levels meant that both technical design and finance were required in an unusually short timescale. In the meantime, agreement was reached for the continued landfilling at Foxhall, using Foxhall IV, by applying an enhanced attenuation layer using bentonite in the base of the landfill and adopting a closed-cell method of filing to minimise water infiltration. It was also recommended that temporary plastic capping of each individual phase of Foxhall IV should be considered before the final levels and permanent capping were installed.
CAPPING OF FOXHALL II
(This section has not been included here, but can be found in the full version of this paper in the journal of The Institution of Water & Environmental Management, 1991, Vol.5., No. 6 December.)
COLLECTION, TREATMENT AND DISPOSAL OF CONTAMINATED GROUNDWATER
The complete scheme for dealing with contaminated groundwater continues to be developed, as works are now underway on the site. The overall layout is shown on Fig 3, and a flow chart of the principal steps in the system is shown in Fig. 4.
Fig. 3. Layout of Scheme
The groundwater is intercepted by the cut-off trench, which is designed in six discrete sections, each of which can have its collected water designated as clean or dirty. Where the water is acceptable for discharge, the clean water will be passed through the continuous monitoring station, which has been agreed with the NRA, prior to discharge via a flow meter to the Mill River. Continuous monitoring has been agreed for ammoniacal nitrogen, nitrate and conductivity. Other contaminants are unlikely to pose a problem due to the efficiency of the attenuation of such as the organics in the sand aquifer and unsaturated zone beneath the landfills.
Where the water quality is not acceptable for direct discharge, the water will be pumped to the irrigation plot through a flow meter for monitoring the application of contaminated water requiring treatment. The irrigation plot constructed on Foxhall II will then allow evaporation and absorption to take place, although under certain circumstances and particularly when winter runoff is to be expected. This runoff will be
monitored for conductivity an sent to the continuous monitoring station if the conductivity limit is not exceeded. If the conductivity indicates that further treatment is required, the water can be either re-circulated from the runoff sump situated at the low point of the perimeter collection ditch around Foxhall II or discharged to the percolation area (Fig. 5).
The percolation area is designed to give complete attenuation and absorption of ammonium ions by the same processes that occur beneath the landfills in the unsaturated zone. The percolation plot will be operated on a "paddy field" basis, whereby short-term flooding will take place over discrete areas to allow an even and controlled distribution of the water to the plot. Detailed monitoring will take place around and within the percolation area to ensure that the loadings are kept such that no significant deterioration in groundwater quality will take place.
The groundwater cut-off trench will contain a "Trammel" type drain installed down to and keyed into the London Clay (Fig. 6). It will be noted that in addition to the perforated collector drain operating within each section of the interception trench, a pair of pipelines will also be present, one available to carry dirty water from further up-gradient and one to carry clean water. Construction work to date has shown that the clay surface varies markedly over very short distances. In some location sit will be necessary to form a clay barrier beneath the pipe, such that the groundwater is fully intercepted, and the pipe maintains a steady fall towards the main collection sump. In general the cut-off drain will be three or four metres deep, although in places this will increase to six or seven metres.
When completed, the cut-off drain will allow any surface water runoff during periods of high rainfall to travel over the surface and discharge to the Mill River, without being intercepted by the groundwater system.
The cut-off drain has been designed for a maximum long-term flow of 1200 m3/day, although all the pipework and pumping equipment has been sized to take a flow rate of up to 2000 m3/day, should it be necessary to extend the length of the cut-off drain in the future. The design allows water to back up if the pump fails and the drain can no longer be allowed to collect water (Fig. 7). This will allow a period of over 24 hours during which emergency maintenance can take place, without any discharge of the contaminated groundwater being allowed to occur to the Mill River. The ability to allow water levels to vary behind the interception trench means that no storage lagoon is required in the vicinity of the main chamber. Such a lagoon is required as potentially undesirable as it would create visual intrusion at the Mill River, at would be potentially at risk from floods.
The irrigation system has been designed using a grid of pop-up sprinklers, which are placed at 20 m centres. A total of 300 sprinklers is therefore required across the whole of the site and these are connected in lines of 13 running north to south. The sequence of spraying will be by using two lines of sprinklers at any one time and rotating these at regular intervals to prevent surface runoff. The pipework is buried 700 mm below the surface (Fig. 2), and the use of pop-up sprinklers is likely to minimise the risk of icing during frosty weather. However, there will be occasions when the air temperature is below zero and the irrigation system will not be able to function. On these occasions it may be necessary to divert the contaminated groundwater directly to the percolation plot.
Monitoring will take place after the system has been installed to establish the efficiency of nitrogen removal, although literature research on the efficiency shows that there is the capacity to deal with all the anticipated ammonia within the irrigation plot of Foxhall II for the cut-off length being constructed.
The percolation plot was defined at a late stage, as the identified area first required access agreement from the landowner and then was subject to a detailed investigation to establish the underlying groundwater flow and London Clay surface. This investigation showed that there was no saturated zone beneath the percolation plot area. It was therefore important to ensure that the chosen location would not lead to the water being returned to beneath Foxhall II, where it would never then leave the water-treatment system. More recent delays have been incurred due to the discovery of archaeological remains from the Iron Age in the vicinity of the percolation plot, and archaeological excavations are underway at present. The percolation plot area will require surface treatment, and it has been decided to remove the topsoil prior to ripping to give maximum contact with the soil and ensure good drainage. Regular moniroing will take place for the build-up of any contaminants.
CONCLUSIONS
- The current construction of the extensive remedial works required to protect water resources at the Foxhall landfill sites has emphasised the essential need for good quality monitoring at existing and closed landfills constructed on the dilute and attenuate basis. Although the current work programme has had to work to a tight time schedule, and contamination will only be treated just in advance of deterioration of the Mill River taking place, no warning would have been provided if Suffolk County Council had not invested in a good network of groundwater monitoring boreholes many years ago. At closed Landfills such as Foxhall I and Foxhall II it is clear that although a long time period is required for leachate to reach the saturated zone, the time between contamination occurring and its arrival in surface water can be relatively short, with a rapid spread of the contamination plume.
- With the forthcoming proposed waste regulation authority function taking responsibility for the impact of such closed landfills, it is clear that the quality of the monitoring data at former dilute and attenuate sites must be reviewed with some urgency. Studies at other landfills within the Suffolk area have shown that a delayed period of up to 20 years between closure and the onset of groundwater contamination can be anticipated. The benefit of remedial measures is proportional to the time between their installation and that of the predicted impact. Early recognition of potential adverse impacts is therefore essential.
- The work over the past few years between the consultants and Suffolk County Council has demonstrated the need for close co-operation between a waste disposal authority and their consultants in such complicated problems. The County Council has provided an essential local involvement in the project team with respect to monitoring, surveying, contract administration, and the provision of the resident engineer for Foxhall groundwater interception and treatment works. The consultants have in the meantime concentrated on developing the options for remedial works into outline and detailed designs as necessary.
- After completion of the existing contract period, a re-evaluation will take place regarding the need for an extension to the cut-off drain to intercept the groundwater from the remainder of Foxhall II. The site investigation work beneath the proposed percolation plot has shown that the groundwater from Foxhall II may move in a south-westerly as well as a south-easterly direction due to the presence of the domed London Clay to the south of the site. It will therefore be necessary to consider whether this limited leakage to the south-west presents any threat to surface water, particularly bearing in mind the recent capping of the site, and the anticipated reduction in infiltration that will result.
- Later stages of landfilling at Foxhall have relied upon raising the attenuation capacity of the unsaturated layer using bentonite. This work has taken account of the recommendations in Waste Management Paper 26(4). The effect of this treatment will be measured in future years using both unsaturated zone and saturated zone monitoring. It is likely, however, that most existing landfill sites in Suffolk will need an impermeable capping, while all future sites are likely to be developed as containment sites.
- The work described in this paper is costly in terms of capital and revenue costs, and it is expected that the groundwater abstraction system will need to operate for many years. As the Foxhall site is developed in accordance with DoE advice, there is a case for financial assistance in the form of supplementary credit approvals to be made available.
ACKNOWLEDGEMENTS
The authors acknowledge the assistance of numerous colleagues at Suffolk County Council and Aspinwall & Company in the development and implementation of the remedial measures discussed in this paper. They also wish to acknowledge the helpful and professional advice and co-operation provided by the NRA, who have been consulted closely regarding the scope of remedial work required.
REFERENCES
- Department of the Environment, Refuse Disposal (Report of the Working Party on Refuse Disposal), Sumner Report, HMSO, 1971.
- Department of the Environment, Hazardous Wastes in Landfill Sites, HMSO, 1978.
- Cartwright, K., Griffin R. A., and Gilkeson, R. H., Migration of landfill leachate through glacial tills. Groundwater, 1977, 15, 4, 294-305.
- Department of the Environment, Waste Management Paper 26 – Landfilling Waste, HMSO, 1986.
DISCUSSION
An extensive discussion was held after the presentation of this paper, a record of which can be found in the full version of this paper in the journal of The Institution of Water & Environmental Management, 1991, Vol.5., No. 6 December.
