Groundwater flow from disused landfills in the UK Chalk

Transcription

1 Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe Groundwater Quality 2001 Conference held at Sheffield, UK. June 2001). IAHS Publ. no Groundwater flow from disused landfills in the UK Chalk G. M. WILLIAMS, D. J. NOY & R. D. OGILVY British Geological Survey, Keyworth, Nottingham NG12 5GG, UK gmw(5>,bgs.ac.uk Abstract To assess of the impact of old landfills on groundwater quality, two domestic waste sites in sand and gravel workings above the Middle Chalk have been investigated. Initial drilling and a 2-D resistivity survey failed to detect a significant pollution plume in the Chalk. However, water level monitoring in more closely spaced boreholes has revealed a high degree of seasonal variability in groundwater flow direction, while a 3-D resistivity tomography survey has identified the spatial variability in leachate drainage below the waste. The pulsed release of leachate from specific areas of the landfill has been modelled whilst changing hydraulic heads at the flow boundaries. The results illustrate how the tortuous migration pathway enhances lateral dispersion and dilution of the plume. The study highlights the need for a detailed and accurately surveyed network of boreholes for continuous water level and groundwater quality monitoring in high transmissivity fractured aquifers. Key words aerial survey; Chalk aquifer (UK); domestic landfill; modelling; monitoring; resistivity tomography; transient How INTRODUCTION While modern day landfill sites are engineered to contain waste and leachate, there are many completed orphan landfills overlying major aquifers in UK, which are poorly monitored and pose an unknown pollution risk. In order to assess the impact of old landfills on groundwater quality, the Environment Agency instigated a desk survey of 102 landfills containing controlled waste (NRA, 1995). As a result, the domestic landfill at Thriplow, Cambridgeshire, which overlies the major Chalk aquifer (Tester & Harker, 1982) was selected for further investigation to provide guidelines for managing similar sites. Operated on the "dilute and disperse principle" the site was landfilled in two phases. Phase 1 was deposited between 1957 and 1977 without compaction, is still uncapped, and contains raw and pulverized domestic waste. Phase 2 was deposited between 1981 and 1987 and is capped above ground level with clay (Klinck et al., 1998). A detailed investigation was launched with the aim of characterizing the plumes emanating from the landfills, identifying controls on leachate migration and attenuation, and developing a well-constrained groundwater flow and reactive mass transport model to describe the existing, and future plume extent. SITE INVESTIGATION The initial site investigation involved a two-dimensional (2-D) resistivity survey around the landfill perimeter which did not detect a pollution plume (Williams et al,

2 554 G. M. Williams et al. 2000). Boreholes drilled outside the site similarly did not intersect any significant groundwater pollution, but showed a low hydraulic gradient in a generally northwesterly direction. Boreholes in the landfill showed that the waste was heterogeneous and in places contained highly polluting leachate. Furthermore, interpretation of aerial photographs taken over a period of time indicated the original excavations to be very variable in depth, with a range of excavated material/overburden covering the quarry floor (Klinck et al, 1998). 3-D RESISTIVITY TOMOGRAPHY In order to determine where the waste was deepest and where leachate may be expected to be released into the Chalk aquifer, a resistivity tomography survey was conducted over the landfills. Recent advances in resistivity survey design, field instrumentation and mathematical inversion theory, now permit a full 3-D solution to be obtained providing the resistivity survey is undertaken in several discrete rectangular blocks. Measurements were obtained using a pole-dipole electrode configuration with a dipole spacing of 10 m, a station interval of 5 m, and a line spacing of 10 m giving a depth of investigation of ~35 m. The results were modelled using a finite difference algorithm to calculate the electrical potentials for a theoretical subsurface resistivity distribution. The difference between the observed data and the finite difference model was minimized using an Occam-type, smoothness-constrained least-squares inversion scheme (De Groot-Hedlin & Constable, 1990; LaBrecque et al, 1999). In this way, the smoothest model that best fits the observed data was determined. The results were imported into a 3-D visualization package to facilitate the volumetric rendering of the subsurface resistivity distribution (Ogilvy et al, 1999). The geometric form of specific features can then be visualized in 3-D space, simply by slicing the tomogram, setting the levels of opacity, and rotating the image in real-time. The tomogram for part of the landfill is shown in Fig. 1. Few natural materials exhibit an intrinsic resistivity of <15 ohm.m and hence the opaque volume is considered to reflect the distribution of wet waste, leachate and bedrock invaded by leachate. The results confirm that the landfill does not occupy a single quarry, but is comprised of several discrete pits, all of which were filled and then covered by a thin layer of topsoil. The resistivity values in excess of 60 ohm.m are indicative of uncontaminated Chalk or dry waste. Boreholes show the base of the pits to be about 8-10mb.g.l. but the low resistivity values extending below this depth suggest that leachate has migrated into the Chalk bedrock. This interpretation is supported by hydrochemical analysis of pore water extracted from cores beneath the centre of the resistivity low which gave chloride concentrations >1000 mg T 1 at a depth of ~12 m b.g.l. A narrow zone of low resistivity near station co-ordinate y = 160 m, is indicative of a leachate plume migrating from Pit 1 in a northwest direction and is consistent with the direction of the regional hydraulic gradient. Figure 2 shows a horizontal slice taken from the 3-D tomogram at a depth of 12.6 m b.g.l. overlaid on the topographical map. This depth slice lies within the top of the Chalk and the two low resistivity zones are considered to reflect high leachate concentrations. These zones of <6 ohm.m correlate well with old Chalk pits identified

3 Groundwater flow from disused landfills in the UK Chalk 555 Concealed waste pits Deplli (m) z hi I wt E hi base Leachate plume? 120 Y 80 Y ResisfivilytOhni.m) i I i. I! I i I i.l 1 ui_l_l_l Opaque Irom 1.2 lo!5 Fig. 1 3-D inverted resistivity model of part of the Thriplow landfill. 448 N A JR Outline of old Chalk Quarry 1046; > TP 14:! Outline of old Chalk Quarry 1969! < 100 m i-l Resistivity ohm.m Fig. 2 Resistivity image of the landfill at 12.6 m below ground level. previously in aerial photographs, and most probably correspond to natural leachate drainage sinks within the landfill. These "hot spots" were used as source terms in the transient solute transport model described in the next section.

4 556 G. M. Williams et al. MODELLING SOLUTE TRANSPORT FROM SPECIFIC SOURCE AREAS The regular monitoring of water levels in boreholes has revealed that periodically the waste is saturated and that the magnitude and direction of the hydraulic gradient varies considerably (Williams et al., 2000). Contouring head data for specific times suggests that heads from around the perimeter of the site can be approximated quite well by a simple planar surface. This observation has been developed to provide both initial and boundary conditions for the flow equations of a 2-D finite element porous medium flow model. The heads at each observation time were first approximated by a leastsquares fitted planar surface, and variations in the regional hydraulic gradient over time are shown in Fig 3. It can be seen that the direction of flow can vary by almost 80 over the course of a year. In addition, the changes in the magnitude of the gradient imply changes of flow rate across the site by factors of up to 2.5. Groundwater gradient azimuth Groundwater gradient magnitude Time (Days) Time (Days) Fig. 3 Variation of the direction and magnitude of the regional groundwater gradient with time after the first observation in January The coefficients from these regional gradient fits were used to evaluate the heads around the boundaries of the finite element grid. The coefficients that were obtained at each observation time were interpolated using cubic spline functions to obtain a smoothly varying groundwater flow field for a period spanning the two years of observations. To illustrate the nature of the flow field obtained from this model, two pairs of particle pathlines were traced from the centre of two of the source zones identified from the resistivity survey, one pair of lines starting at the beginning of the modelled period and a second pair starting 12 months later. These paths are plotted in Fig. 4 and variation of the flow direction is clearly demonstrated. It can also be seen that the pair of paths that start 12 months later follow quite different lines from those started at the beginning of the model. The flow field is thus varying from year to year as well as seasonally, and this has major effect on contaminant plume development. The solute transport advection-dispersion equation for a conservative tracer was then solved, subject to the time-varying flow field that has been generated above, using the same finite element grid. Several potential source zones identified from the resistivity survey were included in the model as trial input locations for a conservative solute tracer. Rather than introduce solute continuously, a pulse was used that was released at the beginning ofthe simulated period (January 1998) and decayed with a half-life of approximately three months, followed by a second similar pulse released during the winter

5 Groundwater flow from disused landfills in the UK Chalk 557 Transient regional gradient model - Pathlines Fig. 4 Pathlines traced from two potential source zones in the landfill showing the effect of the transient regional flow field on contaminant migration. Fig. 5 Contoured solute concentrations after 500-days of model simulation of transport in the transient regional groundwater flow regime from multiple pulsed source zones, together with three modelled borehole response plots. of Figure 5 shows a contour plot ofthe solute concentrations after 500 days of simulation. It can be seen that a complex plume has developed containing several regions of high solute concentration, moving away from the site in a generally northward direction.

6 558 G. M. Williams et al. It is of interest to see how monitoring boreholes might register the migration of these plumes. Figure 5 also shows histories of concentration against time taken from the model at the location of three of the boreholes located along the northwestern boundary of the site. It can be seen that the three locations register widely varying responses, with different shapes of curves and different breakthrough times. These complex, multi-peak breakthrough curves have been developed as a result of the interaction of the flow field variations with the pulsed and spatially dispersed input zones. It may also be noted that some of the peaks in these curves are quite sharp which implies that it will be necessary to sample monitoring boreholes relatively frequently if peak concentrations are to be captured. CONCLUSIONS The study illustrates that the variable flow directions expected in high transmissivity fractured aquifers can confound the detection of pollution plumes. Even when a landfill covers a wide area, leachate release may be quite localized and difficult to find. 3-D resistivity tomography has proved a particularly useful technique in defining leachate within the landfill, and transient flow modelling demonstrates that groundwater quality variations in a borehole can be quite complex due to varying direction and magnitude of groundwater flow. The study highlights the need for a wide network of boreholes in which to determine groundwater flow directions and the requirement for high accuracy topographic surveying to avoid artefacts in the inferred hydraulic gradient. Water levels and water quality also need to be monitored at a high frequency to enable the flow direction and contaminant breakthrough curves to be accurately captured. Acknowledgements This work was funded jointly by the Environment Agency and the British Geological Survey and is published by permission of the Director of the British Geological Survey (Natural Environment Research Council). REFERENCES De Groot-Hedlin, C. & Constable, S. (1990) Occam's inversion to generate smooth, two-dimensional models from magnetolelluric data. Geophysics 55, Klinck, B. A., Ogilvy, R. D. & Boland, M. P. (1998) Landfill characterisation using aerial photography and geophysical techniques. European J. Environ. Engng Geophysics 3, LaBrecquc, D..1., Miletto, M., Daily, W., Ramirez, A. & Owen, L. (1996) The effects of noise on Occam's inversion of resistivity tomography data. Geophysics 61, National Rivers Authority ( 1995) the effects of old landfill sites on groundwater quality. R&D Report 569/3/A. Ogilvy, R. D., Meldrum, P. I. & Chambers, J. E. (1999) Imaging of industrial waste deposits and buried quarry geometry by 3-D resistivity tomography. European J. Environ. Engng Geophysics 3, Tester, D. J. & Marker, R. J. (1982) Groundwater pollution investigations in the Great Ouse basin: solid waste disposal. Water Pollution Control, Williams, G. M., Boland. M. P., Higgo, W., Ogilvy, R. D., Klinck, B. A., Weallhall, G. P., Noy, D. J Trick,.1. K Davis,.1. A., Williams,!,. A.. Leader, R. U. & Hart, P. A. (2000) Effect of old landfills on groundwater quality Phase 2 Investigation ofthe Thriplow landfill Environment Agency R&D Technical Report P20I.