KIRSTEN RÃGGE, J0RN K. PEDERSEN, BENT SKOV,. ANJA FOVERSKOV & THOMAS H. CHRISTENSEN. Institute of Environmental Science & Engineering!
Groundwater Quality: Remediation and Protection (Proceedings of the Prague Conference, May 19951. IAHS Publ. no. 225, 1995.
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Assessment of the spatial variability in leachate migration from an old landfill site
PETER KJELDSEN, POUL L. BJERG, PIA WINTHER, KIRSTEN RÙGGE, J0RN K. PEDERSEN, BENT SKOV, ANJA FOVERSKOV & THOMAS H. CHRISTENSEN Institute of Environmental Science & Engineering!Groundwater Research Centre, Technical University of Denmark, Building 115, DK-2800 Lyngby, Denmark
Abstract Investigations of the pollution of groundwater from old landfills have in most cases focused on delineating the pollution plume and only in very few cases on the landfill as a source to groundwater pollution. Landfills often cover large areas. Spatial variations in leachate composition may have great impact on the location of the main pollution plume in the downstream aquifer. Grindsted landfill in Denmark was investigated by sampling leachate beneath the landfill and in groundwater at the borders of the landfill. A pronounced variability in leachate quality and leakage patterns from the landfill was observed. Also variations in local groundwater flow directions were found. These observations are very important for delineation of the groundwater pollution and for proper choice of remedial action activities, related both to the plume and to the landfill.
INTRODUCTION Investigations of the pollution of groundwater from old landfills have in most cases focused on delineating the pollution plume, and have only in very few cases been dealing with the landfill as a source of groundwater pollution. Investigations of leachate composition at old and new landfills are numerous (cf. the review by Christensen et al., 1994). In many cases, the investigations have been based on only one or very few leachate samples from each landfill, and the representativeness of such few samples is difficult to evaluate. In order to evaluate the spatial variability of leachate composition, a high number of sampling points is needed. One of the few investigations on this subject concludes that pronounced irregular spatial variations are found (Assmuth, 1992). The observed spatial variations in leachate compositionmainly reflect differences in waste composition and infiltration of water through the top cover of the landfill. Landfills often cover large areas. Landfill sizes in the range of 10-20 ha are common. Pronounced spatial variations in leachate composition may have great impact on the location of the main pollution plume in the downstream aquifer. A better designation of locations where high strength leachate is generated may lead to a more cost-effective plume delineation. Remedial actions of groundwater pollution from landfills are often very costly, and prolonged pump-and-treat solutions have in many cases been the only alternative, since treatment of the high volumes of waste contained in landfills is not cost-effective.
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However, on landfills where large spatial variations in leachate composition have been observed, more differentiated remedial action solutions treating only hot spot areas, could be considered. Assessing the variability in leachate composition and leachate migration from old landfills needs an integrated approach. Historical information (including old maps, aerial photographs, interviews, etc.) creates a very valuable basis for understanding the variability (Kjeldsen, 1993; Rugge & Ahlert, 1992). Also information on the hydrology of the landfill and the adjacent part of the polluted aquifer is needed (Kjeldsen, 1993). The purpose of this paper is to assess the spatial variability in leachate migration from an old landfill by use of historical information, groundwater sampling and analysis, and hydrological investigations. Further details are given in Kjeldsen et al. (1995a, b). The investigation was carried out at Grindsted landfill which is located in Mid-Jutland, Denmark on a glacial outwash plain. The annual precipitation is of the order of 700-900 mm. The upper aquifer consists of 8-12 m sandy deposits with a lower boundary of Miocene clay. The water table is located 1-3 m below ground surface (outside the landfilled area). The resulting distribution of the pollutants in the leachate plume downstream the Grindsted landfill has been further investigated by Bjerg et al. (1994), Rùgge et al. (1994) and Holm et al. (1994).
MATERIALS AND METHODS The composition of the waste and the progress of the waste disposal through the years of landfilling were investigated by use of aerial photographs (from the years 1954, 1964, 1971, 1976 and 1980), and by interviews of former workers at the landfill. Three different types of wells were installed: piezometers, profile wells and leachate wells. Piezometers were installed for measuring groundwater table elevations. The location of the piezometers close to the landfill is shown in Fig. 1. Profile wells were installed for investigating the water quality in the groundwater as a function of depth and were placed along the downstream borders of the landfill. Further details on the profile well installation are given in Lyngkilde & Christensen (1992a). The locations of the 17 profile wells are shown in Fig. 1. Leachate wells were installed for investigating the spatial variability in water quality of the leachate (see Fig. 1). Grindsted landfill has no bottom liner and the waste is placed on the ground. Therefore, no saturated waste layer containing leachate exists. Instead the uppermost 35 cm of the underlying groundwater was taken as representative of the leachate quality at the specific location. Details of the wells and sampling system are given in Kjeldsen et al. (1995a). Investigation of spatial variability, based upon results from the leachate wells, assumes that the water sampled from the small screens is representative for the composition of leachate in a certain area surrounding the leachate well. In order to investigate the spatial variability of leachate parameters on a small scale, 13 leachate wells were placed within an area of 16 m2. The bottom of the screens were all placed exactly 35 cm below groundwater table. The small scale variability (SSV) field is located between leachate well L6 and L9 as shown in Fig. 1.
Spatial variability in leachate migration from an old landfill site
X Piezometer
• Profile well Iso potential line
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O Leachate well &.
Streamline
\ I—^—i
°
Fig. 1 (a) Location of leachate wells, profile wells, the piezometers close to the landfill, the small scale variability (SSV) field, groundwater elevations (observed on 4 March 1993) and streamlines, (b) location of the 13 wells in the SSV field.
The leachate wells served also as piezometers. Samples from the leachate wells and the profile wells were analyzed for basic inorganic parameters, non volatile organic carbon (NVOC) and aromatic hydrocarbons (benzene, toluene, ethylbenzene and xylene) by GC-analysis on pentane extractions. Standard methods for the analysis of the different parameters were used. Details on equipment, sampling and analytical methods are given in Lyngkilde & Christensen (1992a, b).
RESULTS AND DISCUSSION Waste disposal Grindsted landfill is placed on the original ground surface in an area with heath and scattered groups of trees. The landfill was started around 1930 and landfilling was stopped at the end of the year 1977. The final area of the landfill as reached in late 1977 is 10 ha. The total mass of landfilled waste is roughly estimated to 300 000 tonnes, with respectively 10, 65 and 25% of the total mass disposed in the periods 1930-1962, 19631973 and 1974-1977. The solid waste was deposited in a more random manner. In approximately the last 5 years of operation, the solid waste was deposited on top of older waste, especially in the eastern part of the landfill. By this operation, the eastern part (making up two-third of the total landfill area) ended up as a "dome-shaped" hill generally raising from the ground surface at 40 m a.m.s.l. at the northern, eastern and southern borders to a
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maximum height at 48.2 m a.m.s.l. The western part which has a height of 2.5 to 4 m, generally sloping in western direction. Leachate composition On the basis of 43 leachate samples taken from the 31 leachate wells, the landfill has been divided into four areas according to leachate strength. The division is primarily based on the indicator parameters chloride, ammonia and NVOC. Figure 2 shows the four areas: a strong leachate area (7% of landfill area), a medium leachate area surrounding the strong leachate area (10% of landfill area), a medium leachate area at the southern part of the landfill (20% of landfill area) and finally a weak leachate area covering the rest of the landfill (63%). The strong and medium leachate areas in the northern part closely coincide with the location of liquid waste lagoons as appearing on the old aerial photographs. The medium leachate area in the southern part coincides with the location where municipal solid waste was deposited in the last 2 years of operation (1976-1977).
I
1
1
0
50
100
1 1 150
200 m
Strong Leachate Medium Leachate (north) Medium Leachate (south) Weak Leachate
Fig. 2 Division of the landfill in four areas with different leachate strength.
In Fig. 3, the concentration contours for chloride in the SSV-field are shown. The figure shows that some variation in chloride concentrations occurs, but it is obvious that the variation is not totally random. The highest concentrations are found in the southern part closest to the strong leachate area (see Fig. 2). Similar results are found for NVOC and ammonia. In Table 1, a simple statistical evaluation of the variation observed is given. In the table the average values and the coefficients of variation (C V) for chloride and NVOC are shown for all samples pooled together, for specific areas (strong leachate and weak leachate areas) and for the SSV-field. The table shows that the variability (as described by the CVs) is lower in the small scale as compared to larger scales. The observed differences, which are the basis for the division of the landfill in four areas, are therefore not only a result of local small scale differences. A more detailed description of the composition of the leachate in the four areas is given in Kjeldsen et al. (1995a).
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A 150
/
\
V
N
250
WEE >300
0 1m Fig. 3 Concentration contours for chloride in the small scale variability (SSV) field.
Figure 4 shows the concentration contours for the five most frequently found aromatic hydrocarbons (BTEX) together with the sum of the five compounds (Total BTEX). The high concentrations of BTEX are generally found in the north eastern part of the landfill, where the disposal of solid and liquid industrial waste took place. Very large differences were observed between concentrations in this area and the weak leachate area, where the total BTEX concentration in most places is below 10 \x,g l"1, even below 1 fig l"1 in some areas. The highest observed total BTEX concentration is over 15 OOO /xg l"1. On average, a factor 100-1000 is observed between concentrations in the weak leachate area and the industrial waste disposal area. In medium leachate from the southern area only slightly elevated BTEX-concentrations are observed, Table 1 Statistical evaluation of spatial variability in leachate composition. The number of samples in the statistical evaluation is given for each area as an index number. Concentrations are in mg l"1. All samples52
Strong leachate13
Weak leachate20
SSV-field13
13-3200
200-3000
13-132
100-330
Average
470
1300
48
240
CV
1.7
0.4
0.6
0.3
16-2900
100-2900
16-86
38-66
Average
330
960
43
53
CV
1.6
0.8
0.5
0.3
Parameter Chloride Range
NVOC Range
a
Non-volatile-organic-carbon.
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Fig. 4 Concentration contours for the five aromatic hydrocarbons, benzene, toluene, ethylbenzene, m/p-xylen and o-xylene. Also contours for the sum of the five compounds are shown. The concentration ranges are given in jxg l"1.
probably reflecting the BTEX content in municipal solid waste, which is the major waste type disposed of in this area. Comparison of the different contour plots given in Fig. 4 shows that the proportional contribution of each hydrocarbon to the total content is very different from place to place. Toluene is the dominant compound in the southwestern corner of the industrial waste disposal area, while ethylbenzene is the dominant compound in the far northern part. This probably reflects differences in hydrocarbon content of the industrial waste types deposited in the area. This very significant variability in leachate strength, which is seen in the general leachate components and especially in the aromatic hydrocarbons, is important in order to understand the formation of the groundwater pollution plume. For planning of future remedial actions the obtained knowledge is also of great benefit, since a more differentiated remedial action plan directed mainly towards the hot spot area could be set up.
Groundwater flow The behaviour of the leachate pollution plume generated in the groundwater zone is governed by the variability in leachate concentrations and the groundwater flow directions. On a regional scale, the groundwater flow is directed to the northwest. The groundwater flow is, however, on a local scale, much more diverse. The streamlines
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based on isopotential lines for the groundwater table as measured in piezometers on 4 March 1993 (cf. Fig. 1) show that the flow direction in the northeastern part of the landfill area is headed north, and in the southeastern part to the west or southwest. Other measurements of water table elevations show that some seasonal differences in flow directions exist (cf. Kjeldsen et al, 1995b). At the eastern border a local water divide is present. This divides the groundwater flow from upstream to the north or the south of the landfill. The shape of the isopotential lines can be interpreted as the superposition of a local water table mound underlying the "dome-shaped" eastern part of the landfill and the "original" groundwater surface sloping to the northwest. The presence of water table mounds underlying landfills has often been reported in the literature (Van Duijvenbooden & Kooper, 1981; MacFarlane et al., 1983; Kjeldsen, 1993). The reason for the formation of the water table mound at Grindsted landfill has not yet been clearly identified. Observations, however, indicate that the hydraulic conductivity of the underlying aquifer general is lower than in the region around the landfill, and that the infiltration is higher in the border regions of the "dome-shaped" area. Both observations could be important factors in the formation of the groundwater table mound (see also Kjeldsen et al., 1995b). The observed diverging flow conditions generally lead to a wider spreading of the leachate than expected from the regional linear flow conditions. At the same time, no unpolluted upgradient groundwater is flowing under the landfill. This minimizes the dilution of the leachate plume.
Leachate migration patterns Figure 5 shows a three-dimensional sketch of contour lines for chloride along the northern and western borders of the landfill and inside the landfill. The flow directions in the groundwater beneath the landfill visualized as streamlines in Fig. 1, connect the leachate and the border concentrations. At the northern border good accordance is seen between the leachate concentrations and the border concentrations. The diverging flow conditions existing in the northern part of the landfill (cf. Fig. 1) result in a spreading of the leachate infiltrated from the hot spot (strong leachate area and the surrounding medium leachate area) throughout the entire northern border. The highest concentrations at the northern border are observed close to the bottom of the upper aquifer. This is probably due to vertical flow introduced by density differences between leachate and groundwater. Another possible explanation of the observed vertical flow of leachate may be locally high infiltration, that causes a vertical transport of leachate coming from upstream. At the western border, the picture is more complicated. In the southern end of the border, relatively high chloride concentrations are observed. These originate from the medium leachate area located in the southern part of the landfill. Very low BTEX concentrations in this part of the border (data not shown) are in accordance with the low BTEX concentrations observed in the southern part of the landfill. In the northern part of the border relative low chloride concentrations are observed with small spots of higher concentrations (>200 mg l"1). Analysis of the streamline locations in this area shows that the small spots of elevated concentrations may originate from leachate
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Fig. 5 Three-dimensional presentation of the chloride concentrations in leachate and in groundwater at the downstream landfill borders.
infiltrated in the southwest part of the hot spot area. However, no clear correlation between the BTEX concentrations in the hot spot area and at the northwestern border (data not shown) exists. Future detailed investigations will examine the groundwater flow behaviour in this region. In general, a fairly good accordance is obtained between leachate concentrations and border concentrations as predicted from streamline locations, considering that seasonal streamline variations exist.
CONCLUSIONS Significant spatial variability in leachate concentrations was observed. In approximately two-third of the landfill area very low concentrations for almost all parameters (including specific organic compounds) were found. A hot spot was found with concentrations of the inorganic components between 20-40 times higher than in the low concentration area. For the BTEX compounds even bigger differences were observed (between 100-1000 higher concentrations in the hot spot as compared to the low concentration area). The observed variability was partly explained from information on waste disposal operation as obtained from aerial photographs and interviews, which emphasizes the usefulness of historical data. Information on spatial variability in leachate concentrations is very important, especially for large landfills as basis for a delineation of the groundwater pollution plume and for choice of proper remedial actions.
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A groundwater surface mound at the eastern part of the landfill was observed. The mound played a significant role in the leachate migration. The main reason for the formation of the mound has not yet been found. By use of the streamlines, as determined from groundwater surface mapping, a relationship between leachate infiltration areas and leakage patterns at the borders of the landfill could be established. A fairly good correlation between concentrations in leachate and in groundwater at the landfill borders was obtained in this way. Pronounced vertical transport of leachate in the upper aquifer was observed, probably as a result of forces driven by differences in leachate and groundwater densities.
REFERENCES Assmuth, T. (',1992) Distribution and attenuation of hazardous substances in uncontrolled solid waste landfills. Waste Management & Research 10, 235-255. Christensen, T.H., Kjeldsen, P., Albrechtsen, H.-J., Heron, G., Nielsen, P.H., Bjerg, P.L. & Holm, P.E. (1994) Attenuation of landfill leachate pollutants in aquifers. Critic. Rev. in Environ. Sci. & Technol. 24, 119-202. Bjerg, P.L., Riigge, K., Pedersen, J.K. & Christensen, T.H. (1994) Distribution of redox sensitive groundwater quality parameters downgradient of a landfill (Grindsted, Denmark). Accepted for publication in Environ. Sci. & Technol. Holm, J. V., Riigge, K., Bjerg, P.L. & Christensen, T.H. (1994) Occurrence and distribution of pharmaceutical organic compounds in the groundwater downgradient of a landfill (Grindsted, Denmark). Accepted for publication in Environ. Sci. & Technol. Kjeldsen, P. (1993) Groundwater pollution source characterization of an old landfill. J. Hydrol. 142, 349-371. Kjeldsen, P., Winther, P. & Andersen, J.S. (1995a) Variability in leachate composition at an old municipal landfill (Grindsted, Denmark). In preparation. Kjeldsen, P., Bjerg, P.L., Riigge, K., Pedersen, J.K. & Christensen, T.H. (1995b) Variability in leachate migration from an old municipal landfill (Grindsted, Denmark). In preparation. Lyngkilde, J. & Christensen,T.H. (1992a) Redox zones of a landfill leachatepollutionpIumeCVejen, Denmark)./. Contam. Hydrol. 10, 273-289. Lyngkilde, J. & Christensen, T.H. (1992b) Fate of organic contaminants in the redox zones of a landfill leachate pollution plume (Vejen, Denmark). J. Contam. Hydrol. 10, 291-307. Macfarlane, D .S., Cherry, J.A., Gillham, R.W. & Sudicky, E.A. (1983) Migration of contaminants in groundwater at a landfill: a case study. 1. Groundwater flow and plume delineation./. Hydrol. 63, 1-29. Rugge, C D . & Ahlert, R.C. (1992) Ground and aerial survey of a peninsular landfill. Wat. Res. 26, 519-526. Riigge, K., Bjerg, P.L. & Christensen, T.H. (1994) Distribution of organic compounds from municipal solid waste in the groundwater downgradient of a landfill (Grindsted, Denmark). Accepted for publication in Environ. Sci. & Technol. Van Duijvenbooden.W. & Kooper, W.F. (1981) Effects on groundwater flow and groundwater quality of a waste disposal site in Noordwijk, The Netherlands. In: Quality of Groundwater (ed. by W. VanDuijvenbooden.P. Glasbergen&H. Van Lelyveld) (Proc. Int. Symp., Noordwijkerhout, The Netherlands, 23-27 March 1981). Elsevier, Amsterdam, 253-260.
