Groundwater flow modelling for old landfill with

Annals of Warsaw Agricultural University – SGGW Land Reclamation No 35a, 2004: 229-236 (Ann. Warsaw Agricul. Univ. – SGGW, Land Reclam. 35a, 2004)

Groundwater flow modelling for old landfill with vertical barrier EDWARD WIENCŁAW EUGENIUSZ KODA Department of Geotechnical Engineering, Warsaw Agricultural University – SGGW

Abstract: Groundwater flow modelling for old landfill with vertical barrier. The paper presents the numerical modelling of groundwater flow for the old sanitary landfill surrounded by the vertical bentonite barrier. FEMWATER numerical program was used for the flow solution. The modelling was aimed at the assessment of the vertical barrier effectiveness of groundwater protection in landfill surroundings area. These calculations were performed for Radiowo sanitary landfill located nearby Warsaw. This object has been existing since 1962, and the permission for its exploitation is valid to the end of the year 2005. Remedial works of the landfill has been conducted since 1998 and they includes: the vertical bentonite barrier, the leachates drainage system, the leachate re-circulation system, the mineral cover, the degassing system as well as the regulation of water relations in surroundings. The local monitoring program includes chemical analyses of leachates, surface and groundwater as well as groundwater level observations. The results of the groundwater monitoring were used for verification and tare of the numerical model. Influence of the vertical barrier on the groundwater flow is also analysed. Key words: sanitary landfill, modelling, bentonite walls

groundwater

INTRODUCTION Old sanitary landfills were often founded in places, which could not be used for other purposes, for example in wetlands or at the sites of disused quarries (high groundwater level and

permeable subsoil). Particular attention, during remedial works on old landfills, must be paid to protection against groundwater pollution by leachate (Koda 1999). This is connected with the design of cut-off wall barriers, supplemented by peripheral drainage and leachate re-circulation system. Development of a proper cut-off wall barrier should be proceeded by the numerical analysis of groundwater flow (Koda, 1994) and the monitoring design for the protection effectiveness control. Numerical modelling with the use of FEMWATER program are often applied for hydroengineering solutions (Kamiński et al., 1998) and to solve transport of pollutants problems (Wiencław et al., 1999, 2000). The numerical modelling of the groundwater flow for Radiowo landfill (Fig. 1) is analysed. The cut-off wall bentonite barrier was constructed there for groundwater protection on surroundings (Koda and Stępień, 2001). The numerical modeling was performed both for subsoil without and with the vertical barrier. The analysis was used for the effectiveness assessment of this solution. Additionally, the analysis of groundwater monitoring results allowed to estimate the effectiveness of the protection system (Golimowski and Koda, 2001).

230 E. Wiencław, E. Koda

Stream

L A N D FI L L

Lipowska Woda stream

Cut-off wall Border of model 100 m

FIG.1. Map of territorial scope of the model with its numerical mesh

Assessment of remedial works effectiveness on water quality ... 231

SITE CHARACTERISTICS Radiowo landfill is localized in the north-western part of Warsaw. It was established in 1962 and any protection system was installed there. Radiowo localisation is presented in Fig. 1. Mixed municipal solid wastes were disposed there up to 1991. Since 1992 only non-composted wastes, i.e.: glass, plastics, textiles, scrap and metals, have been stored there. It covers the area of approximately 15 ha and it is almost 60 m high. Since 1998 remedial works have been carried out on the landfill. They include among others: forming and planting of the slopes, stability reinforcement solution (lateral reinforcements and berms), mineral capping, a bentonite cut-off wall and peripheral leachate drainage limited of groundwater pollution. It is expected that the landfill will be closed in 2005. The landfill subsoil consists of sandy soils, the thickness of 2-5 m, locally to the depth of 20 m. In the upper part there are dense sands, in the deeper part there are well graded sands (from dense to coarse). This layer forms the first groundwater level with the groundwater table at the depth of 0-2 m b.s.l. The water supply of this layer mainly results from infiltration of precipitation and the water inflow from the forest area located in the south-eastern part. Dewatering trenches from the northeastern and western part as well as Lipkowska Woda stream (from the north) compose the local drainage of the first groundwater level. Leachate from the landfill and rain water from the compostory area are pumped to the landfill slopes (re-circulation system).

The vertical bentonite barrier surrounding the landfill was constructed in the years 1999-2001. The barrier of the width of 0.6 m, was performed at the depth of 2 m below the clayey soils roof, i.e. 3.5-22.0 m below the surface level. This aquitard layer consists of boulder and Tertiary clays. THE MODEL OF GROUNDWATER FLOW IN SURROUNDING AREA The numerical model of the groundwater flow was prepared with the use of GMS/FEMWATER software (GMS 2000; Lin et al., 2000). The basis of FEMWATER flow model is the three-dimensional solution of the task of groundwater flow, described by Richards equations. The solution of numerical model is based on the scheme of the finite element method. The numerical modelling described in the paper was aimed at the assessment of the vertical barrier influence on hydrogeological conditions on Radiowo landfill area. Richards differential equation, used in FEMWATER program, to describe groundwater flow is as follows:

k r k s h  z   q  F

H t

(1)

where: kr – relative hydraulic conductivity, ks – intrinsic conductivity tensor of saturation zone, d - differential water capacity F

dh

h – water pressure head, q – internal source/sink discharge, t – time, θ – volume water capacity.


Generally, it can be assumed, that: F, θ and kr are the functions depending on h, while in the saturation zone the function F is very small (approach zero in FEMWATER), θ is equal soil porosity and kr=1. Fig.2 presents the generated model mesh (vertical/horizontal = 10/1), while assigned boundary conditions for the task is shown in Fig. 3. The analysed technical solution for the model simulation consists of the protection system concerning the vertical barrier to stop leachate outflow from the landfill. The model mesh consists of 8903 elements and 5289 nodes. The total area covered by model is approximately 88 ha, including 16 ha of the landfill area. In the numerical model, taking into account geological deposits in subsoil and hydrogeological conditions in landfill surroundings (Koda, 1999), four landfill subsoil materials were distinguish. Beginning from the older

(deep deposited) to younger (occurred directly below surface level), these are: - Tertiary clays (I), with coefficient of hydraulic conductivity ks=5x10-9m/s, - sandy clays (Gp), with coefficient of hydraulic conductivity ks=1x10-7m/s, - variety grained sands (Pd//Ps) , with coefficient of hydraulic conductivity ks=5x10-4m/s, - dense sands (Pd), with coefficient of hydraulic conductivity ks=5x10-5m/s. The parameter values of unsaturated zone, demanded in the numerical model, for sands were accepted on the basis of the literature data (Carsel and Parrish, 1988). They were, as follows: - differential water capacity, F=0.0725, - volume water capacity θ, reached from θ=0.045 for h=-4m to θ=0.36 for h=0, - relative hydraulic conductivity, reached from kr=0 for h=-4m to kr=1 for h=0.

Bentonite barrier

FIG.2. The 3-D GMS numerical mesh for Radiowo landfill.

Groundwater modelling for old landfill with vertical barrier233

It was assumed, that parameter values of bentonite material in the unsaturated zone are similar as for clays. These parameters as follows: H = 17,0-17,3 m - F=0.00725, - θ, reached from θ=0.07 for h=-4m to H = 17,0-23,2 m θ=0.36 for h=0, H = 17,3-23,4 m - kr, reached from kr=0.0885 for h=4m to kr=1 for h=0. Leachate re-circulation area On the situation map, the model q = 3.17e m/s outside borders is overlapped with surface streams on the landfill surroundings. These streams were shaped in the numerical model as assigned in Dirichlet’s boundary conditions. The constant hydraulic gradient equals to the water table level in these streams, reached from 17 (in the northern part of area) to 23.45 m a.s.l. (in the southern part of area). Neuman’s boundary conditions q = 1.58e m/s (qN=3.17x10-8m/s) were assigned in the H = 23,2-23,4 m model for the part of the landfill area where the leachate and rain water from FIG. 3. Boundary conditions for the the compostory plant are pumped (recirculation system). In the case of the numerical model of Radiowo landfill. intensive precipitation, Neuman’s boundary condition was also taken into Municipal wastes, with hydraulic -5 consideration in the model, on the basis conductivity coefficient ks=1x10 m/s, on pumping stations, i.e. on the landfill are placed in the model centre as well as -10 slopes of q =3.17x10 m/s, while on N in the upper part of model. It is the surroundings area it was assumed, that parameter values of -9 wastes in the unsaturated zone can be qN=1.58x10 m/s. Initial conditions the same as for sandy soils. These were assumed from preliminary calculations for the exemplary parameters were assumed as follows: projection, in which leachate pumping - F=0.00588, on the landfill surface is not provided. - θ, reached from θ=0.057 for h=-60m Numerical calculations were carried to θ=0.41 for h=0, out for the two following examples: - kr, reached from kr=0 for h=-60m to inflow of polluted water (rekr=1 for h=0. circulation of leachate and rain water Along the landfill border, there is from compostory area) on the the vertical bentonite barrier (cut-off landfill surface, where the vertical wall) with hydraulic conductivity -10 barrier has not been constructed yet, coefficient of ks=5x10 m/s. -8

-10

qn = 3.17e m/s

n

-8

n


- inflow of polluted water on the landfill surface, where vertical barrier has already been constructed. MODELLING RESULTS The results of the groundwater flow numerical simulation were worked out as the groundwater contour map (Figs. 4 and 5) and the maps of groundwater table level changes caused by the vertical barrier (Fig. 6). The shape of groundwater level and its changes, presented on the maps, are suitable for stable situation of groundwater flow conditions.

FIG. 5. The groundwater contour map for subsoil with the vertical barrier.

FIG. 4. The groundwater contour map for subsoil without the vertical barrier.

Due to the vertical barrier construction, the groundwater table level in the landfill area was also changed. On the landfill, the groundwater level will increase from approximately 0.25m in the southern part to more than 0.75m in the northwestern part of the landfill. On the surrounding area, the groundwater level may increase or decrease, depending on the zone of area (Fig. 6). The largest decrease of the groundwater level (ca. 1m) will take place in the northern part of the area, directly close to the vertical barrier. However, small increase of the groundwater level (ca. 0.2m), will occur in the part close to the south part of the

Groundwater modelling for old landfill with vertical barrier235

landfill. The vertical barrier composes the obstacle for groundwater flow from the southern to the northern direction.

results of the numerical modeling for Radiowo landfill, presented in the paper, proved the isolation role of the vertical bentonite barrier. When the vertical barrier wall was closed, the quality of the groundwater (the first layer) on Radiowo landfill surroundings has been essentially improved. Further reclamation attempts should mainly be concentrated on: finishing the shape forming of the landfill body and biological reclamation of its slopes and introduction of the degassing system. REFERENCES

FIG. 6. The contour map of the groundwater level changes caused by the vertical barrier.

CONCLUSIONS The influence of the vertical bentonite barrier on the groundwater flow on the landfill surroundings is observed in the frame of the local water quality monitoring. For the final assessment of the vertical barrier efficiency, long-term monitoring observations are necessary. The numerical modelling method is useful for investigation of the vertical barrier influence on the groundwater flow. The

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KODA E. 1999: Remediation of old municipal waste landfills. Proc of the 3rd International Conference on Waste Management. 335-355, Poznań. KODA E., STĘPIEŃ M. 2001: Zastosowanie pionowych przesłon przeciwfiltracyjnych wokół składowisk odpadów. Proc. of the 4th International Conference on Waste Management, Poznań-Piła, 333-344. LIN H.C.J., RICHARDS D.R., YEN G.T., CHENG J.R., CHENG H.P. and JONES N. 2000: FEMWATER – A ThreeDimensional Finite Element Computer Model for Simulating DensityDependent Flow and Transport in Variably Saturated Media. Version 3.0, Technical Report CHL. U.S. Army Engineer Waterways Experiment Station. Vicksburg, MS. WIENCŁAW E., ZAJĄC M., KAMIŃSKI R. 1999: Hydrogeological model of contaminant transport for sewage treatment plant at forest environment. ROK CXLIII SYLWAN, No 10, 21-26. WIENCŁAW E., ZIEMBIKIEWICZ M. 2000: Assessment of the Time of Transport of Pollutants trough the Poorly Permeable Layer. Annals of Warsaw Agricultural University - SGGW, Land Reclamation, No 31, 33-40.

Streszczenie: Modelowanie przepływu wód gruntowych w rejonie starego wysypiska z pionową przesłoną przeciwfiltracyjną. W artykule przedstawiono numeryczne modelowanie przepływu wód gruntowych w rejonie starego składowiska odpadów komunalnych odizolowanego pionową przesłoną

przeciwfiltracyjną. Do modelowania wykorzystano program numeryczny GMS/FEMWATER (USA). Celem modelowania była ocena efektywności pionowej przesłony przeciwfiltracyjnej w ochronie wód gruntowych na terenach przyległych do składowiska. Obliczenia przeprowadzono dla składowiska Radiowo k/Warszawy. Składowisko Radiowo powstało w 1962 roku, a jego zamknięcie planowane jest w 2005 roku. Od 1992 roku składowisko jest miejscem deponowania odpadów z kompostowni systemu Dano. Od 1994 roku na obiekcie prowadzone są prace rekultywacyjne obejmujące m.in. wykonanie: bentonitowej przesłony przeciwfiltracyjnej, systemu drenażowego odcieków, systemu recyrkulacji odcieków, kształtowanie skarp i dróg technologicznych, mineralnego przykrycia powierzchni, systemu odgazowania oraz modernizację rowów melioracyjnych na terenach przyległych. Monitoring lokalny wód obejmuje analizy fizyko-chemiczne odcieków, wód podziemnych i powierzchniowych oraz pomiary poziomu zwierciadła wód gruntowych. Wyniki obserwacji monitoringowych wykorzystano do weryfikacji i tarowania modelu numerycznego. W artykule przeanalizowano wpływ pionowej przesłony na warunki przepływu wód w rejonie składowiska.

MS. recived October 14, 2003 Autor’s adresses: Edward Wiencław Zakład Hydrogeologii, Katedra Geoinżynierii Wydział Inżynierii i Kształtowania Środowiska SGGW 02-787 Warszawa, 166 Nowoursynowska St. Poland Eugeniusz Koda Zakład Geotechniki, Katedra Geoinżynierii Wydział Inżynierii i Kształtowania Środowiska, SGGW 02–787 Warszawa, 166 Nowoursynowska St. Poland