Use of a groundwater flow model to explain strong upward gradients in rock formation underneath the Mercier site

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NOTES Use of a groundwater flow model to explain strong upward gradients in rock formation underneath the Mercier site DENIS ISABEL, PIERRE GELINAS, AND JACQUES LOCAT Groupe de recherche en giologie

1 NOTES Use of a groundwater flow model to explain strong upward gradients in rock formation underneath the Mercier site DENIS ISABEL, PIERRE GELINAS, AND JACQUES LOCAT Groupe de recherche en giologie de I'inginieur, Dipartement de GPologie, Universite Laval, Sainte-Foy, Que., Canada GIK 7P4 Received October 18, 1990 Accepted April 7, 1992 The groundwater pollution case at Mercier is a very complex one. Groundwater flow modeling has been a valuable tool in the assessment of this large environmental problem. However, due to the complexity of the hydrogeological setting, the modeling has been performed with various simple case models in lieu of a large complex model. Here we report the results of one of these piecewise modeling tasks that proved very useful in the explanation of the strong upward gradients observed in the bedrock aquifer. These results and their interpretation prove the usefulness of the piecewise modeling strategy in this case. Key words: ground water modeling, finite elements. Le cas de pollution de l'eau souterraine a Mercier est tres complexe. La modelisation de 1'Ccoulement de l'eau souterraine a CtC un outil valable pour proceder a 1'Cvaluation de cet important problkme environnemental. Cependant, a cause de la complexite du cadre hydrogeologique, la modclisation a ete realisee avec des modeles de divers cas simples au lieu d'un grand modkle complexe. Dans cet article, I'on presente les resultats d'une de ces t2ches de modelisation fragmentee qui s'est revelie trb utile pour expliquer les forts gradients ascendants observes dans I'acquifere du lit rocheux. Ces rcsultats et leur interpretation prouvent I'utilitC de la strategic de modelisation fragmentee dans ce cas. Mots clis : modelisation de I'eau souterraine, elements finis. [Traduit par la redaction] Can. Geotech. J. 29, (1992) 1. Introduction Numerical groundwater flow and pollution modeling is now a common tool in the assessment of environmental problems (Bear and Verruijt 1987). In fact, the recent years of model development produced a very large number of groundwater flow and pollution models, and research is still going on in this field as pollutant-soil-water systems are better known and as more powerful numerical techniques are made available. We report here the use of a simple groundwater flow model to help understand one of the numerous problems encountered during the assessment of the decontamination process at the Mercier site. Modeling can be a valuable tool for the understanding of the past and future history of groundwater and pollutant flow. Due to the complexity of this site's geology and the source's composition, the problem has to be divided into smaller pieces, each one being manageable by simpler models. Each of these smaller modeling problems can thus give valuable knowledge which adds to the detailed evaluation of the effectiveness of the restoration process at Mercier. But, prior to choosing and using these models, we have to collect data on the hydrogeologic properties of geological units and on the observed behavior of groundwater and pollutants at the Mercier site. 2. The Mercier case The Mercier site is located a few kilometres south of the town of Mercier on the south shore of the St Lawrence River in the Montreal metropolitan area. From 1968 to 1972, an Printed in Canada / lmprime au Canada estimated quantity of between and m3 of liquid organic wastes was dumped without control into two abandoned sand and gravel pits. The location of the site is illustrated in Fig Hydrogeological properties of geological units The Quaternary geology of the Mercier area is very complex. For the purpose of groundwater modeling, we will recognize five different units, namely, the underlying bedrock, the discontinuous till layer, the central sand and gravel unit, the lateral silty sand units, and the overlying silty clay unit. Figure 2 shows the relative positions and connections between the units as they appear on the surface, whereas Fig. 3 shows their idealized hydrogeologic setting in a section perpendicular to the sand and gravel ridge. The following data have been extracted from previously published reports (Foratek International Inc. 1982; HydrogCo Canada Inc. 1981; Martel 1988; Poulin 1977) and, where possible, are related to our own field data Bedrock hydrogeology The rock formation underlying the Quaternary deposits of the area is recognized as a very productive aquifer. Many large-capacity wells are drawing water from it, some less than 10 km from the contamination site. Reported hydraulic conductivities are erratic within this unit. It is believed that the higher hydraulic conductivity observed locally results from the occurrence of more densely fractured zones. It is also reported that the top 3 m of bedrock is often more intensely fractured and thus shows a higher hydraulic conductivity.

2 LEGEND Sand and gravel M Clay Ti L FIG. 2. Surficial Quaternary geology of the Mercier area. FIG. 1. Location of the Mercier site. The average deep bedrock hydraulic conductivity is in the range of 10-~-10-' m/s, with some local maxima of m/s and a matrix conductivity of lo-'' m/s. The storativity of this unit is low, in the range of lop4. Our own data, from packer tests performed by Environment Canada (Lapcevic 1988) in the seven rock holes prior to the installation of permanent sampling devices, show that the small-scale hydraulic conductivity of rock ranges from to lo-" m/s. From examination of these results we found that variations between the mean values of each hole are as important as variations between various depths within a single hole. This latter observation is in accordance with the already accounted existence of a large lateral variability of the hydraulic conductivity of the bedrock. We must stress the fact that our data set does not establish the existence of a more fractured and conductive top layer in the rock unit at the contaminated site (GClinas et al. 1988). Unfortunately, the construction of the rock holes does not allow proper testing of the top portion of the rock unit. The protective metal casing has been lowered a short distance into the rock formation and cemented in place. The extent of this cement plug is believed to seal some of the top fractures near the hole. Therefore, packer-test results in the top part of rock holes cannot be taken as truly representative data, and testing began below the first 1.5 m of rock. A detailed inspection of the rock cores showed the existence of some open horizontal fractures (GClinas et al. 1989). This can produce a very large anisotropy in the gross hydraulic conductivity of this unit Till layer hydrogeology The bedrock is overlain by a till layer in an erratic pattern. This till is very dense and hard and should not have a large porosity. Hydraulic conductivity ranges from to lo-' m/s in this unit. Depth and extent of this unit are not well known, but it can play a significant role where present because of its location between two high-conductivity units. Its proper modeling is thus a major challenge in this site investigation Hydrogeology of the sand and gravel unit The sand and gravel unit is a somewhat heterogeneous linear structure emerging from the clay plain of the Mercier area. Its geological origin, the accumulation of sediments at the submerged edge of the ice shelf, helps to explain its heterogeneous character, a common feature of glaciomarine deposits. The reported hydraulic conductivities of this unit range from to lop5 m/s as evaluated from pumping tests. It constitutes a very good aquifer despite its limited lateral extent. The small-scale hydraulic conductivities of this unit are most likely to be much variable, since soil samples from our monitoring wells have shown a quite variable granulometric content. In some places, we encountered layers of loose gravel arranged in an open packing which caused problems of excessive loss of drilling fluid and filter sand. In previous studies, this unit also showed a large storativity, in the range of This storativity is reported to drop between 0.03 and 0.06 where the unit is under partially confined conditions. This large storativity, in a free-surface aquifer, implies that the effective porosity is quite large. This has many consequences on the velocity of the groundwater flow and on transport of droplets and lenses of complex organic nonaqueous phase liquid mixtures.

CAN. GEOTECH. J. VOL. 29, 1992 Sand & gravel FIG. 3. Idealization of a perpendicular cut through the sand and gravel ridge. Note that the vertical exaggeration is approximately 20. 2.2.4.

3 CAN. GEOTECH. J. VOL. 29, 1992 Sand & gravel FIG. 3. Idealization of a perpendicular cut through the sand and gravel ridge. Note that the vertical exaggeration is approximately Hydrogeology of the silty sand unit The silty sand unit covers each side of the sand and gravel ridge. It can be followed as far as 700 m from the ridge's centreline, under the silty clay unit. The reported hydraulic conductivities range from to m/s Hydrogeology of the silty clay unit The silty clay unit covers most of the area, excluding the top of the sand and gravel ridge. This marine clay has a hydraulic conductivity of 10-~-10-'~ m/s as measured from consolidation tests in previous studies. It acts as an aquiclude formation but can also contribute slightly to the groundwater recharge of the underlying units Observed hydrogeologic behavior The pattern of groundwater flow at the Mercier site has changed many times because of the inconsistent operation of the groundwater treatment plant. A strong downward gradient of 0.3 was reported in the silty clay unit, suggesting the slow replenishment of underlying draining units like the silty sand and top fractured bedrock units. Also, in the bedrock formation under the sand and gravel ridge, an upward gradient was noticed. The top part of the bedrock unit was at the same potential as the sand and gravel unit while the potential of the deep bedrock was 3-5 m higher. Prior to the plant's operation, water moved toward the Sainte Martine area as evidenced from the rock piezometric map (Poulin 1977). The original regional groundwater flow system was governed by the widespread presence of the bedrock leaky aquifer that was slowly replenished by the overlying silty clay unit. The sand and gravel ridge acted as a draining mechanism preventing groundwater from flowing directly toward the Chiiteauguay River where the bedrock outcrops. In fact, the ridge redirected the groundwater flow toward the Sainte Martine area. Large-capacity water wells and locally higher hydraulic conductivities of bedrock can also be held partly responsible for this diversion phenomenon. Moreover, a possible horizontal anisotropy in the rock's gross hydraulic conductivity, which can be produced by the occurrence of a family of parallel vertical open fractures, can also contribute to the same diversion phenomenon. This latter hypothesis is not yet verified but deserves serious consideration. Since the beginning of the cleanup operations, the regional groundwater flow pattern is quite unchanged. But, in the vicinity of the contaminated site and extraction wells, a hydraulic trap is created by the sustained local drawdown of the sand and gravel aquifer. This trap is effective on a large scale, since in the ridge axis, the hydraulic potential gradient is reversed up to 1.5 km downstream from the wells. We should note that the pumping rate of the groundwater treatment plant is not uniform. Shutdowns and fluctuations of the pumping rate are frequent and induce some oscillation of the groundwater level in the sand and gravel aquifer at the site. We monitored the effect of a 4-day shutdown of the plant on the water levels in our network of observation wells. Despite the large pumping rate (60 L/s), stopping and starting of the pumps did not result in large water-level fluctuations. At 60 m from the pumping wells, a maximum recovery of 50 cm was observed in the sand and gravel unit, whereas no change in level was noticed at 300 m. A tentative interpretation of this recovery and drawdown data with the classic Theis method yielded a hydraulic conductivity of approximately m/s. This figure is an approximate one, given the fact that pumping was resumed at an unstable rate, but agrees with the previously stated range of values observed in the sand and gravel unit. The storativity cannot be safely evaluated from this data, but the small extent of the recovery can only be accounted for by a large storativity, which also agrees with previous data.

4 FIG. 4. Finite-element mesh used to model the steady groundwater flow in the domain. The bedrock unit did not show a significant response to this pumping shutdown. Only the top part of the rock formation followed, with some attenuation, the trend of the overlying units. The deeper part of the bedrock showed no response at all, and the previously observed upward gradient stayed mostly unchanged. From a modeling point of view, the Mercier area case is thus a very complex one. As already stated, full-scale modeling of the whole system represents a tremendous task and would not yield many more benefits compared with the use of many smaller models to assess subsets of this large contamination problem. This is particularly true given the relative scarcity of available field data in comparison with the scale of the problem. 3. Modeling of the local upward gradient in the bedrock unit Although modeling is not a major objective of the whole Mercier study, it is, however, a major tool to be used in the fulfillment of other objectives. Various models help us to understand groundwater and pollutant behavior, and models are also of much help for the assessment of the decontamination process. One of the most puzzling hydrogeological observations made at the Mercier site is the occurrence of a quite strong upward gradient in the bedrock unit. A difference of more than 2 m is observed between piezometers at the rock surface and piezometers 10 m deep into the rock unit. Although this is not so surprising since the ridge area is known to be a regional groundwater discharge area and is experiencing vigorous pumping, it is nevertheless very difficult to explain why the drop in hydraulic head is so concentrated in the top part of the bedrock Model used To search for an explanation of this observation, we used a finite-element model of our own, applied to the modeling of a vertical cross section of the ridge system. This finiteelement model is used to resolve the Laplace equation in a two-dimensional domain: [I] div ( - [K] grad H) = 0 where div is divergence operator, [K] is hydraulic conductivity tensor (in metres per second), grad is gradient operator, and His hydraulic potential (in metres). The two-dimensional Laplace equation can be used to model the steady-state spatial distribution of hydraulic potential in a vertical slice of saturated porous media. The groundwater flow field can be obtained from this distribution of potential by a direct application of Darcy's law: [2] < q > = - [K] grad H where <q> is groundwater flow vector (in metres per second). We used the Galerkin weighting method together with a network of first-order triangular elements to numerically resolve the Laplace equation of steady-state groundwater

5 700 CAN. GEOTECH. J. VOL. 29, 1992 TABLE 1. Properties of hydrogeologic units I Bulk hydraulic Range of elements Unit conductivity (m/s) numbers in Fig. 4 Bedrock 1-39 Fractured rock '-1 10 Till layer lo-' a, Sand and gravel lo Send & Gravei Silty sand 10 -' T~ll -- ij Silty clay lo-' Fractured rock ij Bedrock g 100- ij Sand & Gravel Till Hydrau'c poler: iiol (rn) Fractured rock Bedrock FIG. 6. Vertical distribution of hydraulic potential (effect of till vertical conductivity KV) I I I I 1 I I I r I Hydraulic poienlia (rn) FIG. 5. Vertical distribution of hydraulic potential (first calculation). flow in a two-dimensional domain with known boundary conditions. Dimensions used for the modeled domain did not exactly correspond to the real shape of any given section of the glaciomarine deposits, since we worked on a schematic representation of this complex hydrogeological system. We also used the symmetry of the system to simplify our finiteelement network. The groundwater flow is radial around the three purging wells, and this cannot be taken into account in a two-dimensional vertical model. But we managed this difficulty by replacing whose wells by a fictitious drainage system with a hydraulic potential equivalent to the mean pumping level of the wells. Finally, the effect of the special top boundary condition introduced by the water table drawdown around the wells was first evaluated with an iterative correction of the position of this boundary. We found that this correction had a negligible effect on the distribution of hydraulic potential at depth. The effect of the water table drawdown was thus neglected, and the top impervious boundary was assigned an arbitrary position. A sketch of the resulting network is shown in Fig. 4. It includes 65 nodes and 102 triangular elements. Six different groups of elemental properties are used to represent the various hydrogeological units previously described. Table 1, when used in conjunction with Fig. 4, explains the position and properties of those units. No flow boundaries were set on upper and lower limits of the domain, whereas constanthead boundary conditions were set on right and left limits. The modeled domain represents only the right half of the ridge system because of the symmetric construction of the idealized problem Modeling results When we first tried to model the local flow field, we did not observe the upward gradient in our calculations. Obviously, part of our characterization of the hydrogeological properties of the system was wrong. This led us to a series of trial and error calculations which resulted in a better understanding of the local hydrogeology. Our first calculations were based on previously known hydrogeological data as presented earlier. These first calculations used the properties presented in Table 1. Figure 5 shows some of the results obtained with these parameter values. This vertical distribution of hydraulic head is taken from a vertical line 100 m distant from the pumping centre. The absence of a strong upward gradient in these results was a source of much speculation. We first tested the effect of a more or less discontinuous till layer on this vertical potential distribution. This hypothesis was modeled with various vertical hydraulic conductivity values. The results of this series of calculations are summarized in Fig. 6. A vertical hydraulic conductivity of about lo-' m/s is needed to obtain, from calculations, a vertical hydraulic gradient as strong as the one observed in the field. Is it a realistic value? To explain this conductivity, a discontinuous till layer with a conductivity of lop7 m/s in a formation having a conductivity of m/s has to overlay about 90% of the area. This seems a reasonable figure, since the till layer was encountered in many of our drilling sites. Further speculations came from the fact that the top fractured rock subunit was not evidenced from our own drilling logs. What was the influence of this hypothesized fractured layer on our previous calculations? We repeated those calculations with the same bulk hydraulic conductivity of

6 NOTES I 1 Sand & Gravel 1 Till FIG. 7. Vertical distribution of hydraulic potential (without the fractured rock layer). m/s for the entire bedrock unit. These results are presented in Fig. 7. From these, it seems that a fractured rock layer is not essential to the occurrence of the strong vertical gradient. Our modeling work thus does not substantiate the existence of this layer. A last subject of speculation was the effect of an expected large anisotropy in the hydraulic conductivity of the bedrock unit. Calculations were once again repeated with a vertical conductivity of m/s and a horizontal conductivity of 10-~-10-* m/s in the bedrock unit. The modeled asymmetry of the conductivity tensor did not change the calculated vertical gradient. Thus, the modeling does not substantiate the existence of this hypothesized anisotropy either. With this insight from the modeling results, we now recall that in a heterogeneous system, the flow energy will be dissipated mainly in the most resistive formations. So, if the gradient is well established in the bedrock unit, then this unit must be less permeable than the other units. Our series of simulations agrees with this explanation because they result in a simulated till layer with such high hydraulic conductivity that we must conclude that it is discontinuous. 4. Conclusions Our modeling of the groundwater flow in a transect of the Mercier system leads us to the conclusion that the occur- rence of the observed string upward,hydraulic gradient depends on the discontinuous occurrence of the observed till layer that covers about 90% of the area. The hypothesized existence of a fractured rock layer and also the hypothesized occurrence of an anisotropy in the hydraulic conductivity of the bedrock proved to have no consequence for the upward gradient. Therefore, these assumptions cannot be rejected or accepted on the basis of the presented calculations. This modest modeling experiment did not give us a complete understanding of the groundwater flow at the Mercier site. But, despite its limited scope, it allowed the proper valorization of field data by pointing out the fact that the observed upward gradient was caused by the occurrence of a discontinuous till layer. This till layer was identified in some borehole logs, but the modeling experiment revealed its significance. This discontinuous till layer limits the exchange of water and pollutants between the sand and gravel water-table aquifer and the rock-confined aquifer. This finding will have to be taken into account by any pollution-remediation design. This case of numerical groundwater model use proves that one does not need a very complex model to achieve useful modeling results when coping with complex field data. Bear, J., and Verruijt, A Modeling groundwater flow and pollution. D. Reidel Publishing Company, Dordrecht, The Netherlands. Foratek, International Inc ~tude hydrogeologique de faisabilite du captage des eaux contaminees de la nappe aquifere de Ville Mercier. Report 514, Foratek International Inc., Montreal. Gelinas, P., Isabel, D., Locat, J., et al Aquifer decontamination for toxic organics: the case of Mercier, Quebec. Groupe de recherche en geologie de l'ingenieur, DCpartement de geologie, UniversitC Laval, report GGL HydrogCo Canada Inc HydrogCologie et contamination des eaux souterraines, Ville Mercier. Ministere de 1'Environnement du QuCbec. Lapcevic, P.A Results of borehole packer tests at the Ville Mercier groundwater treatment site. National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ont. Martel, R Groundwater contamination by organic compounds in Ville Mercier: new developments. Proceedings, NATO/CCMS pilot study of remedial action and technologies for contaminated land and groundwater. November 7-11, Bilthoven, The Netherlands. Poulin, M Groundwater contamination near a liquid waste lagoon, Ville Mercier, Quebec. M.Sc. thesis, Department of Earth Sciences, University of Waterloo, Waterloo, Ont.

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