Numerical simulation of groundwater flow in regional rock aquifers, southwestern Quebec, Canada M. Nastev · A. Rivera · R. Lefebvre · R. Martel · M. Savard
Abstract The St. Lawrence Lowlands platform, Quebec, Canada, is a densely-populated area, heavily dependent on groundwater resources. In 1999, the Geological Survey of Canada initiated a large-scale hydrogeological assessment study over a 1,500 km2 region northwest of Montreal. The objectives were to define the regional groundwater flow, and to give quantitative estimates of the groundwater dynamic parameters and of the available groundwater resources. The applied approach consisted of defining the hydrogeologic framework, hydraulic properties of the aquifer units, and groundwater dynamic components. Lower Paleozoic sedimentary rocks represent regional aquifer units. Coarse Quaternary fluvio-glacial sediments locally overlay the rock sequence and constitute an interface aquifer unit. Fine marine sediments confine most of the regional aquifers. Collected GIS based information was synthesized in a finite element numerical model. The regional saturated steady-state flow was calibrated under current stress conditions assuming an equivalent porous medium approach. Water budget calculations show that the total groundwater flow in regional aquifers amounts to 97.7 Mm3 /y. Infiltration from precipitation provides 86.6% of the groundwater supply, while 9.6% comes from subsurface inflow and the remaining 3.8% is induced recharge from surface waters. Discharge from regional aquifers occurs through flow to streams (76.9%), groundwater withdrawal (18.4%), and underground outflow (4.7%). R´esum´e La plateforme Lowllands du Saint-Laurent, Qu´ebec, Canada, est une aire dens´ement peupl´ee, d´ependant grandement des ressources en eau souterraine. En 1999, le Service G´eologique du Canada a initi´e une e´ tude hydrog´eologique a` grande-´echelle sur 1500 km2 au Received: 24 April 2003 / Accepted: 18 January 2005 / Published online: 2 July 2005 C Springer-Verlag 2005
M. Nastev () · A. Rivera · M. Savard Natural Resources Canada, Geological Survey of Canada, 880 Ch. Sainte-Foy, suite 840, Quebec, QC, G1S 2L2, Canada e-mail:
[email protected] R. Lefebvre · R. Martel Institut National de Recherche Scientifique INRS-ETE, Quebec, QC, Canada Hydrogeology Journal (2005) 13: 835–848
Nord-Ouest de Montr´eal. Le sobjectifs ont e´ t´e de d´efinir la dynamique de l’´ecoulement r´egional, et de donner des estimations quantitatives des param`etres dynamiques des ressources disponibles en eau souterraine. L’approche utilis´ee consista a` d´efinir le cadre hydrog´eologique de travail, les propri´et´es hydrauliques des unit´es aquif`eres, et les composantes dynamiques des eaux souterraines. Les roches s´edimentaires du Pal´eozo¨ıque Inf´erieur repr´esentent les unit´es aquif`eres r´egionales. Les s´ediments marins fins confinent la plus grande partie des aquif`eres r´egionaux. Les informations de base, collect´ees dans un SIG, ont e´ t´e synth´etis´ees dans un mod`ele num´erique aux e´ l´ements finis. L’´ecoulement permanent r´egional, en zone satur´ee, a e´ t´e calibr´e en conditions de stress en assumant une approche de milieu poreu e´ quivalent. Les calculs du bilan hydrologique montrent que l’´ecoulement total des eaux souterraines dans les aquif`eres r´egionaux atteind 97,7 Mm3 /an. L’infiltration a` partir des pr´ecipitations apporte 86.6% de l’eau souterraine exploit´ee; sachant que 9,6% proviennent d’´ecoulement de subsurfaces, et que les 3,8% restants proviennent de la recharge via les eaux de surface. Le d´ebit pomp´e des aquif`eres r´egionaux apparaˆıt a` travers l’´ecoulement des cours d’eau, le rabattement des eaux souterraines (18.4%), et l’´ecoulement ascendant (4.7%). Resumen La plataforma de Tierras Bajas San Lorenzo, Quebec, Canad´a, es un a´ rea densamente poblada que depende fuertemente de recursos de agua subterr´anea. En 1999 el Servicio Geol´ogico de Canad´a inici´o un estudio de evaluaci´on hidrogeol´ogica a gran escala sobre un a´ rea de 1,500 km2 en la regi´on noroeste de Montreal. Los objetivos fueron definir el flujo regional de agua subterr´anea y aportar estimados cuantitativos de los par´ametros din´amicos de agua subterr´ane y de los recursos disponibles de agua subterr´anea. El enfoque aplicado consisti´o en definir el marco hidrogeol´ogico, propiedades hidr´aulicas de las unidades acu´ıferas, y los componentes din´amicos de agua subterr´anea. Rocas sedimentarias del Paleozoico Inferior representan unidades regionales de acu´ıferos. Sedimentos fluvio-glaciares Cuaternarios gruesos sobreyacen localmente la secuencia rocosa y constituyen una unidad acu´ıfera interfacial. Sedimentos marinos finos confinan la mayor´ıa de acu´ıferos regionales. Informaci´on colectada DOI 10.1007/s10040-005-0445-6
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basada en SIG se sintetiz´o en un modelo num´erico de elemento finito. El flujo regional saturado en r´egimen permanente se calibr´o bajo condiciones de stress asumiendo un enfoque de medio poroso equivalente. Los c´alculos de balance h´ıdrico muestran que el flujo total de agua subterr´anea en acu´ıferos regional alcanza 97.7 Mm3 /a˜no. Infiltraci´on a partir de lluvia aporta el 86.6% del abastecimiento al agua subterr´anea, mientras que el 9.6% proviene de entradas subsuperficiales y el restante 3.8% consiste de recarga inducida a partir de aguas superficiales. La descarga proveniente de acu´ıferos regionales ocurre a trav´es de flujo a r´ıos (76.9%), utilizaci´on de agua subterr´anea (18.4%), y salida subterr´anea (4.7%). Keywords Sedimentary rocks . Groundwater flow . Numerical modeling
Introduction The physiographic region of the central St. Lawrence Lowlands platform extends on both shores of the St. Lawrence River. Most of Quebec’s population and industrial activities are located within this region. Groundwater use for domestic, agricultural and industrial purposes is high, particularly in the area northwest of Montreal. Here, fractured aquifers serve as a major source of water for some 250,000 residents. In 1999, the Geological Survey of Canada (GSC) initiated a large-scale hydrogeological assessment study in this region. The regional aquifer system consists of a sedimentary sequence of Cambrian sandstone, dolomite, and limestone that is, in turn, underlain by Precambrian crystalline rocks (Globensky 1987). Occasional deposits of sand and gravel at the base of the Quaternary sequence also contribute to the regional transmissivity. Regional aquifers are confined in most of the study area by thick clay-rich deposits. The sedimentary rocks are characterized by relatively high permeability, and have long been known to provide large groundwater yields (Simard 1978; Simard and Des Rosiers 1980). In the last decade, however, discords have occurred among groundwater users for the rights to the groundwater resource, particularly in highly exploited areas. The lack of knowledge about the groundwater flow and of the availability of the resource precluded the formulation of suitable groundwater management plans. One of the major components of the GSC hydrogeological assessment study was the development of a reliable numerical model. The objectives were to define the regional groundwater flow and to give quantitative estimates of the groundwater dynamic parameters. One important aspect was the definition of the spatial distribution of coarse sediments overlying sedimentary rocks. This interface aquifer unit is a particular geological feature of the region (Simard 1978; Ageos and INRS-Eau 1998). The numerical model was also used for the estimation of the regional impacts of current and future groundwater use, which is beyond the scope of this paper. The first stage of the modeling study was the description of the hydrogeological framework. The definition of the hydraulic properties of the regional aquifer Hydrogeology Journal (2005) 13: 835–848
units was based on the field measurements and interpretations presented by Nastev et al. (2002). Regional recharge rates were estimated by Hamel (2002). Other groundwater dynamic components such as water level fluctuations, withdrawals, and groundwater inflow and outflow are discussed by Savard et al. (2002). The study proceeded with the development of the conceptual model of regional groundwater flow. The FEFLOW software—Finite element subsurface flow system (Dierch 1998a, b), was used to simulate the 3-dimensional saturated regional groundwater flow. The calibration of the model parameters was conducted under steady-state flow conditions. The numerical model was then used to simulate the groundwater flow under the current stress conditions. This manuscript is the second paper describing the hydrogeological components of the GSC investigation project. The first paper presents the hydraulic properties of regional aquifers (Nastev et al. 2004). The discussion presented herein is limited to the applied methodology, results, and conclusions of the numerical modeling analysis. Full details of the numerical modeling aspects are detailed in Nastev et al. (2002).
Study area The 1,500 km2 study area is located in south-western Quebec, north-west of Montreal (Fig. 1). Its natural limits are Rivi`ere du Nord and the Laurentian Plateau to the north, and Rivi`ere des Outaouais, Lac des Deux Montagnes and Rivi`ere des Mille ˆIles to the south. The drainage basin of the Rivi`ere Mascouche forms the eastern boundary. The southern limit is a segment of the St. Lawrence River and represents a regional discharge boundary. The current geomorphology is controlled by the lithology and structure of the bedrock and by the glacial movements and subsequent sedimentation under marine and fluvial conditions. Steep hillsides, relatively wide valleys and flat terraces characterize the topography of the region. The Laurentian Plateau dominates the northern portion of the study area. The Oka and Saint-Andr´e Hills are located to the south overlooking the Rivi`ere des Outaouais and the Lac des Deux Montagnes. Terrain altitudes range from 22 m above sea level (masl) at the shore of the Rivi`ere des Mille ˆIles to 228 masl at the summit of the Oka Hills. The higher altitudes are typically associated with rock outcrops or rock overlain by thin till layers. At lower altitudes, thick marine stratigraphic units cover bedrock depressions filled at the base with glacial deposits of variable thickness (Ross et al. 2001). The surface water is drained mainly through Rivi`ere du Nord, Rivi`ere Mascouche, Rivi`ere du Chˆene and Rivi`ere du Chicot (Fig. 1). Minor streams drain the rest of the study area (∼25%) directly to the southern boundary. Parts of the drainage basin of the Rivi`ere du Nord in the Laurentian Plateau are located outside the study area as it is focused primarily on the sedimentary platform. The basin of the Rivi`ere Mascouche is also only partially included as municipal limits define the study area to the east. There, most of the riverbeds are cut in the low permeability surficial DOI 10.1007/s10040-005-0445-6
837 Fig. 1 Location of study area with the digital elevation model as background. The co-ordinates are in the UTM NAD83 system
sediments, which hinder the interaction between surface waters and the regional bedrock aquifers. The study area is located within the Great Lakes and St. Lawrence climatic region. It is characterized by a temperate climate with cold wet winters and hot wet summers (Environment Canada URL). Monthly air temperatures are lowest in January with a mean of –11.4◦ C and peak in July with a mean of 20.9◦ C. Ground frost generally develops in late November and lasts until mid-March when the spring snowmelt begins. The total annual precipitation varies between 930 and 1,130 mm, with some 20% in the form of snowfall. The monthly precipitation is relatively uniform, with highest averages recorded in September (101 mm), and lowest in February (63 mm). This relatively abundant precipitation results in a positive water budget for the region.
Hydrogeological framework
Hydrostratigraphy Four major hydrostratigraphic units have been identified within the regional flow system: fractured rocks, glacial and fluvio-glacial deposits, marine and glaciolacustrine fine sediments, and fluvial and littoral sediments. A brief description of these hydrostratigraphic units is given below.
Fractured rock The regional bedrock geology consists of generally flat lying sedimentary strata of the Lower Paleozoic period (Globensky 1987; Salad-Hersi et al. 2002). The bedrock geology in the study area and typical cross sections are shown in Fig. 2. The lower sedimentary formation consists of Cambrian rocks with siliciclastic dominated units of the Potsdam Group. This sandstone unit is divided into two formations, the Covey Hill at the base and Cairnside at the top. The Covey Hill Formation is formed of coarsegrained feldspathic sandstone, poorly graded and poorly cemented. Its thickness exceeds 500 m. Diagenetic alter-
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ation of the top several meters of the Covey Hill Formation has locally degraded the sandstone into a loose, oxidized and highly-permeable breccia. The Cairnside Formation is a homogeneous quartzitic well-cemented sandstone of up to 250 m in thickness (Globensky 1987). The sandstones are overlain by the Lower to Middle Ordovician carbonate-dominated Beekmantown Group, a succession of mostly dolostone, limestone, quartzose carbonate, and subordinate siltstone and shale, with a total thickness of up to 460 m (Bernstein 1992; Salad-Hersi et al. 2002). The Beekmantown Group is divided into three extensive formations, including basal Theresa, intermediate Beauharnois, and upper Carillon. The Theresa Formation contains interstratifications of quartz arenite, dolomitic sandstone and dolostone. The Beauharnois Formation contains dolostone with bioclastic and oolitic interbeds. The Carillon Formation consists mostly of cyclic packages of laminated, fine dolostone and limestone, as well as siltstone and shale. No well developed karst systems have been identified in these formations (Salad-Hersi et al. 2003). The uppermost sedimentary formations are the limestones of the Chazy, Black River and Trenton Groups (Globensky 1987). The Chazy Group is formed of sandstone at the base changing to shale and dolostone in the upper portion. Its maximal thickness in the region can reach 100 m. The Black River Group is relatively thin, at most 30 m, and represents a succession of dolostone and limestone. The Trenton Group, up to 250 m thick, is composed of clayey limestone, interstratified in its upper part with thin beds of shale. The Lower Paleozoic sedimentary strata are underlain by Grenvillian metamorphic rocks of Precambrian age, which form the Laurentian Plateau to the north and the outer rims of the Oka and St. Andr´e Hills to the south. At the latter locations, Cretaceous carbonatite rocks intruded the central rind of the hills. The sedimentary sequence has been gently folded during the Appalachian Orogenesis as shown on the cross-sections in Fig. 2. Few major geologic structures occur in the region (Globensky 1982, 1987). The major fault of Lachute, located at the northern limit of the study area, separates the
DOI 10.1007/s10040-005-0445-6
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Fig. 3 Spatial distribution of fine marine and glacio-lacustrine sediments (clay) and glacial and fluvio-glacial deposits (till)
Fig. 2 Regional map of bedrock geology and cross sections (modified from Rocher et al. 2001)
St. Lawrence Lowlands from the Grenvillian geological province.
Quaternary deposits In the study area, the bedrock is generally covered by unconsolidated Quaternary formations. They include from the base: glacial and fluvio-glacial deposits, and Champlain Sea clays, which are occasionally overlain by fluvial and littoral sands (Ross et al. 2001). During the most recent Wisconsinan glacial period, between 21,000 to 12,000 years ago, the entire region was covered with ice. As a result of successive advances and retreats, the glaciers laid down a relatively continuous layer of glacial and fluvio-glacial deposits (tills) over the bedrock. The thickness of this unsorted and unstratified material ranges from several meters to several tens of meters. The grain size distribution is very variable ranging from clay to boulders. Based on the analysis of 34 samples, Hamel (2002) reported average composition of 5% clay, 39% silt, 33% sand and 23% gravel. Most of the coarser materials are derived from the underlying sedimentary rocks, and to a lesser degree from the metamorphic rocks. Close to the surface, the tides and waves of the Champlain Sea reworked the till. The fine matrix has been flushed away and at some locations blocks and boulders are visible at the surface. In the vicinity of the Rivi`ere des Outaouais and Rivi`ere du Nord, the till sequence has been partially eroded during the formation of the river channels. Occasionally, coarse sediments are found at the base of the Quaternary sequence. These sediments were laid down when the sub-glacial melt water eroded partially or completely the earlier glacial sequence, and deposited coarse outwash sand and gravel and, in some places, bouldery gravels. Hydrogeology Journal (2005) 13: 835–848
Marine and glacio-lacustrine sediments constitute most of the land surface in the region (Fig. 3). The Champlain sea event, which took place after the meltdown and retreat of the glaciers, resulted in thick layers of fine clayey deposits, silts and fine sands (Ross et al. 2001). The general thickness of this unit is between 10 and 20 m, but it can occasionally reach up to 100 m in the deep rock depressions. The marine retreat permitted the formation of fluvial and littoral sediments attaining a thickness of several meters at certain locations. Spatially, sandy layers occur as extensive sheets (Th´er`ese-de-Blainville) or as long and narrow channels (along the Rivi`ere du Nord). These deposits have high permeability and constitute perched water-table aquifers. They are usually used for private wells and well points, and to a lesser extent for municipal water supply. Saint-Canut, located near the Rivi`ere du Nord, and Oka, Pointe-Calumet and Sainte-Marthe-sur-Lac on the shore of the Lac des Deux Montagnes draw water from this unit (Fig. 3).
Regional groundwater flow A schematic hydrostratigraphic section for the region is shown in Fig. 4. The regional aquifer system is formed of sandstone and carbonate rocks (limestone and dolomite), irregularly overlain by coarse sediments. Groundwater flow in sedimentary rocks is primarily through fractures, joints and bedding planes. Flow in the primary pore space is significantly lower. While the density and the degree of interconnection of fractures are highly variable, the most permeable units are generally found at the top of the rock sequence (Nastev et al. 2004). Besides the intrinsic weakness of the poorly consolidated sandstone, alteration and extensive fracturing in the uppermost several meters were often observed in limestone and dolostone as well (Rocher et al. 2001; Lemieux 2002). Karanta (2002), identified and characterized a high permeability rock sequence immediately beneath Quaternary sediments in the vicinity of Sainte-Anne-des-Plaines and Saint-Janvier municipal wells (Fig. 3). Deeper sedimentary horizons are characterized by a lower density of interconnected fractures and are less permeable (Nastev et al. 2002). The interface aquifer unit DOI 10.1007/s10040-005-0445-6
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ing layer which are filled with coarser matrix sediments, or in sections where fine sediments were completely flushed away (Fig. 4).
Hydraulic heads
Fig. 4 Conceptual model for the regional groundwater flow. The arrows represent the direction of the groundwater flow
consisting of sand and gravel is found between the clayrich sediments and the underlying sedimentary rocks. These permeable deposits are generally distributed in channels a few meters thick along the bedrock depressions and are hydraulically connected with the underlying rock aquifers. However, their thickness and spatial distribution are still poorly understood, although their presence has often been observed on many occasions in the study area (Simard 1978). Two municipalities, Saint-Benoˆıt and Saint-Hermas, draw their water supply exclusively from this unit. The existence of buried rock depressions filled with coarse sediments was also confirmed by seismic geophysical profiles (Ross et al. 2001). On top of the regional aquifer system, the Quaternary aquitard consists of unsorted till, which due to the high content of fine-grained materials yields small amounts of water. Locally, intermediate water bearing lenses of coarser matrix deposits are found within the till layer, but the observed yields are only sufficient for domestic water supply. In most of the study area, low permeability clay-rich sediments overlie tills. In the first 3–5 m from the ground surface, clays can be weathered permitting limited infiltration (Nastev et al. 2002). At higher altitudes, generally above 80 masl, the occurrence of clayey sediments is irregular, and they are relatively thin or absent. Here, the regional aquifers are under water table conditions or are semi-confined by shallow till (Figs. 3 and 4). These areas are denoted in the following text as ‘recharge areas’, as most of the regional recharge occurs there (Hamel 2002). After reaching the regional aquifer, the vertically percolating water moves horizontally towards the longitudinal axis of the rock depressions, which usually coincides with the flow direction of the existing streams. Discharge to streams occurs only through the occasional windows in the confinHydrogeology Journal (2005) 13: 835–848
In 1974, the Quebec Ministry of Environment established a program for monitoring long-term groundwater fluctuations in the regional aquifers (Savard et al. 2002). Currently, water level fluctuations are measured continuously in 12 wells using digital data-loggers, and in 20 wells on a monthly basis. These measurements provide useful information of the aquifer’s behavior with time. Groundwater fluctuations observed in two typical bedrock wells under natural flow conditions are shown in Fig. 5. Water levels vary seasonally and locally as a result of the aquifer storage capacity and the dynamic interaction between the groundwater inputs and outputs. Water levels rise during recharge events following infiltration from precipitation. Although monthly precipitation is distributed nearly evenly throughout the year, the annual maximum is generally observed in late March and in April. It is related to the spring snowmelt, when the soil moisture is at maximum. A second peak, of lesser amplitude than the first one, is usually observed in November or at the beginning of December, when the evapotranspiration is very low. Water levels decline as a consequence of groundwater discharging to surface water, or due to evapotranspiration when the water table is close to the ground surface. The annual minimum usually occurs in late August, when the evapotranspiration is at maximum. The long-term results obtained from the analysis of 20 well hydrographs show that the average annual fluctuations are more pronounced for water table aquifers, 2.4 m, than for confined aquifers, 1.4 m. One monitoring well intercepting a water table aquifer at Oka Hills even showed a maximal annual fluctuation of 8.1 m. The year-to-year fluctuations did not show any significant increasing or decreasing trend over the 30 year period under consideration. This is an indication that regional aquifers are in pseudo steady-state flow conditions.
Fig. 5 Long-term hydraulic head fluctuations in two typical bedrock wells DOI 10.1007/s10040-005-0445-6
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In addition to long-term water-table fluctuations, potentiometric data were measured in 570 wells and piezometers intercepting rock aquifers, providing a snapshot distribution of the hydraulic heads. Groundwater levels were measured during the regional surveys in 1999 and 2000. As the calibration of the numerical model is intended to reproduce steady-state flow conditions, the representative measurements of the average hydraulic heads took place from May to July. The interpolated potentiometric surface, based on water level measurements, is shown in Fig. 8a together with the model simulated potentiometric surface, which is discussed later in the text. The regional groundwater flow originates primarily from the north and discharges to the south of the study area. Groundwater flows radially from topographic highs, and often follows the same general direction as the major streams. Groundwater elevations attain on average 60-80 masl in the recharge areas, but can reach as much as 140 masl at the Oka Hills and the Laurentian Plateau. The regional aquifers discharge at approximately 20 masl to the southern model boundary. Steep hydraulic gradients characterize the Oka and Saint-Andr´e Hills. The influence on the hydraulic heads of the important groundwater withdrawal at the Saint-Eustache quarry can be observed at the confluence of the Rivi`ere Chicot and Rivi`ere des Mille ˆIles. Elsewhere, pumping effects are subdued due to lower withdrawal rates. Fault related features, where present, do not appear to influence the configuration of the potentiometric surface.
Groundwater dynamic parameters The steady-state flow assumption implies equal input and output rates and neglects changes in aquifer storage. Although, as shown in Fig. 5, aquifers are in seasonal transient conditions reflected by the fluctuation of the potentiometric surface, the recharge and discharge rates may be considered constant on an annual basis and under long-term equilibrium conditions. The groundwater is replenished annually in the recharge areas and aquifers are sufficiently permeable to transmit the infiltrated water towards the discharge areas. Fully transient conditions may be underway in low permeability units, such as seepage through the clay-rich confining unit. Transient flow conditions probably also occur in deeper low permeability bedrock layers as a result of the Champlain Sea retreat, and in proximity of the pumping wells where water levels may still decline due to pumping. In these cases, the steady-state assumption may lead to an error, but it is judged of minor importance for the regional groundwater flow and is neglected. Water supply to regional aquifers comes from infiltration from precipitation and from groundwater inflow across the northern boundary. Groundwater naturally discharges to surface waters, exits the aquifers as underground outflow, and is withdrawn for human purposes. Streams in the region are generally gaining streams, as water levels there are lower than the hydraulic heads in the underlying aquifers, and water moves upward from aquifers to streams. The contribution of groundwater to streamflow is defined as Hydrogeology Journal (2005) 13: 835–848
baseflow. In the study area, surficial aquifers and regional aquifers contribute simultaneously to the baseflow. In certain areas, excessive withdrawal rates lower the groundwater levels below that of the surface waters. In those cases, groundwater discharge ceases in river reaches affected by pumping, and the stream begins to lose water to regional aquifers. Such induced recharge is occurring in the vicinity of the Saint-Eustache rock quarry, at the confluence of the Rivi`ere du Chicot and Rivi`ere des Mille ˆIles, where the water table is approximately 40 m below the stream levels.
Recharge Recharge is the major component of the groundwater budget and an important calibration parameter for the numerical simulation of groundwater flow. In the study area, the extensive low permeability clayey layers generally hamper infiltrating water. The accumulated perched water moves laterally through the shallow sandy deposits. Significant distances may be attained in this way before the infiltrated water is evacuated from the underground flow system mainly through seepage to local depressions, or as runoff to surface waters. In cases where laterally moving water intercepts windows filled with relatively coarse deposits, downward flow occurs to regional aquifers. It is, however, difficult to estimate this recharge component. For this study, the total recharge to the regional aquifers will be considered. It comprises both the seepage component from the perched aquifers and the direct recharge occurring in the areas of till and rock outcrops. Recharge areas are shown in Fig. 3. The local recharge rates were evaluated from direct infiltration measurements in the field, by the water table fluctuation method, and by the soil-water budget method (Hamel 2002). In addition, hydrograph separation allowed estimation of the total baseflow contribution for two gauged streams, Rivi`ere du Nord and Rivi`ere du Chˆene. Pan lysimeters, installed at four sites with till outcrops, yielded infiltration values from 194 to 218 mm/y. Recharge values obtained from head fluctuations measured in 15 observation wells vary from 131 to 225 mm/y depending on the storage coefficient used in the computations. The water budget was calculated for various sites with the standard mass balance equation and based on the data from nine meteorological stations located in the region. The possible combinations included farmland or woodland, poor to good drainage conditions, and various surface slopes, and produced results in the range of 139-248 mm/y. These estimates only apply to the recharge areas as outlined above, covering approximately 20% of the study area. The overall recharge rate was estimated to vary between 40 and 100 mm/y, depending on the values for the parameters considered and the estimation method (Hamel 2002).
Subsurface inflow and outflow Regional aquifers do not represent a closed hydrogeologic system. Groundwater inflow to regional aquifers occurs from the Laurentian Plateau at the northern boundary of DOI 10.1007/s10040-005-0445-6
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the study area. There, the bed of the Rivi`ere du Nord is cut at various intervals in thick clay sediments (Fig. 3). The groundwater inflow takes place below the aquitard unit. Initially, it is most probably directed southward towards the rock depression at the junction between the Laurentian Plateau and the Paleozoic platform. Groundwater flow then continues westward parallel to the surface water flow, until reaching favorable conditions for discharging into the Rivi`ere du Nord. To evaluate the inflow rate, a classical flow-net analysis was performed (Savard et al. 2002). Although most of the parameters such as physiographic framework, potentiometric surface, hydraulic properties, etc., vary from one site to another, a typical vertical cross section was chosen as representative for the average flow conditions: average hydraulic head gradient of 0.02, average hydraulic conductivity for fractured rocks of 2.6×10−5 m/s, and average hydraulic conductivity for deeper solid rock units of 1×10−8 m/s to represent the regional flow system. Darcy’s flow equation yielded a groundwater inflow rate from the Laurentian Plateau equal to 1×10−5 m3 /m2 /s for the fractured rock, and of 1.8×10−9 m3 /m2 /s for the less fractured underlying rocks. To the east, the watershed of the Rivi`ere Mascouche bounds the study area. Riverbed materials are composed of a 10–30 m thick clay layer. It is believed that negligible hydraulic interaction exists between surface waters and regional aquifers at this boundary. The groundwater outflow from the domain takes the downstream direction of the Rivi`ere Mascouche, and will be quantified during the calibration of the numerical model.
Groundwater use Due to the lack of precise measurements, the current groundwater use is still only approximately known. The few reliable data are those for larger users such as municipal aqueducts, rock quarries and bottling companies. For other uses, an estimation is made based on typical water needs per person (private wells), and using data from the Quebec Ministry of Agriculture for various irrigation and farming needs (Nastev et al. 2002). The estimated distribution of the withdrawal rate among various users is given in Table 1. These withdrawals indicate a relatively high use of groundwater resources in the region, estimated at 18 Mm3 /y (million cubic meters per year). The total withdrawal rate normalized for the study area is equivalent to 14.8 mm/y. Most of the available groundwater (51.6%) is withdrawn from rock quarries. The Saint-Eustache quarry, located at the confluence of the Rivi`ere Chicot and Rivi`ere des Mille ˆIles, is one of the largest quarries in Canada. The pumping rate at this quarry alone amounts to 8.2 Mm3 /y, or approximately 45% of the total withdrawal rate in the region. The water quantities pumped at municipal and private wells are used mostly for human needs. They amount to 31.1% of the total withdrawal, while agriculture accounts for 14.1% of the groundwater use, and the balance is attributed to bottlers and golf courses.
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Table 1 Repartition of the annual groundwater withdrawal rates from the regional rock aquifers (modified from Nastev et al. 2002) Groundwater use
Annual withdrawal rate Mm3 /y %
mm/y
Municipal aqueducts Private wells Irrigation Livestock Golf courses Bottlers Rock quarries Total
3.4 2.2 1.2 1.3 0.1 0.5 9.3 18.0
2.8 1.8 1.0 1.1 0.1 0.4 7.6 14.8
18.8 12.3 6.8 7.3 0.5 2.8 51.6 100
Regional hydraulic properties The transmissivity, horizontal hydraulic conductivity, and storage coefficients, for the interface aquifer and for the fractured rock aquifers, were estimated by the interpretation of field hydraulic tests. A brief analysis of the results is presented below. The reader is referred to Nastev et al. (2004) for a detailed description of the regional hydraulic properties.
Interface aquifer Results from 12 aquifer tests (6 multi-well pumping tests and 6 pumping tests without observation wells) and 13 slug tests were used to derive the permeability properties of the sand and gravel filled channels found at the base of the Quaternary deposits. As the horizontal hydraulic conductivity is usually log-normally distributed (Freeze and Cherry 1979), the geometric mean was used as an estimate of the average hydraulic conductivity, Km =7.8×10−4 m/s, with a sample standard deviation of the logarithms of 0.47. This relatively high and uniform hydraulic conductivity implies that, when present, these sediments represent a major conduit for groundwater flow. The storage coefficient estimated from the 6 multi-well pumping tests under confined conditions was found to vary between 2×10−5 and 7×10−3 .
Fractured rock aquifer Particular effort has been applied to the characterization of the hydraulic properties of the fractured rocks. First, it was necessary to define their spatial variation and also the thickness of the water bearing sequence in which most of the groundwater flow occurs. Because of the high degree of fracturing, an equivalent porous medium was assumed to be adequate for the interpretation of the aquifer tests. The field testing program consisted of multiple-scale measurements of the hydraulic conductivity: – Data from 12 multi-well pumping tests were collected. These tests involved 1 pumped well and up to 10 observation wells. The Theis solution for groundwater flow in a simple isotropic aquifer with non-leaky confined conditions was used for the analysis of the drawdown vs. elapsed time curves on log-log scales (Kruseman and de Ridder 1994);
DOI 10.1007/s10040-005-0445-6
842 Table 2 Summary of hydraulic conductivity results (after Nastev et al. 2002). The average values are computed as geometric means
No
Aquifer test
Number of tests Hydraulic conductivity K (m/s) min max mean
1 2 3 4
Packer test (entire well) Specific capacity Single-well pumping test Multi-well pumping test Total (1+2+3+4)a
20 132 23 12 179
3.8×10−8 3.5×10−8 1.4×10−6 1.0×10−6 3.5×10−8
1.6×10−4 4.9×10−4 6.7×10−4 7.9×10−3 7.9×10−3
4.8×10−6 2.8×10−5 3.1×10−5 1.2×10−4 2.6×10−5
Variance σ 2 logK 0.81 0.69 0.62 0.74 0.79
a
To avoid the re-sampling, for 8 wells, where more than 1 testing method was applied, only the larger scale test result was retained
– For 23 constant-rate pumping tests, water levels were continuously observed in the production wells only. The Cooper-Jacob straight-line fitting method was applied on drawdown or recovery data plotted as log time functions; – Specific capacity data (pumping rate and final drawdown) were compiled from water well records in the drillers’ database compiled by the Quebec Ministry of Environment. Following the elimination of potentially erroneous data (duration
