Modeling the evolution of a regional fractured-rock aquifer system in southern Quebec following the last deglaciation 1Marc Laurencelle, 1René Lefebvre, 2Christine Rivard, 2Michel Parent, 1Pierre Ladevèze, 1Châtelaine Beaudry, 1Marc-André Carrier & 2Nicolas Benoit 1Institut national de la recherche scientifique, Centre Eau Terre Environnement, Quebec, Quebec, Canada

2Natural Resources Canada, Geological Survey of Canada (GSC), Quebec, Quebec, Canada

ABSTRACT The ~9 000 km2 Montérégie-Est study area covers the watersheds of the Richelieu and Yamaska rivers. Numerical modeling of its fractured-rock aquifer system aims to provide an understanding of current groundwater flow dynamics, but also of its evolution through recent geologic time. The conceptual model of the aquifer system is based on an extensive regional characterization. The simulation approach involves long cross-sectional 2D conceptual and numerical models extending from the Appalachians (SE) to the St. Lawrence River (NW). Alternative conditions are simulated to assess the impacts of geological events: glacier melting, invasion of the aquifer by Champlain Sea water, and subsequent system adjustments up to present-day conditions. These simulations provide insight into processes that led to the current distribution of brackish groundwater and the ongoing aquifer desalinization process. RÉSUMÉ La région d’étude couvre ~9 000 km2 en Montérégie Est et inclut les bassins versants des rivières Richelieu et Yamaska. La modélisation numérique du système aquifère rocheux régional a pour but d'améliorer la compréhension de la dynamique actuelle des eaux souterraines, mais aussi de l'évolution de cette dynamique au cours des temps géologiques récents. Le modèle conceptuel de la région d’étude est basé sur une caractérisation régionale détaillée. L’approche de simulation implique des modèles conceptuels et numériques en coupes verticales 2D allant des Appalaches (SE) jusqu’au fleuve Saint-Laurent (NO). La simulation de différentes conditions permet d’évaluer l’effet des événements géologiques récents : fonte glaciaire, invasion de l’aquifère par les eaux saumâtres de la Mer de Champlain, ajustements subséquents jusqu'aux conditions actuelles. Ces simulations permettent de comprendre les mécanismes ayant mené à la présente distribution d’eaux saumâtres dans la région et à la désalinisation, toujours en cours, du système aquifère. 1 INTRODUCTION This study aims to understand the changes in regional aquifer dynamics following major recent geological events: ice sheet melting and sea water invasion. Figure 1 shows the location of the study area, in Montérégie-Est (~9 000 km2), where the regional fractured-rock aquifer system underwent a complex Late Quaternary glacial history that left a sparse discontinuous sedimentary record, as well as a ~2 200 km2 zone of brackish groundwater. The presence and areal extent of this brackish water can be used as an indicator of historical aquifer dynamics. The study area also has the advantage of having high-quality data available.

The regional aquifer characterization of Montérégie-Est was carried out as part of Quebec’s systematic groundwater resources assessment program (Palmer et al. 2011). This study provided detailed information on geography (topography, hydrography, land use), geology (surficial sediments, bedrock lithologies and structures, fracturing), hydrogeology (hydraulic properties, flow, recharge), and hydrogeochemistry (water types, geochemical evolution, groundwater residence time) (Carrier et al. 2013).

The impact of Late Quaternary events is studied by combining numerical simulations of present-day

hydrogeological (steady-state) conditions and historical palaeo-hydrogeological (transient) conditions, using present-day hydrogeochemical observations as constraints.

Figure 1. Location of the study area (in red) 2 DESCRIPTION OF THE STUDY AREA Montérégie-Est is located in southern Quebec (Figure 1) and covers two watersheds (Figure 2): those of the Yamaska River (4 800 km2) and Richelieu River (24 000 km2). The latter is transboundary and extends ~20 000 km2 into the U.S. As most of the subwatersheds on the USA side flow towards Lake Champlain, for which Richelieu River is the outlet, it was decided to restrict the

study area to the Quebec region (~9 000 km2) for the purpose of the present study.

Two main geological provinces underlie the study area: the St. Lawrence Platform and the Appalachians (MNRF 2010) (Figure 2). Their main lithologies comprise shales, dolomites, limestones, sandstones, and volcanites. These provinces are separated by “Logan’s Line”, which is a complex ~SW-NE sublinear network of SE-dipping thrust faults (Globensky 1987). Appalachian rock units underwent significant orogenic deformation, including folding, faulting, and fracturing, as well as low-grade metamorphism (Castonguay et al. 2010), whereas sedimentary rocks of the St. Lawrence Platform underwent little deformation (Globensky 1987). In addition, Cretaceous igneous activity emplaced Monteregian intrusives in both provinces. The seven Monteregian Hills were exposed later due to differential erosion of the less resistant host rocks.

Figure 2: Study area main geographic features

The study area may be divided into four physiographic domains. The St. Lawrence Lowlands (0-60 m above mean sea level) coincide with the Platform, and has a flat topography. The Appalachian Uplands (>200 m) have a high, hilly topography, with major valleys aligned perpendicular to the Appalachian thrust front (~Logan's Line). In between, the Appalachian Foothills (60-200 m) represent an intermediate domain where topography rises gradually towards east, where it also becomes more hilly. Finally, Monteregian Hills cut through these domains, with summits at heights of 170 to 410 m above surrounding areas. The surficial sediment cover is Late Quaternary in age and its depositional history is essentially related to the last glaciation-deglaciation events, since the last glaciation removed most preexisting sediments. 3 QUATERNARY EVENTS AND STRATIGRAPHY Surface conditions changed significantly throughout the millennia. During the last 700 ka, orbitally driven climatic fluctuations caused ice sheets to grow and decay with a

periodicity of ca. 100 ka (Cronin 2010). The Wisconsinan glaciation (80-10 ka BP) was the last period of major glacial cover in North America (Fulton and Prest 1987). During the Last Glacial Maximum (LGM, 23-19 ka BP), Canada was almost completely covered by the Laurentide Ice Sheet (Clark and Mix 2002). Ice thickness reached as much as 5 km west of current Hudson Bay, but only about 2 km over the study area, according to the ICE-5G ice sheet reconstruction model (Peltier 2004). Furthermore, loading by the Laurentide Ice Sheet depressed the upper crust by as much as 1 km (Peltier 1998), while global mean sea level was lowered to a level of -120 m relative to present during LGM (Peltier and Fairbanks 2006). Then, North America was progressively deglaciated, from 0% land recovery at 19 ka BP to almost 100% at 6 ka BP (Dyke 2004). As the southwestern part of the St. Lawrence Valley was deglaciated, it was briefly occupied by Glacial Lake Candona (fresh glacial meltwater) until Champlain Sea invasion (seawater coming from the Atlantic Ocean) (Parent and Occhietti 1999). The Champlain Sea flooded the St. Lawrence Valley for ~1 800 years (13.1-11.3 ka BP), that is until postglacial rebound led to the replacement of Champlain Sea water by freshwater, thus reverting the St. Lawrence Valley to a lacustrine environment. Subsequently, the drainage system in the lowland evolved towards its present-day configuration through continued glacial isostatic adjustment.

Ice cover had an important influence on groundwater flow dynamics. Among the processes involved, sub-ice-sheet forced recharge, due to basal melting combined with ice loading, was a key one (Lemieux et al. 2008; Person et al. 2007). Glacial loading and unloading also contributed to the generation of overpressures and underpressures, that are still preserved today in some locations, such as in the Michigan Basin (Person et al. 2012). Similarly, water bodies that extensively flooded the St. Lawrence Valley during the last deglaciation produced high hydraulic heads on the top of the underlying aquifer system, thus inducing strong vertical gradients that bolstered infiltration (Lemieux et al. 2008). Among those water bodies, Champlain Sea was special in that it constituted a long-lasting saltwater input to the regional aquifer system, unlike the preceding and succeeding freshwater lakes.

Figure 3. Conceptual hydrostratigraphy of the study area

These Late Quaternary events also replaced preexisting sedimentary sequences with new ones. The present-day sediment cover is indeed made of glacial (till; G), fluvioglacial (eskers; FG), glaciolacustrine / glacio-

(NW) (SE)

marine (fine-grained units; GL / GM), and Holocene fluvial (coarse-grained units; F) materials that were deposited since the last glaciation (Figure 3). A ubiquitous thin (~5 m) layer of till covers bedrock throughout the region. In the northern part of the Lowlands, the surficial sediment cover is thick (~15-30 m) and largely dominated by Champlain Sea marine silts and clays. In its southern part (not illustrated in Figure 3), the surficial cover is thinner (~5-15 m) and dominated by till and littoral sediments instead of marine silt and clay due to the combined effect of higher bedrock elevation and lower marine limit in this area. The Appalachian Foothills are also dominated by tills, but these have been partly reworked by wave action along the palaeo-shorelines of the Champlain Sea. Littoral sediments (sand and gravel; L) were deposited as well. In the Appalachian Uplands, a thin (~5 m), discontinuous till covers bedrock. Overall, the aquifer potential of unconsolidated deposits is low, except locally where coarse-grained sequences are present. 4 CHARACTERIZATION RESULTS Analysis of structural data resulting from the interpretation of acoustic televiewer images and outcrop fracture mapping provided an insight into the structural setting of each geological context (Crow et al. 2013; Ladevèze et al. 2013). Overall, fractures are concentrated in the first 30 m below the top of bedrock, and they share a NNE to NE average strike, which is parallel to major geological structures (e.g. Logan's Line).

In the St. Lawrence Platform, most fractures are subhorizontal but some vertical fractures are also present (Figure 4). This would suggest an orthogonal fracture network in which permeability parallel to bedding is consistently higher than across bedding. However, hydraulic aperture of some vertical fractures (not yet quantified) may potentially be larger than that of subhorizontal fractures, thereby counterbalancing their relative scarcity.

Figure 4. Initial conceptual model of the regional fracture pattern in the study area (after Ladevèze et al. 2013)

In the Appalachians, two quasi-orthogonal fracture sets also coexist, but dips are much higher (~45°) as a consequence of deformations in this context, that also added significant variability in fracture orientations (Figure 4). Hence, in the Appalachians, structural controls on permeability are much more complex at the local scale, yet the use of a representative elementary volume (REV)

will greatly simplify the definition of hydraulic conductivity tensors for both Appalachian and St. Lawrence Platform rocks.

Statistical analyses on the horizontal hydraulic conductivity (Kh) values of the fractured-rock aquifer (from specific capacity data) showed that Kh depends on the depth in bedrock (z) rather than geological formations or contexts (Laurencelle et al. 2011). Consequently, the regional fractured-rock aquifer was defined as an equivalent single-continuum porous medium with Kh decreasing with depth in bedrock following a Kh(z) permeability-depth relationship similar to the one developed by Jiang et al. (2010). Figure 5 shows all available Kh(z) values (grey dots); the median of log10 Kh for each depth interval (black squares); and the model representing Kh(z) within the study area (black curve).

Figure 5. Horizontal hydraulic conductivity systematic decrease with depth within the study area

Equation 1a, adapted from Jiang et al. (2010), describes this nonlinear model, in which λ(z), describing the decreasing trend of fracture frequency with depth, follows the same explicit function as Jiang's (Equation 1b). Parameter definitions and optimal values for Equation 1a can be found in Table 2. Since very limited data are available on vertical hydraulic conductivity (Kv), it was assumed to follow a constant anisotropy ratio (Kh/Kv) and defined according to Equation 2.

[1a]

[1b]

[2]

Besides the fractured-rock aquifer, the aquifer system comprises other components, which were characterized, including the surficial sediment cover, recharge to the aquifer, and the geochemistry of groundwater. Geostatistical modeling based on interpreted borehole logs and near-surface seismic surveys provided a hydrostratigraphic framework for Quaternary sediments, which was used, notably to infer confining conditions of the fractured-rock aquifer (Carrier et al. 2013). Spatially distributed recharge was estimated using the HELP infiltration model (Carrier et al. 2013; Schroeder 1994).

Groundwater dynamics was explored qualitatively in the hydrogeochemical study by Beaudry (2013), which provided information on the origins, geochemical evolution, and residence time of groundwater (e.g. data for Figure 10b below). In particular, a vast zone (2 200 km2) of brackish groundwater was delineated in the north-western part of the study area (Beaudry et al. 2011) (Figure 2). 5 MODELING APPROACH Considering the present understanding of the study area plus its Late Quaternary history, a multi-stage numerical modeling approach was developed to assess the relative importance of various palaeo-hydrogeological processes in the past evolution of the regional aquifer system. However, as understanding current dynamics of the regional groundwater system was a prerequisite (since calibration data are all contemporary), a hydrogeological model reflecting present-day conditions of the regional aquifer system was developed prior to tackling the palaeo-hydrogeological questions.

First, a 2Dxz vertical geological cross-section was developed to depict the regional fractured-rock aquifer, its overlying sediment cover, and the main rivers/lakes, along a 90-km line extending from the St. Lawrence River south shore (Contrecœur; 45.86°N, 73,26°W; 8 mamsl) to the eastern limit of the study area in the Appalachian Uplands (Lake Stukely; 45.37°N, 72.27°W; 389 mamsl) (Figure 2).

Figure 6. Geological cross-section used for the numerical model of the regional aquifer system. Its geographical location is shown on the inset map

Hydrostratigraphic data at trace location defined surficial sediment geometries, while the regional fractured-rock aquifer was considered a single continuum extending hundreds of meters below these sediments (Figure 6). Each of the Quaternary hydrostratigraphic units was discretized into several layers of laterally varying thicknesses (0.05-5 m), while the rock aquifer was discretized into tens of layers with increasing thickness (0.5-50 m) down to a depth of ~500 m (i.e. arbitrarily set model base). Longitudinal discretization generated elements with a uniform width of ~100 m. Kh and Kv were assigned homogeneously within a given Quaternary unit (Table 1), while they followed Equations 1 and 2 with parameter values from Table 2, within the fractured-rock aquifer. No-flow boundary conditions were assigned to the

lateral (at xmin and xmax) and bottom (zdeepest) faces of the numerical model, whereas both fixed water-table and fixed inflow types of boundary conditions were tested on the upper limit of the model. The fixed water-table Table 1. Hydraulic conductivity values assigned to the Quaternary hydrostratigraphic units

Hydrostratigraphic unit Kh (m/s) Kv (m/s) Sandy unit 4×10-5 4×10-6

Clayey unit 2×10-9 4×10-11

Till unit 2×10-6 1×10-7

Table 2. Parameters defining the regional trend of hydraulic conductivity within the fractured-rock aquifer

Parameter description Symbol Value K value at depth zero1 K0 3.9×10-5

Reference depth term1 zc 16.6

Effective aperture term1 br/b0 2×10-6

Vertical anisotropy ratio2 Kh/Kv 500 1for use with Equation 1a 2for use with Equation 2 boundary condition (i.e. hydraulic head of surface nodes equal to ground elevation) was preferred over the fixed inflow boundary condition as a way to mimic recharge, given the fact that the water table is close to the ground surface in this area, and because this condition lets the flow simulator compute distributed fluxes (inflows & outflows) at the upper boundary, without the need of any a priori knowledge of them. However, these computed inflows were compared with HELP spatially-distributed recharge estimates, during model calibration, to verify model fluxes.

FLONET/TR2 (Molson and Frind 2010) was selected for modern-time simulations, since this simple, efficient and robust numerical simulator is designed to solve 2Dxz steady-state flow problems; it calculates flow nets as well as advective-dispersive groundwater age (also known as age mass). 6 PRELIMINARY MODELING RESULTS Calibration of the modern-time numerical model is still ongoing. Simulation outputs presently available are therefore considered preliminary semi-quantitative exploratory results. Nonetheless, some interesting findings emerge from their analysis.

Firstly, K vertical anisotropy ratio as well as K rate of decrease with depth exert a major control on the groundwater flow patterns. Globally, higher Kh/Kv ratios result in longer, more "regional" flow paths (Figure 7a/7c), while lower ratios result in shorter, more "local" flow paths (Figure 7b/7d). Similarly, a slower decrease of K with depth results in deeper, also more "regional", flow paths (Figure 7c/7d). This latter finding echoes that of Jiang et al. (2009), who demonstrated that "the decrease in K with depth inhibits the development of regional flow systems". The actual importance of regional flow in the rock aquifer thus strongly depends on the spatial distribution of K.

vertical exaggeration: 140x

N

Logan's Line

Therefore, apparent groundwater age at discharge zones also depends on K parameters. Indeed, the more regional flow paths there are towards a discharge zone, the older its groundwater is. Hence, groundwater age near discharge zones may be used as a calibration target. Locations of the main recharge/discharge zones appear, however, insensitive to those parameters defining the K field. Furthermore, results suggest the presence of a few stagnation points at depths and locations that vary according to K parameters; their occurrence appears related to the uneven Appalachians topography.

Secondly, the influence of the sediment cover in the model was assessed. Results show that the clayey unit (marine clays and silts) has the most important influence

on location and intensity of recharge and discharge. Where it is thick enough (>10 m), this unit is an effective barrier to flow, thereby preventing recharge or discharge. This fact, coupled with the very low topographic gradients of the St. Lawrence Lowlands, also explains why regional flow is virtually inactive in the flat confined area west of Richelieu River. Local- to intermediate-scale flow cells would appear in the northern Lowlands if Quaternary units were not integrated into the model, again using a fixed water-table condition at upper boundary (Figure 8). This conceptual result shows that groundwater dynamics was likely much more active within the rock aquifer before the build-up of the "new" Quaternary sediment cover.

7a. optimal model (Kh/Kv = 500; zc = 16.6)

7b. lower anisotropy (Kh/Kv = 100; zc = 16.6)

7c. slower decrease of Kh(z) (Kh/Kv = 500; zc = 83.0)

7d. combination of b and c cases (Kh/Kv = 100; zc = 83.0)

Figure 7. Controls of the main hydraulic parameters defining hydraulic conductivity of the fractured-rock aquifer on the regional groundwater flow patterns using four different sets of values for Kh/Kv and zc

Figure 8. Simulated groundwater flow patterns in the regional fractured-rock aquifer if the sediment cover is not integrated into the numerical model

Besides, the clayey unit induces excessive confined conditions underneath, in some locations where the thickness, continuity, or impermeability of the unit was overestimated in the numerical model. Such deviations occur mainly in valleys where the clayey unit is of significant thickness (Figure 9b).

Similarly, it was found that smoothing of hydrostratigraphic surfaces (e.g. to get a simpler topographic profile) often distorts groundwater interactions with the surface, particularly where hydrostratigraphic unit thicknesses vary abruptly. In the same way, limited resolution of the hydrostratigraphic framework probably impedes simulations from being fully representative.

To alleviate these "stratigraphic" issues, some manual corrections will be applied to the hydrostratigraphic cross-section to ensure that it is representative of the surficial geology not only at trace location but also in its vicinity. In particular, improving the geometries of surficial sediment layers where there are significant discontinuities in the clayey unit or an outcropping of the fractured-rock aquifer (e.g. in the Yamaska River) will result in more realistic recharge, discharge, and seepage fluxes across recharge zones, riverbeds, and lower slopes of hills.

Moreover, although this numerical model is preliminary, its simulation results can be interpreted in relation with the regional setting and distinctive features. According to these results, groundwater flow systems east of the Noire River discharge almost entirely within the Appalachians, and the Noire River valley itself receives local- to regional-scale flow (Figure 9). In the same way, groundwater flow systems west of the Noire River discharge entirely in the Yamaska River. These two flow patterns thus appear to be separate. However, Figure 10 shows that deeper, slower, older (> 5 ka) waters can travel further in the regional rock aquifer until it reaches the Yamaska River. Therefore, these major rivers probably discharge mixtures of young (< 100 a) and old (> 100 a) groundwater.

Furthermore, comparison of simulated groundwater ages against isotopic ages and water groups confirms

northern Lowlands

that the calibration of the numerical model has not yet been completed. Indeed, simulated ages look globally too young compared to isotopic ages from groundwater dating (Figure 10). Also, one sample of tritium-free, very old (19 14C ka BP) groundwater does not agree at all with simulated age (<100 a) at this location. In this respect, it would be interesting to add more tritium-free groundwater samples to the geochemical dataset, to better constrain the hydraulic parameters, deeper in the regional aquifer.

Nonetheless, the simulated groundwater dynamics at shallow depths (0-100 m below surface) is generally consistent with groundwater geochemical evolution as inferred from water groups (see in Beaudry 2013 for details), except around Logan's Line where simulated ages are too young. This suggests that Logan's Line fault network (or other major structural features) may play a

significant hydrogeologic role in the aquifer system by enhancing upward flow in its denser fracture network. Therefore, integration of relevant structural features into the numerical model should at least be attempted, to account for this plausible process.

Interestingly, the zone of brackish groundwater (group M3) was properly simulated west of Yamaska River, in the St. Lawrence Lowlands, according to the very old (> 20 ka BP) simulated groundwater ages, indicating virtually stagnant groundwater (Figure 10). The two young age-mass plumes in this part of the numerical model (left part of Figure 10) are probably the consequence of excessive local asperities in the fixed water-table boundary condition or in the thickness of hydrostratigraphic units (Figure 9). These inconsistencies will likely be dampened with the manual correction of the "hydrostratigraphic" model.

9a. Computed flow net and distributed horizontal hydraulic conductivity (Kh) field

9b. Elevation profiles, vertical fluxes, and hydraulic heads

Figure 9. Preliminary results using the modern-time numerical model for steady-state flow simulation

10a. simulated groundwater age (by age mass transport)

Noire River

Yamaska River

Uplands Appalachian Foothills St. Lawrence Lowlands / Platform

Uplands Appalachian Foothills St. Lawrence Lowlands / Platform

> 20 ka (~stagnant water)

Logan's Line Logan's Line

Noire River

Yamaska River

10b. geochemical data along the profile

Water groups*: - Champlain Sea (CS) invasion area: CS*, M3, M2, LL (oldest to youngest; CS* & M3 = brackish zone) - High altitude areas: A2, A3, A1* (oldest to youngest) - Uncertain origins: M1 (long residence time, possibly upflowing along major faults / dykes) - displayed along the ground elevation profile using the color scheme from Beaudry (2013) - groups marked with an asterisk (*) are not present in this cross-section Sampled boreholes: - well head location, projected onto the cross-section: brown circle crosses (some are located in valleys…) - open depth interval: brown segments (to scale; relative to ground elevation at well location) - horizontal distance to the cross-section: black segments (relative scale, from 0 to 18 km) Isotopic ages from groundwater dating: - tritium (3H): triangles (for presence, > 0.0 TU) vs. black filled-in circles (for absence, 0.0 TU) - uncorrected radiocarbon age (14C): displayed in thousands of radiocarbon years BP (14C ka BP)

Figure 10. Preliminary (a) results using the modern-time numerical model of the regional aquifer system (i.e. rock aquifer & sediment cover) for a quasi-steady state groundwater age simulation, and (b) a synthesis of interpreted geochemical data (water groups and isotopic ages) from wells within 20 km of the cross-section (data from Beaudry 2013) 7 CONCLUSIONS AND FUTURE WORK Overall, modeling results to date emphasize the importance of accurate parameterization and calibration of the numerical model for modern conditions prior to running any palaeo-hydrogeological simulation. In particular, this study showed that the parameters defining the hydraulic conductivity within the fractured-rock aquifer exert a major control on simulated regional groundwater flow patterns and apparent groundwater age. In addition to transient "historical" boundary conditions, which are to be defined as time series, changes in geometries and hydraulic properties of the sediment cover and of the shallow bedrock with time may also have to be integrated into the model, to reflect the actual evolution of confining conditions of the regional rock aquifer due mainly to sedimentation and hydromechanical processes.

In the next few months, the focus of this research will be placed on the validation of the "modern-time" numerical model, and on further development of the multi-stage approach for palaeo-hydrogeological modeling of the regional aquifer system under study. On the one hand, the 2Dxz vertical geological cross-section will be revised to ensure it is as representative as possible of the regional setting. Boundary conditions and hydraulic properties will be revised as well, during the model calibration. On the other hand, "historical" aspects of the problem will be quantified: i) well-documented time series representing historical conditions will be defined; ii) conceptual models will be developed to illustrate the expected hydrogeological effects of these historical

conditions on the regional aquifer system; iii) simple simulations will be run to identify the controlling factors for optimal parameterization of the "historical" problem. Then, more elaborate palaeo-hydrogeological models will be developed to simulate the important steps in the evolution of the regional aquifer system since the last glaciation. Ultimately, it should be possible to reconstitute past changes in the regional aquifer system. In this regard, simulating the evolution of the brackish groundwater zone, from its inception until present (or even beyond), will be our main objective. ACKNOWLEDGEMENTS The authors would like to acknowledge the financial contribution of Quebec's Ministère du Développement durable, de l’Environnement, de la Faune et des Parcs (MDDEFP), of the Geological Survey of Canada (GSC), and of the many regional partners who contributed to the PACES program. Many thanks also go to all members of the PACES Montérégie Est research team, who have provided an invaluable input to this research. A special thank you goes to Dr. John Molson (Université Laval) for his help and interest in this work. Authors also gratefully acknowledge the financial support of NSERC.

Color scheme for the water groups

(A2)

(A3)

(LL)

(M1)

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