El ámbito de la proyección

Prior to building the numerical groundwater flow and transport model, a conceptual geologic model, based upon the field observations and available data, was developed. The glacial-outwash channel is incised into the till and extends from the Bayfield River north beyond the watershed boundary to the Maitland River. The conceptual model was extended beyond the Tricks Creek watershed to include the northern portion of the outwash aquifer lying in the Bridgewater Creek watershed (Fig. 3-2). The conceptual model comprised three main surfaces: the ground surface (Fig. 3-4), constructed from a 10-m horizontal-resolution digital-elevation model supplemented with ground-elevation

data collected during surveys of monitoring locations; the till surface (Fig. 3-5),

constructed from the ground surface digital-elevation model on the exposed till moraines and domestic water well records and borehole data within the outwash channel; and the bedrock surface (Fig. 3-6), constructed from information contained in water wells records in the area. The outwash channel is in-filled with glacial outwash comprising coarse sand and gravel of varying thickness. The channel is incised into the till more deeply on the western side where the aquifer is up to 25-m-thick (Fig. 3-5). Approximately 1 km north of the Tricks Creek – Bridgewater Creek watershed boundary, the outwash aquifer thins and is absent for approximately 900 m before reappearing and continuing on to the Maitland River (Fig. 3-2). The aquifer is ~1.8-km-wide within the Tricks Creek

watershed and narrows to the north where it is 650-m-wide near the Maitland River. The till varies in thickness across these watersheds and, with the exception of the glacial outwash channel, it is at or near the ground surface (Fig. 3-4). The Bayfield River has eroded into the till, and at the confluence with Tricks Creek, the till is only 4-m-thick. Throughout most of the outwash channel, the till varies between 15 and 20-m-thick. At the Maitland River, the river has eroded the till and exposed the bedrock along portions of the river bottom. To the east of the outwash channel, the till varies between 20 and 40- m-thick and to the west it varies between 50 and 75-m-thick.

Figure 3-4 Ground surface elevation overlain by the stream network (light blue), Bridgewater Creek watershed in the north (dark blue), and Tricks Creek watershed in the south (dark blue). The boundary of the glacial-outwash is shown by the dashed line.

Figure 3-5 Till surface elevation overlain by Bridgewater Creek watershed in the north (dark blue), and Tricks Creek watershed in the south (dark blue). The glacial- outwash channel is the distinct north-south trending feature incised into the surface of the till moraine. The white dots indicate the location of water well record and borehole data used to construct the till surface.

Figure 3-6 Bedrock surface elevation overlain by Bridgewater Creek watershed in the north (dark blue), and Tricks Creek watershed in the south (dark blue). The white dots indicate the location of water well record and borehole data used to construct the bedrock surface.

The base of the conceptual model corresponds to the till-bedrock interface (Fig. 3- 6). The bedrock surface dips gently to the west and ranges in elevation from 210 m asl, along the western edge of the Tricks Creek watershed, to over 270 m asl at the eastern tip of the Bridgewater Creek watershed. Beneath the outwash aquifer, the bedrock surface ranges between ~235 m asl along the eastern edge to ~220 m asl along the western edge.

The conceptual model comprises two major hydrostratigraphic units. These are the unconfined glacial-outwash aquifer and the underlying clay-till aquitard. The till- bedrock interface is interpreted as the base of the conceptual model such that the bedrock is considered only a boundary. From the geologic descriptions in the water well records for the area, the till is relatively uniform, and thus homogeneous hydraulic and thermal properties were assigned to this layer. Based on observations during drilling and measurements of aquifer hydraulic and thermal properties, the glacial outwash is

heterogeneous. An examination of exposed faces of outwash in aggregate pits, within the Tricks Creek watershed, indicates that there is a gradual transition from gravel and sand with cobbles, in the northern portion of the watershed, to fine and medium sand in the southern portion near the Bayfield River. Down-stream fining is a common transition in glacial outwash-channel deposits (Koteff, 1974; Barnett, 1992), and observations at this site are in agreement with the conclusion that this outwash channel formed

approximately 13,000 years ago during a glacial advance and drained southward (Cooper and Fitzgerald, 1977).

The surface-water divide between the Tricks Creek and Bridgewater Creek

watersheds (Fig. 3-2) is derived from the surface topography. Groundwater divides do not always coincide with watershed boundaries established from topography. As is shown later through the use of a 3-dimensional groundwater-flow model for the outwash aquifer, the groundwater divide in the outwash is approximately 1.1 km north of the surface-water divide. This groundwater divide is implemented as a model boundary in the

implementation of the numerical model in subsequent sections.

Finally, the outwash aquifer is bounded laterally by till moraines. This creates a setting in which recharge may enter the glacial-outwash aquifer by (1) direct infiltration of precipitation falling on the ground surface within the highly permeable aquifer, (2) infiltration, at the lateral till-aquifer contact, of unchanneled overland flow and shallow subsurface flow from the upland till moraines, and (3) leakage from tributary streams that originate on the till moraines and flow on to the outwash aquifer as channeled flow (Morrissey et al., 1988). The conceptual model (Fig. 3-7) accounts for recharge entering the aquifer by each of these mechanisms.

Figure 3-7 Conceptual hydrogeological model of groundwater and surface water flow in a valley-filled glacial-outwash aquifer incised into a till moraine.