Recharge Through a Regional Till Aquitard: Three‐Dimensional Flow Model Water Balance Approach

Abstract

In southern Ontario, vertical leakage through a regionally extensive till is the primary source of recharge to underlying aquifers used for domestic and municipal water supply. Since leakage is largely controlled by the bulk hydraulic conductivity (K) of the aquitard, accurate estimates of K are necessary to quantify the resource. Considerable controversy exists regarding estimates of K for this aquitard, which vary according to the scale of the test method. For the till matrix, estimates from core samples and slug tests consistently range from 10−11 to 10−10 m/s. Isotopic evidence (3H), on the other hand, indicates that nonmatrix structures such as sand lenses, erosional surfaces, joints, and fractures significantly enhance till permeability. This is confirmed by slug test, pump test, recharge, and water balance studies, which show that K varies over seven orders of magnitude (10−12 to 10−5 m/s). To provide a regional estimate of bulk K and a reliable estimate of vertical recharge through the Northern Till, a numerical ground water flow model was constructed for the Duffins and Petticoat Creek drainage hasin. The model was calibrated to measurements of hydraulic head and estimates and measurements of base flow throughout the basin. This model demonstrates that the vertical hydraulic conductivity (Kv) for the Northern Till ranges from 5 × 10−10 to 5 × 10−9 m/s, values that are up to 2.5 orders of magnitude greater than matrix K estimates. Regional recharge through the Northern Till is estimated to range from 30 to 35 mm/a.

Groundwater Recharge Assessment in the Chateauguay River Watershed

“…The 3D model was further simplified for a maximum of three stratigraphic units for each vertical profile. The initial saturated hydraulic conductivities (K s ) were assigned on the basis of values reported by previous studies carried out in the watershed (Dufresne, 1979;Hamel, 2002;Pontlevoy et al, 2004;Technorem, 1998) and from data collected from the literature (de Marsily, 1986;Fetter, 1980;Freeze and Cherry, 1979;Gerber and Howard, 2000). The K s value was used as the main calibration parameter in the infiltration simulations.…”

“…Airport, Montreal. The Canadian Climate Normals provided the quarterly relative humidity for the period 1971-2000(Environment Canada, 2004b. To evaluate the observed differences in climate records, Thiessen polygons were used to define individual areas of influence around each of the climate stations.…”

A 3-dimensional geological model of the Oak Ridges Moraine area, Ontario, Canada

“…This low permeability unit partially confines ORM aquifers. Thin sandy interbeds within Halton Till allow recharge of ∼200-300 mm a-1 (Gerber and Howard, 2000). The unit yields domestic supply aquifers, and provides pathways for groundwater flow (springs) of ∼5-10 l/s from underlying ORM sediment.…”

Characterizing the hydrogeology of a complex clay-rich aquitard system using detailed vertical profiles of the stable isotopes of water

“…The bulk hydraulic conductivity ðKÞ of aquitards reflects that of the geologic matrix material (typically , 1 £ 10 210 m/s; Gerber and Howard, 2000;Hendry, 1982;Keller and van der Kamp, 1986;McKay et al, 1993). Such low K values result in groundwater flow rates on the order of 1 m per 1000 years (Hendry, 1988;Keller et al, 1989;Remenda et al, 1996), to , 1 m per 10,000 years (Hendry and Wassenaar, 1999;Shaw and Hendry, 1998).…”