| Lower deposits aquifer |
| Lower sediment is formed of poorly exposed, interbedded lake sediment and till. It comprises sand, silt, clay, till, and distinctive organic-rich and fossil-bearing beds. The unit overlies the bedrock and is covered by Newmarket Till, Oak Ridges Moraine sediments and Halton Till. The aquifer is confined by the Newmarket Till aquitard. However, the till is incised and sometimes eroded entirely, resulting in local high-conductivity windows between upper and lower aquifer systems. The unit is characterized by sandy formations with good hydraulic conductivity and aquifer potential. Lower sediment hosts a number of key regional aquifers that have prolific yields; however, the aquifer-aquitard system is poorly known. A part of the groundwater percolating through Oak Ridges Moraine sediments recharges lower sediments aquifers. In the lower sediments, groundwater discharges as springs along deep river valleys where the river has eroded into or beneath the Newmarket Till or it can migrate through the complete sediment column to discharge at Lake Ontario. Lower sediments groundwater is used for municipal water supply and transmits inter-watershed flow. Groundwater quality is good, however the lowest formation of the unit may have water-quality problems related to methane and dissolved organic carbon. |
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| Newmarket Till aquitard |
| In ascending order, the stratigraphic units of the Oak Ridges Moraine are: Paleozoic bedrock, Lower deposits, Newmarket Till, Oak Ridges Moraine sediments, and Halton till. Newmarket till forms an areally extensive sheet, the upper contact of which is a highly irregular erosional surface marked by incised channels and drumlins. Newmarket till is a thick aquitard, characterized by low hydraulic conductivity. Flow through the Newmarket Till aquitard is attributed primarily to interconnections among various heterogeneities of the unit. Flow within the diamicton matrix itself is considered to be minor. In some places, the till is incised, and sometimes eroded entirely, by a network of sand- and gravel-filled channels forming productive aquifers. The sediments filling the channels are Oak Ridges sediments. In other places, the channels breach the till entirely and form high-conductivity windows connecting upper and lower aquifer systems. |
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| Oak Ridges Moraine sediments aquifer |
| In ascending order, the stratigraphic units of the Oak Ridges Moraine (ORM) are: Paleozoic bedrock, Lower deposits, Newmarket Till, Oak Ridges Moraine sediments, and Halton Till. Oak Ridges Moraine sediments overlie a regional unconformity (cut on Newmarket Till) with infill tunnel channels and forms a prominent east-west ridge. Channels infill with coarse glaciofluvial sediments form prolific regional aquifers. ORM sediments consist mostly of silt and fine sand, but also include large gravel seams and minor clay and diamicton. The aquifer is unconfined, except where it underlies Halton Till. Hydraulic conductivity is good and the aquifer potential is one of the highest of the country. ORM sediment forms a major recharge area. Water percolates (200-400mm/ yr) through the moraine ridge and mostly discharges from dozens of headwater stream springs sourced in the ORM. Groundwater that does not discharge from headwater stream springs can move deeper through the ORM into lower aquifers, then discharge as springs along deep river valleys where the river has eroded into or beneath the Newmarket Till. It can also migrate through the complete sediment column to discharge at Lake Ontario. These discharge pathways are controlled by the distribution of permeable channel and subaqueous fan structures, by confining aquitards, and by regional gradients with up to 300 m head differential to Lake Ontario. The ORM sediment aquifer is exploited for domestic uses due to high yields and good water quality. |
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| Halton Till aquitard |
| In ascending order, the stratigraphic units of the Oak Ridges Moraine are: Paleozoic bedrock, Lower deposits, Newmarket Till, Oak Ridges Moraine sediments, and Halton till. The latter is predominantly a massive to laminated mud unit with low stone count and local interbeds of sand and gravel. Along the upper flanks of the ORM the unit is 1-5 m thick and southward can increase in thickness up to 30 m. Halton Till forms a discontinuous aquitard. This low permeability unit partially confines ORM aquifers and restricts recharge. Groundwater is fresh, but very hard. Iron concentrations are above the aesthetic objective. |
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