Aquifers and aquitards along section A-B [PDF, 68.3 kb, viewer]

Aquifers and aquitards along section C-D [PDF, 50.0 kb, viewer]

The Precambrian highlands are often covered by only a thin layer of unconsolidated sediments, except in depressions and along valleys, as indicated on the Drift Thickness map. Similarly, Paleozoic bedrock located at an altitude above 100 metres a.s.l, are often outcropping or covered by only a thin layer of unconsolidated sediments. Consequently, overburden aquifers over these rocks are generally localized, highly controlled by the surface topography, and are not a major source of groundwater, except along river valleys that are covered by ice-contact, glaciofluvial and related granular sediments.

The Paleozoic lowlands are generally covered by unconsolidated sediments (drift) that can attain more than 100 metres thick ( Maps - GeoServ ). These sediments range from granular materials, that can form important aquifers, to thick deposits of fine materials that are acting as major aquitards. In groundwater studies, it is important to understand the hydrogeological characteristics and distribution of these materials. The hydrogeological characteristics ( Maps - GeoServ ) were derived from the surficial geology map, which was simplified, based on the nature of surficial materials by grouping geological formations based on the nature of materials ( Maps - GeoServ ), mainly grain size, rather than on their genesis. It is also important to study the sequence of geological events that deposited the various layers of sediments (stratigraphy) since this determines how the various aquifers are connected. Although detailed studies are required to define accurately the flow of water in surficial materials at the local level, it is possible to infer regional patterns in groundwater flow based on the nature, distribution, stratigraphy, and topography of surficial materials as discussed below.

Till aquifers
As shown in the stratigraphic sections A-B [PDF, 68.3 kb, viewer] and C-D [PDF, 50.0 kb, viewer], the lower overburden stratigraphic unit is a till deposited directly over the bedrock, corresponding to a compact layer of sand, silt and clay with some gravel, overlain by a less compact layer of sand, gravel and boulders in areas below ~200 metres a.s.l. (Champlain Sea level). This unit can attain 20 metres thick and forms a permeable layer at the surface becoming less permeable at depth, as the till becomes more compact and in area where clay content is more important. The till layer generally forms a good aquifer and recharge is greatest in areas where the till outcrops at the surface, as indicated on the Surficial Material map, or where overlain by a permeable layer, as shown in sections A-B and C-D. In areas where the till is confined by a layer of relatively impermeable clay, artesian pressure can bring the water level close to ground surface.

Glaciofluvial aquifers:
On the Surficial Materials map and on sections A-B and C-D, glaciofluvial sediments also include reworked glaciofluvial material by wave action of the Champlain Sea. This unit, which is composed of coarse sand, gravel and cobbles up to 40 metres thick, forms very permeable aquifers and can be good sources of well water. Where glaciofluvial deposits outcrop, they can form excellent recharge zones as this geological unit was generally either deposited directly on bedrock or is in contact with the permeable glacial till. A typical example of a glaciofluvial ridge, located near the Ottawa International Airport, can be seen in the centre of section A-B.

Marine aquitard
Following deglaciation, a period of marine invasion (maximum elevation 200-220 metres) deposited a layer of clays and clayey silts that can attain 100 metres thick. These massive marine deposits are generally saturated with water but are of low permeability, thus forming an aquitard between the underlying till aquifers and aquifers in overlying surficial deposits. Where marine deposits outcrop, water penetrates very slowly into the soil and tend to flow as surface runoff. Wells dug into marine deposits have very poor yield due to the slow migration of water in this material.

Surficial sand aquifers.
Surficial sandy deposits were created during Champlain Sea regression as wave action reworked the outcropping till and fluvioglacial materials and redeposited the finer material as beach deposits. A second source of sandy material comes from deltaic sediments deposited during the later sequence marine regression, and finally in modern river channels. In upland areas where sand deposits overly bedrock, glaciofluvial deposits or till, they form good aquifer recharge zones. In many instances, the deltaic sandy deposits (see Surficial Geology map) overly marine clays and form localized shallow flow systems (sections A-B and C-D). Groundwater in these aquifers generally flows in the direction of topographic lows, discharges into the surface drainage system and provides little recharge to till or bedrock aquifers.

Quality of overburden aquifers.
Surface water quality analyses indicate that total dissolved solids are generally acceptable. Saline water has been reported in till aquifers covered by a thick layer of marine clay, however, in many cases the salinity can be caused by contamination from shaly bedrock aquifers. Wells reported as being fed by bedrock aquifers can also contain water coming from till aquifers, mainly if the well casing is not well anchored into the bedrock or if the bedrock surface is fractured. Aquifers exposed at the surface are most susceptible to contamination. Where marine aquitards are present, underlying aquifers may be protected from surface contamination. In rural farming areas and in areas of closely spaced lots where households are using septic tank systems, overburden aquifers are susceptible to contamination by waste disposal. In areas of thin overburden, contamination can extend to bedrock aquifers especially where the overburden consists of granular material.