Bedrock formations can be grouped into five main aquifers, based on the quality and yield of each aquifer. The characteristics of each aquifer are closely related to the nature of host formations as shown on the Bedrock Aquifers map ( Maps - GeoServ ).

Sound Precambrian rocks are relatively impermeable to ground water; movement of water is restricted to cracks, joints and fractures, or solution channels in limestone. Except for limestone bedrock, the groundwater yield of Precambrian aquifers is generally limited. Since the quality of water deteriorates with depth and the yield does not increase, many of the wells are shallow (<20m), and yield is often dependent on the recharge capacity of the overlying Quaternary deposits. Water supply in Precambrian bedrock is highly variable. A method used by drillers to improve well yield is to increase bedrock fractures around the drilled hole, with the use of explosives.

The quality of groundwater is also highly variable, due to the complexity of bedrock stratigraphy. Mineral content generally increases with depth and areas of elevated mineralization and sulfur occur randomly alongside good quality aquifers.

Paleozoic aquifers:
Even though the stratigraphy of the Paleozoic rocks is relatively simple compared to the Precambrian bedrock, the study of bedrock aquifers in these younger rocks is complicated by the numerous faults and block displacement throughout the region, and the migration of groundwater within the cracks and fractures. As a general principle the quality and quantity of groundwater is very good in the older and lower formations and deteriorates gradually from the base of the stratigraphic sequence towards the younger formations, corresponding also to an increase in organic matter in rock formations. Except for formations south of the Carp Ridge, bedrock formations located at the surface are progressively younger going east ( Maps - GeoServ ), consequently, the quality and quantity of groundwater decrease going east.

Although, some general guidelines can be established regarding the quality and quantity of groundwater for each bedrock formation, there are significant regional variations. These variations are due to the presence or absence of bedrock fractures at the site where the well is drilled, or to the mixing of water from different aquifers. Groundwater coming from different aquifers can be mixed within a well if drilled through more than one formation, or if water from one formation flows to an other formation through fractures or cracks. The samples provided in the Water Chemistry Database give an indication of the chemistry of the water found at that location, but chemical composition can vary within the same formation at different locations.

Faults and fault blocks play an important role in regional pattern of groundwater flow. Yield and chemical properties of water often differ substantially with the crossing of fault boundaries due to several factors: the bedrock stratigraphy can change completely due to fault block displacement (Sections A-B and C-D), the bedding which is normally nearly horizontal can dip steeply within a fault zone, and the presence of calcite and related mineralization along faults can create barriers to groundwater flow..

Stratigraphic Section Along Line A-B [PDF, 74.4 kb, viewer]

Stratigraphic Section Along Line C-D [PDF, 56.5 kb, viewer]

Covey Hill and Nepean aquifers.
The sandstone and conglomerate rocks of the Covey Hill and Nepean formations provide the best aquifers in the region, yields exceeding domestic requirements. Groundwater flows through a series of vertical and horizontal sets of joints, and the joints located at the bottom and top of the formations yield the most water. The formations outcrop sporadically along the margins of the Paleozoic rocks and south of Ottawa ( Maps - GeoServ ), but generally lie at great depth, being at the bottom of the Paleozoic sequence.

March and Oxford aquifers.
These rocks consist mainly of dolostone, grading from sandy at the base of the March formation to mainly dolomitic in the Oxford formation. These rocks are relatively impermeable, consequently, water flows through joints at different depth giving several aquifers. Water quality and quantity is generally good, with low average mineral content compared to other formations, but local variations are frequent. In places, these aquifers can have high mineral content and sulphide odour may occur.

Rockliffe, Gull River, Bobcaygeon, Verulam and Lindsay aquifers.
The stratigraphy from this group is relatively complex, ranging from sandstone and shale for the Rockliffe formation grading to limestone for the Gull River and Bobcaygeon formations and to calcareous shale for the Verulam and Lindsay formations. Each formation contains interbeds of rocks similar to the adjacent formation. Aquifers in these rocks are restricted to seams that are in the calcareous shale layers. The massive crystalline limestone layers generally contains little to no water, aquifers in other formations are just sufficient for domestic purposes. Water is of potable quality in the western part but the quality deteriorates eastward with high mineral content and sulfur odour.

Eastview, Billings, Carlsbad and Queenston aquifers.
These formations are characterized by the predominance of fossiliferous and petroliferous (Eastview and Billings) shale and limestone. Supply is generally sufficient for domestic use with thick sections with no water. The quality is generally poor with mineral content in excess of accepted levels; aquifers commonly yield saline and sulfurous water.