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Environmental Services>Hydrogeology & Hydrogeochemistry>ORM Experience>Overview

Hydrogeology and Hydrogeochemistry
Oak Ridges Moraine Hydrogeology Study 1 : Overview

The Oak Ridges Moraine is a regionally significant hydrogeologic feature, and source of drinking water for more than 200,000 people north of the Greater Toronto Area. Approximately 70% of this population is serviced by municipal communal well fields and 30% by private wells. The Oak Ridges Moraine is a significant groundwater resource area for storage and recharge for supply of high quality drinking water and stream baseflow. This abstract provides an overview of the regional, hydrogeological resource management issues in the Oak Ridges Moraine and suggested planning considerations, presented to the Oak Ridges Moraine Technical Working Committee2 in 1996.

Presently, some major well fields in the Moraine are approaching sustainable yield and some are being mined. There is evidence that old 'softer' residual waters are being replaced with new 'hard' water recharge at some locations. Well field pumping decreases static levels (hydraulic head) and causes the movement towards and the mixing of upper Moraine aquifer waters with deep channel aquifer residual waters. However, measurable impact of pumping of the major deep aquifers has not been observed in stream flow records. Static level drawdowns in the central upper Moraine aquifers may induce increased snowmelt and stormwater flood recharge. Therefore local headwater stream baseflow emerging on the Moraine margins is not measurably affected, i.e. flood flows and evapotranspiration losses are reduced.

Although the groundwater quality in the Oak Ridges Moraine Aquifer system is generally good to excellent, the upper aquifers are unconfined and at risk to contamination from existing and future intensive land uses in the Moraine. Recharge increases with elevation from about 300 mm/yr on the lowlands flanks to about 400 mm/year at the higher elevations of the Moraine. The upper aquifers provide "reservoir" storage and recharge to the deeper aquifers along the Moraine axis and discharge to the headwater streams at the Moraine margin. Protection and maintenance of groundwater quality in the upper aquifers will protect the deeper aquifer and discharge to the headwater streams.

The approximate 275 m asl contour has been accepted as the 'toe of slope' definition for the Oak Ridges Moraine, and corresponds, for the most part, to a natural boundary between the high density urban forms encroaching from the south and the sparse rural developments historically characteristic of the Moraine.

A pervasive and widespread threat to the regional quality of the shallow Oak Ridges Moraine aquifers is road salt employed for highway de-icing. Stormwater and road salt seepage into the upper aquifers results in increases to chloride, sodium and hardness levels that exceed drinking water objectives and may render the water aesthetically unacceptable for a variety of domestic uses. Road salts are also a problem in combination with septic tank effluents in village plumes and impair a number of domestic and communal supplies on the Moraine. Road salts have also impacted surface waters and there is an increasing trend of chloride concentrations in most streams emerging from the Moraine.

Urban development, even when serviced with sanitary sewers, is a source of a wide variety of contaminants conveyed and recharged in storm runoff. In general, large communal sewage disposal systems are considered environmentally inappropriate for new residential development in the Oak Ridges Moraine upland area and headwater catchments above 275 m asl. There is little evidence to suggest that existing dispersed private individual soil effluent absorption systems in low density residential developments are adversely impacting the Oak Ridges Moraine aquifers.

As a general planning principle, lake based water supply expansion to Oak Ridges Moraine areas should be limited to below the 275 m asl elevation contour. The Moraine environment, i.e. at elevations above 275 m asl, should be protected from major new high density urban and transportation development.

The surface watershed divides in the Oak Ridges Moraine often differ substantially from the groundwater divides. Indeed there are localized areas within the Moraine where lateral groundwater flow direction may be opposite to surface flow direction and opposite to groundwater flow direction at alternate depths. Most existing literature water budget and stream base flow estimates closing to surface watershed boundaries should be regarded as 'suspect'.

Environmental databases related to the Moraine are often not adequately maintained, integrated and cross referenced by source agencies. At present, historical long-term environmental monitoring data at best is virtually inaccessible and at worst has been lost by the administrative processes of the multi-jurisdictional agencies which operate within the Moraine. Many old monitoring records appear to have been lost, destroyed, redistributed with administrative mergers, lost on key employee retirement, or buried in archives. Often the only source of continuous long term monitoring information are the production wells and the existing large landfill sites. Comprehensive integrated environmental information management systems should continue to be implemented.



1 Hunter and Associates and Raven Beck Environmental Ltd. (1996). Executive Summary and Technical Report, Hydrogeological Evaluation of the Oak Ridges Moraine Area, Part of Background Report No. 3 for the Oak Ridges Moraine Planning Study. Prepared for the Oak Ridges Moraine Technical Working Committee.

2 Oak Ridges Moraine Technical Working Committee: Ministry of Natural Resources; Geological Survey of Canada; Ministry of the Environment and Energy; Ministry of Municipal Affairs; Regional Municipality of Peel, York and Durham; Town of Caledon; Metropolitan Toronto and Region Conservation Authority; Ganaraska Region Conservation Authority; Urban Development Institute of Ontario; Aggregate Producers Association of Ontario; Federation of Ontario Naturalists; Conservation Council of Ontario and the STORM Coalition.