Open File Report 1965-10

Open File Report 1965-10

OFR 1965-10

Evaluation of the Grande Prairie Alluvial-Terrace Water Supply

Author(s) Lennox, D.H. Date 1965-07-01

This report presents the general conclusions from a detailed study of the available aquifer-test and recharge-test data for a number of test sites along the Wapiti River south of Grande Prairie. Each test is reported on individually. The results as a whole suggest that the alluvial terrace on the north side of the river, which is presently used as a source of water supply for Grande Prairie, is unsuitable for that purpose under natural conditions. Under natural conditions water cannot be induced to flow into the aquifer from the river when water is pumped from the aquifer and, consequently, it is not possible to reduce the high iron concentration which has been a continual problem during the two years that the city has made use of the aquifer. River water can be introduced artificially into the aquifer by pumping it into a recharge pit but, although there is a tendency for iron concentration to be reduced, the reduction may not be sufficient. In addition, the costs of operating this artificial-recharge system might be excessive, because, besides the additional pumping installations required, there would be no need for periodic cleaning and maintenance of the recharge pits. An infiltration gallery projecting out under the river is a possible alternative but test results indicate that a layer of fine sediment permeates the sands and gravels of the river bottom. Such a layer would tend to inhibit the free flow of river water into the gallery and there could be major maintenance costs associated with its operation.

It is suggested that an alternative well worth investigation is the groundwater potential in the bedrock underlying the alluvial terrace. A successful bedrock supply could eliminate or minimize treatment problems. Such supplies have proved successful elsewhere in Alberta under similar conditions. If a bedrock supply is not feasible, experiments could be made with an infiltration gallery or an attempt could be made to design an efficient artificial-recharge system. The possibilities of direct intake from the river or from an oxbow lake located at the north side of the terrace are ignored in the report but they might take precedence over the infiltration gallery or artificial recharge. The investigation of a bedrock supply should, however, be given prime consideration.

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