Geohydrology of the metropolitan Winnipeg area as related to groundwater supply and construction

An extensive confined aquifer, currently pumped at the rate of 3 billion gallons (13.6 billion l) per year, occurs in the fractured and jointed upper 50 ft (15 m) of the thick Paleozoic carbonate rock sequence underlying metropolitan Winnipeg. The karstic bedrock surface is mantled by 30 (9 m) to 200 ft (60 m) of glacial drift. The drift is composed of 20 ft (6 m) of till overlain by lacustrine sediments, mainly clays, that average 40 ft (12 m) in thickness. Two minor water-bearing zones occur in the drift, one in the silt deposits in the upper 5 to 15 ft (1.5 to 4.5 m) of the lacustrine unit and the other in the top few feet of the till. Moderately permeable sandstones interbedded with shales underlie the carbonate rock at depths between 250 and 800 ft (75 and 240 m). The sandstones are aquifers that contain saline water. A partially confined glaciofluvial aquifer covering 60 square miles (155 square km) occurs to the northeast of the city. The karstic bedrock surface, a controlling parameter of the groundwater flow systems, slopes towards the Red River Valley from recharge areas located in uplands along the borders of the Red River Basin.Groundwater has been a source of water supply in the Winnipeg area since the early 1800's. The Upper Carbonate aquifer, which during the first two decades of the twentieth century supplied all of the city's water requirements, has a transmissibility ranging between 2000 and 200 000 gallons (24.8 and 2480 m 3 /m/day)/ft/day. Total groundwater pumpage was 10 7 gallons (4.5 × 10 7 l)/day at the time groundwater usage was curtailed in 1919 following the completion of an aqueduct from the Lake of the Woods. The water in the aquifer is generally fresh to brackish; however, south of the Assiniboine River the water is brackish to saline and is not potable. The groundwater which in most places has a temperature of 39° to 43° F (3.9 to 6.1° C) is used mainly for commercial cooling purposes. Pumpage varies from 5 × 10° gallons (22.7 × 10 6 l)/day in the winter months to 10 7 gallons (4.5 × 10 7 l)/day during the summer air-conditioning period.Groundwater withdrawals have created a major drawdown cone in the central industrial area of metropolitan Winnipeg. Elevations of the piezometric surface range between 800 and 1500 ft (240 and 450 m) in the recharge areas and are below 700 ft (210 m) in the drawdown cone. The Winnipeg drawdown cone is at the centre of a lateral radial flow system. The aquifer also loses water by natural discharge into the Assiniboine and Red Rivers and into the Red River Floodway.Under natural conditions the saline water-bearing Upper Sandstone aquifer is isolated hydraulically from the Lower Carbonate aquifer by the 10 to 20 ft (3 to 6 m) thick upper shale unit of the Winnipeg formation. However where open well bores penetrate the sandstone aquifers, higher hydraulic heads force saline water up into the carbonate aquifers.Hydrostatic pressure in the Upper Carbonate aquifer affects construction works that intercept the aquifer or initiate hydraulic fracturing by unloading the confining layers. Groundwater discharge from this aquifer has affected a number of excavation projects. The largest construction groundwater difficulties encountered in the metropolitan Winnipeg area occurred during the construction of the Red River Floodway. Groundwater flows from the aquifer interfered with the excavation of the lower sections of the channel and a number of structures. Groundwater seepage was first encountered on this project when artesian water discharged into a test pit that had been excavated to a depth of 20 ft (6 m) below the piezometric surface of the aquifer. Inflows commonly exceeded several hundred gallons/min. Groundwater control during construction of the floodway inlet control structure required a grout curtain 4000 ft (1220 m) long in the Upper Carbonate aquifer and the drift, and an extensive dewatering system. Knowledge of the groundwater regime prior to construction facilitated excavation and construction, and eliminated claims for unforeseen conditions.