A Hydrogeological Study of the Three Hills Area, Alberta

A program of groundwater exploration, reconnaissance, and research was carried out in the Three Hills area in 1965 and 1966. The area of study covers 204 square miles of the rolling grass lands of the entral Alberta Plains, 50 miles east of the Rocky Mountain Foothills. Information concerning the geology and groundwater was obtained from eological reports, oil-company records, a water-well survey, and ifteen drilled and tested bore holes, providing an average density of points of various observations of approximately 1.5 per square mile.

The freshwater-bearing rocks consist of cross-bedded, interbedded, and lenticular, Upper Cretaceous argillaceous siltstones and andstones of river channel and floodplain origin, overlain by a veneer of lacustrine and glacial clays and sands ranging from 0 to 110 feet in thickness. The strata dip westward at 15 ft/mi and hydrogeologic phenomena indicate the sporadic existence of east-west oriented minor faults.

A supply of potable groundwater of 45 gpm from two sites has been located for the Town of Three Hills, with an additional 65 gpm indicated but requiring further testing. The average, renewing potential of groundwater in the area is estimated to be between 20 and 60 gpm per square mile. The total dissolved solids content in the groundwaters ranges from approximately 400 ppm to over 10,000 ppm, the majority being in the range of 1000 to 2000 ppm. The most common type of water is sodium bicarbonate-sulfate. Water-storing and ransmitting properties of the rocks are poor, generally permitting the development of only 0 to 5 gpm capacity wells. Development of wells with capacities of 5 to 50 gpm is possible only in isolated and regularly distributed areas a few square miles in extent at most, while single wells with a sustained production of over 50 gpm can probably not be established.

Due to the heterogeneity of the rock formations, pumping-test data can not be interpreted by known standard methods. However, a combination of type-curve solutions and geological interpretation made possible the estimation of long-term safe yields of proposed permanent production wells. The transmissibility calculated from the early parts of the time-drawdown curves is believed to represent the aquifer into which the well is drilled, while values having ransmissibility dimensions may be obtained from late portions of the pumping test, and are used for safe-yield calculations. Such a value is called 'equivalent transmissibility' and is thought to be characteristic of the average water- transmitting properties of the ntire rock volume traversed by the front of the cone of depression up to the time the field-data curve is used for calculation. On continued pumping, the successive values are expected to decrease approaching he average value of the basin transmissibility, thus that calculated from regional flow-system analyses.

In order to facilitate the analysis of regional groundwater flow a wo-dimensional electric model has been designed, constructed, and applied. With the use of conducting paper and a multi-electrode arrangement, the distribution and intensity of the natural, hydraulic recharge and discharge become solutions rather than prior imposed constraints of the flow problem. In a homogeneous geologic environment flow lines converge toward regions of a steeply sloping water table regardless of whether these occur in areas of recharge or discharge. Abrupt changes of chemical quality of groundwater have been explained by the analysis of the electric cross sections.

The depth of intensive flow in the Three Hills area has been found to be a function of the topography. A correlation between rock permeability and flow intensity is suggested by assuming that a differential internal chemical and physical weathering is brought bout by different intensities of groundwater flow. The possibility of reduction in the bentonites' swelling properties by exchanging ortions of their sodium ions with the groundwater-transported calcium ions is postulated.

The area of study is divided into two, approximately equal parts: (1)an area of downward groundwater flow; and (2) an area of upward groundwater flow. The natural basin (dynamic reserves) is estimated to be 1.35 inches per year, or 9 per cent of the precipitation. The average velocity of groundwater flow is approximately 0.1 feet per day, and the average rock permeability in the basins is 450 gpd/square foot or 310 darcys.