Chemistry and Hydrology of Groundwater in the Vicinity of Waterton Reservoir, Alberta

This report describes the influence of the filling of Waterton reservoir in the spring of 1965 on the chemistry and hydrology of groundwater in the vicinity of the reservoir, and attempts to estimate the amount of underground outflow from the reservoir through the Cochrane Valley aquifer, a buried bedrock valley aquifer hydraulically connected to the reservoir, into the Belly River.

Prior to the filling of the reservoir, samples of groundwater were obtained from wells and springs in an area of approximately 120 square miles and analyzed for the major constituents. Statistical analysis and trend-surface fitting of the chemical data showed that fresh groundwater predominates in the unconsolidated sediments of the topographically low areas surrounding the major water courses.

Two years after the filling of the reservoir a second set of groundwater samples was obtained from the same sources and analyzed. Although it could be shown that, on the average, a statistically significant change to a fresher groundwater had occurred, the spatial distribution of the change contradicts the assumption that it is caused by the filling of the reservoir; it is shown that the change is likely the result of an increase in precipitation in the years between the two samplings.

The hydrological investigation has been limited to the aquifer in the Cochrane buried bedrock valley; the aquifer, which reaches a thickness of over 90 feet locally, consists predominantly of clean gravel with occasional thin sand lenses and is covered by approximately 100 feet of till.

The rise of the level of the reservoir to its operating level during the spring of 1965, as well as later fluctuations, was followed almost instantaneously by a rise in the hydraulic head in the aquifer, which was monitored by a set of 5 piezometers.

From the measured average gradient of the hydraulic head and the estimated total overflow through flowing wells and springs, an average hydraulic conductivity of 2,755 igpd/ft (imperial gallons per day per foot) and an average transmissibility of 143,000 igpd/ft were calculated; these figures are close to those determined from a pump test.

In 1969 a number of large relief wells had been drilled to lessen the pressure in the aquifer and the danger of uncontrolled outflow; the combined free flow from these wells and other flowing wells in the area was 4,750 gallons per minute.

A comparison of the average hydraulic gradients in the portion of the aquifer between the reservoir and the flowing wells in 1965 and 1969 indicated that a sizeable portion of the total outflow was not derived from the reservoir, but supplied by leakage from the till or bedrock or both; it is further concluded that there is no significant underground outflow into the Belly River.