NEW APPROACHES TO THE EVALUATION OF GROUNDWATER RESOURCES IN CANADIAN PRAIRIE WATERSHEDS

Groundwater is a critical source of fresh water in the Canadian Prairies. The Paskapoo Aquifer, covering 65000 km² of central to southern Alberta, is the most significant supply of groundwater in the Prairie Provinces. With 100% of the surface water rights in the Bow River and the Oldman River watersheds already allocated and a large population growth, there will be a growing demand on this heavily utilized groundwater resource. This vital resource must be properly managed to maintain a strong economy.

Well yield in Alberta is evaluated using the Q₂₀ method, which assumes a homogeneous aquifer of infinite extent. However, the Paskapoo sandstone formation was deposited in fluvial channels resulting in a highly complex and heterogeneous aquifer. As a result, conventional methods of evaluating well yields have resulted in alarming declines in aquifer levels in many cases. One of these case studies has been examined in detail.

Regulated groundwater pumping for municipal water supply in Irricana, Alberta, a small hamlet 39 km north-east of Calgary, caused local water levels to drop over 10 meters in 21 years, forcing the municipality to abandon the wells and switch to surface water supply. Water level decline due to pumping was calculated to be significantly smaller at the time of well yield evaluation. A 48-hour pumping test was conducted on one of the now abandoned water wells, and the aquifer response was monitored in 7 observation wells. Barometric pressure and earth tide loading effects were removed from drawdown data and the residual drawdown levels were used to mathematically extend the length of the pumping test to 21 days. The analysis displayed properties of a strip-channel aquifer.

These results were verified with a geological interpretation of the surrounding area. Sub-surface formation logs of the local water wells and geophysical gamma-radiation logs of the local gas production wells were used to create sand-fraction maps of the surrounding area. The geological analysis also displays an aquifer of finite extent. The combined hydrogeological and geological analysis can provide a more accurate interpretation of the aquifer than the current conventional methods. This process can potentially result in a more realistic well yield analysis, which will mitigate the possibility of excessive groundwater extraction.