4. Zehner Aquifer, Existing Water Balance and Safe Yield
Since 1988, Maathuis and van der Kamp’s study has been the guideline for other studies to infer the Zehner aquifer boundary, water balance and groundwater flow in the Regina East area. Regina Aquifer Management Plan (Part A- Allocation Plan) commissioned by the Saskatchewan Water Corporation (SaskWater) in 1989 had inferred the safe yield for the Zehner aquifer as 4800 dam3/year (Clifton Associates Ltd., 1989). This yield is from the difference between the estimated natural recharge and discharge mentioned in the 1988 water balance (Table 1). It is referred to as inflow less natural outflow. At present, WSA accepts this as the safe yield for allocation limits within the Zehner aquifer.
Table 1 is the detailed water balance after Clifton Associates Ltd., (1989) report. Total inflow to the Zehner aquifer is assumed to be at 6,750 dam3/year. The estimated infiltration rate of 4,000 to 6,000 dam3/year was evaluated by multiplying 2.5 to 4 cm/year over the 150 Km2 area of the Zehner aquifer. Recharge through depression-focused recharge was assumed to be 1,000 to 2,000 dam3/year over the entire aquifer. Contribution from Condie aquifer east section was assumed to be relatively small through the underlying thick till at 0 to 500 dam3/year.
Natural discharge through springs and flowing wells was estimated at 300 to 600 dam3/year and inter aquifer flow through the hydraulic barrier separating the Regina aquifer at 1,000 to 1,500 dam3/year. This value from the Zehner aquifer into the Regina aquifer was based on the hydraulic gradient and transmissivity in the area of the landfill. Due to the high hydraulic head of the Pilot Butte South well (Zehner aquifer) compared to the Pilot Butte North well (Condie aquifer) it was inferred that there is an upgradient flow from the Zehner aquifer to the Condie aquifer in this vicinity (B-Zone) and was estimated to be from 0 to 500 dam3/year. As per the 1990 water use consumption rates, the water withdrawn through production wells was taken as 5,696 dam3/year. This water balance indicates that the change in storage within the Zehner aquifer is in negative –896 dam3/year because the outflow volume is greater than the inflow volume. The natural outflow through the aquifer was 1,950 dam3/year and when subtracted from the natural inflow of 6,750 dam3/year it gives the safe yield of 4,800 dam3/year. Hence, inflow less natural outflow.
Table 1: Existing water balance for Zehner aquifer (After Maathuis & van der Kamp, 1988, Clifton Associates Ltd., 1989)
