Spatial and temporal variations in Groundwater chemistry of shallow aquifers in the Sand River area, Alberta

Groundwater samples from shallow (less than 80 m) wells in the Sand River area (NTS 73L) were collected and chemically analyzed in August 1974 and in March 1975 to assess the magnitude of seasonal variation of groundwater composition. The samples were collected along a west-east traverse that essentially paralleled the regional topographic slope and the inferred regional groundwater flow direction. The means and standard deviations (frequency histograms) of the concentration differences (in mg/l) between summer and winter samplings were comparable in magnitude and showed that no statistical difference could be found between the two suites of samples. The variable shape and range of the frequency distributions, however, showed that a simple statistical averaging approach was not adequate as a way of interpreting the temporal variations. Where the frequency distributions were asymmetrical, as in the case of sodium magnesium and chloride, the trend was for a higher summer than winter concentration. When the differences were plotted along the traverse, generally positive winter-summer concentration differences were found in drift and bedrock aquifers associated with the Wapiti Formation, while mainly negative shifts of the concentration differences were found in the thin drift aquifer underlain by the Lea Park Formation. Positive winter to summer shifts in the area of Wapiti Formation subcrop are likely the result of dilution and mixing. Negative shifts, on the other hand, were found primarily in the area underlain by the Lea Park Formation and were explained as due to flushing of salts from the soil and unsaturated zones during summer recharge. The groundwaters were very close to chemical equilibrium with respect to the minerals aragonite, calcite, dolomite, magnesite, siderite amorphous silica and Ca-montmorillonite. These observations tend to support the belief that chemical variability of groundwater composition is constrained by equilibrium reactions with aquifer minerals. Carbonate and clay minerals appeared to be particularly important in controlling the composition because of the consistency of chemical equilibrium and the fact that these minerals are found everywhere in the drift and bedrock aquifers of the region.