Natural temporal variations in the chemistry of shallow groundwater, Athabasca Oil Sands area, Alberta

Abstract

Significant natural variation of the chemistry of shallow groundwater was observed from 1977 through 1979 in three wells located in the Athabasca Oil Sands area, Alberta. The wells are between 5 and 8 m deep and are located in boreal forest far from any direct influence by man.The coefficient of variation of total dissolved solids for the well sampled monthly for 35 months was 34%, while those for the two wells sampled bi-monthly were 21 and 11%. The coefficient of variation for individual constituents was generally higher than the above values.An annual pattern of variation in shallow groundwater chemistry is recognized. Calcium, magnesium, and bicarbonate reach lowest annual concentration in the spring and rise rapidly by early summer. Concentration of these ions gradually decreases through fall and winter. This cycle is related to the abundance of dissolved carbon dioxide in recharge water and is controlled to a great extent by the abundant muskeg.Spring and fall are typically times of highest sulfate concentrations. This is coincident with recharge events and is related to leaching of sulfur compounds. High chloride during winter is related to slower rates of groundwater flow and the consequent increased opportunity for release of ions from chloride-bearing minerals.Information from other wells in the Athabasca Oil Sands area indicates that the coefficient of variation of total dissolved solids with respect to time generally decreases with depth. Coefficients of variation might be expected to be as high as 35% at depths of 10 m; the range declines to a relatively constant 4% at depths greater than 150 m.