The chemistry of surface water, precipitation, and ground water in Filson Creek watershed was studied to evaluate the kinetics of dissolution of silicate minerals in a natural watershed underlain by troctolite. Annual chemical budgets in 1977 showed net losses of dissolved silica, bicarbonate, calcium, magnesium, and sodium of about 31.0, 19.6, 4.8, 3.8, and 2.1 kg per hectare per year, respectively, and net gains of dissolved potassium and sulfate of 0.4 and 8.9 kg/ha/yr. On a molar basis, about 1.5 times as much magnesium was lost as was calcium, even though the amount of mafic minerals in till and bedrock underlying the watershed is about 10 times less than the amount of calcic plagioclase. Molar concentrations of magnesium in base flow and ground water were about the same as concentrations of calcium and sodium. These field data indicate that magnesium loss from the mafic minerals may be faster than either calcium or sodium loss from plagioclase.

Stoichiometric “reconstruction” of primary minerals from base-flow water chemistry and kaolinite was done as a first approximation to evaluate the rates of loss. The calculations indicated that the release of magnesium from mafic minerals would have to be more than ten times faster than the release of calcium and sodium from plagioclase to account for the amount of magnesium in base flow. These calculations compare favorably with experimental linear rates of cation and silica release from forsterite, enstatite, augite, and labradorite.

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