STABLE CHLORINE ISOTOPES IN SEDIMENT PORE WATERS OF LAKE ONTARIO AND LAKE ERIE

摘要

The lower Great Lakes are a large and relatively pure source of water. Potential pollution has always been considered to come from surface sources such as direct effluent or river transport. Recent investigations have shown a direct link between groundwater input and possible contaminant sources. Land based groundwater geochemical surveys in southern Ontario and New York State have shown the existence of highly saline fluids in the shallow environment. These fluids are believed to have their origin in the deep sedimentary rock environment. In general, our initial study of lake sediments has dealt with interstitial water quality to determine the source of the water. This paper uses stable chlorine isotopes to identify the source of the water and the different transport mechanisms that move the solutes into the sediments. Sixty-two cores up to 20 metres in length were collected using the Canadian scientific ship Limnos from the Canadian Center for Inland Waters, Burlington, Ontario. Sites were selected according to sediment availability. Linear features which appear to be the result of neo-tectonic movement of the sediments in recent time have been determined in the lake sediments. Progressive increases with depth of Na and Cl (considered conservative elements) was found in all cores. The general increase with depth indicates ion movement from a higher concentration under the sediments. Numerous elements show a similar increase with depth, although some metals are undetectable and others have variable concentration profiles. Plots of ~(18)O versus D/H show that at least four different sources of water exist in the sediments. The four possible sources are lake water, connate water, fresh groundwater and saline bedrock water. In all cases shallow sections of the core had water with isotopic values close to that of the lake (~(18)O = -6.5, ~2D = -50). The trend with depth could be the result of mixing with fresh groundwater or relict glacial meltwater and some horizons had values of the formations below the lakes. Michigan and Appalachian basin formation water signatures for Cl isotopes have a range of 0.3-2.3‰. Measurements of this parameter in the interstitial waters of Lake Ontario sediments indicate that the formation brines are entering the lake bottom sediments. The Cl isotope values in some of the cores are much higher than that found in any of the saline fluids of the three basins. Mechanisms will be postulated as to how these values are produced in the cores. Na, Cl and Cl isotope profiles show that the dominant mechanism is diffusion. However in some areas where no isotope shift is found, it is reasonable to believe that advection, albeit very slow, is occurring.