HYDROLOGY AND HYDROGEOCHEMISTRY OF THE SOUTH FORK, WRIGHT VALLEY, SOUTHERN VICTORIA LAND, ANTARCTICA.

摘要

Complex, dynamic hydrogeochemical systems--involving the transport and fractionation of water and solutes--occur in the rocks, surficial materials, and surface waters of the south fork of Wright Valley, situated in the ice-free region of Southern Victoria Land, Antarctica. All of these systems have features reflective of the regional polar desert environment, and particularly of the extreme cold and aridity.;Don Juan Pond is a large, unique CaNa-Cl brine pond situated in a closed basin at the topographically lowest point of the south fork. The floor of the basin is a discharge zone for grounwater brines confined in an underlying dolerite aquifer. Intermittent, freshwater streams fed by melting ground ice enter the basin late in the austral summer. Evaporation and sublimation are, with aerosols, the sole means whereby water leaves the basin. The pond is in a precarious hydrologic equilibrium; the cessation of either streamflow or groundwater flow would cause the pond to go dry.;Rapid, transient fluctuations of fluid pressure in the aquifer were recorded at a borehole drilled in the basin. The fluctuations are explained in terms of existing theories for the behavior of fluids in stressed porous media; they are interpreted as evidence of movements in a rock glacier lying near the pond. The characteristics of the fluctuations suggest that the causative movements were small and abrupt and involved a large part of the rock glacier.;The major element chemistry of the brines is controlled by phase relations in the aqueous system CaCl(,2)-NaCl, for which the important solid phases are antarcticite, halite, hydrohalite, and ice. Evaporation--which forces the precipitation of halite and antarcticite--and dilution control the chemistry of the pond during the austral summer; temperatures as low as -50(DEGREES)C probably control its chemistry during winter. The ground-water brines are less saline than, but otherwise similar in composition to, the brines in the pond. During the austral summer, the groundwater temperature is near the mean annual air temerature of -18(DEGREES)C; the concentrations of solutes are such that, at this temperature, the groundwater is at or near equilibrium with ice.;Groundwater flow systems overlying impermeable frozen ground occur intermittently in soils along the axis of the south fork and sustain small, intermittent ponds. The flow systems transport solutes, effecting the accumulation of salts in topographic lows. Spatial segregation of elements occurs during the translocation of solutes and is facilitated by evaporation and freezing. Relatively insoluble sulfates tend to accumulate in the upper parts both of flow systems and of soil profiles, relatively soluble chlorides in the lower parts.;The stable isotopic behavior of water in Don Juan Pond is described using a modified version of the evaporation model of Craig and Gordon (1965) and Sofer and Gat (1972, 1975). During evaporation, as seen on a (delta)('18)O-(delta)D diagram, the isotopic composition of the pond evolves along a straight line, as predicted by the model. Late in the summer, mixture with stream water deflects the composition of the pond away from the evaporation line. When corrections are made for the effect of the chemistry of the brines upon the activities of isotopic species, calculations made using the model show that the isotopic behavior of the pond is compatible with groundwater discharge; the calculations also suggest that the bond evaporates into a very dry atmosphere containing moisture that is very light isotopically. In terms of the model, Don Juan Pond is in neither a stationary isotopic state nor a steady mass state.