DENSITY-DRIVEN GROUNDWATER-FLOW IN CLOSED DESERT BASINS - FIELD INVESTIGATIONS AND NUMERICAL EXPERIMENTS

With a numerical model, Duffy and Al-Hassan demonstrated that free con vection of brine could occur under the playa lake in a closed basin. S ince then, continued effort has been made to further our understanding of the role of free convection in closed basin hydrology. In this pap er we present the results of 5 years of field investigation in Pilot V alley, Nevada, of a literature search for field studies in closed basi ns around the world, and of numerical simulations to examine the effec t of heterogeneity on free convection and the effect of climate change s. Observations along the divide-playa transect revealed evidence of f orced convective flow and the existence of a hinge line, The salinity profile and the inferred flux suggest that diffusion is the dominant s olute transport mechanism under the field sites, but the stable isotop es (O, H) suggest that free convection of playa brine also occurs. We propose a model that explains these observations. First, free convecti on could occur farther out on the playa where the density of surface b rines is higher. Second, once free convective flow penetrates the surf ace sediments, it could migrate laterally in the more permeable layers of alluvium. Third, free convection could occur during periods of tim e when brines pond on the surface for extended periods. The search for field studies of groundwater processes in other closed basins leads t o a few cases where free convection has occurred on a basin scale. Bas ed on this survey, we discuss the geologic and climatic conditions tha t favor free convection. Numerical simulations are carried out to exam ine the effect of basin-scale heterogeneity in hydraulic conductivity. The results indicate that heterogeneity changes the pattern of flow b y creating multiple convection cells. Finally we address how a closed basin may respond to climate change, Climate changes alter the salinit y of surface brines and the freshwater recharge, both acting as 'bound ary conditions' for subsurface processes. Using dimensional analysis a nd numerical simulations, we evaluate the effect of a warming and dryi ng trend on free convection, and how the geologic features of a closed basin affect the response. (C) 1997 Elsevier Science B.V.