Near Vast Bodies of Water, Land Lies Parched
South Elgin, Illinois, which gets as much rain as Seattle each year, is one of communities in six-county region bordering Lake Michigan that is stunned by new study warning of serious water shortages within twenty years; photos; report surprises people who live near lake system containing one-fifth of world's surface water, but not few companies betting that water will soon be commodity more precious than oil; factors in accelerated water shortages include general warming trend, sprawl covering sponge of land that replenishes nation's vast underground reservoirs, and growing demands of agriculture and cities; problem that has long concerned arid West now extends to unlikely places like Florida, where saltwater creeps into aquifers, and Pacific Northwest, where too little water has been promised to too many competing interests; several Southwestern cities could go dry in 10-10 years, but number of towns in New England and well-watered half of Midwest could also run out in generation (M)