Present acid forming emissions to the atmosphere have the potential to alter significantly the chemistry of rain, snow, and surface water of weakly buffered lakes in the Upper Midwest. Average precipitation pH from field measurements during 1979-1983 declined from west to east from 4.8, 4.6, and 4.3 along a cross-section of sites in Minnesota, Wisconsin, and Michigan respectively where 990 lake and stream sampling sites were studied. Measurements of weakly buffered lakes show a parallel decline in lake water pH with the lowest values measured, 5.1, 4.6 and 4.4, respectively in the same regions. Correspondingly, the percentage of lakes sampled with little or no acid neutralizing capacity (ANC) was found to increase from 0 to 4 and 13 percent, respectively. The geographic patterns in ionic composition of airborne acids and bases, and the resultant surface water concentrations are compared. The differences in AFC of airborne inputs from west to east, and differences in in-lake processes explain the observed acidity of weakly buffered lakes across the region. (Copyright (c) 1986 by D. Reidel Publishing Company.)