Measured changes in surface water chemistry in areas that have experienced short-term changes in chemical constituents in response to changes in mineral acid inputs are available from a number of sources. The spatial distributions of lake-water chemical variables across a longitudinal gradient in the upper Midwest for low- Acid neutralizing capacity groundwater recharge seepage lakes provides a good example of the use of space-fortime substitution to evaluate acidification dose-response relationships. The basic concept of critical load is relatively simple, as the threshold concentration of pollutants at which harmful effects on sensitive receptors begin to occur. The majority of the critical loads work to date has been conducted in Europe. A number of documents have been prepared in conjunction with the United Nations Economic Commission for Europe critical loads research efforts. A simplistic and generalized attempt to quantify critical loads for sulphur and nitrogen was presented at the Skokloster workshop, based on a long-term mass-balance approach.