The Wisconsin Early Response Lakes (WERL) program was a long-term monitoring effort between 1987-2000 designed to better assess trends in acid-base chemistry of sensitive lakes in Wisconsin. Study lakes were located in the northeast, north-central and northwest parts of the state across the west to east gradient of increasing deposition acidity. Within each region, three types of lakes were monitored: stratum 1 were low alkalinity (≤0 µeq/L), clear lakes; stratum 2 were low alkalinity humic lakes; and stratum 3 were slightly higher alkalinity (>~20 µeq/L), clear lakes. The lakes were mainly seepage lakes. Lakes were sampled in the autumn for major ion chemistry.