Role of Bedrock and Surficial Geology in Determining the Sensitivity of Thunder Bay Area Lakes to Acidification

Abstract

During 1979–82, samples from 331 lakes in the Thunder Bay area were analyzed for alkalinity, conductivity, and pH. Eight lakes (2.4%) were extremely sensitive to acidification (alk. < 40 μeq/L), 259 (78.2%) had moderate to low sensitivity (alk. 40–500 μeq/L) and 64 (19.4%) were not sensitive (alk. >500 μeq/L). Conductivity was <35 μS/cm in 91 (27%) lakes and 43 (13%) lakes had a pH of < 6.5. Lake buffering capacity was related to bedrock and surficial geology. Glacial lacustrine and outwash deposits were associated with higher alkalinities on sensitive bedrock terrain, whereas glacial ground and end moraines had little effect. All lakes with calcarous material in their watershed had elevated alkalinities, regardless of surficial geology. Geological sensitivity mapping that combines both bedrock and surficial geology can aid managers in predicting lake sensitivity.