The water quality graphs were inspected and erroneous values were removed. There were questionable values for pH, filterable residue, and air and water temperature. Total chromium, copper, lead, and zinc exhibited high values between 1986 and 1991 due to the failure of preservative vial lid liners which resulted in sample contamination. Mercury data were not plotted because all detectable values were likely due to contamination as well (Pommen, 1994). Observed pH values between 1986 and 1988 were much lower than the rest of the pH data due to laboratory method control problems at that time and have been removed from the data base. Quality assurance issues are discussed in further detail in the next section.