The water quality graphs were inspected and erroneous values were removed. There were questionable values for total aluminum, total dissolved nitrogen and pH. Total chromium, copper, lead, and zinc had 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 (Pommen, 1994). Values for pH between 1986 and 1988 were much lower than the rest of the pH data due to laboratory control problems at that time. Quality assurance issues are discussed in further detail in the next section.