Assessment Comments
Assessment is based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2007 and 2008.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are "not assessed" due to a lack of information upon which to base an assessment. The fish consumption uses are assessed (monitored) as "partially supported" due to the issuance of a fish consumption advisory at this lake in 2008.
EXPLANATION: Fish consumption uses are assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Grade Lake in 2007 and 2008. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. The fish contaminant data generated from the 2007 RAFT sampling conducted at Grade Lake show that levels of mercury are sufficiently high for concern and thus justify follow-up monitoring. Results from the 2007 sampling show that the level of mercury in the sample of largemouth bass fillets (0.338 ppm) exceeds the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol. According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory. Follow-up sampling conducted in 2008 showed that levels of mercury in largemouth bass (0.303 ppm) again exceeded the 1 meal/week trigger level (0.30 ppm). Thus a one meal per week fish consumption advisory was issued for Grade Lake and the fish consumption uses are assessed as “partially supported.”
Levels of primary contaminants in the composite sample of channel catfish fillets were all below advisory trigger levels (mercury: 0.0222 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm).
