Grade Lake IA 04-LDM-6311

Fish consumption uses are assessed (monitored) as “partially supported” based on results of U.S.EPA/DNR fish contaminant (RAFT) monitoring at Grade Lake in 2007, 2008, and 2012. 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 showed that, while levels of contaminants in bottom-feeding fish were low, levels of mercury in predator fish (largemouth bass) were sufficiently high for concern and thus justified follow-up monitoring. Levels of primary contaminants in the 2007 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). Results from the 2007 sampling, however, showed that the level of mercury in the sample of largemouth bass fillets (0.338 ppm) exceeded the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol. According to DNR’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 were thus assessed as “partially supported.”Follow-up sampling conducted in 2012 showed that levels of mercury in largemouth bass (0.61 ppm) again exceeded the 1 meal/week trigger level (0.30 ppm). Thus, the consumption advisory and impairment of fish consumption uses remain.