Big Woods Lake IA 02-CED-466

Assessment Comments

Assessment remains based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1996. This lake is not part of the 2000-2004 ISU statewide lake survey.

Basis for Assessment

SUMMARY:  The fish consumption uses of this new lake remain assessed (evaluated) as "fully supported" based on results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 1996.   Other beneficial uses remain "not assessed" due to the lack of information upon which to base an assessment.

EXPLANATION:  Fish consumption uses remain assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Big Woods Lake in 1996.   Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).  

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.   Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa.   In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol.   This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol).   Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest impairment of the fish consumption uses.  

Although this scenario does not apply to the fish contaminant data generated from the 1996 RAFT sampling conducted at this lake, levels of contaminant are sufficiently high for concern and justify follow-up monitoring.   The levels of at least one contaminant from past monitoring now exceeds one or more advisory trigger levels, thus suggesting the need for additional (follow-up) monitoring to determine whether a consumption advisory should be issued.   The composite samples of fillets from channel catfish and largemouth bass had generally low levels of contaminants.   Although assessed as “fully supported” for this reporting cycle, the level of at least one contaminant does, however, now exceed one or more advisory trigger levels:  the level of mercury in the sample of largemouth bass fillets (0.232 ppm) exceeds the one meal per week trigger level of 0.20 ppm.  

According to the IDNR/IDPH advisory protocol, this single result from the 1996 sampling at Big Woods Lake 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.   But, this elevated level does indicate a concern and the need to conduct additional monitoring to better define contaminant levels in fish from this river segment.   Thus, follow-up monitoring will be conducted in 2007 to better determine (1) levels of mercury in the edible portions (fillets) of fish in this lake and (2) whether a one-meal-per-week consumption advisory needs to be issued.