Assessment Comments
Assessment is based on results an IDNR investigation of a fish kill in June 2005 and on fish contaminant monitoring in 2006 and 2007.
Basis for Assessment
SUMMARY: The general uses of this Black Pit (aka, Pierce Pond) in Mason City are assessed (evaluated) as “partially supported” due to a fish kill in June 2005. The kill was attributed to natural causes. This impairment is appropriate for Category 3b of Iowa’s 2006 Integrated Report. Fish consumption uses are considered "fully supported." Although a fish consumption advisory was issued for this lake by IDNR in July 2006 due to high levels of mercury, subsequent monitoring showed lower levels of this contaminant; thus, this consumption advisory was rescinded by IDNR in August 2007.
EXPLANATION: The fish kill occurred on or before June 16, 2005 and was attributed to natural causes (spawning stress). An estimated 47 fish were killed including largemouth bass (1), bluegill (24), and crappies (22). No estimate of the value of the fish killed was provided.
According to Iowa’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winter kill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
A fish consumption advisory was issued for this waterbody in July 2006 based on results of IDNR fish contaminant monitoring in May and June, 2006 (samples were analyzed by UHL). This monitoring was conducted in response to results of a 2005 investigation by U.S. EPA, and follow-up investigations by IDNR Field Office 2 (Mason City), of two buildings and surrounding property near Black Pit were mercury oxide battery manufacturing operations were conducted in the past. Levels of mercury in the composite samples of largemouth bass were 0.24 ppm in the May 2006 sample and were 0.23 ppm in the June 2006 sample. Based on the IDNR/IDPH trigger level for mercury of 0.2 ppm for a 1-meal/week advisory, a consumption advisory was issued for this lake by IDNR in July 2006 (levels of mercury in samples of channel catfish and panfish collected in 2006 were below the 0.2 ppm trigger level). See http://www.iowadnr.gov/fish/news/consump.html for more information on the IDNR/IDPH consumption advisory protocol. Additional sampling conducted in May 2007 showed that the level of mercury in a composite sample of largemouth bass fillets was below the 0.2 ppm trigger level. In addition, the IDNR/IDPH advisory trigger level for mercury was changed to 0.3 ppm in 2007 to be consistent with U.S. EPA's 2001 recommendation to states for adopting a tissue-based water quality criterion for mercury into state water quality standards (see http://www.epa.gov/waterscience/criteria/methylmercury/merctitl.pdf). Based on the 2007 sampling that showed mercury levels below the 0.2 ppm, and the change to a advisory trigger level of 0.3 ppm for mercury, the consumption advisory for Black Pit was rescinded by IDNR in August 2007. Thus, fish consumption uses are considered "fully supported" at this waterbody.
