Assessment Comments
Assessment remains based on (1) results of a fish kill investigation in 2005 and (2) results of fish tissue monitoring in 2006 and 2007.
Basis for Assessment
Note: This waterbody has been added to Iowa's current surface water classification (http://www.iowadnr.gov/water/standards/files/swcdoc2.pdf) and is classified for Class B(LW)[lake/wetland] aquatic life uses and Class HH (fish consumption) uses.
SUMMARY: The general uses of this Black Pit (aka, Pierce Pond) in Mason City remain 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 2014 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levels of mercury, subsequent monitoring showed lower levels of this contaminant; thus, this consumption advisory was rescinded by IDNR in August 2007.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.
This is the same assessment as that developed for the 2008, 2010, and 2012 assessment/listing cycles. The continuance of the IR Category 3b listing is based on IDNR's assessment methodology that states the following: 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 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., winterkill), the assessment type will be considered “evaluated” and will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation. Thus, this assessment segment will remain in Category 3b of Iowa’s Integrated Report.
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 at that time 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.
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 remain assessed as "fully supported" at this waterbody.
