Assessment Comments
Assessment is based on: (1) the listing of fish consumption advisories for the state of Nebraska, (2) results of Iowa State University (ISU) lake surveys in from 2004-2007, (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of fish kill investigation in April 2004.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses remain assessed (monitored) as "not supported" due primarily to high levels of algal turbidity that reduce water transparency and create aesthetically objectionable conditions. Data from the ISU and UHL lake surveys suggests that very large populations of cyanobacteria violate Iowa's narrative criterion protecting against nuisance aquatic life. The Class B(LW) aquatic life uses are assessed (monitored) as "partially supporting" due to levels of dissolved oxygen that violate the state water quality standard. Nuisance blooms of algae, nutrient loading to the water column, and the occurrence of a fish kill in April 2004 also remain concerns at Carter Lake. Fish consumption uses remain assessed (monitored) as "partially supported" due to the existence of a fish consumption advisory issued by the state of Nebraska. The sources of data for this assessment include (1) the listing of fish consumption advisories for the state of Nebraska, (2) results of Iowa State University (ISU) lake surveys in from 2004-2007, (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of fish kill investigation in April 2004.
Note: A TMDL for algae/algal toxins, chlorophyll a, total phosphorus, total nitrogen, and pH at Carter Lake was prepared by IDNR and the Nebraska Department of Environmental Management and approved by EPA in 2007. Because the Section 303(d) impairment for dissolved oxygen was not addressed in the TMDL, this waterbody is placed into IR Category 5a (impaired; TMDL required) for the 2008 and current, 2010, assessment/listing cycles and.
EXPLANATION: Results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program suggest that the Class A1 uses at Carter Lake are assessed (monitored) as “not supported.” Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 81, 76, and 79 respectively for Carter Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Carter Lake in the hypereutrophic category. These values suggest extremely high levels of chlorophyll a and suspended algae in the water, extremely poor water transparency, and extremely high levels of phosphorus in the water column.
The level of inorganic suspended solids is also very high at this lake and suggests that non-algal turbidity contributes to the impairment at this lake. The median inorganic suspended solids concentration at Carter Lake was 8.0 mg/L, which was the 28th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Carter Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (72.7 mg/L) was also the 5th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values for the 2004-2008 period. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “not supported” based on information from the IDNR Fisheries Bureau, results from the ISU and UHL lake surveys, and results of a fish kill investigation in 2004. Information from the IDNR Fisheries Bureau indicates that the fish community and water quality are negatively impacted by the presence of common carp. Results from the ISU and UHL lake surveys show poor water quality at Carter Lake. The ISU and UHL data show 6 violations of the Class B(LW) criterion for dissolved oxygen in 25 samples (24%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore constitute an impairment of the Class B(LW) uses. Results of the ISU and UHL surveys show 3 violations of the Class A1,B(LW) criterion for pH in 25 samples (12%). Based on IDNR’s assessment methodology, these violations are not significantly greater than 10% and therefore do not constitute an impairment of the Class B(LW) uses of Carter Lake. Data from these surveys from 2004-2008 also show 2 violations of the Class B(LW) criterion for ammonia in 24 samples. Based on IDNR’s 2010 Section 303(d) listing methodology these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(LW) aquatic life uses. Because, however, more than one violation of the chronic criterion for ammonia occurred, this potential impairment will be added to Iowa's list of waters in need of further investigation.
Note regarding identification of ammonia impairments: Based on consultation with other Region 7 states and U.S. EPA Region 7 staff in 2007, IDNR's methodology for assessing impairments due to violations of chronic criteria for toxic parameters (e.g., ammonia and toxic metals) was changed. Prior to the 2008 listing cycle, IDNR followed a U.S. EPA recommendation (U.S. EPA 1997b, page 3-18) that more than one violation of a water quality criterion for a toxic pollutant in an abundant data set indicates an impairment of aquatic life uses. Because no state in Region 7 collects ambient data with the frequency necessary to accurately identify compliance with a chronic criterion (i.e., to allow calculation of a short-term (4-day or 30-day averages)), Region 7 states concluded that another assessment approach (e.g., the 10% rule) was more appropriate. Thus, for the 2008 and 2010 listing cycle, IDNR identified aquatic life impairments for toxic parameters when significantly more than 10% of the samples exceeded a chronic criterion.
Although attributed to "natural causes", the occurrence of a fish kill in this lake in April 2004 is consistent with the assessment of aquatic life uses as “not supported.” This kill occurred on April 22, 2004. The cause of the kill was identified as disease (virus). An estimated 5,000 fish were killed; the kill affected only yellow bass. No estimate of the value of the fish killed was provided. The fish kill portion of this assessment is the same as that developed for the 2008 assessment/listing cycle. The continuance of the IR Category 3b listing for the fish kill is based on IDNR's 2010 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 (2006-2009) 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, the fish kill impairment will remain in Category 3b of Iowa’s 2010 Integrated Report.
Fish consumption uses remained assessed (monitored) as "partially supported" due to the continuation of a fish consumption advisory for this lake issued by the state of Nebraska due to high levels of PCBs (for more information, see the following web site: http://www.deq.state.ne.us/SurfaceW.nsf/Pages/FCA. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. According to IDNR’s assessment methodology, the existence of a restricted consumption advisory indicates that fish consumption uses should be assessed as “partially supported.”
