Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of IDNR/U.S. EPA turtle tissue (RAFT) monitoring in 2010.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to violations of the state’s criterion for pH and aesthetically objectionable conditions caused by algae blooms. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the state criterion for pH. Fish consumption uses are assessed (monitored) as "fully supported" based on results of turtle tissue monitoring in 2010. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of IDNR/U.S. EPA turtle tissue (RAFT) monitoring in 2010.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Lake Hendricks are assessed (monitored) as “partially supported” due to violations of Iowa’s water quality criterion for pH and also due to aesthetically objectionable conditions caused by algae blooms. Using the median values from these surveys from 2006 through 2010 (approximately 16 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 64, and 70 respectively for Lake Hendricks. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Hendricks in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.
While the TSI values for Lake Hendricks are below the impairment trigger of 65 for the 2012 reporting cycle, Lake Hendricks was listed as "partially supporting" the Class A1 uses for the 2010 reporting cycle due to algal turbidity. Based on IDNR's assessment methodology, median-based TSI values for both chlorophyll-a and Secchi depth must be 63 or less for two consecutive summer seasons before a lake can be removed from the state’s Section 303(d) list (IR Category 5). Therefore, Lake Hendricks remains assessed as "partially supported" for the 2012 assessment cycle.
The level of inorganic suspended solids was low at this lake and does not suggest impairment due to non-algal turbidity. The median inorganic suspended solids concentration at Lake Hendricks was 2.0 mg/L, which was the 24th lowest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Lake Hendricks. These data show that cyanobacteria comprised only 25% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (11.6 mg/L) was also the 34th lowest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to a high frequency of violations of Iowa’s criterion for pH. Based on data from the ISU and UHL lake surveys from 2006 through 2010, there were no violations of the Class B(LW) criterion for ammonia in 16 samples or dissolved oxygen in 16 samples. The pH data, however, show 7 violations in 16 samples (44%). Based on IDNR’s assessment methodology these results are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class A1,B(LW) uses of Lake Hendricks. The pH violations at Lake Hendricks are likely due to high primary productivity at this lake and do not reflect the input of pollutants to this lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR turtle contaminant (RAFT) monitoring at Lake Hendricks in 2010. The composite samples of shoulder tissue from snapping turtle had low levels of contaminants. Levels of primary contaminants in the composite sample of snapping turtle shoulder were as follows: mercury: 0.132 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. 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. The turtle contaminant data generated from the 2010 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
