Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
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. Large populations of cyanobacteria also contribute to an impairment due to nuisance aquatic life. 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 remain “not assessed.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau.
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 nuisance algal blooms. Using the median values from these surveys from 2004 through 2008 (approximately 19 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 69, and 67 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 high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.
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 1.5 mg/L, which was the 12th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Lake Hendricks, which contributes to impairment at this lake. These data show that cyanobacteria comprised 75% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (36.5 mg/L) was also the 27th 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 from 2004-2008. 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. Information from the IDNR Fisheries Bureau also suggests that algal blooms are a problem at this lake.
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 2004 through 2008, there were no violations of the Class B(LW) criterion for dissolved oxygen in 19 samples. There was one violation of the Class B(LW) criterion for ammonia in 19 samples, however based on IDNR’s assessment methodology the occurrence of a single violation of the ammonia criterion does not constitute an impairment of the Class B(LW) uses. The pH data, however, show 9 violations in 19 samples (47%). 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 are “not assessed” due to the lack of recent information upon which to base an assessment. The most recent fish contaminant monitoring at this lake was conducted in 1994 as part of the U.S. EPA/IDNR fish tissue (RAFT) monitoring program. Although results of the 1994 RAFT monitoring indicated very low levels of contaminants and “full support” of the fish consumption uses, these data are now considered too old (greater than 10 years) to characterize current water quality conditions.
