Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in the summer of 2008, (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) EPA/DNR fish tissue (RAFT) monitoring in 2004.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to aesthetically objectionable conditions caused by poor water clarity due mainly to non-algal turbidity. Violations of the Class A1,B(LW) criterion for pH also contribute to the impairment of Class A1 uses at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the state’s pH criterion and impacts of nutrients and sediment at this lake. The Class C (drinking water) uses are "not assessed" due to a lack of information upon which to base an assessment. Fish consumption uses are assessed (evaluated) as “fully supported.” Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in the summer of 2008, (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) EPA/DNR fish tissue (RAFT) monitoring in 2004.
Note: A TMDL for siltation and nutrients at Lake Darling was prepared by IDNR and approved by EPA in 2003. Because, however, the Section 303(d) impairment due to indicator bacteria (aka, pathogens) was not addressed in the TMDL, this waterbody was placed in IR Category 5a (impaired; TMDL required) for the 2006, 2008, 2010 assessment/listing cycles. Due to changes in IDNR's assessment methodology, the indicator bacteria is suggested for de-listing and Lake Darling is moved to IR Category 4a for the 2012 assessment/listing cycle.
Note 2: Lake Darling was drawn down in 2009 as part of a restoration project at this lake. It remained drawn down in 2010. It is expected to return to normal pool in 2011.
EXPLANATION: Results of IDNR beach monitoring from 2008 suggest that the Class A1 uses are “fully supported." Levels of indicator bacteria at Lake Darling beach were monitored once per week during the primary contact recreation season (May through September) of 2008 (26 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Lake Darling beach, the geometric mean from 2008 was below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 81 E. coli orgs/100 ml in 2008. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 19% in 2008. These results are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “full support” of the Class A1 uses.
Results from the ISU lake survey and the UHL ambient lake monitoring program also suggest that the Class A1 (primary contact recreation) uses for Lake Darling be assessed (monitored) as “not supported” for the 2012 reporting cycle. Using the median values from these surveys from 2006 through 2008 (approximately 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 73, 62, and 78 respectively for Lake Darling. According to Carlson (1977) the Secchi depth and total phosphorus values place Lake Darling in the hypereutrophic category, while the chlorophyll a value places Lake Darling in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column.
The level of inorganic suspended solids was very high at Lake Darling, and suggests that non-algal turbidity contributes to the impairment at this lake. Based on data from the ISU and UHL surveys in 2006 through 2008, the median concentration of inorganic suspended solids was 8.0 mg/L, which was the 26th highest concentration of the 134 lakes monitored by these programs.
Data from the 2006-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Darling. These data show that cyanobacteria comprised 56% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (17.0 mg/L) was the 54th lowest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” based on results from the ISU and UHL lake surveys, and information from the IDNR Fisheries Bureau. Data from the ISU and UHL lake surveys from 2006-2008 show no violations of the Class B(LW) criteria for ammonia in 17 samples and one violation of the Class B(LW) criterion for dissolved oxygen in 17 samples (6%). The data show 4 violations of the Class A1,B(LW) criterion for pH in 17 samples (24%). Based on IDNR’s assessment methodology, these violations are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the aquatic life uses at this lake.
Lake Darling is undergoing a restoration project that includes both watershed and in-lake restoration activities. The project is expected to be completed in 2011. The IDNR Fisheries Bureau expects that the algae and turbidity problems will be reduced due to these extensive restoration activities.
The Class C (drinking water) uses remained "not assessed" due to a lack of water quality information. The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of these surveys from 2006-08 show nitrate levels at this lake are low (maximum = 6.7 mg/l; median = 1.7 mg/l) relative to the drinking water MCL (10 mg/l), these data are not sufficient for developing a valid assessment of the Class C uses.
Fish consumption uses are assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Lake Darling in 2004. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.092 ppm; total PCBs: 0.09 ppm; and technical chlordane: 0.049 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.11 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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 fish contaminant data generated from the 2004 RAFT sampling conducted in this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
