Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in the summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (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) 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 levels of indicator bacteria that exceed Iowa’s water quality standard, and aesthetically objectionable conditions caused by poor water clarity due mainly to non-algal turbidity. Large populations of cyanobacteria also contribute to an impairment due to nuisance aquatic life. 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 (monitored) as “fully supported.” Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in the summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (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) 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 and 2008 assessment/listing cycles where it remains for the 2010 assessment/listing cycle.
EXPLANATION: Results of IDNR beach monitoring from 2006 through 2008 suggest that the Class A1 uses are "not supported." Levels of indicator bacteria at Lake Darling beach were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (20 samples), and 2008 (26 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day 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”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At Lake Darling beach, the geometric means of 9 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml: 3 of 24 geometric means violated in 2006, 5 of 16 geometric means violated in 2007, and 1 of 22 geometric means violated in 2008. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 11% in 2006, 30% in 2007 and 19% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, the exceedences of the geometric mean criterion suggest impairment (nonsupport) of the Class A1 (primary contact recreation) uses.
Lake Darling was sampled as part of IDNR’s Safe Lakes Program, which aims to identify sources of bacteria to selected beaches where bacteria levels have consistently violated the state water quality criteria. The Safe Lakes Program found that geese and livestock were the likely sources of bacteria to Lake Darling. This program also noted that while a large amount of work has been done in the watershed, bacteria have been found to exit retention ponds when they discharge.
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 2010 reporting cycle. Using the median values from these surveys from 2004 through 2008 (approximately 26 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 65, and 77 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 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 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 2004 through 2008, the median concentration of inorganic suspended solids was 9.5 mg/L, which was the 21st highest concentration of the 132 lakes monitored by these programs.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Lake Darling, which contributes to impairment at this lake. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (32.9 mg/L) was also the 34th highest of the 132 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 2004-2008 show no violations of the Class B(LW) criteria for ammonia in 25 samples and one violation of the Class B(LW) criterion for dissolved oxygen in 26 samples (4%). The data show 8 violations of the Class A1,B(LW) criterion for pH in 26 samples (31%). 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 2004-08 show nitrate levels at this lake are low (maximum = 6.7 mg/l; median = 1.4 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 (monitored) 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. 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 at Lake Darling 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.
