Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results of the IDNR-county voluntary beach monitoring program in 2010 and 2012, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2002 and 2012.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to aesthetically objectionable conditions caused by elevated levels of algae (chlorophyll a). An additional impairment exists due to violations of the Class A1 criteria for pH. The Class B(LW) (aquatic life) uses are assessed (monitored) as "partially supported" due to violations of the Class B(LW) criteria for pH. Fish consumption uses are assessed (monitored) as “fully supported.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results of the IDNR-county voluntary beach monitoring program in 2010-2012, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2002 and 2012.
EXPLANATION: Results of IDNR city/county beach monitoring from 2010 and 2012 suggest that the Class A1 uses are “fully supported." Levels of indicator bacteria at Lake Pahoja beach were monitored once per week during the primary contact recreation seasons (May through September) of 2010 (18 samples) and 2012 (9 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 2014 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 Pahoja beach, the geometric means from 2010 and 2012 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 50 E. coli orgs/100 ml in 2010, and 11 E. coli orgs/100 ml in 2012. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 17% in 2010 and 0% in 2012. None of these are 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.
For the 2014 assessment/listing cycle, results from the ISU and UHL lake surveys indicate that the Class A1 (primary contact recreation) uses of Lake Pahoja are assessed (monitored) as “partially supported.” Using the median values from these surveys from 2008-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 66, 68, and 75 respectively for Lake Pahoja. According to Carlson (1977) the Secchi depth and chlorophyll a values place Lake Pahoja in between the eutrophic and hypereutrophic categories while the total phosphorus value places Lake Pahoja in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and very high levels of phosphorus in the water column.
Based on data from the ISU and UHL lake surveys, the level of inorganic suspended solids was moderately high at this lake but does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Pahoja was 5.1 mg/L, and ranked 80th of the 134 monitored lakes.
Data from the 2008-2012 ISU and UHL surveys suggest a moderate population of cyanobacteria exists at Lake Pahoja, which contributes to the aesthetically objectionable conditions caused by algae blooms at this lake. These data show that cyanobacteria comprised 93% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (63.2 mg/L) and ranked 125th of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2008-2012. 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. 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 “partially supported” due to violations of the Class B(LW) criteria for pH. Results of the ISU and UHL lake surveys from 2008-2012 show no violations of the Class B(LW) chronic criterion for ammonia in 14 samples and no violations for dissolved oxygen in 14 samples. Data for pH show 5 violations of the Class A1,B(LW) criterion for pH in 14 samples (36%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment of the Class B(LW) uses of Lake Pahoja. The pH violations at Lake Pahoja are likely due to the high levels of primary productivity at this lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Lake Pahoja in 2002. The composite samples of fillets from channel catfish had low levels of contaminants: mercury: 0.036 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.030 ppm. A predator sample was not collected as part of the 2002 RAFT sample. Typically, samples of both bottom-feeding fish (e.g., common carp) and predator species (e.g., largemouth bass) are collected at RAFT status sites such as the 2002 sampling at Lake Pahoja. Predator species, however, are sometimes difficult to capture, and RAFT status samples occasionally contain only the bottom-feeder sample. 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 2002 RAFT sampling conducted at Lake Pahoja 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.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Lake Pahoja in 2012. The composite samples of tissues plugs from largemouth bass had low levels of mercury (0.203 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 2012 RAFT sampling conducted at Lake Pahoja 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.
