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, (4) results of the IDNR-county voluntary beach monitoring program in 2008, 2009, and 2010, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2002.
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. The pH impairments of the Class A1 and Class B(LW) uses are new for this lake. Fish consumption uses are assessed (evaluated) as “fully supported.” 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, (4) results of the IDNR-county voluntary beach monitoring program in 2008, 2009, and 2010, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2002.
EXPLANATION: Results of IDNR city/county beach monitoring from 2008 through 2010 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 2008 (13 samples), 2009 (10 samples), and 2010 (12 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 Pahoja beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 105 E. coli orgs/100 ml in 2008, 36 E. coli orgs/100 ml in 2009, and 74 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 23% in 2008, 10% in 2009 and 25% in 2010. 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.
NOTE: Lake Pahoja was listed as impaired for indicator bacteria for the 2008 and 2010 assessment and listing cycles. Based on the updated methodology and results of sampling from 2008 through 2010, Lake Pahoja no longer should be assessed as impaired. Therefore, the bacteria impairment is suggested for de-listing for the 2012 assessment/listing cycle.
For the 2012 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 2006 through 2010 (approximately 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 68, and 73 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 relatively low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Pahoja was 3.0 mg/L, which was the 55th lowest of the 134 monitored lakes.
Data from the 2006-2010 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 41% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (19.8 mg/L) was the 66th lowest of the 134 lakes sampled.
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 2006 through 2010 show 1 violation of the Class B(LW) chronic criterion for ammonia in 17 samples. Based on IDNR’s listing methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(LW) aquatic life uses. Data from the ISU and UHL surveys show no violations of the Class B(LW) criterion for dissolved oxygen in 17 samples. Data for pH 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 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.
