Assessment Comments
Assessment is based on: (1) results of the IDNR beach monitoring program in the summers of 2010-2012, (2) results of the statewide survey of Iowa lakes conducted from 2009 through 2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of EPA/DNR fish contaminant (RAFT) monitoring in 2004.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of the state water quality criteria for indicator bacteria. The Class B(WW-1) (aquatic life) uses are assessed (monitored) as “fully supported” although information from the IDNR Fisheries Bureau suggests that algae blooms are a concern at this lake. Fish consumption uses are assessed (evaluated) as “fully supported.” Sources of data for this assessment include (1) results of the IDNR beach monitoring program in the summers of 2010-2012, (2) results of the statewide survey of Iowa lakes conducted from 2009 through 2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of EPA/DNR fish contaminant (RAFT) monitoring in 2004.
EXPLANATION: Results of IDNR beach monitoring from 2010 through 2012 suggest that the Class A1 uses are “not supported." Levels of indicator bacteria at Big Creek Lake beach were monitored once per week during the primary contact recreation seasons (May through August) of 2010 (18 samples), 2011 (24 samples), and 2012 (16 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 1: The IDNR TMDL for the impairment due to indicator bacteria at this lake was approved by U.S. EPA in August 2011. Because this TMDL addresses all of the impairments identified for this lake, this lake is moved from IR Category 5a (impaired; TMDL required) to IR Category 4a (impaired; TMDL approved).
NOTE 2: 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 Big Creek Lake beach, the geometric means from 2011 and 2012 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was greater than the Iowa water quality standard of 126 E. coli orgs/100ml in 2010. The geometric mean was 143 E. coli orgs/100 ml in 2010, 38 E. coli orgs/100 ml in 2011, and 27 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 55% in 2010, 29% in 2011 and 6% in 2012. The results from 2010 and 2011 are significantly greater than 10% of the samples and therefore also suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest the Class A1 uses are “not supported.”
Results from the ISU and UHL lake surveys, however, suggest full support of the Class A1 uses of 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 48, 50, 65 respectively for Big Creek Lake. According to Carlson (1977) the Secchi depth value places Big Creek Lake in the mesotrophic category, while the chlorophyll a value places Big Creek Lake in the eutrophic category. The index value for total phosphorus places Big Creek Lake in between the eutrophic and hypereutrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids was extremely low at this lake and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Big Creek Lake (1.6 mg/L) was ranked 2nd among the 134 lakes sampled by the ISU and UHL lake surveys.
Data from the 2008-2012 ISU and UHL surveys suggest a small population of cyanobacteria exists at Big Creek Lake, which does not suggest an impairment at this lake. These data show that although cyanobacteria comprised 99% of the phytoplankton wet mass at this lake, the median cyanobacteria wet mass (12.6 mg/L) was ranked 35th of the 134 lakes sampled.
The Class B(WW-1) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. Information from the IDNR Fisheries Bureau suggests that algae blooms are getting worse and becoming a concern at this lake. The ISU and UHL lake survey results show good chemical water quality at Big Creek Lake. During 2008-2012 there were no violations of the Class B(WW-1) criterion for ammonia (17 samples) or pH (17 samples). There were two violations of the Class B(WW-1) criterion for dissolved oxygen in 17 samples (12%). These violations are not significantly greater than 10% of the samples and therefore based on IDNR's assessment methodology do not suggest impairment of the Class B(WW-1) uses of Big Creek Lake.
Fish consumption uses are assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Big Creek Lake in 2004. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. 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.
