Big Creek Lake IA 04-UDM-1242

Results of IDNR beach monitoring from 2012 through 2014 suggest that the Class A1 uses are "Fully Supported" Levels of indicator bacteria at Big Creek Lake Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (16 samples), 2013 (24 samples) and 2014 (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 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or 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).

At Big Creek Lake Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 28 E.  coli orgs/100 ml in 2012, 32 E.  coli orgs/100 ml in 2013 and 85 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 6% in 2012, 13% in 2013 and 13% in 2014.   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 "Fully Supported" of the Class A1 uses. While the levels of indicator bacteria for Big Creek Lake Beach are not significantly greater than 10% of the samples and do not exceed the state's single season geometric mean criterion of 126 E.  coli orgs/100 ml for the 2016 reporting cycle, Big Creek Lake Beach was listed as "partially supporting" the Class A1 uses for the 2014 reporting cycle due to indicator bacteria.   Based on IDNR's assessment methodology, indicator bacteria violations must be not violation for two consecutive assessment cycles before a lake can be removed from the state’s Section 303(d) list (IR Category 4a).    Therefore, Big Creek Lake Beach remains assessed as "partially supported" due to violations of the criterion for indicator bacteria for the 2016 assessment cycle.       

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Big Creek Lake are assessed (monitored) as "fully supported" based on information from the ISU lake survey. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 52, 56, and 61 respectively for Big Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Big Creek Lake in the Eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, very good water transparency, and moderately high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was extremely low at Big Creek 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 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at Big Creek Lake. These data show that cyanobacteria comprised 96% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (12.9 mg/L) was ranked 39th of the 138 lakes sampled.

The Class B(WW-1) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU lake surveys.   Based on information from the IDNR Fisheries Bureau, large cyanobacteria blooms have formed on the lake every summer in recent years and it has deterred recreation. Additionally issues in the watershed and well established common carp population have led to decreased water quality. Results of the ISU lake survey from 2010-2014, however, show there were no violations of the criterion for ammonia in 15 samples(0%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and no violations of the criterion for pH in 15 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(WW-1) uses of Big Creek Lake.

The level of support of the Class HH-fish consumption uses is changed from “fully supporting” to “not assessed” due to the age of the data upon which the previous assessment was based.  The previous assessment was based on results of U.S. EPA / IDNR fish tissue (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.