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, and (4) results of the IDNR-UHL beach monitoring program in 2000, 2001, and 2002.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency primarily due to large amounts of suspended sediment, but also due to algae blooms that violate Iowa's narrative criterion protecting against nuisance aquatic life. Violations of the state water quality standard for indicator bacteria also contribute to the impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to high levels of non-algal turbidity and siltation impacts at this lake. The Class C (drinking water) uses are “not assessed.” Fish consumption uses are “not assessed.” 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, and (4) results of the IDNR-UHL beach monitoring program in 2000, 2001, and 2002.
Note: A TMDL for siltation at Bob White Lake was prepared by IDNR and approved by EPA in 2001. Because the Section 303(d) impairment for indicator bacteria was not addressed in the TMDL, this waterbody was placed into IR Category 5a (impaired; TMDL required) for the 2004, 2006, 2008, 2010 and current (2012) assessment/listing cycles.
EXPLANATION: Results of IDNR beach monitoring from 2000 through 2002 suggest that the Class A1 uses are assessed (evaluated) as "not supported" (this beach was not monitored for indicator bacteria during recreational seasons of 2003 through 2010). 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). Levels of indicator bacteria at Bob White Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2000 (18 samples), 2001 (16 samples), and 2002 (29 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) all thirty-day geometric means for 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 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”. 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 3-35of U.S. EPA 1997b).
At Bob White Lake beach, the geometric means for 8 of the 12 thirty-day periods during the summer recreation season of 2001 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml; none of the geometric means exceeded this standard during the recreational seasons of 2000 (14 geometric means) or 2002 (25 geometric means). Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was significantly greater than 10% in 2002 (4 of 18 samples, 22%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (“nonsupport”) of the Class A1 primary contact recreation uses due to geometric mean values that exceed Iowa’s Class A1 criterion.
Results of the ISU lake survey and UHL ambient lake monitoring program also suggest that the Class A1 uses are “not supported” at Bob White Lake due to poor water transparency primarily due to high levels of suspended sediment, but also due to algae blooms that create an impairment due to nuisance aquatic life. Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 77, 64, and 79 respectively for Bob White Lake. According to Carlson (1977) the index values for Secchi depth and total phosphorus place Bob White Lake in the hypereutrophic category, while the value for chlorophyll a places Bob White Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately 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 median concentration of inorganic suspended solids is extremely high and contributes to the impairment at Bob White Lake. Results from the ISU and UHL lake surveys show that the median level of inorganic suspended solids in Bob White Lake from 2006-2010 was 17.6 mg/L, which was the 4th highest concentration of the 134 lakes monitored by these programs.
Data from the 2006-2010 ISU and UHL surveys suggest a large population of cyanobacteria exists at Bob White Lake, which causes a nuisance aquatic life impairment at this lake. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (81.4 mg/L) was the 7th highest 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 for the 2006-2010 period. 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. The assessment category for 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 (evaluated) as “partially 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 siltation and non-algal turbidity cause an impairment of the Class B(LW) uses at Bob White Lake. While populations of gizzard shad have decreased recently, large populations of common carp still remain a concern at this lake. The ISU and UHL lake survey results show relatively good chemical water quality at Bob White Lake. During 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 22 samples. There were three violations of the Class B(LW) criterion for dissolved oxygen in 22 samples (14%), and one violation of the Class A1,B(LW) criterion for pH in 22 samples (5%). Based on IDNR’s assessment methodology, these violations are not significantly greater than 10% of the samples and therefore do not constitute an impairment of Class B(LW) uses at Bob White Lake.
The Class C (drinking water) uses are “not assessed” due to the lack of recent information upon which to base an assessment. 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 the ISU and UHL surveys from 2006-10 show that nitrate levels are low at this lake (maximum value = 7.3 mg/l; median = 0.3 mg/l) compared to the MCL (10mg/L), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
