Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2004, 2005, and 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to aesthetically objectionable conditions caused by algal turbidity. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” The Class C (drinking water) uses are “not assessed.” The fish consumption uses are “not assessed.” Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2004, 2005, and 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR beach monitoring at Lake Ahquabi from 2004 through 2006 suggest that the Class A1 uses should be assessed (monitored) as “fully supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2004 (16 samples), 2005 (17 samples), and 2006 (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) 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 3-35 of U.S. EPA 1997b).
At Lake Ahquabi beach, the geometric means of all 37 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. These results suggest generally low levels of indicator bacteria at this lake. None of the 49 samples collected during recreational seasons of 2004, 2005, and 2006 exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. Thus, these results do not suggest an impairment of the Class A1 uses of Lake Ahquabi.
Results from the ISU and UHL lake surveys, however, suggest that the Class A1 uses at Lake Ahquabi are “partially supported” due to the occurrence of aesthetically objectionable conditions caused by nuisance algae blooms. Using the median values from these surveys from 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 66, and 66 respectively for Lake Ahquabi. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Ahquabi in between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is relatively low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Ahquabi was 2.5 mg/L, which was the 29th lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Ahquabi, which contributes to occasional nuisance algae blooms and the impairment at this lake. These data show that cyanobacteria comprised 94% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (20.2 mg/L) was the 62nd highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau, results from the ISU and UHL lake surveys, and results of physical/chemical monitoring associated with the IDNR-UHL beach monitoring program. The ISU and UHL lake survey results show good chemical water quality at Lake Ahquabi. During 2002-2006 there were no violations of the Class B(LW) criterion for ammonia (16 samples), or dissolved oxygen (22 samples). There was one violation of the Class A1,B(LW) criterion for pH in 23 samples (4%). Based on IDNR’s assessment methodology these violations were not significantly greater than 10% of the samples and therefore do not constitute an impairment of the Class B(LW) uses of Lake Ahquabi. The physical/chemical monitoring results from the IDNR-UHL beach monitoring program showed one violation of the Class B(LW) criterion for dissolved oxygen in 48 samples (2%). Based on IDNR’s assessment methodology this violation does not suggest impairment of the Class B(LW) uses of Lake Ahquabi. Results of pH monitoring through the IDNR-UHL beach monitoring program show 9 violations of the Class B(LW) criterion for pH in 49 samples (18%). Combined with the ISU and UHL surveys these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses of Lake Ahquabi.
The Class C (drinking water) uses remain "not assessed" due to lack of water quality information on 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. Results of these surveys from 2002-06 show that nitrate levels are usually very low at this lake (median = 0.1 mg/l; mean = 0.5 mg/l) although levels can, at times, reach higher levels (maximum value for 2002-2006 = 10.0 mg/l). These data are, however, 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 fish contaminant monitoring in this lake.
