Lake Ahquabi IA 04-LDM-1080
Warren County S14T75NR24W 4 mi SSW of Indianola.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trophic
- Eutrophic
- Trend
- Stable
- Created
- 10/15/2018 3:06:32 PM
- Updated
- 4/5/2019 1:38:02 PM
The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to aesthetically objectionable conditions caused by algal turbidity and violations of the state's criteria for indicator bacteria. 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 assessed (monitored) as “fully supported." Sources of data for this assessment include (1) results of IDNR/UHL beach monitoring from 2014 through 2016, (2) results of the statewide survey of Iowa lakes conducted from 2012 through 2016 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2007.
Results of DNR beach monitoring from 2014 through 2016 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Lake Ahquabi Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2014 (18 samples), 2015 (20 samples) and 2016 (15 samples), as part of the DNR beach monitoring program. According to DNR’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). At Lake Ahquabi Beach, the geometric means from 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 78 E. coli orgs/100 ml in 2014, 90 E. coli orgs/100 ml in 2015 and 21 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 28% in 2014, 45% in 2015 and 7% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014 and 2015. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses. For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Lake Ahquabi are assessed (monitored) as "partially supported" based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 64, and 61 respectively for Lake Ahquabi. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Ahquabi in the Eutrophic category. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor 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. Although the index value for chlorophyll a is below the impairment trigger of 65, Lake Ahquabi was listed as partially supporting its Class A1 uses due to aesthetically objectionable conditions. Based on DNR's methodology, the median TSI value for chlorophyll a must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). Therefore, Lake Ahquabi were remain listed as "partially supported" for the 2018 assessment/listing cycle. The level of inorganic suspended solids was relatively low at Lake Ahquabi, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Lake Ahquabi (2.6 mg/L) was ranked 55th among the 138 lakes by the ISU lake survey. Data from the 2012-2016 ISU lake survey suggest a moderately large population of cyanobacteria exists at Lake Ahquabi. These data show that cyanobacteria comprised 83% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (17.3 mg/L) was ranked 70th of the 138 lakes sampled. The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2012-2016 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 DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Lake Ahquabi.