Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2008, 2009, and 2010, (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 2007 and turtle tissue monitoring in 2009.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to levels of indicator bacteria that exceed Iowa’s water quality standard, and violations of the state water quality criterion for pH. Algae levels that are approaching the impairment level also contribute to the impairment at this lake. The Class B(LW) uses are assessed (monitored) as “partially supported” due to violations of the state water quality criterion for pH. Fish consumption uses are assessed (monitored) as "fully supported" based on results of fish tissue (RAFT) sampling in 2007. Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2008, 2009, and 2010, (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 2007 and turtle tissue monitoring in 2009.
Note: A TMDL for nutrients and siltation at Lake MacBride was prepared by IDNR and approved by EPA in 2005. Because, however, the Section 303(d) impairment due to indicator bacteria at this lake was not addressed in the TMDL, this lake is placed in IR Category 5a (impaired; TMDL required).
EXPLANATION: Results of IDNR beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “partially supported." Levels of indicator bacteria at Lake MacBride beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (25 samples), 2009 (25 samples), and 2010 (19 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: 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 Lake MacBride beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 13 E. coli orgs/100 ml in 2008, 23 E. coli orgs/100 ml in 2009, and 86 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2008, 4% in 2009 and 32% in 2010. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2010 and therefore suggests impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “partial support” of the Class A1 uses. Bacteria levels at Lake Macbride beach had been low from 2005 through 2009, however levels increased in 2010. This is likely due to rainfall and the timing of sample collection. In 2010, there were multiple rain events on Sundays and Mondays which influenced the regularly scheduled sampling conducted on Mondays.
Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses should be assessed (monitored) as "fully supported/threatened" due to increasing levels of algae (chlorophyll a). Using the median values from these surveys from 2006 through 2010 (approximately 24 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 62, and 61 respectively for Lake Macbride. According to Carlson (1977) the Secchi depth value places Lake Macbride at the upper end of the eutrophic category while the chlorophyll a and total phosphorus values place Lake Macbride in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column. The chlorophyll TSI value had been increasing over the past assessment/listing cycles (2004: 55, 2006: 56, 2008: 62, 2010: 64) and was approaching the trigger level (65) in 2010. Lake MacBride was therefore listed as "fully supported/threatened" for the 2010 assessment/listing cycle where it will remain for the 2012 assessment.
The level of inorganic suspended solids was low at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Lake Macbride was 2.0 mg/L, which was the 27th lowest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Lake Macbride, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 82% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (21.5 mg/L) was also the 63rd highest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to a high frequency of violations of Iowa’s criterion for pH. Based on data from the ISU and UHL lake surveys from 2006 through 2010, there were no violations of the Class B(LW) criterion for ammonia in 23 samples or of the Class B(LW) criterion for dissolved oxygen in 24 samples. There were, however, 4 violations of the Class A1,B(LW) criterion for pH in 24 samples (17%). Based on IDNR’s assessment methodology these results are not significantly greater than 10% of the samples and therefore do not constitute an impairment of the Class A1 and B(LW) uses of Lake Macbride. However, for the 2010 assessment/listing cycle, Lake Macbride was assessed as "partially supporting" due to significant violations of the pH criteria. According to IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. Therefore Lake Macbride will remain assessed as "partially supported" due to violations of the pH standard for the 2012 assessment/listing cycle.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Lake MacBride in 2007 and turtle tissue monitoring in 2009. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.179 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.0627 ppm. Levels of primary contaminants in the samples of shoulder tissue from snapping turtle were as follows: mercury: 0.113 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. 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 and turtle contaminant data generated from the 2007 and 2009 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
