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 “not supported” due to levels of indicator bacteria that exceed Iowa’s water quality standard, and violations of the state water quality criterion for pH. 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 remain “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 from 2004 through 2006 suggest that the Class A1 uses are "not supported." Levels of indicator bacteria at Lake Macbride beach were monitored once per week during the primary contact recreation seasons (May through September) of 2004 (22 samples), 2005 (23 samples), and 2006 (28 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 Macbride beach, the geometric means of 6 thirty-day periods during the summer recreation season of 2004 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. No geometric means violated this criterion in 2005 or 2006. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was not significantly greater than 10% in any recreation season: 2004: 14%, 2005: 4%, 2006 4%. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A1 (primary contact recreation) uses.
Results from the ISU and UHL lake surveys show good water quality at Lake Macbride. 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 60, 62, and 60 respectively for Lake Macbride. According to Carlson (1977) the Secchi depth, and total phosphorus values place Lake Macbride at the upper end of the eutrophic category while the chlorophyll a value places 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 relatively low levels of phosphorus in the water column.
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 31st lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Lake Macbride, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 72% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (12.4 mg/L) was also the 45th lowest of the 132 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 2002 through 2006, there were no violations of the Class B(LW) criterion for ammonia in 16 samples and one violation of the Class B(LW) criterion for dissolved oxygen in 22 samples (5%). Based on IDNR’s assessment methodology, this single violation of the Class B(LW) criterion for dissolved oxygen is less than 10% of the samples and therefore does not constitute an impairment of the aquatic life uses of Lake Macbride. Data from the ISU and UHL surveys, however, show six violations of the Class A,B(LW) criterion for pH in 23 samples (26%). Based on IDNR’s assessment methodology these results are significantly greater than 10% of the samples and therefore constitute an impairment (partial support/monitored) of the Class B(LW) uses of Lake Macbride. Results of physical/chemical monitoring associated with the IDNR-UHL beach monitoring program also suggest “partial support” of the Class B(LW) uses of Lake Macbride. Results from 2004 through 2006 show two violations of the Class B(LW) criterion for dissolved oxygen in 70 samples (3%), and 8 violations of the Class A,B(LW) criterion for pH in 70 samples (15%). Combined with the ISU and UHL lake survey data there are 14 violations of the pH criterion in 93 samples. These results are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class B(LW) uses at Lake Macbride.
Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring at this lake. The most recent fish contaminant monitoring was conducted in 1991 as part of the U.S. EPA/IDNR (RAFT) program. Although results of the 1991 RAFT monitoring at Lake MacBride indicated very low levels of contaminants and “full support” of the fish consumption uses, these data are now considered too old (greater than ten years) to characterize current water quality conditions.
