Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2006, 2007, and 2008, and (5) results of the EPA/DNR fish contaminant monitoring program (RAFT) in 1994, 2006, and 2008.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of the Class A1 criterion for indicator bacteria and violations of the Class A1 criterion for pH. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the Class B(LW) criterion for pH. Fish consumption uses are assessed (monitored) as “partially supported” due to the issuance of a fish consumption advisory at this lake. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2006, 2007, and 2008, and (5) results of the EPA/DNR fish contaminant monitoring program (RAFT) in 1994, 2006, and 2008.
EXPLANATION: Results of IDNR county beach monitoring from 2006, 2007, and 2008 suggest that the Class A1 uses are assessed (monitored) as "not supported." Levels of indicator bacteria at Mormon Trail Lake beach were monitored approximately once per week during the primary contact recreation season (May through August) of 2006 (16 samples), 2007 (13 samples), and 2008 (11 samples) as part of the IDNR county beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. 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”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At Mormon Trail Lake beach, the geometric means of 6 thirty-day periods during the summer recreation seasons of 2006 through 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. Four of 12 geometric means exceeded the criterion in 2006, 2 of 6 geometric means exceeded in 2007, and none of the 3 geometric means exceeded in 2008. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 6% in 2006, 15% in 2007, and 18% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest nonsupport of the Class A1 (primary contact recreation) uses.
Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Mormon Trail Lake are “partially supported” due to violations of the Class A1 criterion for pH. Additional results of the ISU and UHL lake surveys, however, suggest "full support" of the Class A1 uses. Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 55, 59, and 57 respectively for Mormon Trail Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Mormon Trail Lake in the eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, very good water transparency, and relatively low 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 Mormon Trail Lake was 2.6 mg/L, which was the 40th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a moderate population of cyanobacteria exists at Mormon Trail Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 84% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (14.0 mg/L) was the 64th highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” based on results from the ISU and UHL lake surveys. The ISU and UHL lake surveys show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 25 samples and no violations of the Class B(LW) criterion for dissolved oxygen in 25 samples. There were 3 violations of the Class A1,B(LW) criterion for pH in 25 samples (12%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A1 and Class B(LW) uses of Mormon Trail Lake. However, Mormon Trail Lake was assessed as “partially supporting” in the 2008 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on 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. The pH violations at Mormon Trail Lake likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (monitored) as “partially supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Mormon Trail Lake in 1994, 2006, and 2008. 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. Levels of primary contaminants in 1994 were generally low in the fish samples collected. The level of mercury in the sample of largemouth bass (0.34 ppm), however, exceeds the IDNR/IDPH trigger level of 0.30 ppm for a one meal per week consumption advisory. According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory. Follow up sampling was conducted in 2006 at Mormon Trail Lake. The level of mercury in a sample of largemouth bass was 0.565 ppm. This again exceeded the IDNR/IDPH trigger level; therefore a one meal per week consumption advisory was issued for Mormon Trail Lake. Follow up sampling was conducted again at Mormon Trail Lake in 2008. The level of mercury in a sample of largemouth bass was 0.22 ppm, which is below the one meal per week consumption advisory. Two consecutive samplings with mercury levels below the advisory level are necessary for removal of the fish consumption advisory and for results to indicate "full support" of the fish consumption uses. The level of mercury in a sample of walleye, however, was 0.361 ppm, which is above the one meal per week advisory level. Therefore, the fish consumption uses at Mormon Trail Lake remain assessed as "partially supported."
