Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2006, and (5) results of the EPA/DNR fish contaminant monitoring program (RAFT) in 1994 and 2006.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (evaluated) 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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2006, and (5) results of the EPA/DNR fish contaminant monitoring program (RAFT) in 1994 and 2006.
EXPLANATION: Results of IDNR county beach monitoring from 2006 suggest that the Class A1 uses are assessed (evaluated) as "not supported." Because only one year of data for indicator bacteria has been collected for this lake the assessment is considered evaluated (lower confidence) as opposed to monitored (higher confidence). 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) as part of the IDNR county 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 Mormon Trail Lake beach, the geometric means of 4 out of 12 thirty-day periods during the summer recreation season of 2006 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. However, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was less than 10% in 2006 (6%). 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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 52, 55, and 56 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 moderately high 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 3.6 mg/L, which was the 58th lowest of the 132 monitored lakes.
Data from the 2002-2006 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 83% of the phytoplankton wet mass at this lake. However, the median cyanobacteria wet mass (13.0 mg/L) was also the 49th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” based on information from the IDNR Fisheries Bureau and results from the ISU and UHL lake surveys. The ISU and UHL lake surveys show that during 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 17 samples and no violations of the Class B(LW) criterion for dissolved oxygen in 23 samples. However, there were 5 violations of the Class A1,B(LW) criterion for pH in 23 samples (22%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment (partial support/monitored) of the Class A1 and Class B(LW) uses of Mormon Trail 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 and 2006. 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.
