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, and (4) results from the IDNR-county voluntary beach monitoring program from 2004 through 2006.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.” The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses are “not assessed.” 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, and (4) results from the IDNR-county voluntary beach monitoring program from 2004 through 2006.
EXPLANATION: Results of IDNR county beach monitoring from 2004 through 2006 suggest that the Class A1 uses are assessed (monitored) as "fully supported." Levels of indicator bacteria at Oldham Lake beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2004 (13 samples), 2005 (13 samples), and 2006 (11 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 Oldham Lake beach, the geometric means of all 19 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was also less than 10% in all of the recreation seasons: 2004: 0%, 2005: 8%, 2006: 0%. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest full support of the Class A1 (primary contact recreation) uses.
Results from the ISU and UHL lake surveys also suggest that the Class A1 uses at Oldham Lake are “fully supported.” 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 56, 63, and 63 respectively for Oldham Lake. According to Carlson (1977) the Secchi depth value places Oldham Lake in the eutrophic category, while the chlorophyll a and total phosphorus values place Oldham Lake 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 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 Oldham Lake was 2.4 mg/L, which was the 26th lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Oldham Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised only 56% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (10.0 mg/L) was also the 32nd lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “fully 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, or for dissolved oxygen in 22 samples. There were 3 violations of the Class A1,B(LW) criterion for pH in 23 samples (13%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses at Oldham Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
