Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) results from the IDNR-county voluntary beach monitoring program in 2008, 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 poor water transparency caused mainly by inorganic suspended solids that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. Algal turbidity also contributes to the impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Excessive growth of aquatic macrophytes at this shallow lake, however, remains a concern. Fish consumption uses are “not assessed” due to a lack of information upon which to base an assessment. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) results from the IDNR-county voluntary beach monitoring program in 2008, and (4) information from the IDNR Fisheries Bureau.
Note: A TMDL for turbidity at Spring Lake was prepared by IDNR and approved by EPA in 2006. Because the primary Section 303(d) impairment identified for the 2012 assessment/listing cycle (turbidity) is addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved) for the 2012 cycle.
EXPLANATION: Results of IDNR county beach monitoring from 2008 suggest that the Class A1 uses are assessed (evaluated) as "fully supported." Levels of indicator bacteria at Spring Lake beach were monitored approximately once per week during the primary contact recreation season (May through August) of 2008 (5 samples) as part of the IDNR county beach monitoring program. Because a limited number of samples were collected in 2008 these data are considered not sufficient to accurately characterize current water quality conditions, therefore the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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 Spring Lake beach, the geometric mean from 2008 was below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 16 E. coli orgs/100 ml in 2008. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2008. These results are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “full support” of the Class A1 uses.
Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Spring Lake are “not supported.” Using the median values from these surveys from 2006 through 2010 (approximately 16 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 70, 64, and 65 respectively for Spring Lake. According to Carlson (1977) the Secchi depth value places Spring Lake in the hypereutrophic category, while the chlorophyll a and total phosphorus values place Spring Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is very high at this lake and suggests impairment due to non-algal turbidity. The median inorganic suspended solids concentration at Spring Lake was 7.3 mg/L, which was the 29th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Spring Lake, which does not suggest an impairment due to nuisance aquatic life at this lake. These data show that cyanobacteria comprised only 22% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (9.2 mg/L) was also the 20th lowest of the 134 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. Information from the IDNR Fisheries Bureau suggests that nuisance aquatic macrophytes remain a concern at this lake. The ISU and UHL lake surveys show that during 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 16 samples, or dissolved oxygen in 16 samples. There were 2 violations of the criterion for pH in 16 samples (12%). However, based on IDNR’s assessment methodology these violations were not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses at Spring Lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
