Willow Lake IA 06-BOY-1612

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) information from the IDNR Fisheries Bureau, and (4) results of the IDNR-county voluntary beach monitoring program in 2008, 2009, and 2010.

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 remain “not assessed” due to the lack of fish contaminant monitoring at this lake.   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) information from the IDNR Fisheries Bureau, and (4) results of the IDNR-county voluntary beach monitoring program in 2008, 2009, and 2010.  

EXPLANATION: Results of IDNR city/county beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “fully supported."  Levels of indicator bacteria at Willow Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (14 samples), 2009 (14 samples), and 2010 (14 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) 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 Willow Lake beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 31 E.  coli orgs/100 ml in 2008, 20 E.  coli orgs/100 ml in 2009, and 16 E.  coli orgs/100 ml in 2010.   The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.  coli orgs/100 ml) was 14% in 2008, 0% in 2009 and 0% in 2010.   None of these are 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.   Willow Lake was assessed as “not supporting” in the 2008 assessment/listing cycle due violations of the geometric mean criterion.   Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles with no geometric mean violations and also without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on bacteria violations.   Because there were no geometric mean violations or significantly greater than 10% of the samples exceeding the single-sample maximum for both the 2010 and the 2012 assessment/listing cycles, the bacteria impairment is suggested for de-listing for the 2012 assessment/listing cycle.  

Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at Willow Lake are “fully supported.”  Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 50, 46, and 53 respectively for Willow Lake.   According to Carlson (1977) the Secchi depth and total phosphorus values place Willow Lake in the eutrophic category, while the chlorophyll a value places Willow Lake in the mesotrophic category.   These values suggest extremely low levels of chlorophyll a and suspended algae in the water, very good water transparency, and low levels of phosphorus in the water column.  

The level of inorganic suspended solids is low at Willow Lake and suggests that non-algal turbidity does not cause water quality problems.   The median inorganic suspended solids concentration at Willow was 2.0 mg/L, which was the 44th lowest of the 134 monitored lakes.

Data from the 2006-2010 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Willow Lake, which does not contribute to the impairment at this lake.   These data show that cyanobacteria comprised 71% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (11.8 mg/L) was the 35th lowest of the 134 lakes sampled.  

The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  Data from the ISU and UHL lake surveys show relatively good chemical water quality at Willow Lake.   Results from these surveys show that during 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 21 samples, dissolved oxygen in 22 samples, or pH in 22 samples.   Information from the IDNR Fisheries Bureau also suggests that Willow Lake has good water quality.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.