Blue Lake IA 06-WEM-1728

Assessment Comments

Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008 (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), 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 clarity mainly caused by large amounts of suspended solids, but secondarily caused by the presence of aesthetically objectionable blooms of algae.   The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported.”  Fish consumption uses remain “not assessed.”  Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008 (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.

Note:  A TMDL for the algae and turbidity impairments at Blue Lake was prepared by IDNR and approved by EPA in January 2009.   Because the 2009 TMDL covers both impairments, this waterbody is moved from IR Category 5b to Category 4a (impaired; TMDL approved) for the 2008 cycle.  

EXPLANATION: Results of IDNR beach monitoring at Blue Lake from 2006 through 2008 suggest that the Class A1 uses should be assessed (monitored) as “fully supported.”  Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (15 samples), and 2008 (13 samples) as part of the IDNR 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 Blue Lake beach, the geometric means of all 41 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 were below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   These results suggest generally low levels of indicator bacteria at this lake.   Also, there were no violations of Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml)in 2006 and 2007 and the percentage of samples exceeding this criterion was not significantly greater than 10% in 2008 (23%).  Therefore, these results do not suggest an impairment of the Class A1 uses of Blue Lake.

Results of the ISU lake survey and UHL ambient lake monitoring program however suggest that the Class A1 uses are “not supported” at Blue Lake due to poor water transparency mainly due to high levels of suspended sediment and secondarily aesthetically objectionable algae blooms.  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 72, 66, and 68 respectively for Blue Lake.   According to Carlson (1977) the index value for Secchi depth places Blue Lake in the hypereutrophic category, while the values for chlorophyll a and total phosphorus place Blue Lake in between the eutrophic and hypereutrophic categories.   These values suggest 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 median concentration of inorganic suspended solids is very high and contributes to the impairment at Blue Lake.   Results from the ISU and UHL lake surveys show that the median level of inorganic suspended solids in Blue Lake from 2004-2008 was 16.7 mg/L, which was the 7th highest concentration of the 132 lakes monitored by these programs.

Data from the 2004-2008 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Blue Lake, which does not contribute to impairment at this lake.   These data show that cyanobacteria comprised only 15% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (3.6 mg/L) was also the 19th lowest of the 132 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau.   Information from the IDNR Fisheries Bureau suggests that Blue Lake has a poor recreational fishery and a large carp population.   Results from the ISU and UHL lake surveys suggest "full support" of the Class B(LW) (aquatic life) uses.  The ISU and UHL lake survey results show good chemical water quality at Blue Lake.   During 2004-2008 there were no violations of the Class B(LW) criterion for dissolved oxygen (25 samples).   There was one violation of the Class B(LW) criterion for ammonia (25 samples) and one violation of the Class A1,B(LW) criterion for pH (25 samples).   Based on IDNR’s assessment methodology, these violations are not greater than 10% of the samples and therefore do not suggest an impairment of water quality at Blue Lake.  

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