Brushy Creek Lake IA 04-UDM-1276

Assessment Comments

Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2004, 2005, and 2006 (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 “fully supported.”  The Class B(LW)(aquatic life) uses are assessed (monitored) as “fully 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 2004, 2005, and 2006 (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.

EXPLANATION:  Results of IDNR beach monitoring at Brushy Creek Lake from 2004 through 2006 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 2004 (16 samples), 2005 (22 samples), and 2006 (28 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) 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 Brushy Creek Lake beach, the geometric means of all 54 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.   These results suggest generally low levels of indicator bacteria at this lake.   Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E.  coli orgs/100 ml) was not significantly greater than 10% in any of the years (2004: 8%, 2005: 14%, 2006: 0%).   These results do not suggest an impairment of the Class A1 uses of Brushy Creek Lake.

Results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program also show “full support” of the Class A1 uses of Brushy Creek Lake.   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 49, 61, and 57 respectively for Brushy Creek Lake.   According to Carlson (1977) the Secchi depth value places Brushy Creek Lake in the mesotrophic category while the chlorophyll a and total phosphorus values place Brushy Creek Lake in the eutrophic category.   These values suggest moderately high levels of chlorophyll a and suspended algae in the water, exceptional water transparency, and relatively low levels of phosphorus in the water column.  

The level of inorganic suspended solids is also low at this lake and does not suggest impairment due to high non-algal turbidity.   The median inorganic suspended solids concentration at Brushy Creek Lake was 2.0 mg/L, which was the 19th lowest of the 132 monitored lakes.

A small population of cyanobacteria exists at Brushy Creek Lake, which does not cause impairment at this lake.   Data from the ISU and UHL lake surveys show that the median cyanobacteria wet mass at Brushy Creek Lake (4.7 mg/L) was the 18th lowest of all 132 monitored lakes and that cyanobacteria only account for 39% of the total phytoplankton wet mass.

The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau, results from the ISU and UHL lake surveys, and results of physical/chemical monitoring done by IDNR’s beach monitoring program.   The ISU and UHL lake survey results show good chemical water quality at Brushy Creek Lake.   During 2002-2006 there were no violations of the Class B(LW) criterion for ammonia (16 samples), dissolved oxygen (23 samples), or pH (23 samples).  .   The physical/chemical data associated with the beach monitoring data from 2004 through 2006 at Brushy Creek Lake also show no violations of the Class B(LW) criteria for dissolved oxygen (62 samples) or pH (63 samples).    

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