Lake Greenfield IA 05-NOD-1402

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), and (3) information from the IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY: The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.”  The Class C uses are “not assessed” due to the lack of information upon which to base an assessment.   Fish consumption uses remain “not assessed” due to a 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 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), and (3) information from the IDNR Fisheries Bureau.

EXPLANATION: Results from the ISU and UHL surveys suggest good water quality at Lake Greenfield.   Using the median values from these surveys from 2002 through 2006 (approximately 20 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 59, and 67 respectively for Lake Greenfield.   According to Carlson (1977) the Secchi depth and chlorophyll a values place Lake Greenfield at the upper end of the eutrophic category.   The value for total phosphorus places Lake Greenfield in between the eutrophic and hypereutrophic categories.   These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and high levels of phosphorus in the water column.  

The levels of inorganic suspended solids at this lake were relatively low and do not suggest an impairment due to non-algal turbidity.   The median level of inorganic suspended solids in Lake Greenfield (2.2 mg/L) was the 25th lowest median of the 132 lakes sampled by the ISU and UHL programs.

Data from the 2002-2006 ISU and UHL surveys suggest a moderate population of cyanobacteria exists at Lake Greenfield, which does not suggest an impairment at this lake.   These data show that cyanobacteria comprised 85% of the phytoplankton wet mass at this lake.   The median cyanobacteria wet mass (18.6 mg/L) was the 67th highest of the 132 lakes sampled.   NOTE: The City of Greenfield applies about 400 pounds of copper sulfate to Lake Greenfield about every three weeks during the summer months (Water Web), which may affect the amounts of algae in the lake and detected by routine sampling.

The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on results of the ISU and UHL lake surveys.  Data from these surveys from 2002-2006 show no violations of the Class B(LW) criteria for ammonia in 14 samples, dissolved oxygen in 19 samples, or pH in 19 samples.  

The Class C (drinking water) uses are not assessed due to the lack of recent information upon which to base an assessment.   The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate.   While the results of the ISU and UHL surveys from 2002-06 show that nitrate levels are very low at this lake (maximum value = 0.3 mg/l; median = 0.1 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.  

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