Lake Greenfield IA 05-NOD-1402

Results from the ISU survey suggest good water quality at Lake Greenfield. Using the median values from these surveys from 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 62, and 68 respectively for Lake Greenfield. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Greenfield in the Eutrophic category. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column. The data show no violations of the Class A1 criterion for pH in 15 samples.

The level of inorganic suspended solids was relatively low at Lake Greenfield, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Lake Greenfield (2.5 mg/L) was ranked 39th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at Lake Greenfield. These data show that cyanobacteria comprised 45% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (12.4 mg/L) was ranked 38th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on results of a fish kill investigation in July of 2012.   The fish kill occurred on or before July 9, 2012.  An estimated 500 fish were killed, primarily adult bluegill, in the sediment basin at south end of Greenfield Lake.   Results of the investigation indicate that a large algae die-off caused the kill.    According to the IDNR assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2012-2014) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired.”  If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”  Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5).   Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation. 

The Class B(LW) (aquatic life) uses, however, are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), 2 violations of the criterion for dissolved oxygen in 15 samples(13%), and no violations of the criterion for pH in 15 samples(0%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Lake Greenfield.

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 lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU surveys from 2010-2014 show that nitrate levels are low at this lake (maximum value = 1.2 mg/l; median = 0.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.   EPA/IDNR fish contaminant (RAFT) monitoring at Lake Greenfield in 2007.   The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants.   Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.0201 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.    Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.0633 ppm.   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.   The fish contaminant data generated from the 2007 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.