Lake Greenfield IA 05-NOD-1402

For the 2018 reporting cycle results from the ISU statewide survey of lakes as a part of the ambient lake monitoring program show moderately water quality at Lake Greenfield. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 65, and 73 respectively for Greenfield Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Greenfield Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and very high levels of phosphorus in the water column.

The level of inorganic suspended solids was relatively low at Greenfield Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Greenfield Lake (2.5 mg/L) was ranked 52nd among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a moderate population of cyanobacteria exists at Greenfield Lake. These data show that cyanobacteria comprised 25% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (15.9 mg/L) was ranked 67th 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. Thefish kill occurred on or before July 9, 2012. An estimated 500fish were killed, primarily adult bluegill, inthesediment basin at south end of Greenfield Lake. Results of the investigation indicate that a large algae die-offcaused the kill.According to the DNR 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 DNR 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 are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and no violations of the criterion for pH in 15 samples(0%). Based on DNR'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 Greenfield Lake.

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 2012-2016 show that nitrate levels are extremely low at this lake (maximum value = 1.2 mg/l; median = 0 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.