Arbor Lake IA 03-NSK-864
Poweshiek County S20T80NR16W W edge of Grinnell
- Cycle
- 2016
- Release Status
- Final
- Overall IR
- 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
- Trophic
- Hypereutrophic
- Trend
- Stable
- Created
- 5/5/2016 3:13:40 PM
- Updated
- 7/11/2016 9:10:42 AM
The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency and the presence of aesthetically objectionable conditions caused by algal blooms. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported”. Fish consumption uses are assessed (evaluated) as “fully supported.” Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2010 through 2014 by Iowa State University (ISU) and (2) information from the IDNR Fisheries Bureau.
For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Arbor Lake are assessed as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms based on information from the ISU lake survey. 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 70, 68, and 74 respectively for Arbor Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Arbor Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and very 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 high at Arbor Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Arbor Lake (9.6 mg/L) was ranked 116th 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 Arbor Lake. These data show that cyanobacteria comprised 74% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (12.4 mg/L) was ranked 37th of the 138 lakes sampled. The Class B(LW) (aquatic life) uses 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%), 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 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 Arbor Lake. Fish consumption uses were “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 2001 and 2005. While these results suggest that levels of contaminants were low at Arbor Lake, they are now too old (greater than 10 years) to be used for an assessment Note: A TMDL for siltation and nutrients at Arbor Lake was prepared by IDNR and approved by EPA in 2003. Because all Section 303(d) impairments identified in the 2016 assessment/listing cycle (algal growth, nutrients, and siltation) are addressed by the TMDL, Arbor Lake remains in IR Category 4a (TMDL approved). Sources of impairments are attributed to multiple sources including Agriculture, Internal nutrient cycling (primarily lakes), Urban Runoff/Storm Sewers and Naturals sources