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), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation in July 2005.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency and violations of the state’s criterion for pH. Nuisance algae blooms (cyanobacteria) also contribute to the impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to a fish kill that occurred in July 2005 and violations of the state’s pH criterion. Sediment, nutrients, turbidity, and algae are also concerns at this lake. Fish consumption uses are “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), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation in July 2005.
Note: A TMDL for siltation at Arrowhead Pond was prepared by IDNR and approved by EPA in 2001. Not all Section 303(d) impairments identified for the 2006 assessment/listing cycle or the current (2008) cycle (algae and pH), however, are addressed in the TMDL. Thus, this waterbody remains in IR Category 5a (impaired; TMDL required) for the 2008 assessment/listing cycle.
EXPLANATION: For the 2008 reporting cycle, the Class A1 (primary contact recreation) uses for Arrowhead Pond are assessed (monitored) as “not supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. 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 70, 68, and 76 respectively for Arrowhead Pond. According to Carlson (1977) the Secchi depth and total phosphorus values place Arrowhead Pond in the hypereutrophic category, while the chlorophyll a value places the lake between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were relatively low and do not suggest impairment due to non-algal turbidity. The median level of inorganic suspended solids in Arrowhead Pond (3.0 mg/L) was the 38th lowest of all 132 lakes sampled by ISU and UHL.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Arrowhead Pond, which contributes to impairment at this lake. These data show that cyanobacteria comprised 94% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (48.9 mg/L) was also the 24th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values for the 2002-2006 period. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
The poor water transparency and high levels of chlorophyll a and cyanobacteria indicate impairment to the Class A1 (primary contact) use through the presence of algae blooms that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” based on information from the IDNR Fisheries Bureau, results from the ISU and UHL lake surveys and the occurrence of a fish kill in July 2005. Siltation, excessive nutrient loading to the water column, and the presence of large populations of cyanobacteria are concerns at Arrowhead Pond. The ISU and UHL lake surveys data from 2002-2006 show no violations of the Class B(LW) criteria for ammonia in 14 samples, and one violation of the criterion for dissolved oxygen in 19 samples. Three of the 20 pH samples exceeded the Class A1,B(LW) criterion for pH. Based on IDNR’s methodology, however, these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A1 and Class B(LW) uses of Arrowhead Pond. However, Arrowhead Pond was assessed as “partially supporting” in the 2006 assessment/listing cycle due to significant violations of the pH criteria and therefore remains “partially supported” due to the continued violations. Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on pH violations. The pH violations at Arrowhead Pond likely reflect the excessive primary productivity at the lake and do not reflect the input of pollutants into this lake.
A fish kill that occurred in this lake on July 26, 2005 also suggests "impairment" of the Class B(LW) uses. The kill was identified as a summer kill due to low levels of dissolved oxygen related to an algal die-off. According to the IDNR investigation, approximately 500 bluegill, 1,500 yellow bass, and 600 channel catfish (including all cage catfish at this lake) were killed. The estimated value of the fish killed was $39,000. 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 (2004-2007) 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., winter kill), 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.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
