Lake Pahoja IA 06-BSR-1532

NOTE: A TMDL for impairments at Lake Pahoja due to algae and pH was prepared by Iowa DNR and approved by the U.S. EPA in 2016.  Thus, the impairments of Class A1 uses due to algae and pH, and the impairment of the Class B(LW) uses due to pH, are moved from IR Category 5a (impaired and TMDL required) to IR Category 4a (impaired; TMDL not required).    

Results of IDNR beach monitoring from 2012 through 2014 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Lake Pahoja Recreation Area Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (9 samples), 2013 (8 samples) and 2014 (11 samples), as part of the IDNR beach monitoring program.   According to IDNR’s assessment methodology two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) the geometric mean of the samples from each recreation season of the three-year assessment period are less than the state's geometric mean criterion of 126 E.  coli orgs/100 ml and (2) not more than 10% of the samples during any one recreation season exceeds the state's single-sample maximum value of 235 E.  coli orgs/100 ml. If a sampling season geometric mean exceeds the state criterion of 1000 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, if a sampling season geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period and/or if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa's single-sample maximum value of 235 E.  coli orgs/100 ml, the Class A1 uses should be assessed as “partially supported.” This assessment approach is based on U.S.   EPA guidelines (see pgs 3-33 to 3-35 of U.S.   EPA 1997b).

At Lake Pahoja Recreation Area Beach, the geometric means from 2012 and 2013 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml. The geometric mean from 2014, however, was greater than the Iowa water quality standard of 126 E.  coli orgs/100ml and therefore suggests impairment of the Class A1 uses.   The geometric mean was 11 E.  coli orgs/100 ml in 2012, 11 E.  coli orgs/100 ml in 2013 and 186 E.  coli orgs/100 ml in 2014.   The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E.  coli orgs/100 ml) was 0% in 2012, 0% in 2013 and 45% in 2014.   The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2014.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

For the 2016 assessment/listing cycle, however, the Class A1 (primary contact recreation) uses of Lake Pahoja are assessed (monitored) as "not supported" due to poor water transparency, aesthetically objectionable conditions caused by algae blooms and due to frequent violations of the state criterion for pH 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 67, 69, and 85 respectively for Lake Pahoja. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Lake Pahoja in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, poor water transparency, and extremely high levels of phosphorus in the water column. The data show 4 violations of the Class A1 criterion for pH in 15 samples (27%).

The level of inorganic suspended solids was relatively low at Lake Pahoja, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Lake Pahoja (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 Pahoja. These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (19.8 mg/L) was ranked 64th 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 September of 2012.   The fish kill occurred on or before September 14, 2012. An unknown number of fish were killed on June 7, 2011, and the kill was attributed to natural causes (low DO and elevated levels of ammonia), thus placing Lake Pahoja in category 3b: waters in need of further investigation.  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 are assessed (monitored) as "not supported" due to violations of the Class A1,B(LW) criterion for pH. Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 4 violations of the criterion for pH in 15 samples(27%). Based on IDNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Lake Pahoja.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring at Lake Pahoja in 2012.   The composite samples of tissues plugs from largemouth bass had low levels of mercury (0.203 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 2012 RAFT sampling conducted at Lake Pahoja 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.

Note: A Fisheries renovation in 2013 was preformed to remove common carp. Water quality has improved significantly since renovation.  Aquatic plants have returned.  Nuisance cyanobacteria blooms still present but less frequent. Water quality improvement is expected in coming years.