Assessment Comments
Assessment is based on: (1) results of the IDNR beach monitoring program from 2010-2012, (2) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL),(4) information from the IDNR Fisheries Bureau, and (5) IDNR fish contaminant monitoring in 2013.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due levels of indicator bacteria that exceed the state water quality standard. Aesthetically objectionable conditions due to low water clarity also cause an impairment at this lake. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau that suggests that siltation remains a problem at Rock Creek Lake. The Class C (drinking water) uses are “not assessed” due to a lack of information upon which to base an assessment. Fish consumption uses remain (monitoring) “fully supported” based on fish tissue monitoring in 1998 and 2013. Sources of data for this assessment include: (1) results of the IDNR beach monitoring program from 2010-2012, (2) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) IDNR fish contaminant monitoring in 2013.
Note: A TMDL for siltation and nutrients at Rock Creek Lake was prepared by IDNR and approved by EPA in 2001. Because the Section 303(d) impairment due to indicator bacteria (aka, "pathogens") was not addressed in the TMDL, this waterbody was moved from IR Category 4a to Category 5a (impaired; TMDL required) for the 2006, 2008, 2010, and 2012 assessment/listing cycles and will remain in Category 5a for the 2014 assessment/listing cycle.
EXPLANATION: Results of IDNR beach monitoring from 2010-2012 suggest that the Class A1 uses are “partially supported." Levels of indicator bacteria at Rock Creek Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2010 (20 samples), 2011 (16 samples), and 2012 (15 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 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported.” Also, 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2014 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Rock Creek Lake beach, the geometric means from 2010-2012 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 68 E. coli orgs/100 ml in 2010, 41 E. coli orgs/100 ml in 2011, and 20 E. coli orgs/100 ml in 2012. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 25% in 2010, 6% in 2011 and 7% in 2012. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2010. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “partial support” of the Class A1 uses.
Rock Creek Lake was sampled as part of IDNR’s Safe Lakes Program, which aims to identify sources of bacteria to selected beaches where bacteria levels have consistently violated the state water quality criteria. The Safe Lakes Program found high levels of bacteria in the watershed, especially in the northern and eastern portions, that were likely a significant source of the high bacteria levels at the beach. This program also found that bacteria samples were higher on the west side of the beach, which is shallower and muddier.
Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Rock Creek Lake are “partially supported.” Using the median values from these surveys from 2008-2012(approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 67, 62, and 71 respectively for Rock Creek Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Rock Creek Lake in between the eutrophic and hypereutrophic categories, while the total phosphorus value places Rock Creek Lake in the hypereutrophic category. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, poor water transparency, and very high levels of phosphorus in the water column.
The level of inorganic suspended solids is high at this lake and may contribute to the impairment at Rock Creek Lake. The median inorganic suspended solids concentration at Rock Creek Lake was 10.5 mg/L, and ranked 112th of the 134 monitored lakes.
Data from the 2008-2012 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Rock Creek Lake, which does not suggest impairment at this lake. These data show that cyanobacteria comprised 66% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (18.4 mg/L) and ranked 68th of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the IDNR Fisheries Bureau, and results from the ISU and UHL lake surveys. Information from the IDNR Fisheries Bureau suggests that sedimentation is still a concern at the upper end of the lake. The ISU and UHL lake surveys, however, suggest relatively good chemical water quality at Rock Creek Lake. Data from these surveys show that from 2008-2012 there were no violations of the Class B(LW) criterion for ammonia in 14 samples and no violations of the Class A1, B(LW) uses for pH in 14 samples. There was one violation of the Class B(LW) criterion for dissolved oxygen in 14 samples (7%). Based on IDNR's methodology, this violation is not significantly greater than 10%, and therefore, suggests "full support" of the Class B(LW) uses.
The Class C uses remained "not assessed" due to a lack of monitoring data. The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU and UHL surveys from 2006-10 show that nitrate levels are low at this lake (maximum value = 5.5 mg/l; median = 1.1 mg/l) relative to the MCL (10 mg/L), these data are not sufficient for developing a valid assessment 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 Rock Creek Lake in 2013. The composite samples of fillets from largemouth bass and channel catfish had low levels of contaminants. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.116 ppm. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: <0.05 ppm; total PCBs: <0.06 ppm; and technical chlordane: <0.02 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 2013 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.
