Rock Creek Lake IA 03-NSK-865

Results of IDNR beach monitoring from 2012 through 2014 suggest that the Class A1 uses are "partially supported." Levels of indicator bacteria at Rock Creek Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2012 (15 samples), 2013 (25 samples) and 2014 (17 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 Rock Creek Beach, the geometric means from 2012, 2013 and 2014 were all below the Iowa water quality standard of 126 E.  coli orgs/100 ml.   The geometric mean was 19 E.  coli orgs/100 ml in 2012, 20 E.  coli orgs/100 ml in 2013 and 42 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 7% in 2012, 8% in 2013 and 18% in 2014.   None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses.   According to IDNR's assessment methodology and U.S.  EPA guidelines, these results suggest "Fully Supported" of the Class A1 uses. However, in the 2014 assessment/listing cycle, the Class A1 uses were listed as "partially supported" due to geometric mean and single sample maximum exceedances.  According to IDNR's methodology, lakes must meet water quality standards for two consecutive assessment/listing cycles to suggest de-listing.  Therefore, the Class A1 uses will remain listed as "partially supported" for the 2016 cycle. 

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.

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Rock Creek Lake are assessed (monitored) as "partially supported" due to poor water transparency 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 65, 59, and 71 respectively for Rock Creek Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Rock Creek Lake in between the Eutrophic and the Hypereutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, 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.

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, 2012, and 2014  assessment/listing cycles and will remain in Category 5a for the 2016 assessment/listing cycle.  

The level of inorganic suspended solids was high at Rock Creek Lake, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Rock Creek Lake (9 mg/L) was ranked 112th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Rock Creek Lake. These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (26 mg/L) was ranked 83rd of the 138 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 lake surveys.   Information from the IDNR Fisheries Bureau suggests that sedimentation is still a concern at the upper end of the lake.   The ISU lake survey, however, suggest relatively good chemical water quality at Rock Creek Lake. 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 Rock Creek 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 2010-2014 show that nitrate levels are relatively low at this lake (maximum value = 9.3 mg/l; median = 0.9 mg/l), these data are not sufficient for developing a valid assessment of support 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.