Rock Creek Lake IA 03-NSK-865

Results of DNR beach monitoring from 2014 through 2016 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 2014 (17 samples), 2015 (22 samples) and 2016 (17 samples), as part of the DNR beach monitoring program. According to DNR’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 2014, 2015 and 2016 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 42 E. coli orgs/100 ml in 2014, 98 E. coli orgs/100 ml in 2015 and 79 E. coli orgs/100 ml in 2016. The percentage of samples exceeding Iowa's single-sample maximum criterion (235 E. coli orgs/100 ml) was 18% in 2014, 32% in 2015 and 6% in 2016. The number of samples exceeding the single-sample maximum criterion was significantly greater than 10% in 2015. According to DNR's assessment methodology and U.S. EPA guidelines, these results suggest "partially supported" of the Class A1 uses.

Rock Creek Lake was sampled as part of DNR’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 2018 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 2012-2016 (approximately 15 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, chlorophyll a, and total phosphorus values all place Rock Creek Lake in between the Eutrophic and the Hypereutrophic categories. 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 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 DNR 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, 2014, and 2016 assessment/listing cycles and will remain in Category 5a for the 2018 assessment/listing cycle.

The level of inorganic suspended solids was relitivley low at Rock Creek Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Rock Creek Lake (8.7 mg/L) was ranked 116th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a moderately large population of cyanobacteria exists at Rock Creek Lake. These data show that cyanobacteria comprised 81% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (18.3 mg/L) was ranked 81st of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” based on information from the DNR Fisheries Bureau, and results from the ISU lake surveys. Information from the DNR Fisheries Bureau suggests that sedimentation is still a concern at the upper end of the lake. The ISU lake survey, however, suggests relatively good chemical water quality at Rock Creek Lake. Results of the ISU lake survey from 2012-2016 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 DNR'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 2012-2016 show that nitrate levels are moderately high at this lake (maximum value = 9.3 mg/l; median = 1.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.