Assessment Comments
Assessment is based on: (1) results of the IDNR-UHL beach monitoring program from 2006 through 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due levels of indicator bacteria that exceed the state water quality standard. 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 (evaluated) “fully supported” based on fish tissue monitoring in 1998. Sources of data for this assessment include: (1) results of the IDNR-UHL beach monitoring program from 2006 through 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
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 and 2008 assessment/listing cycles and will remain in Category 5a for the 2010 assessment/listing cycle.
EXPLANATION: Results of IDNR-UHL beach monitoring from 2006 through 2008 suggest that the Class A1 uses are "not 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 2006 (28 samples), 2007 (17 samples), and 2008 (24 samples) as part of the IDNR-UHL beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day 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”. 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 Lake beach, the geometric means of 15 thirty-day periods during the summer recreation seasons of 2006, 2007 and 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml: 12 of 24 geometric means exceeded in 2006, 0 of 13 geometric means exceeded in 2005, and 3 of 17 geometric means exceeded in 2008. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 32% in 2006, 12% in 2007 and 17% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, the exceedences of the geometric mean criterion suggest impairment (nonsupport) of the Class A1 (primary contact recreation) uses at Rock Creek Lake.
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, however, suggest that the Class A1 uses at Rock Creek Lake are “fully supported.” Using the median values from these surveys from 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 64, and 67 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 hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is moderately high at this lake but does not suggest an impairment due to non-algal turbidity. The median inorganic suspended solids concentration at Rock Creek Lake was 5.5 mg/L, which was the 48th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Rock Creek Lake, which does not suggest impairment at this lake. These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (22.7 mg/L) was the 44th highest of the 132 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 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 25 samples, or dissolved oxygen in 25 samples, and one violation of the Class B(LW) criterion for pH in 25 samples (4%). Based on IDNR’s assessment methodology this violation is less than 10% of the samples and therefore suggests “full support” of the Class B(LW) uses at Rock Creek Lake.
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 2004-08 show that nitrate levels are low at this lake (maximum value = 6.6 mg/l; median = 3.0 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 “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 1998. While these results suggest that levels of contaminants were low at Rock Creek Lake, they are now too old (greater than 10 years) to be used for an assessment.
