Assessment Comments
Assessment is based on: (1) results of the IDNR-UHL beach monitoring program from 2004 through 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 1998.
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 (monitored) as “fully supported.” 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 2004 through 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (4) information from the IDNR Fisheries Bureau, and (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 1998.
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 assessment/listing cycle and will remain in Category 5a for the 2008 assessment/listing cycle.
EXPLANATION: Results of IDNR-UHL beach monitoring from 2004 through 2006 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 2004 (22 samples), 2005 (23 samples), and 2006 (28 samples) as part of the IDNR-UHL 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) all thirty-day geometric means for 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 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.” 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).
At Rock Creek Lake beach, the geometric means of 39 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml: 13 of 18 geometric means violated in 2004, 14 of 19 geometric means violated in 2005, and 12 of 24 geometric means violated in 2006. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was greater than 10% in the following recreation seasons: 2004: 41%, 2005: 39%, 2006: 32%. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results 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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 63, 63, and 65 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 moderately high levels of phosphorus in the water column.
The level of inorganic suspended solids is 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 5.9 mg/L, which was the 48th highest of the 132 monitored lakes.
Data from the 2002-2006 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 78% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (25.5 mg/L) was the 49th highest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported” based on information from the IDNR Fisheries Bureau, results from the ISU and UHL lake surveys, and results of physical/chemical monitoring associated with the IDNR-UHL beach monitoring program. The ISU and UHL lake surveys suggest relatively good chemical water quality at Rock Creek Lake. Data from these surveys show that from 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 17 samples, or dissolved oxygen in 23 samples, and one violation of the Class B(LW) criterion for pH in 21 samples (5%). 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.
Results of physical/chemical monitoring associated with the IDNR-UHL beach monitoring program also suggest relatively good chemical water quality at Rock Creek Lake. Data from this program show no violations of the Class B(LW) criterion for dissolved oxygen in 69 samples and 11 violations of the pH criterion in 69 samples (13%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest that the Class B(LW) uses are “fully supported” 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 2002-06 show that nitrate levels are low at this lake (maximum value = 6.6 mg/l; median = 2.0 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 assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Rock Creek Lake in 1998. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. The fish contaminant data generated from the 1998 RAFT sampling conducted at Rock Creek Lake showed that levels of all contaminants were below advisory trigger levels, thus suggesting “full support” of fish consumption uses.
