Assessment Comments
Assessment is based on (1) results from the IDNR beach monitoring program from 2002-04 (2) ISU lake surveys from 2000-04, (3) information from the DNR Fisheries Bureau, (4) ISU reports on lake plankton, 2000-05, and (5) results of EPA/IDNR fish contaminant monitoring in 1998.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to levels of indicator bacteria at this lake’s beach area that exceed state water quality standards. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Fish consumption uses are assessed (evaluated) as "fully supported" based on fish contaminant monitoring conducted in 1998. The Class C (drinking water) uses remain "not assessed" due to lack of information upon which to base an assessment. Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2002, 2003, and 2004, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information on plankton communities collected from 2000 through 2005 for the ISU lakes survey, and (5) results of U.S. EPA / IDNR fish tissue monitoring in 1998.
Note: A TMDL for siltation and nutrients at Rock Creek Lake was prepared by IDNR and approved by EPA in 2001; thus, this lake was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. 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.
EXPLANATION: Results of IDNR beach monitoring at Rock Creek Lake from 2002 through 2004 suggest that the Class A uses are "not supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (30 samples), 2003 (29 samples), and 2004 (23 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 A (primary contact recreation) uses: (1) all five-sample, 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 A uses should be assessed as “not supported”. Also, if 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 A uses should be assessed as “partially supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Rock Creek Lake beach, the geometric means of 19 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml: 3 of 26 geometric means violated in 2002, 3 of 25 geometric means violated in 2003, and 13 of 19 geometric means violated in 2004. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
Despite the non-support of Class A uses suggested by results of beach monitoring, results from the ISU surveys at Rock Creek Lake from 2000 through 2004 suggest relatively good water transparency and “full support” of the lake’s Class A uses. Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 63, and 62, respectively. According to Carlson (1977), the index values for all three parameters are in the range between eutrophic and hyper-eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus and chlorophyll-a and marginally good water transparency.
The general agreement between the TSIs for phosphorus, chlorophyll, and Secchi depth suggests that non-phosphorus limitations—which appear to occur at most Iowa lakes—do not occur at Rock Creek Lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 48, thus suggesting that phosphorus (versus nitrogen) is the nutrient that limits production of suspended algae at this lake.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at Rock Creek Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 20% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (43 mg/l) was the 23rd lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Rock Creek Lake.
The levels of inorganic suspended solids at this lake, however, are moderately high and suggest a potential impact on water transparency. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level at Rock Creek Lake was 6.2 mg/l; this was the 54th highest of the 131 lakes sampled. Thus, the somewhat elevated TSI values for chlorophyll-a and Secchi depth, and the moderately high levels of inorganic suspended solids, suggest concerns regarding the full support of the Class A (primary contact) uses due to lowered water transparency. The data collected at this lake over the five-year period, however, do not suggest any worsening tends in water transparency: the TSI for total phosphorus, chlorophyll-a, and Secchi depth for the 2000-2002 period are identical to the respective TSI values calculated for the 2002-2004 period.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 50%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Rock Creek Lake (9.5 mg/l) was the 50th lowest of the 131 lakes sampled. This level is in the lower half of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Based on information from the IDNR Fisheries Bureau, the Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". The ISU lake survey data suggest relatively good chemical water quality at Rock Creek Lake. Results of monitoring during summers of 2000 through 2004 show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected, or for pH in the 13 samples collected.
The Class C uses remained "not assessed" due to a lack of data for Class C water quality parameters. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are low at this lake (maximum value = 7.6 mg/l; median = 2.7 mg/l), these data are not sufficient for developing a valid assessment of support 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. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 1998 RAFT sampling conducted at Rock Creek Lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
