Assessment Comments
Assessment based on results of (1) IDNR beach monitoring program from 2002 through 2004, (2) ISU lake survey from 2000-04, (3) surveys by IDNR Fisheries Bureau, (4) ISU reports on lake plankton communities from 2000-05, and (5) results of U.S. EPA/IDNR fish tissue monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to (1) levels of indicator bacteria that exceed state criteria and (2) to high levels of pH. The Class B(LW) aquatic life uses are assessed (monitored) as "partially supported" also due to high levels of pH. The Class C (drinking water) uses remain "not assessed" due to a lack of monitoring information. Fish consumption uses are assessed (evaluated) as "fully supported" based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 1996. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) IDNR/UHL beach monitoring from 2002 through 2004, (3) surveys by IDNR Fisheries Bureau, (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (5) results of U.S. EPA / IDNR fish tissue monitoring in 1996.
EXPLANATION: Results of IDNR beach monitoring at Lake Geode 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 (31 samples), 2003 (29 samples), and 2004 (22 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 Lake Geode beach, the geometric means of 5 of the 25 thirty-day periods during the summer recreation season of 2003 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. None of the geometric means exceeded this standard during the recreational seasons of 2002 or 2004. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was greater than 10% in the 2003 recreation season (31%). No more than 10% of the samples exceeded this standard during the recreational seasons of 2002 (10%) and 2004 (5%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
In contrast to results of IDNR beach monitoring, results from the ISU statewide survey of Iowa lakes from 2000-04 suggest “full support” of the Class A (primary contact recreation) uses of Lake Geode. Using the median values from this survey from 2000-04 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 55, 52, and 45, respectively. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the middle to lower range of eutrophic lakes. The index value for Secchi depth is in the middle range of mesotrophic lakes. These index values suggest exceptional water quality for an Iowa impoundment: very low levels of phosphorus, very low production of suspended algae, and exceptional water transparency.
Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for Lake Geode is 98. This ratio is high for Iowa lakes and indicates an excess of total nitrogen.
The levels of inorganic suspended solids at this lake are extremely low relative to other Iowa lakes and do not suggest the potential for either impairing designated uses or contributing to in-lake turbidity. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000-04 was 5.2 mg/l; the median level at Lake Geode was 1.9 mg/l.
In terms of all Iowa lakes sampled, data from the ISU survey show very small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that while Cladoceran taxa (e.g., Daphnia) comprised about 45% of the dry mass of the zooplankton community of this lake, the average per summer sample mass of Cladoceran taxa (59.4 mg/l) was the 43rd lowest level of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Geode Lake.
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 (over 75%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Geode Lake (14.1 mg/l) was the only the 55th highest of the 131 lakes sampled. This level is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large population of bluegreen algae at this lake does not, however, suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels greater than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to 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).
The Class B(LW) aquatic life uses are assessed as "fully supported" based on information from the DNR Fisheries Bureau. Results of chemical water quality monitoring conducted as part of the ISU lake survey, however, suggest an impairment of the Class B(LW) uses. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Four of 15 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.5; minimum = 7.9 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria. Thus, these results suggest an impairment (partial support/monitored) of the Class A and Class B(LW) uses of this lake.
The Class C (drinking water) uses remained "not assessed" due to the lack of water quality information. 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 2000-04 show that nitrate levels at this lake are low relative to the drinking water MCL (10 mg/l) (maximum sample value = 4.5 mg/l; median = 2.1 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 Lake Geode in 1996. 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 an impairment of fish consumption uses. Although this scenario does not apply to the fish contaminant data generated from the 1996 RAFT sampling conducted in at Lake Geode, levels of contaminant are sufficiently high for concern and thus justify follow-up monitoring. Results from the 1996 sampling show that the level of mercury in the sample of largemouth bass fillets (0.348 ppm) exceeds the 1 meal/week trigger level (0.20 ppm) as defined in Iowa’s revised (2006) fish consumption advisory protocol. According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory. But, this elevated level does indicate a concern and the need to conduct additional monitoring to better define contaminant levels in fish from this river segment. Thus, follow-up monitoring will be conducted in 2006 to better determine (1) levels of mercury in the edible portions (fillets) of largemouth bass from Lake Geode and (2) whether a one-meal-per-week consumption advisory needs to be issued.
