Three Mile Lake IA 05-GRA-1371

Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, (3) U.S. EPA / IDNR fish tissue (RAFT) monitoring in 2000, and (4) surveys by the IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting."  This lake has very good water quality relative to other Iowa lakes.   [Note:  this lake is not designated for Class A (primary contact recreation) uses in the Iowa Water Quality Standards.]  Fish consumption uses are assessed as "fully supporting" based on results of fish contaminant monitoring in 2000.   The Class C (drinking water) uses remain “not assessed” due to lack of information upon which to base an assessment.   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) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of U.S.  EPA/IDNR fish contaminant monitoring in 2000.

EXPLANATION:  The water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported."  Using the median values from Iowa State University’s survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 59, 54, and 50, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake at the upper range of eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes, and the index value for secchi depth is at the boundary between mesotrophic and eutrophic lakes.   These index values suggest somewhat elevated levels of phosphorus, very low (and somewhat less than expected) levels of chlorophyll-a, and very good to exceptional water transparency.  

According to Carlson (1991), the occurrence of a low TSI value for chlorophyll-a and secchi depth relative to that for total phosphorus indicates that algal production is limited by zooplankton grazing, nitrogen limitation, or some factor other than phosphorus.   Results of ISU monitoring, however, do not immediately suggest non-phosphorus limitations on algal production at this lake.   Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Threemile Lake is 26.   This ratio does not suggest a strong likelihood that algal production at this lake is limited by nitrogen availability.  

The levels of inorganic suspended solids at this lake are extremely low and do not suggest the potential for impairing designated uses.   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 Threemile Lake was 2.0 mg/l; this is the 7th lowest median level of the 131 lakes sampled.  

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.   Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 50% 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 (42 mg/l) was the 21st lowest of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Threemile 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 (approximately 65%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Threemile Lake (12.2 mg/l) was the 66th lowest of the 131 lakes sampled.   This level is typical for the Iowa lakes sampled.   The presence of a moderately large 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).  

Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected (7%) during summers of 2000 through 2004.   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), however, a violation frequency of less than 10% for conventional parameters such as dissolved oxygen nonetheless suggests "full support" of aquatic life uses.   Thus, the percentage of violations of the dissolved oxygen criterion at this station does not suggest an impairment of aquatic life uses at this lake.   Also, one of 15 samples (7%) violated the Class A,B(LW) criterion for pH (maximum = 9.1; minimum = 7.5 pH units).   This violation likely reflects primary productivity at Threemile Lake and does not reflect the input of pollutants into this lake.

The Class C (drinking water) uses remain "not assessed" due to lack of water quality information upon which to base such an assessment.   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 are very low at this lake (maximum value = 1.2 mg/l; median = 0.2 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.  

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Threemile Lake in 2000.   The composite samples of fillets from channel catfish and walleye had low levels of contaminants.   The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.   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 2000 RAFT sampling conducted at Threemile Lake:  the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.