Lake Cornelia Use Attainability Analysis REVISED DRAFT Prepared for Nine Mile Creek Watershed District January 2010 Prepared by Barr Engineering Co. Lake Cornelia Use Attainability Analysis Updated Draft Prepared for Nine Mile Creek Watershed District January 2010 4700 West 77th Street Minneapolis, MN 55435-4803 Phone: (952) 832-2600 Fax: (952) 832-2601 Executive Summary Overview This report describes the results of the Use Attainability Analysis (UAA) for Lake Cornelia in Edina, MN. The UAA provides the scientific foundation for a lake-specific best management plan that will permit maintenance of, or attainment of, the intended beneficial uses of Lake Cornelia. The UAA is a scientific assessment of a water body’s physical, chemical, and biological condition. This study includes both a water quality assessment and prescription of protective and/or remedial measures for Lake Cornelia and the tributary watershed. The conclusions and recommendations are based on historical water quality data, the results of an intensive lake water quality monitoring in 2004 & 2008, and computer simulations of land use impacts on water quality in Lake Cornelia using watershed and lake models calibrated to the water quality data sets. In addition, best management practices (BMPs) were evaluated to compare their relative effect on total phosphorus concentrations and Secchi disc transparencies (i.e., water clarity). Management options were then assessed to determine attainment or non-attainment with the lake’s beneficial uses. Nine Mile Creek Watershed District Water Quality Goals The Nine Mile Creek District Water Management Plan (Barr, 2007) lists the NMCWD goals for both the North and South basins of Lake Cornelia as Level III, with the desired use listed as fishing and aesthetic viewing. The NMCWD goal was quantified using the standardized lake rating system termed the Carlson’s Trophic State Index (TSI). This index considers the lake’s total phosphorus, chlorophyll a, and Secchi disc transparencies to assign a water quality index number reflecting the lake’s general fertility level. The rating system results in index values between 0 and 100, with the index value increasing with increased lake fertility. Total phosphorus, chlorophyll a, and Secchi disc transparency are key water quality indicators for the following reasons. • Phosphorus generally controls the growth of algae in lake systems. Of all the substances needed for biological growth, phosphorus is typically the limiting nutrient. • Chlorophyll a is the main photosynthetic pigment in algae. Therefore, the amount of chlorophyll a in the water indicates the abundance of algae present in the lake. • Secchi disc transparency is a measure of water clarity, and is inversely related to the abundance of algae. Water clarity typically determines recreational-use impairment. P:\Mpls\23 MN\27\2327634\WorkFiles\Lake Cornelia\Report_2009Update\Text\UAA_LakeCornelia_January2010.docx i All three of the parameters can be used to determine a TSI. However, water transparency is typically used to develop the TSI (trophic state index based on Secchi disc transparency) because people’s SD perceptions of water clarity are often directly related to recreational-use impairment. The TSI rating system results in the placement of a lake with medium fertility in the mesotrophic trophic status category. Water quality trophic status categories include oligotrophic (i.e., excellent water quality), mesotrophic (i.e., good water quality), eutrophic (i.e., poor water quality), and hypereutrophic (i.e., very poor water quality). Water quality characteristics of lakes in the various trophic status categories are listed below with their respective TSI ranges: 1. Oligotrophic – [20 < TSISD < 38] clear, low productive lakes, with total phosphorus concentrations less than or equal to 10 μg/L, chlorophyll a concentrations of less than or equal to 2 μg/L, and Secchi disc transparencies greater than or equal to 4.6 meters (15 feet). 2. Mesotrophic – [38 < TSISD < 50] intermediately productive lakes, with total phosphorus concentrations between 10 and 25 μg/L, chlorophyll a concentrations between 2 and 8 μg/L, and Secchi disc transparencies between 2 and 4.6 meters (6 to 15 feet). 3. Eutrophic – [50 < TSISD < 62] high productive lakes relative to a neutral level, with 25 to 57 μg/L total phosphorus, chlorophyll a concentrations between 8 and 26 μg/L, and Secchi disc measurements between 0.85 and 2 meters (2.7 to 6 feet). 4. Hypereutrophic – [62 < TSISD < 80] extremely productive lakes which are highly eutrophic and unstable (i.e., their water quality can fluctuate on daily and seasonal basis, experience periodic anoxia and fish kills, possibly produce toxic substances, etc.) with total phosphorus concentrations greater than 57 μg/L, chlorophyll a concentrations of greater than 26 μg/L, and Secchi disc transparencies less than 0.85 meters (2.7 feet). The NMCWD’s management strategy typically is to “protect” lakes similar to Lake Cornelia. According to the NMCWD Water Management Plan, “protect” means “to avoid significant degradation from point and nonpoint pollution sources and from wetland alterations, in order to maintain existing beneficial uses, aquatic and wetland habitats, and the level of water quality necessary to protect these uses in receiving waters.” The five specific goals are outlined below. The Water Quantity Goal for Lake Cornelia is to provide sufficient water storage of surface runoff during a regional flood, the critical 100-year frequency storm event. This goal is attainable with no action. The Water Quality Goal for Lake Cornelia currently is a Level III classification level, which is generally intended for fishing and aesthetic viewing. The Minnesota Department of Natural P:\Mpls\23 MN\27\2327634\WorkFiles\Lake Cornelia\Report_2009Update\Text\UAA_LakeCornelia_January2010.docx ii Resources (MDNR) stocks the lake annually with approximately 300 - 400 bluegill for the Fishing in the Neighborhood Program. The specific NMCWD goal for Level III classification is to achieve and maintain a TSI between 60 and 70. SD The Aquatic Communities Goal for Lake Cornelia is to achieve a water quality that will result in a diverse and balanced native ecosystem. The MDNR did not include Lake Cornelia’s fisheries-use classification in the 1992 report, An Ecological Classification of Minnesota Lakes with Associated Fish Communities (Schupp, 1992). Since the MDNR did not specify the ecological classification for Lake Cornelia there is no specific fisheries related TSI goal. However, the MDNR stocks the lake with bluegills as part of the Fishing in the Neighborhood program, and it is the goal of the NMCWD to achieve water quality that will result in a diverse and balanced native ecosystem. The Recreational-Use Goal for Lake Cornelia is to achieve water quality that supports the functions of the lake and maintain a balanced ecosystem. Lake Cornelia is a wildlife lake generally intended for wildlife habitat, aesthetic viewing and runoff management. Since the MDNR stocks the lake with bluegill for the Fishing in the Neighborhood Program, a reasonable recreational use goal would be to achieve Level III water quality. The Wildlife Goal for Lake Cornelia is to protect existing beneficial wildlife uses. The wildlife goal can be achieved with no action, especially if the wetlands and natural land surrounding the lake remain intact. In additional to the goals set by the NMCWD, the MPCA has developed assessment methodologies, conducted extensive sampling of lakes, and ultimately derived ecoregion-based lake eutrophication standards for deep and shallow lakes for total phosphorus, chlorophyll-a, and Secchi depths (MPCA, 2008). For shallow lakes in the North Central Hardwood Forests (NCHF) ecoregion (where Lake Cornelia is located), the total phosphorus standard established by the MPCA is 60 μg/L, which serves as the upper threshold for lake water quality. The chlorophyll-a and Secchi disc standards are listed as less than 20 µg/L greater than 1.0 meters, respectively. Because the water quality in Lake Cornelia does not meet these standards, it is currently listed on the 2020 (draft) 303(d) impaired waters list with an expected total maximum daily load (TMDL) study start date of 2013. Table EX-1 summarizes the 2004 and 2008 water quality in Lake Cornelia as well as the NMCWD and MPCA goals for the lake. P:\Mpls\23 MN\27\2327634\WorkFiles\Lake Cornelia\Report_2009Update\Text\UAA_LakeCornelia_January2010.docx iii Table EX-1 Lake Cornelia Management Table Water Quality, Recreational Use and Ecological Classification of, and Management Philosophies for Lake Cornelia, Referencing Carlson’s Trophic State Index (TSI) Values (Secchi Disc Transparency Basis) MPCA Shallow Lake Water Current Summer Average Water Quality District Management Lake Quality Standards Conditions (TSISD)1 District Water Quality Goal 2 Strategy North Cornelia Year of Record III 2004 2008 Fishing and aesthetic Unspecified viewing [TP] < 60 µg/L 164 µg/L 153 µg/L 105 µg/L ≥ [TP] > 75 µg/L [Chl-a] < 20 µg/L 70 µg/L 51 µg/L 60 µg/L ≥ [Chl-a] > 50 µg/L SD > 1.0 m 0.4 m 0.4 m 0.6 m ≤ [SD] < 1.0 m TSI = 73 TSI = 73 70 ≥ TSI > 60 SD SD SD South Cornelia Year of Record III 2004 2008 Fishing and aesthetic Unspecified viewing [TP] < 60 µg/L 190 µg/L 150 µg/L 105 µg/L ≥ [TP] > 75 µg/L [Chl-a] < 20 µg/L 95 µg/L 61 µg/L 60 µg/L ≥ [Chl-a] > 50 µg/L SD > 1.0 m 0.2 m 0.3 m 0.6 m ≤ [SD] < 1.0 m TSI = 83 TSI = 77 70 ≥ TSI > 60 SD SD SD 1 TSISD Carlson's Trophic State Index score. This index was developed from the interrelationships between summer average Secchi disc transparencies and epilimnetic concentrations of chlorophyll a and total phosphorus. The index results in scoring generally in the range between zero and one hundred. [ District values calculated by Barr Engineering Company (from field data and water quality model predictions). MPCA values taken from the 1994 Clean Water Act Report to the U.S. Congress; and MDNR values taken from Schupp (1992) Minnesota Department of Natural Resources Investigational Report No. 417. An ecological classification of Minnesota lakes with associated fish communities.] 2 District I = Fully supports all water-based recreational activities including swimming, scuba diving and snorkeling. II = Appropriate for all recreational uses except full body contact activities: sailboating, water skiing, canoeing, wind surfing, jet skiing. III = Supports fishing, aesthetic viewing activities and wildlife observation IV = Generally intended for runoff management and have no significant recreational use values V = Wetlands suitable for aesthetic viewing activities, wildlife observation and other public uses. * MPCA and MDNR TSI scores were provided by the agency without evaluation by the District. P:\Mpls\23 MN\27\2327634\WorkFiles\Lake Cornelia\Report_2009Update\Tables\Table 3-1_updated.xls Lake Characteristics Lake Cornelia is located in the north central portion of Edina. The lake is part of a natural marsh area. Lake Cornelia is comprised of a north (North Cornelia) and south (South Cornelia) basin, connected by a 12-inch culvert under 66th Street (with an invert elevation of 859 feet MSL) on the south side of the North Cornelia, and a secondary 12-inch pipe located on the southeast side of the North Cornelia (with an invert elevation of 860.22 feet MSL). The elevation in North Cornelia is ultimately controlled by the outlet structure on South Cornelia. The outflow from South Cornelia discharges over a weir structure at 859.1 feet MSL and flows through an extensive storm sewer system to Lake Edina. North Cornelia North Cornelia has a water surface of approximately 19 acres, a maximum depth of 5 feet, and a mean depth of approximately 3 feet at a normal water surface elevation of 859.1. At this elevation the lake volume is approximately 61 acre-feet. The water level in the lake is controlled mainly by weather conditions (snowmelt, rainfall, and evaporation), by the outlet capacity of the pipe on North Cornelia, and by the elevation of the outlet structure located on South Cornelia. South Cornelia South Cornelia has a water surface of approximately 31 acres, a maximum depth of 7 feet, and a mean depth of 4.2 feet at a normal surface elevation of 859.1. At this elevation the lake volume is approximately 130 acre-feet. The water level in the lake is controlled by the elevation of the weir structure at the south side of the lake. Water Quality Problem Assessment Baseline Lake Water Quality Status The Minnesota Lake Eutrophication Analysis Procedure (MnLEAP) is intended to be used as a screening tool for estimating lake conditions and for identifying “problem” lakes. MnLEAP is particularly useful for identifying lakes requiring “protection” versus those requiring “restoration” (Heiskary and Wilson, 1990). In addition, MnLEAP modeling has been done in the past to identify Minnesota lakes which may be in better or worse condition than they “should be” based on their location, watershed area and lake basin morphometry (Heiskary and Wilson, 1990). Results of MnLEAP modeling done for Lake Cornelia suggest that the lake could achieve “better” water quality than is currently observed (Heiskary and Lindbloom, 1993). For the MnLEAP analysis, Lake Cornelia was treated as a single basin, rather than two separate basins. The results of the various P:\Mpls\23 MN\27\2327634\WorkFiles\Lake Cornelia\Report_2009Update\Text\UAA_LakeCornelia_January2010.docx v models used to estimate the baseline total phosphorus (TP) concentration are summarized in Table EX-2. Vighi and Chiaudani (1985) developed another method to determine the phosphorus concentration in lakes that are not affected by anthropogenic (human) inputs. As a result, the phosphorus concentration in a lake resulting from natural, background phosphorus loadings can be calculated from information about the lake’s mean depth and alkalinity or conductivity. Alkalinity is considered more useful for this analysis because it is less influenced by development of the watershed. Alkalinity data from 2008 was used as well as epilimnetic specific conductivity data collected throughout the summer of 2004 and 2008 to predict the total phosphorus concentration range from natural, background loadings. Finally, the Wisconsin Lake Modeling Suite (WiLMS) model (WI-DNR, 2004) was also used to estimate Lake Cornelia’s water quality under natural (predevelopment) watershed conditions. The WiLMS model (Lake Total Phosphorus Prediction Module) uses an annual time step and predicts spring overturn, growing season mean, and annual average TP concentrations in lakes. The model uses information about the lake and watershed characteristics in conjunction with 13 different published phosphorus prediction regressions to predicted the expected in-lake TP. The historic watershed information for Lake Cornelia was used to estimate the model input parameters and the expected water quality in Lake Cornelia. Table EX-2 Summary of Lake Cornelia Baseline Water Quality Modeling Results Model Lake Expected Water Quality (Total Phosphorus) MnLEAP Lake Cornelia 55 – 97 ug/L (North & South) Vighi and Chiaudani North Cornelia 34 – 40 ug/L (1985) – Alkalinity South Cornelia 34 – 40 ug/L Vighi and Chiaudani North Cornelia 27 – 61 ug/L (1985) – Specific South Cornelia 27 – 66 ug/L Conductivity WiLMS (Canfield and Lake Cornelia 53 – 133 ug/L Bachman (1981)) (North & South) Comparison of the predicted baseline TP concentrations to observed annual average phosphorus concentrations indicates that the water quality in Lake Cornelia is worse than it likely was historically, based on its location, watershed area and lake basin morphometry. The predicted ranges of TP concentrations indicates that the NMCWD Level III classification goal should be attainable, P:\Mpls\23 MN\27\2327634\WorkFiles\Lake Cornelia\Report_2009Update\Text\UAA_LakeCornelia_January2010.docx vi given the appropriate phosphorus loadings to the lake. When compared to the MPCA shallow lakes standard (60 μg/L), the upper end of the Vighi and Chiaudani predicted natural background TP concentration range falls right around the standard, indicating that it may be possible to attain the MPCA shallow lake standard for phosphorus. However, when considering the TP ranges predicted by the MnLEAP and WiLMS models, the MPCA shallow lake standard may be attainable only on the very low end of the expected range of TP concentrations, and is likely not attainable for most conditions. Lake Cornelia Current Water Quality Because recreational use is greatest during the summer (June, July, and August) months, and because it is during these times that algal blooms and diminished transparency are most common, attention is usually focused on summer water quality in the upper (epilimnetic) portions of the lake. The NMCWD conducted intensive water quality monitoring in North and South Cornelia in 2004 and again in 2008. The Metropolitan Council Citizen Assisted Monitoring Program (CAMP) also collected water quality data in North Cornelia in 2003, 2005, 2006, 2007, and 2008. Figure EX-1 shows the historic summer average TP and Chl-a concentrations, and transparency data from 2003 through 2008 for Lake Cornelia. When comparing the three key water quality parameters, North Cornelia has slightly better water quality than in South Cornelia. However, the 2008 summer average TP, Chl a, and transparency place both North and South Cornelia in the hypereutrophic category throughout the summer, meaning that Lake Cornelia is rich in algal nutrients, susceptible to dense algal blooms, and exhibits poor water clarity. Figure EX-2a and Figure EX-2b show the most recent (2008) water quality throughout the monitoring season in North and South Cornelia, respectively. The summer averages of the various water quality parameters for 2004 and 2008 are also summarized in Table EX-1. 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