Lake Yale Hydrologic/Nutrient Budgets and Water Quality Management Plans

Transcription

1 Lake Yale Hydrologic/Nutrient Budgets and Water Quality Management Plans Presentation to the Harris Chain Restoration Council June 2018 Michael J. Perry Lake County Water Authority

2 Scope of Work The primary objectives of this project are to quantify and rank hydrologic and pollutant loadings to Lake Yale, and to identify potential water quality improvement projects. Conducted field monitoring to collect hydrologic and water quality data for use in developing hydrologic and nutrient budgets and identify opportunities for load reductions. The hydrologic budgets include inputs from bulk precipitation, stormwater runoff, inflow from interconnected lakes and canals, and groundwater seepage. The nutrient budgets include inputs from bulk precipitation, stormwater runoff, inflows from interconnected lakes and canals, groundwater seepage, and internal recycling. Conduct a detailed evaluation of sediment characteristics to include physical and chemical characterization of surficial sediments and evaluation of internal phosphorus recycling. Develop bathymetric maps of water and muck depth, along with estimates of water and muck volume. Identify and evaluate water quality improvement projects, and provide recommendations for water quality improvement options.

3 Lake Yale is a 4,044 acre eutrophic shallow lake with an average water depth of approximately 3.7 m. The TMDL document indicates a drainage basin of approximately 15,394 acres (Fulton, et al., 2003). Surface outflow from the lake occurs periodically through the Yale Canal into Lake Griffin. Discharge rates and water elevations in Lake Yale are partially controlled by a fixed crest weir located in the outfall canal. Lake Yale is classified as a Class III waterbody with designated uses of recreation and propagation of a healthy, well-balanced population of fish and wildlife. The watershed area of Lake Yale is largely undeveloped, consisting primarily of agriculture, wetlands, and natural land.

4 Previous Studies During 1995, SJRWMD released Technical Publication SJ95-6 titled External Nutrient Budget and Trophic State Modeling for Lakes in the Upper Ocklawaha River Basin (Fulton, 1995). This document provides external nutrient budgets for each of the Upper Ocklawaha River Basin lakes and includes drainage basin boundaries, land use characterization, estimation of runoff volumes and loadings, and trophic state modeling to corroborate the identified loading inputs. An additional study was conducted by Fulton, et al. (2003) titled Interim Pollutant Load Reduction Goals for Seven Major Lakes in the Upper Ocklawaha River Basin for SJRWMD. This document consists of an expansion of the 1995 study, identifies both hydrologic and nutrient loadings to Lake Yale over the period from , and recommends pollutant load reduction goals to achieve water quality targets in each of the evaluated lakes. Sources included runoff from land uses such as residential, commercial, industrial, mining, open land/recreational, muck farms, pastures, croplands, silviculture, wetlands, and other agriculture. Atmospheric contributions from wet and dry deposition directly on the lake surface were accounted for based upon measurements in the basin.

5 Previous Studies (cont.) The mean annual total phosphorus load over this period was estimated at 1,432.4 kg. The three major sources for phosphorus were dry deposition (25.94%), precipitation (19.74%), and wetlands (14.59%). In this evaluation, pasture, cropland, feeding operations, and other agricultural activities represented approximately 5% of the annual average phosphorus. Total nitrogen was estimated at 23,078.7 kg/yr, with precipitation accounting for approximately 42% of the total load. Permitted industrial or domestic wastewater sources represented less than 4% of the phosphorus load and less than 2% of the nitrogen load to the lake. This 2003 report forms the basis for the TMDL document titled Total Maximum Daily Load for Total Phosphorus for Lake Yale and Yale Canal, Lake County, Florida issued by FDEP, dated August 14, However, neither of the two TMDL studies evaluated nutrient loadings originating from groundwater seepage or internal recycling, which often exceeds runoff generated nutrient loadings in hypereutrophic lakes. This study is intended to re-evaluate and verify the previously identified inputs, while supplementing the analyses to include shallow groundwater seepage and internal recycling.

6 Water Level Elevations and Control Water level elevation data have been collected by SJRWMD in Lake Yale from 1959 to the present. Over the available period of record, water levels in Lake Yale have varied from ft, a difference of approximately 6.7 ft, with an overall mean water elevation of ft. The observed water level fluctuation in Lake Yale represents a relatively large range for a Central Florida lake.

7 Water Depth Contours for Lake Yale (March 16, 2015; Water Elevation = ft).

8 Water Level Elevations and Control

9 Water Level Elevations and Control The most significant structure which controls discharges from Lake Yale is a 24-inch HDPE in an earth berm approximately 50 ft east of CR 452. The current pipe is a replacement for a similar pipe that was washed out during high water conditions from the 2004 hurricane season. Box Culvert Crossing at SR 452. The invert of the HDPE culvert is approximately 58 ft which essentially establishes this as the control elevation for the lake.

10 Water Level Elevations and Control Yale-Griffin Canal Culvert Crossing at Emeralda Road.

11 Relationships Between Rainfall and Water Level Water level data were obtained from the historical water level measurements. Annual rainfall data is based on measurements collected at the Lisbon Meteorological Site which is located approximately mid-way between Lakes Yale, Griffin, and Eustis. A relatively good correlation appears to exist between annual rainfall and water level in Lake Yale, with increases and decreases in water level elevations in Lake Yale generally following annual rainfall patterns. Rainfall in recent years has been insufficient to maintain constant water levels in Lake Yale, resulting in an overall decline in water level.

12 Summary of Available Historical Water Quality Data for Lake Yale

13 Historical Water Quality Monitoring Sites in Lake Yale

14 Trends in Total Nitrogen in Lake Yale from Total nitrogen concentrations have exhibited a moderate degree of variability over time in Lake Yale, with a relatively close agreement between nitrogen concentrations measured by the various agencies. A general trend of increasing nitrogen concentrations is apparent in Lake Yale over the period of record. A jump in total nitrogen concentrations appears to have occurred in Lake Yale during the period from , with measured values before this period ranging from approximately 500-1,200 g/l and nitrogen concentrations following this period ranging from approximately 1,200-3,000 g/l.

15 Trends in Total Phosphorus in Lake Yale from Measured total phosphorus concentrations in Lake Yale have been highly variable over time. A general trend of increasing phosphorus concentrations has occurred in Lake Yale over time. A jump in phosphorus concentrations also occurred in Lake Yale from , similar to the trend observed for total nitrogen. Prior to 1996, the majority of total phosphorus concentrations ranged from approximately 5-25 g/l. After 1998, phosphorus concentrations ranged from 20 g/l to approximately 40 g/l during most events.

16 Trends in Secchi Depth in Lake Yale from Measured Secchi disk depths in Lake Yale have also been highly variable over time. Prior to 1996, measured Secchi disk depths in Lake Yale generally exceeded 1 m during virtually all of the monitoring events. However, after 1998, the majority of measured Secchi disk depths were less than 1 m, although isolated values in excess of 1 m were observed on occasion. The trend line for the average annual Secchi disk depths indicates a negative slope, suggesting that Secchi disk depths are decreasing in Lake Yale over time.

17 Trends in Chlorophyll a in Lake Yale from The observed trends of increases in chlorophyll-a and decreases in Secchi disk depth, beginning in approximately 1994, appear to closely follow the pattern of increasing nutrient concentrations. As chlorophyll-a concentrations increased during the mid- to late-1990s, Lake Yale was converted from a clear, non-turbid oligotrophic lake system to a plankton-dominated, turbid, eutrophic waterbody. The low visibility caused by the elevated algal growth reduces light penetration and limits the ability for submerged vegetation to grow within the lake which provides a continuing feedback mechanism that maintains the altered high nutrient status of Lake Yale.

18 TN/TP Ratios in Lake Yale from The TN/TP ratio data suggest that Lake Yale has existed in a phosphorus-limited condition throughout virtually all of the historical monitoring period. The calculated trend line for changes in TN/TP ratios over time has a positive slope which suggests an increase in TN/TP ratios over time. However the trend is not statistically significant.

19 Trends in Trophic State Index in Lake Yale from

20 What Caused the Changes? The plots of historical concentrations of total nitrogen and total phosphorus in Lake Yale indicate a sudden rapid increase in water column concentrations of both total phosphorus and total nitrogen during the mid-1990s. This relatively rapid spike in nutrient concentrations appears to coincide with the introduction of a large number of grass carp into Lake Yale, beginning in 1987 and continuing to 1994, along with repetitive herbicide applications for control of Hydrilla. The nutrients released from the decomposition of vegetation, combined with excretions of plant material by the grass carp, resulted in an increase in water column nutrient concentrations, beginning in approximately Peak concentrations for total nitrogen and total phosphorus were reached during 1998, with relatively consistent water column concentrations of both total phosphorus and total nitrogen since that time.

21 Impaired Waters Designation Lake Yale was included on the verified list of impaired waters for the Ocklawaha Basin that was adopted by secretarial order on August 28, 2002, with the cause of impairment listed as nutrients. FDEP indicates that Lake Yale is a phosphorus-limited system based on a median TN/TP ratio of 64 during the verified period. Lake Yale was de-listed from FDEP s verified list when a nutrient TMDL was adopted by rule on August 14, The water quality targets for Lake Yale were established by the St. Johns River Water Management District (SJRWMD) based upon estimates of average annual nitrogen and phosphorus loadings to Lake Yale over the period from Phosphorus inputs were quantified from watershed sources, atmospheric contributions, failed septic tanks, and from domestic and wastewater facilities. The mean annual total phosphorus loading over the period from was estimated at 1,432 kg, with approximately 41% contributed by watershed loadings. The estimated annual nitrogen loadings to Lake Yale from are 23,079 kg/yr, of which approximately 34% is contributed by watershed loadings. SJRWMD identified a target in-lake phosphorus concentration of 20 ug/l which would result in an in-lake TSI value less than 50. It was concluded that phosphorus is the limiting nutrient in Lake Yale, and a 10% reduction in phosphorus loadings is necessary to reduce the in-lake TSI to less than 50.

22 Surface Water Monitoring Sites in Lake Yale Used by ERD Five separate monitoring sites were established in Lake Yale to evaluate horizontal variability within the lake and potential water quality impacts from the large agricultural area on the south shoreline of the lake and the large wetland areas located on the east side of the lake. Water quality monitoring in Lake Yale was conducted on a monthly basis, with a total of 12 monitoring events conducted at each of the original four sites and 11 events conducted at Site 5 during the 12-month monitoring program.

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24 Vertical Profiles in Lake Yale Measured ph values exhibited a rapid decrease at a depth of 3 m during each of the field monitoring events, with measured bottom ph values ranging from Measured conductivity values ranged from approximately mho/cm. Below a depth of 3 m, conductivity generally increased, with bottom measurements ranging from mho/cm. The observed increases in conductivity near the water-sediment interface are an indication of internal recycling occurring at this site. Dissolved oxygen concentrations exhibited a gradual decrease with increasing water depth to a depth of 3 m. Above a depth of 3 m, oxygen concentrations in excess of 5 mg/l were observed during all monitoring events. However, below this depth, dissolved oxygen concentrations decreased rapidly to values ranging from 1-2 mg/l. A similar trend is also apparent for dissolved oxygen saturation, with saturation values well in excess of the 38% dissolved oxygen criterion at depths of 3 m or less during all 12 monitoring events. Below a depth of 3 m, oxygen saturation decreases rapidly to values generally less than 10-20%.

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26 Temporal Variability in Measured Values for Alkalinity, Color, Turbidity, and TSS in Lake Yale Samples Collected from March 2015-March No significant seasonal trend is apparent in measured alkalinity concentrations. No seasonal trend is apparent in color concentrations observed in the main portion of Lake Yale, with a peak in color observed during fall conditions at Site 5. A slight trend of increasing turbidity values was apparent during summer conditions, with lower values observed during remaining portions of the year. A slight trend of more elevated TSS concentrations was observed during summer conditions.

27 Temporal Variability in Measured Values for Nitrogen Species in Lake Yale Samples Collected from March 2015-March Measured concentrations of total nitrogen in Lake Yale were moderate to elevated in value A relatively close agreement in measured total nitrogen concentrations was observed between the individual sites and between top and bottom samples during the majority of the monitoring events. Somewhat more elevated values for total nitrogen were observed at Site 5 during approximately half of the monitoring events.

28 Temporal Variability in Measured Values for Phosphorus Species in Lake Yale Samples Collected from March 2015-March A relatively close agreement was observed between phosphorus concentrations at the individual monitoring sites on any given monitoring date, with the exception of more elevated total phosphorus concentrations observed in samples collected near the bottom at Site 4. A slight seasonal pattern is apparent for total phosphorus concentrations in Lake Yale, with slightly higher concentrations during the fall and winter months which is often a sign of internal recycling. Phosphorus concentrations at Site 5 appear to be greater than values measured in open portions of Lake Yale during a majority of the monitoring events.

29 Nutrients vs. Lake Levels These relationships indicate that concentrations of the measured parameters decrease as water level elevations within Lake Yale increase. Increases in water level elevations are generally associated with significant regional rainfall and increased runoff inflows, and the decrease in concentrations observed during periods of higher water level elevations suggests that stormwater runoff is not the most significant variable impacting water quality characteristics in Lake Yale. The observed increases in concentrations with decreasing water depth for the three trophic state variables suggest an internal loading source may be present which becomes a more significant component in determining water column concentrations under low water levels than under high water level conditions where significant dilution may occur from other less concentrated sources. Overall, the relationships between rainfall and water level suggest that internal sources may be more significant in determining water quality characteristics within the lake than inputs of stormwater runoff.

30 Lake Yale Nutrient Budget Components Internal Recycling

31 Bulk Precipitation Characteristics of Lake Yale Estimated annual average volumetric input from direct precipitation of 16,607ac-ft/yr

32 Lake Yale Nutrient Budget Components Internal Recycling

33 Stormwater Runoff During 2008, a study was completed by Inwood Consulting Engineers for Lake County Public Works, titled Lake Yale Basin Study which was designed to identify and reduce stormwater-based phosphorus loadings to Lake Yale. Inwood conducted an inventory of existing drainage systems and structures associated with Lake Yale, developed a pollutant loading model to identify priority areas within the Lake Yale Basin, and developed conceptual designs for selected improvement projects. Priority projects were identified as a result of the Lake Yale Basin study. Monitoring sites in the Lake Yale Drainage Basin

34 Stormwater Runoff On an average annual basis, stormwater runoff contributes approximately 3,012 kg/yr of total nitrogen and approximately 509 kg/yr of total phosphorus to Lake Yale.

35 Lake Yale Nutrient Budget Components Internal Recycling

36 Groundwater Seepage Nutrient influx from groundwater seepage was quantified using a series of underwater seepage meters.

37 Isopleths of Total Nitrogen Influx (ug/m 2 -day) and Total Phosphorus Influx (ug/m 2 -day) from Groundwater Seepage Entering Lake Yale.

38 Groundwater seepage contributes an influx of approximately 732 ug/m 2 -day of total nitrogen to Lake Yale, with an influx of 25.1 ug/m 2 -day for total phosphorus. On an overall average annual basis, groundwater seepage contributes approximately 4,341 kg/yr of total nitrogen and 149 kg/yr of total phosphorus to Lake Yale. Calculated areal loadings of groundwater seepage reflects the mass influx divided by the lake surface area. These values provide a way of comparing seepage loadings between lakes without consideration of lake size. The mean areal seepage loadings of 1.08 kg/ac-yr for total nitrogen and kg/ac-yr for total phosphorus are somewhat lower than areal mass loading rates for groundwater seepage commonly observed by ERD in Central Florida lakes. The relatively low areal loading rates are primarily due to the low nutrient concentrations observed in seepage samples collected from Lake Yale.

39 Lake Yale Nutrient Budget Components Internal Recycling

40 Internal Recycling Sediment Release Experiments Large diameter sediment cores are collected from various locations within the lake and incubated in the laboratory under a variety of conditions to simulate variability in the lake throughout the year. Changes in phosphorus concentrations are measured in the overlying sediments, and this information is extrapolated to an areal release rate within the lake. This is the only method of estimating internal loadings which provides a direct measurement of phosphorus release. This method has been used by ERD on multiple occasions in previous work efforts and was selected as the quantification method for Lake Yale.

41 Locations for Collection of Large Diameter Core Samples in Lake Yale. (water depth provided in parentheses)

42 On an average annual basis, sediment nutrient release contributes approximately 4,889 kg/yr of SRP, 7,981 kg/yr of total phosphorus, and 109,925 kg/yr of total nitrogen to Lake Yale. This information is used to generate an overall nutrient mass balance for Lake Yale.

43 Lake Yale Nutrient Budget Components Internal Recycling

44 Nutrient Losses Nutrient losses from Lake Yale occur primarily as a result of discharges through the lake outfall structures and losses due to aquifer recharge. Calculated Mean Annual Mass Losses From Lake Yale Through The Outfall Structures Calculated Mean Annual Mass Losses From Lake Yale as a Result of Aquifer Recharge Pollutant mass which is not discharged through the outfall structures or lost to deep recharge is assumed to accumulate into the sediments of each lake.

45 Mean Annual Mass Budgets Estimated mean annual mass budgets were developed for total nitrogen, total phosphorus and TSS discharging to Lake Yale The dominant source of nitrogen loading to Lake Yale appears to be bulk precipitation which contributes approximately 64% of the calculated mean annual nitrogen loadings. Approximately 18% of the annual nitrogen loading is contributed by groundwater seepage, with 12% by stormwater runoff and 6% by internal recycling. Contributions of total nitrogen from overland flow contribute less than 1% on an average annual basis. Internal recycling is clearly the largest phosphorus input to Lake Yale, contributing 80% of the estimated annual phosphorus loading. Approximately 13% of the annual loading is contributed by bulk precipitation, with 5% from runoff and 2% from groundwater seepage. Overland flow contributes less than 1% of the annual phosphorous load to Lake Yale.

46 Annual Mass Loadings of Total Nitrogen, Total Phosphorus, and TSS Discharging to Lake Yale.

47 Mass Losses On an average annual basis, approximately 51% of the total nitrogen, 98% of the total phosphorus, and 91% of the TSS loading is retained within the sediments of the lake. The remaining mass loadings for total nitrogen, total phosphorus, and TSS are lost due to the combined effects of outfall discharges and lost to deep recharge

48 Comparison of Estimated Annual Mass Losses of Total Nitrogen, Total Phosphorus, and TSS from Lake Yale.

49 Based upon the field monitoring conducted by ERD, it is apparent that the existing sediment accumulations in Lake Yale contribute a significant phosphorus loading to the lake each year. Water quality within the lake could be improved substantially by reducing the observed phosphorus loadings from internal recycling and shallow groundwater seepage.

50 Probing Locations for Muck Depths in Lake Yale (March 16 and 26, 2015) Accumulated pockets of organic muck are present throughout the entire lake, with muck depths ranging from 1-14 ft. Muck depths of approximately 0-1 ft were observed along most of the shoreline areas, and approximately half of the lake appears to have muck depths of 8 ft or more. Overall, Lake Yale contains approximately 20,725 ac-ft (33,436,333 yd 3 ) of unconsolidated organic sediments. The volume of unconsolidated sediment in Lake Yale is sufficient to cover the entire lake bottom to a depth of approximately 5.2 ft, a value substantially greater than commonly observed by ERD in Central Florida lakes, and one of the deepest mean muck accumulations ever measured by ERD.

51 Sediment Characteristics of Lake Yale Sediment core samples were collected in Lake Yale by ERD to evaluate the characteristics of existing sediments and potential impacts on water quality within the lake. Sediment core samples were collected at 32 separate locations within Lake Yale on March 18, 2015 by ERD personnel. Each of the collected sediment core samples was analyzed for a variety of general parameters, including moisture content, organic content, sediment density, total nitrogen, and total phosphorus. Measured concentrations of total nitrogen in Lake Yale sediments exhibited a relatively high degree of variability in measured values throughout the lake. Sediment total nitrogen concentrations in Lake Yale ranged from 822-4,194 ug/cm 3, with a mean of 2,640 ug/cm 3, similar to sediment nitrogen concentrations commonly observed by ERD in eutrophic Central Florida lakes. The most elevated levels of sediment nitrogen were observed in central portions of the lake in areas which correspond to the largest accumulations of organic muck. Sediment nitrogen concentrations in Lake Yale decrease in shoreline areas, with the majority of shoreline areas exhibiting concentrations ranging from 1,000-2,000 ug/cm 3, particularly on the west, north, and northeast shorelines of the lake.

52 Location of Sediment Core Samples in Lake Yale

53 Sediment Characteristics of Lake Yale Measured concentrations of total phosphorus in Lake Yale sediments exhibited only a moderate degree of variability throughout the lake, with measured concentrations ranging from ug/cm 3 (wet weight), which reflects a moderately elevated value for phosphorus compared with other eutrophic Central Florida lakes. The most elevated levels of sediment phosphorus appear to occur primarily in central portions of the lake in areas with elevated muck accumulations. Sediment phosphorus concentrations appear to decrease in shoreline areas with low muck accumulations, particularly along the west, northern, and northeastern shorelines of the lake.

54 Vegetation Management History Prior to the early 1990s, Lake Yale was characterized by moderate levels of total nitrogen and relatively low levels of both total phosphorus and chlorophyll-a. Mean annual Secchi disk depths over this period ranged from approximately m, and the lake generally exhibited oligotrophic, low color, conditions. Both anecdotal accounts and documented reports indicate that Lake Yale contained an impressive assemblage of emergent and submerged vegetation, and the lake was renowned as a fishing destination. The bottom of the lake consisted primarily of fine sand with minimal accumulations of organic muck. The watershed at that time consisted primarily of natural wetlands and agricultural activities, mostly tree crops, with little significant development. During the mid-1970s, hydrilla began to appear in Lake Yale, and by the mid- 1980s hydrilla had expanded to virtually all areas of the lake, topping out at the water surface over more than 50% of the lake area.

55 Vegetation Management History During 1984, the Florida Game and Freshwater Fish Commission (FGFFC) conducted a chemical application for treatment of hydrilla which virtually eliminated hydrilla from the lake, but by 1990, the hydrilla infestation had returned to approximately 50% of the lake area. During 1987, FGFFC introduced more than 20,000 grass carp into Lake Yale, equivalent to a stocking rate of approximately 5 fish/acre, more than twice the current recommended rate. In addition, herbicide applications of Sonar were conducted on multiple occasions during the early-1990s. The combination of grass carp and repeated herbicide applications eliminated virtually all of the bottom rooted vegetation within the lake and left an accumulation of organic muck from the dead plant material as well as excretions from grass carp consuming the bottom vegetation.

56 Vegetation Management History During the mid- to late-1990s, Lake Yale exhibited a significant trophic state shift, with substantial and sudden increases in water column concentrations of total nitrogen, total phosphorus, and chlorophyll-a, and a subsequent decrease in Secchi disk depth, as Lake Yale transitioned from a clear plant-dominated system to a turbid plankton-dominated system. The resulting increase in algal productivity reduced water column clarity and opportunities for regrowth of submerged aquatic vegetation, and deposits of organic muck began to rapidly accumulate on the lake bottom. No evidence of rooted submerged vegetation was observed in central portions of Lake Yale by ERD during the entire 16-month field monitoring program. In recent years, Lake Yale has experienced a gradual increase in floating tussocks in the southeast portion of the lake which flourish in the elevated nutrient conditions which occur in Lake Yale under present conditions. A combination of mechanical and chemical control methods have been used to remove these floating islands.

57 Summary of Conditions in Lake Yale The plots of historical concentrations of total nitrogen and total phosphorus in Lake Yale indicate a sudden rapid increase in water column concentrations of both total phosphorus and total nitrogen during the mid- 1990s. This relatively rapid spike in nutrient concentrations appears to coincide with the introduction of a large number of grass carp into Lake Yale, beginning in 1987 and continuing to 1994, along with repetitive herbicide applications for control of Hydrilla. The nutrients released from the decomposition of vegetation, combined with excretions of plant material by the grass carp, resulted in an increase in water column nutrient concentrations, beginning in approximately Peak concentrations for total nitrogen and total phosphorus were reached during 1998, with relatively consistent water column concentrations of both total phosphorus and total nitrogen since that time. In general, the water column in Lake Yale was well mixed and well buffered.

58 Summary of Conditions in Lake Yale Overall, total nitrogen concentrations in Lake Yale appear to be relatively evenly distributed and are moderate to slightly elevated in value compared with concentrations commonly observed in Central Florida lakes. Particulate nitrogen concentrations in Lake Yale comprised approximately one-third of the overall nitrogen measured within the lake. The dominant phosphorus species in Lake Yale was particulate phosphorus which comprised approximately 50% of the total phosphorus measured during any given monitoring event. Dissolved organic phosphorus concentrations were generally low in value. Measured SRP concentrations in Lake Yale were moderate in value and relatively similar between surface and bottom measurements. Based upon the field monitoring conducted by ERD, it is apparent that the existing sediment accumulations in Lake Yale contribute a significant phosphorus loading to the lake each year. Water quality within the lake could be improved substantially by reducing the observed phosphorus loadings from internal recycling and shallow groundwater seepage.

59 Summary of Conditions in Lake Yale Internal recycling is clearly the largest phosphorus input to Lake Yale, contributing 80% of the estimated annual phosphorus loading. Approximately 13% of the annual loading is contributed by bulk precipitation, with 5% from runoff and 2% from groundwater seepage. Overland flow contributes less than 1% of the annual phosphorous load to Lake Yale. Overall, the observed total phosphorus concentrations in Lake Yale are indicative of mesotrophic to eutrophic lake conditions.

60 A TMDL for total phosphorus in Lake Yale was developed by FDEP and approved on September 19, The TMDL only considered inputs from stormwater runoff and bulk precipitation and estimated that these two sources contribute 1,432 kg/yr of total phosphorus to Lake Yale. The nutrient budget conducted by ERD includes phosphorus loadings from bulk precipitation, runoff, overland flow, groundwater seepage, and internal recycling. The sum of bulk precipitation and runoff in the ERD annual mass budget is relatively similar to the estimated loading for runoff and bulk precipitation estimated by FDEP. However, the TMDL document didn t recognize and quantify loadings to Lake Yale from groundwater seepage and internal recycling, which together contribute an additional 8,130 kg/yr to Lake Yale.

61 Evaluation of Water Quality Improvement Options

62 Stormwater Retrofit Opportunities at Lake Yale

63 Stormwater Retrofit Opportunities at Lake Yale ERD currently recommends construction of a treatment pond for Subbasins 12 and 13, provided that the project could be constructed at an acceptable cost. The sources of elevated total phosphorus concentrations originating from Sub-basin 14 should also be investigated.

64 Whole Lake Dredging at Lake Yale A whole-lake dredging project would remove the existing organic muck, leaving the original parent bottom material of the lake. This option will eliminate much of the water quality impacts from the existing sediments, with added benefits of increasing water depth, water volume, and lake residence time. In general, increasing residence time within a waterbody increases the opportunity for biological uptake, resulting in better equilibrium water quality characteristics and lower nutrient loadings to downstream waterbodies. A decision to remove accumulated lake bottom sediments generally occurs when there is sufficient evidence that the accumulated sediments are having an adverse impact on habitat, water quality, recreation, or navigation. The existing sediments in Lake Yale do not appear to have a direct impact on recreation or navigation within the lakes, with the possible exception of the southeast portions of Lake Yale, although sediment disturbance by boating could become a problem at low water levels. However, field and laboratory work conducted by ERD has demonstrated an adverse impact of sediments on existing water quality in Lake Yale.

65 Summary of Dredging Design Assumptions and Containment Area Requirements for Lake Yale

66 Estimated Cost for Hydraulic Dredging at Lake Yale The estimated dredging cost for removal of 33,436,333 yd 3 of material from Lake Yale is approximately $334,363,330 which does not include costs associated with land purchase which may be required for the containment area. Assuming that land purchase is required at a unit cost of $20,000/ac, the land cost for the 9,326-ac parcel would be approximately $185,520,000. An additional $2,000,000 is included for engineering, design and testing during the dredging feasibility analysis phase. The estimated total project cost with land purchase is approximately $522,883,330, or $336,363,330 without land purchase. However, the spoils containment area can often be repurposed or sold which may recoup some of the initial land cost.

67 Sediment Inactivation Treatment for Lake Yale Based on the nutrient budget for Lake Yale internal recycling contributes approximately 80% of the phosphorus loading to the lake, with an additional 2% contributed by groundwater seepage. It is possible to control both internal recycling and groundwater seepage through a carefully planned alum treatment application. The goal of a sediment inactivation treatment for Lake Yale is to add sufficient aluminum to provide simultaneous control for both internal recycling and groundwater seepage. Together, these sources contribute approximately 82% of the current total phosphorus loading to the lake. Control of these inputs has the potential to result in substantial improvements in water quality within Lake Yale.

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70 Recommended Management Options for Lake Yale

71 Although not recommended at this time for existing shoreline properties, construction of rear yard berms and swales should be required for all future shoreline development which occurs on Lake Yale. Efforts should also be undertaken to continue management of tussocks on an as-needed basis, preferably using mechanical means to prevent additional accumulation of organic matter on the lake bottom. Maintaining long-term stable and improved water quality in Lake Yale is highly dependent on re-establishment of the submerged vegetation communities, which will likely re-colonize naturally as proposed water quality improvement projects reduce phosphorus concentrations and algal productivity within the lake. One of the most significant components to reduce total phosphorus concentrations in Lake Yale is the proposed sediment inactivation project which will provide significant load reductions for inputs resulting from internal recycling and groundwater seepage, and ERD recommends that this activity be given high priority.

72 What is next? Lake County Water Authority has included $2,202,328 in its FY budget for a Lake Yale Sediment Inactivation project. Will need to look for funding partners for the balance or continue accumulating funds over future fiscal years. Will need to coordinate with Florida Fish and Wildlife Commission and the Florida Department of Environmental Regulation regarding whole lake alum applications. Release RFP s for a Lake Yale Sediment Inactivation project

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