Seasonal Salinity Cycles in Eight Lakes of the Minneapolis/St. Paul Metropolitan Area

ST. ANTHONY FALLS LABORATORY Engineering, Environmental and Geophysical Fluid Dynamics

Project Report No. 485

Dan Murphy and Heinz Stefan

Prepared with support from the

Minnesota Local Roads Research Board though the Center for Transportation Studies University of Minnesota, Twin Cities

October, 2006 Minneapolis, Minnesota

The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, religion, color, sex, national origin, handicap, age or veteran status.

Abstract

Substantial amounts of road salt are spread annually on highways, streets, sidewalks, and parking lots in the northern regions of the U.S. Road salt application is considered an economic necessity to keep roads free of ice for safe winter travel in northern climate zones. Commonly used road salts to deice roads are sodium chloride (NaCl) and calcium chloride (CaCl2). Because of a large difference in cost, NaCl is applied in much larger quantities. Snowmelt runoff containing dissolved road salt feeds many Twin Cities Metro Area lakes, but chloride levels in these lakes have not been studied explicitly. Little is known about the fate of NaCl entering these lakes.

Eight urban lakes in the Minnepolis/St.Paul metropolitan area were therefore studied for 14 months during two winters and the summer in between. Specific conductance profiles were measured at roughly bimonthly intervals in each of the eight lakes. Variations in specific conductance with season and with depth were found in each of the lakes. Specific conductance values varied from 400 to 1800 µS/cm in seven of the lakes and reached a maximum of 3500 µS/cm in the eighth lake which was meromictic. The largest specific conductance values were in late winter and at the bottom of the lakes, and the lowest in late summer near the surface of the lakes.

Chloride concentration was linearly related to specific conductance. Chloride concentration profiles were calculated from specific conductance, and integrated with depth to give the total chloride and the total NaCl content. There was clearly a seasonal pattern in all eight lakes, i.e. an accumulation of NaCl during the winter months and a decrease in total salt content during the summer. Chloride concentrations near the lake bottom in Brownie Lake and Ryan Lake exceeded the chronic MPCA chloride standard of 230 mg/L during the entire 14 months of the field study. Chloride concentrations exceeded the chronic standard near the lake bottom in McCarrons Lake during late spring in 2004 and Medicine Lake during late spring in 2005.

Density profiles were calculated from the temperature and conductance measurements. Density increases due to salinity were typically less than 0.0001kg/m3 and made only a weak contribution to the density stratification. The largest density increase due to salinity was calculated for the bottom of Brownie Lake (∆ρ=0.00135kg/m3). The effect of vertical density gradients on the vertical dispersion coefficient was calculated with and without the effect of salinity. In all lakes the vertical diffusivity changed by less than a factor of 1.8. The vertical mixing coefficients are more strongly affected by temperature than by salinity.

The overall conclusion is that lakes are in the hydrologic pathway of roadway deicing salt. They act as a sink and provide temporary storage of dissolved salt in the winter months when road salt is applied, and they act as a source of salinity in the summer months. One of the eight lakes studied is a meromictic lake, i.e. it has a permanent salt water layer. It is still unknown under what conditions other lakes would act likewise.

1 Table of Contents

LIST OF TABLES...... 3 LIST OF FIGURES...... 3 INTRODUCTION...... 5 FIELD STUDY SITES AND PROCEDURES ...... 6 LAKE CHARACTERISTICS ...... 6 FIELD DATA COLLECTION ...... 7 Measured parameters ...... 7 Specific conductance calculation...... 7 Instrument calibration ...... 7 DATA COLLECTED...... 7 DATA ANALYSIS ...... 9 COMPUTATION OF CHLORIDE CONCENTRATIONS ...... 9 TOTAL CHLORIDE AND SODIUM CHLORIDE MASS IN EACH LAKE ...... 10 AVERAGE CHLORIDE CONCENTRATION IN EACH LAKE...... 11 DENSITY STRATIFICATION ...... 11 VERTICAL DIFFUSIVITY ...... 13 DISCUSSION...... 14 CONCLUSIONS...... 14 REFERENCES ...... 16

2 List of Tables Table 1. Lake Characteristics...... 18 Table 2. Specific conductivity (µS/cm @ 25 ºC ) calibration data...... 18 Table 3. Calibration equations ...... 19 Table 4. Dates of field measurements...... 19 Table 5. Ionic composition of five water samples ...... 20 Table 6. Total estimated chloride (Cl-) mass (metric tons) ...... 21 Table 7. Total estimated sodium chloride (NaCl) mass (metric tons) ...... 22 Table 8. Increase in total lake chloride (Cl-) mass from late summer to late winter (metric tons)...... 23 Table 9. Annual road salt application rates for Mn/DOT and various cities ...... 24 Table 10. Salt loading from lakeshed and increase in lake salt (NaCl) content from late summer to late winter for 2003/04 and 2004/05...... 25

List of Figures Figure 1. Lake Locations ...... 26 Figure 2.a Bass Lake bathymetric map...... 27 Figure 2.b Brownie Lake bathymetric map ...... 28 Figure 2.c Cedar Lake bathymetric map...... 29 Figure 2.d Lake of the Isles bathymetric map...... 30 Figure 2.e Lake Johanna bathymetric map ...... 31 Figure 2.f Medicine Lake (West Bay) bathymetric map ...... 32 Figure 2.g Lake McCarrons bathymetric map ...... 33 Figure 2.h Ryan Lake bathymetric map...... 34 Figure 3.a Bass Lake area vs. depth curve...... 35 Figure 3.b Brownie Lake area vs. depth curve ...... 35 Figure 3.c Cedar Lake area vs. depth curve...... 36 Figure 3.d Johanna Lake area vs. depth curve...... 36 Figure 3.e Lake of the Isles area vs. depth curve...... 37 Figure 3.f Lake McCarrons area vs. depth curve...... 37 Figure 3.g Medicine Lake (SW Bay) area vs. depth curve...... 38 Figure 3.h Ryan Lake area vs. depth curve...... 38 Figure 4. Average specific conductance profiles in 8 lakes...... 39 Figure 5.a Bass Lake - Specific conductance profiles ...... 40 Figure 5.b Brownie Lake - Specific conductance profiles...... 40 Figure 5.c Cedar Lake - Specific conductance profiles ...... 41 Figure 5.d Lake of the Isles - Specific conductance profiles...... 41 Figure 5.e Lake Johanna - Specific conductance profiles...... 42 Figure 5.f. Lake McCarrons - Specific conductance profiles ...... 42 Figure 5.g Medicine Lake (SW Bay) - Specific conductance profiles ...... 43 Figure 5.h. Ryan Lake - Specific conductance profiles...... 43

3 Figure 6.a Bass Lake – Temperature profiles ...... 44 Figure 6.b Brownie Lake – Temperature profiles...... 44 Figure 6.c Cedar Lake – Temperature profiles ...... 45 Figure 6.d Lake of the Isles – Temperature profiles...... 45 Figure 6.e Lake Johanna – Temperature profiles...... 46 Figure 6.f Lake McCarrons– Temperature profiles ...... 46 Figure 6.g Medicine Lake (SW Bay) – Temperature profiles ...... 47 Figure 6.h Ryan Lake – Temperature profiles...... 47 Figure 7. Laboratory analyzed chloride concentration [Cl-] vs. in situ field measurements of specific conductance for selected lake samples...... 48 Figure 8. Estimated total chloride (Cl-) mass in each lake vs. time...... 49 Figure 9. Increase in lake chloride (Cl-) mass from late summer to late ...... 49 Figure 10. Average chloride (Cl-) concentration in each lake vs. time...... 50 Figure 11. Chloride (Cl-) concentration at 1m depth in each lake vs. time ...... 50 Figure 12. Chloride (Cl-) concentration at the deepest point of each lake vs. time...... 51 Figure 13.a Bass Lake - Density profiles...... 52 Figure 13.b Brownie Lake - Density profiles ...... 52 Figure 13.c. Cedar Lake - Density profiles...... 53 Figure 13.d Lake of the Isles - Density profiles...... 53 Figure 13.e Lake Johanna - Density profile...... 54 Figure 13.f Lake McCarrons - Density profiles...... 54 Figure 13.g Medicine Lake (SW Bay) – Density profiles ...... 55 Figure 13.h Ryan Lake - Density profile ...... 55 Figure 14.a Bass Lake – Diffusivity ratio...... 56 Figure 14.b Brownie Lake – Diffusivity ratio ...... 56 Figure 14.c Cedar Lake – Diffusivity ratio...... 57 Figure 14.d Lake of the Isles – Diffusivity ratio...... 57 Figure 14.e Lake Johanna – Diffusivity ratio ...... 58 Figure 14.f Lake McCarrons – Diffusivity ratio...... 58 Figure 14.g Medicine Lake (SW Bay) – Diffusivity ratio...... 59 Figure 14.h Ryan Lake – Diffusivity ratio...... 59 Figure 15. Comparison of increase in total lake salt (NaCl) mass from late summer to late winter (metric tons) and total lakeshed salt loading for 2003/04 and 2004/05...... 60

4 INTRODUCTION

Road salt application is considered an economic necessity to keep roads free of ice for safe winter travel in northern climate zones. Municipalities, counties, state transportation departments and private businesses spread significant amounts of road salt on streets, sidewalks, and parking lots in the northern regions of the U.S. Typical application values average 14 metric tons per road mile per winter in Minnesota on state maintained roads (Arnebeck, 2006). In the Minneapolis/St. Paul Metropolitan Area the Minnesota Department of Transportation (Mn/DOT) alone applies an average of 93,440 metric tons of salt to state maintained roads each winter (Mn/DOT, 2004). The most commonly used road salts used to deice roads are sodium chloride (NaCl) and calcium chloride (CaCl2). Because of a large difference in cost NaCl is applied much more frequently.

However, this practice of applying road salt to winter roads can come at a cost to the environment, especially in urban areas with a high road density. Environmental impacts of chloride on vegetation alongside roads have been documented for distances as far as 200m from the road (Environment Canada, 1999). Organisms in streams and shallow, small lakes are particularly vulnerable to road salt application and chloride pollution (Environment Canada, 1999). That is why chloride standards of 860 mg/L for acute events and 230 mg/L for chronic pollution have been established by the Minnesota Pollution Control Agency (MPCA) for surface waters in Minnesota designated as important for aquatic life and recreation (Minnesota R. Ch. 7050 and 7052). High sodium chloride content will impair the taste and uses of groundwater. The groundwater standard for chloride has therefore been set at 250 mg/L by the Environmental Protection Agency (USEPA, 1992, 1973).

Increases in chloride concentration have been found to occur in streams and lakes of cold regions using road salt for deicing. Runoff of road salt from an interstate highway in New Hampshire tripled chloride concentration in a nearby lake over a period of 23 years (Rosenberry, 1999). Chloride concentrations in Irondequoit Bay, a small bay on Lake Ontario, increased by at least a factor of five between 1950 and 1970 (Bubeck et. al.,1971). An increase in chloride concentrations in the Mohawk River, New York, by a factor of 2.4 from 1952 to 1998 was primarily attributed to road salt use in the watershed (Godwin, Hafner, and Buff, 2003). The increasing salinity of alluvial aquifers in the northern portion of the U.S. has been well documented (Mullaney, 2005).

Excessive chloride levels have already been documented in two Twin Cities Metro Area streams. Shingle Creek was added to the ‘List of Impaired Waters’ by the MPCA for excessive levels of chloride in 1998. Spring runoff containing road salt from winter ice control was determined to be the primary source (Bischoff, 2004). Nine Mile Creek was classified as an Impaired Water for high chloride levels in 2004.

Many urban Twin Cities Metro Area lakes receive snowmelt inflows containing road salt each spring from nearby roads. In very small Brownie Lake (.043 km2 surface area)

5 located west of downtown Minneapolis a chemocline formed near the lake bottom several decades ago (Swain,1984). Surface water chloride concentrations in 13 Metro Area lakes receiving storm sewer runoff were found to peak every spring (Sadecki, 1991). Lakes Boksjon and Nedre Glottern in Sweden, located in a region of heavy road salt use, also exhibited peaks in measured chloride concentration every March and April for three years (Thunqvist, 2003).

Although surface runoff from snowmelt feeds many Twin Cities Metro Area lakes chloride levels in these lakes have not been studied explicitly. Little is known about the fate of NaCl entering these lakes. Few studies have attempted to describe seasonal salinity fluctuations by examining NaCl concentrations over the complete depth of the lake. Is some or all of the NaCl redistributed throughout the water column by vertical mixing processes? Does NaCl accumulate in a lake or does it exit through surface water outflows? Does NaCl in lakes seep into groundwater increasing its salinity?

To begin to address these questions, a field study was devised. The objective of this study was 1) to measure seasonal salinity concentration and temperature profiles with depth in eight Twin cities Metro Area lakes receiving snowmelt water input through storm sewers, 2) to analyze the data in order to a) identify seasonal patterns in salinity stratification, and their causes, b) to estimate the seasonal variation of total chloride content of the eight lakes, c) to estimate seasonal loss rates of chloride to surface water or ground water, d) to determine the salinity effects on stratification stability in the eight lakes.

FIELD STUDY SITES AND PROCEDURES

Lake characteristics

All of the lakes selected for field study are located in the Twin Cities Metropolitan Area (Figure 1). Their hydrologic and bathymetric characteristics are summarized in Table 1. Bathymetric maps (Figures 2a to 2h) were available from the Minnesota Department of Natural Resources (MNDNR). Area vs. depth curves were obtained from these maps (Figures 3a to 3h) and total lake volumes were obtained by integrating the area-depth curves for each lake. Brownie Lake, Cedar Lake, and Lake of the Isles are three of five interconnected lakes referred to as the Minneapolis Chain of Lakes. A number of factors were considered in the lake selection. They included proximity to roads subject to winter ice control, existence of previous studies, ease of access, knowledge of storm sewer outlet locations, and availability of lake level data, bathymetric data and lake water chemical analyses.

6 Field data collection Measured parameters

Measurements of field conductivity, in units of microSiemens per centimeter (µS/cm), and temperature, in units of degrees Celsius (ºC) were recorded using a YSI Model 63 probe. Measurements were taken as a function of vertical distance in the water column every 0.25 – 0.5 meters at approximately the deepest location in each lake. Specific conductance calculation

Specific conductance (SC) was calculated from measured field conductivity and temperature using the following equation:

Conductivity SC (µS/cm @ 25 ºC) = (1) 1+ TC(T − 25)

where TC is the temperature coefficient, T is the measured temperature in degrees Celsius (ºC), and Conductivity is the measured field conductivity in micro Siemens per centimeter (µS/cm). A temperature coefficient (TC) of 0.0191 was used to calculate SC (YSI, Inc.,1998). Instrument calibration

The YSI Model 63 probe was calibrated for SC twice during the measurement period. Measured SC (SCm), in units of micro Siemens per centimeter at 25 degrees Celsius (µS/cm @ 25 ºC) and calculated from probe measurements of conductivity and temperature, were compared with samples of de-ionized water and lab standards of known SC (SCk) (Table 2). Calibration equations were produced using an ordinary linear least squares fit (Table 3). Equation (2) was applied to data from the measurement period t1 to t6 and Equation (3) was applied to data from the measurement period t7 to t9.

SC = 0.99(SCm) + 5.0 (2)

SC = 0.96(SCm) + 17.5 (3)

DATA COLLECTED

Conductivity and temperature profiles were measured nine times (t1 to t9) in each lake over the course of 14 months (Table 4). Vertical profiles of specific conductance (SC) were calculated from raw conductivity and temperature data by the method described in Field Study Sites and Procedures. Every one of the 72 profiles (nine SC-profiles measured in each of the eight lakes) shows a salinity gradient with depth. To summarize this gradient the nine SC-profiles for each lake were averaged and the result plotted in Figure 4. The salinity at 1m depth is about the same in all eight lakes. The salinity

7 gradient with depth is by far the strongest in Brownie Lake and weakest in Bass Lake. A measurement at only one depth in a lake, especially near the surface, is therefore an insufficient indicator of salinity. The second feature is the seasonal change in salinity stratification which is presented in Figures 5a to 5h for each lake in alphabetical order. We shall discuss this feature for each lake in decreasing order of overall lake salinity.

Brownie Lake is a very small, very wind sheltered lake, and by far the most saline of the lakes studied. It has been saline for a significant amount of time, has a very strong chemocline, and is known to be a meromictic lake (Swain, 1984). It shows seasonal salinity variations only in the upper 7m of the water column. Below that depth specific conductance (SC) is above 2500 µS/cm at all times of the year. The seasonal salinity variation is strongest between 2m and 5m depth. Measurements in April (04/09/2004 and 04/10/2005) produced the highest SC-values, and a measurement in October (10/05/2004) the lowest SC-values in the upper 7m of Brownie Lake. Between February and April salinity between 1.5m and 7m depth increased, and from April to October it decreased. The surface mixed layer reached a maximum depth of about 5m in October. An explanation of these observations is as follows: Brownie Lake receives runoff from I394 and other traffic arteries in Minneapolis. Snowmelt runoff carrying road salt is the source of the salinity increase in spring. However, snowmelt runoff does not contain enough salt to make it sink all the way to the lake bottom. Instead the saline road runoff accumulates between 2 and 7m depth. After the snowmelt season freshwater accumulates in the surface layer of the lake which has a discharge to Cedar Lake. The seasonal salinity cycle is essentially a saltwater loading in spring followed by freshwater flushing in the summer. The groundwater connection may be at the level of the surface mixed layer, but not below since that salinity does not vary in time.

Ryan Lake and Lake McCarrons are the next two most saline lakes investigated. Their SC-profiles have a number of communalities. Their maximum lake depths are comparable to Brownie Lake (11 to 15m), but salinity in their bottom layers (2 to 3m thickness) is highly variable with season, as opposed to Brownie Lake. SC-values vary from 1000 to 1800µS/cm at the bottom of Ryan Lake, and from 700 to 1200µS/cm in Lake McCarrons. By comparison SC-values in the ‘active layer’ of Brownie Lake vary from less than 1000 to 2500µS/cm. They highest SC-values at the bottom of Ryan Lake have been measured in February, March and April, the lowest in June, August and October. Similarly the highest SC-values at the bottom of Lake McCarrons have been measured in February and March, and the lowest in April, June, August and October. An explanation of these observations is as follows: Snowmelt runoff containing road salt enters both lakes in early spring, causes underflows to the lake bottom and enriches the bottom layers with dissolved salt. Intermittent density current intrusions at the time of snowmelt runoff have been documented in continuous records of temperature profiles in Ryan Lake (Ellis et al., 1997). During the summer the salinity disappears by an unknown mechanism (seepage into groundwater, adsorption to the sediments, microbial activity).

Medicine Lake (SW Bay) and Lake Johanna have a surface mixed layer down to about 6m depth. SC-values in that layer are quite uniform, and vary between 500 and 700uS/cm. The highest SC-values in the surface mixed layer were measured in February

8 and March 2005, the lowest in June through October. There is a strong salinity gradient between 6 and 12m depth. The gradients vary substantially in time. At the bottom of the lake the highest salinities were measured in March and April 2005, and the lowest in February 2004 and 2005. The bottom layer from 6 to 12m was less saline in spring 2004 than in spring 2005. Compared to Ryan Lake and Lake McCarrons the SC-profiles in Medicine Lake and Lake Johanna are less consistent from one year to another. Also the saline layer seems to be thicker. The Medicine Lake measurements come from a bay, and the interaction with the open lake may be a factor contributing to more variability.

Cedar Lake and Lake of the Isles are both in the Minneapolis chain of lakes. The SC- profiles from the two lakes have some similar features, even though their average depths are substantially different. Lake of the Isles is less deep in the average. The surface mixed layer is less than 5m deep in both lakes; its SC-values vary seasonally from about 500uS/cm in June to October to 700uS/cm in February 2004. Salinity gradients with depth are weaker in Lake of the Isles, probably because the lake is shallower than Cedar Lake. The highest SC-values at the bottom of both lakes were measured in March and April 2005. Both lakes show nice examples of the effect of fall turnover due to surface cooling on the salinity distribution: between August and October a salinity front moves downward in the lake, and surface water salinity increases.

Bass Lake is at the other extreme of the eight lakes. SC-values are mostly constant with depth, except in February and March when slight salinity increases appear in the lowermost 2m of the 7m deep lake indicating salt water intrusions (a gradient at the lake bottom was measured also in August, but cannot be explained). The highest salinities were recorded in February and March 2005. By April SC has already dropped, and a minimum is reached by August. In six of the eight lakes the uppermost 1m water layer shows a drop in specific conductance at one time or another. This is most likely due to freshwater inflow from the watershed or direct precipitation. The two exceptions are Brownie Lake and Ryan Lake, the two most saline lakes.

For reference we also give the measured temperature profiles in Figures 6a to 6h. Temperature stratification in Minnesota lakes, and lakes in general, has been documented for over a hundred years and does not require any discussion here. Temperature and salinity contribute to water density stratification which will be discussed in another section.

DATA ANALYSIS Computation of chloride concentrations

Conductivity is by definition “a measure of the ability of an aqueous solution to carry an electric current. This ability depends on the presence of ions; on their total concentration, mobility, and valence; and on the temperature of measurement” (APHA/AWWA/WEF, 1992). In Minnesota lakes conductivity is often due to NaCl. Sadecki (1991) and Thunqvist (2003) found a good correlation between electrical conductivity and chloride

9 concentration in lakes. To establish a relationship between chloride concentration and specific conductance for the eight lakes investigated a total of five water samples from Brownie Lake and Ryan Lake were analyzed for chloride concentration in the laboratory using a colorimetric analysis by the mercury (II) thiocyanate method (US EPA, 1979). Ionic concentrations of 15 other common constituents, including sodium (Na) and calcium (Ca), were also analyzed (Table 5). An in situ field measurement of specific conductance was taken for each water sample. Samples 1 – 4 were taken from each lake at depths of 2 m and 10 m. A fifth sample was obtained by diluting the 10 m sample from Brownie Lake by fifty percent.

Chloride, sodium, and calcium constituted, on average, 95% of the total ions analyzed for each lake water sample. Of this 95%, chloride constituted 52%, sodium constituted 32%, and calcium constituted 11%. These three dissolved ions represent the two most common road salts, calcium chloride (CaCl2) and sodium chloride (NaCl).

Chloride concentrations were plotted against in situ field measurements of specific conductance (Figure 7). The relationship is very linear. The following equation relating specific conductance (SC in µS/cm) to chloride concentration [Cl- in mg/L] was developed by an ordinary linear regression analysis

[Cl-] = 0.2965(SC) - 55.205 with R2 = 0.991, SE = 88.73 (4)

Equation (4) was applied to measurements of specific conductance in all 8 lakes in order to obtain chloride concentration profiles from measured conductivity profiles.

Total chloride and sodium chloride mass in each lake

The total chloride mass in each lake was estimated from the measured/computed chloride concentration profiles and bathymetric lake characteristics. Lake level fluctuations were assumed to be negligible in this estimation because most lakes have an outflow/overflow structure. Area-depth curves (Figures 3a to 3h) were developed from bathymetric maps (Figures 2a to 2h). Chloride concentration at a given depth was multiplied by the area at that depth and the result was integrated over the total lake depth, to obtain an estimate of the total chloride mass in the lake at the time of the measurement. The integration was a summation of horizontal layers with a thickness equal to the distance between the midpoints of the measurement depths. For the deepest layer at which a chloride concentration was estimated, the vertical distance was bounded above by the midpoint between the two deepest measurement points and below by the bottom of the lake as indicated in the bathymetric map. For the topmost layer, the vertical distance was bounded above by the lake surface and below by the midpoint between the first two conductance measurement points.

The calculated total chloride mass for each lake was tabulated (Table 6), and plotted vs. time in Figures 8. Chloride values were also converted to sodium chloride (NaCl) values (Table 7) using molecular weights, because we assume that the chloride input to a lake is

10 the result of road salt application and snowmelt runoff in the watersheds. There are no other major sources of chloride in the watershed. Salt used in food and water softening drains for the most part through sanitary sewers and wastewater treatment plants to the Mississippi River, not the lakes. The largest salt masses are in the largest, not in the most saline lakes. Cedar Lake and Lake Johanna have peak chloride contents of over 600 metric tons in spring. Brownie Lake and Ryan Lake have less than 100 metric tons. Salt storage in the large lakes is therefore sizeable.

A strong change in total mass from late summer/early fall to late winter/early spring is evident in Tables 6 and 7, and Figure 8. The annual turnover, i.e. the cycle of input and subsequent loss of NaCl is of interest. The chloride mass at the end of winter relative to the mass at the end of summer was calculated as an indicator of seasonal turnover rate. Chloride mass representative of the winters of 2004 and 2005 was obtained by averaging the values for times t1 and t2, and t7 and t8, respectively. Similarly, the chloride mass in each of the 8 lakes in the summer of 2004 was estimated as the average of the values at times t5 and t6 (for Bass Lake the chloride mass only at time t5 was available). The increase in winter is plotted in Figure 9 as a percentage and listed in Table 8 as an absolute value in tons/ year.

Average chloride concentration in each lake

In addition to the total salt content of a lake, its average lake chloride concentration (total chloride mass divided by the total lake volume) is also of interest as an indicator of lake water quality, and as a measure of overall salinity. Average lake chloride concentrations, were plotted vs. time in Figure 10. The difference between lakes and the change with season can be clearly seen. Note that Brownie Lake is above the standard of 230 mg/L for chronic pollution, and Ryan Lake is not far from it in spring. For comparison, and as a measure of salinity stratification, the chloride concentrations at 1m depth below the surface, and at the deepest measurement point of the lake were plotted in Figures 11 and 12. Note that the surface waters in all eight lakes are always below the salinity standard, but chloride levels exceeding the chronic pollution standard were found in the bottom waters of two of the eight lakes (Brownie and Ryan Lake) year-round, and in two more lakes (McCarrons Lake and a bay of Medicine Lake) in late spring.

Density stratification

Water temperature differentials and salinity both contribute to density stratification in a lake. Density stratification means that the density of the water at different depths is not the same. Density stratification is a main cause of vertical water quality gradients in many lakes. It prevents the complete vertical mixing of many lakes in the summer. Only very shallow or very large lakes are exceptions. A “lake geometry ratio” defines as 1/4 A /Hmax, where A is the lake surface area and Hmax is the maximum depth, can be used to estimate if a lake in Minnesota will be stratified in summer (Gorham and Boyce, 1989). When the lake geometry ratio has a value smaller than 3, a lake will have a

11 summer stratification (dimictic lake), when it has a value greater than 5, the lake will mix several times in the summer (polymictic lake).

To describe the vertical differences in water quality, a lake is often thought of as a three- layered system with a surface layer (epilimnion), interlayer (metalimnion) and a bottom layer (hypolimnion). 1-D water quality models describe a lake as a system of multiple horizontal layers of finite (e.g. 1m) thickness. The vertical mixing (diffusion/dispersion) coefficient in a lake is an important parameter in one-dimensional lake water quality models. It is used in Fick’s law to calculate the vertical fluxes of dissolved materials (e.g. nutrients and dissolved oxygen) in a lake. The vertical diffusion (dispersion) coefficient in a stratified lake is known to depend strongly on the vertical density gradient (Henderson-Sellers, 1984). According to Hondzo and Stefan (1994) the vertical mixing coefficient in Minnesota lakes is related to the vertical density gradient. Salinity gradients will contribute to vertical water density gradients, and hence reduce vertical mixing in a lake. Water quality, e.g. dissolved oxygen, can be adversely affected by the reduced mixing. It is therefore appropriate to ask how the observed salinity distribution in a lake affects the vertical transport of materials. Are the vertical material fluxes in a lake increased when road salt is added or are they decreased? To answer this question we have calculated for each day of the field measurements the density profiles and the associated vertical mixing coefficients with and without the measured salinity in each lake.

Water density as a function of temperature was calculated from a formula given by Maidment (1993), attributed to McCutcheon et al. (1993)

2 ρw = 1000 (1 – (T+288.9414) / (508929.2*(T+68.12963))*(T-3.9863) ) (5)

-3 o where ρw is the water density (kg m ) as a function of water temperature T ( C) at atmospheric pressure. Water density as a function of salinity and temperature is defined as 3/2 2 ρs = ρw + AS + BS + CS (6)

-3 where ρs is the water density (kg m ) and S is salinity (parts per thousand = ppt). The coefficients A, B, and C are defined by

A = 0.824493 - 4.0899*10-3*T + 7.6438*10-5 *T2 -8.2467*10-7 *T3 + 5.3675*10-9 *T4 B = -5.724*10-3+ 1.0227*10-4*T - 1.6546*10-6*T2 C = 4.8314*10-4 (7)

In these equations T is again the water temperature (oC).

Results of the density calculations are shown in Figures 13a to 13h for the months of August 2004 and March 2005. The density profiles obtained without salinity (S = 0) are shown as solid lines for comparison. The largest density increase due to salinity was calculated for the bottom of Brownie Lake (∆ρ=0.00135kg/m3). This corresponds to about 40% of the ocean water density increase. Most density increases were less than 0.0001kg/m3.

Vertical diffusivity

The vertical 1-D eddy diffusion/dispersion coefficient in a lake can be estimated from a variety of relationships, one of which is (Hondzo and Stefan, 1994)

-4 0.56 2 -0.43 Dz = 8.17 *10 (A) (N ) (8)

2 where Dz is the vertical diffusion/dispersion coefficient (cm /s), A is the lake surface area in km2, and N is the Brunt-Vaisala frequency (sec-1). N2 is defined as

N2 = (dρ/dz) (g/ρ) (9)

where g is acceleration of gravity (9.81m/s2), ρ is water density(kg m-3), and z is depth (m). It can be seen from equations (7) and (8) that the vertical mixing (diffusion/dispersion) coefficient depends on the vertical density gradient in a lake.

The effect of salinity on the vertical mixing coefficients Dz can be measured by the diffusivity ratio (DR) calculated from equations (7) and (8) as

- 0.43 DR = Dz(S) / Dz(S=0) = ((dρ/dz)(S) / (dρ/dz)(S=0)) (10)

A DR greater than 1 indicates an increase in diffusivity while a DR less than 1 indicates a decrease in diffusivity at a given depth due to salt content. A DR equal to 1 indicates no change in diffusivity due to salt content. The DR is essentially a relative measure of how salinity affects vertical density gradients and associated vertical diffusivities. Although equation (8) was developed for lake hypolimnia only, equation (10) was applied over the total lake depth because it gives the stabilizing effect of a density gradient on vertical mixing regardsless of the forcing mechanism that causes the vertical turbulence. The calculated DRs are given in Figures 14a to 14f for the months of August 2004 and March 2005.

In all eight lakes the calculated vertical diffusivities change by less than a factor of two due to the addition of salt, compared to the pure water conditions. The largest DR values were 1.8 and 1.7 in Brownie Lake and Ryan Lake, respectively. These are also the lakes with the highest salt contents. In Brownie Lake the largest DR values occurred in March near the surface (1 to 4m depth) and in Ryan Lake in August near the lake bottom (at 9 to 11m depth). In all other six lakes the DR values are less than 1.25 at all depths. Vertical diffusivities in the lakes studied are therefore minimally affected by the salt input.

In most of the lakes studied, the density profile and the vertical mixing coefficient are much more strongly affected by temperature than by the salinity.

13 DISCUSSION

Previous studies of road salt runoff effects, e.g. by Bubeck et al. (1971), Rosenberry (1999) and Thunqvist (2003), have shown effects similar to those presented in this report. Clearly lakes are in the hydrologic pathway of deicing salt applied to roads in winter. The amounts of salt turnover in the lakes studied are high. We have a much better idea how salt gets into a lake than how it gets out of a lake. In particular we have not analyzed how much leaves by surface water, how much gets into groundwater, and how much stays in the interstitial waters of the sediment. Adsorption processes and microbial processes are generally not considered for NaCl removal from water. Clearly more analysis is needed.

Another missing link is the connection to road salt applications. During the winters of 2003-2004 and 2004-2005, The Minnesota Department of Transportation (Mn/DOT) estimates that an average of 12.9 metric tons of road salt was applied per road mile for deicing. These values were loosely confirmed with data from various cities in the Shingle Creek Watershed District for the winters of 1995-1996 through 1999-2000 (Table 9).

To relate the amount of road salt that is discharged from a lake in a year to the amount applied to the watershed in a year, we have identified the watershed area for each lake (Figures 16a to 16h), and counted the lane miles in the watershed. The results are given in Table 1. With this information we can give an estimate of the total amount of salt applied in the watershed of each lake in the winters of 2003/2004 and 2004/2005. The Mn/DOT values for the winters of 2003-2004 and 2004-2005 were used to determine the total salt load for all eight lakes in this study because road salt data quality and application rates are highly variable among municipalities. The Mn/DOT values also provide a conservative estimate of road salt loading because, per mile, there are a higher average number of lanes compared to the municipal road dominated lakesheds in this study. This analysis assumes that all road salt applied is NaCl and that in-lake salts consist entirely of NaCl. We have plotted the amount of salt passed through a lake against the amount of salt applied, with the results given in Figure 15 and Table 10.

The estimated lakeshed NaCl load for each lake is much greater than the amount of NaCl passed through each lake. Even if the lakeshed NaCl load estimates are too large, the total lakeshed load is more than enough to explain the flux of salt passing through the lakes. At this time the large difference cannot be explained. A salt flux model will be needed to explain the difference. Some of the salt may accumulate over time in the lakes and some of the NaCl applied to the lakeshed may never reach the lake. There is certainly a lag time between salt application and its detection in the lakes.

CONCLUSIONS 1) Salinity of eight urban lakes in the Minnepolis/St.Paul metropolitan area was studied for 14 months during two winters and the summer in between. Salinity was caused

14 principally by NaCl. Specific conductance profiles were measured at roughly bimonthly intervals in each of the eight lakes. 2) Variations in specific conductance were found with season and with depth in each of the lakes. Specific conductance values varied from 400 to 1800 µS/cm in seven of the lakes and from 490 to 3500 µS/cm in the eighth lake which was meromictic. The largest specific conductance values were in late winter and at the bottom of the lakes, and the lowest in late summer near the surface of the lakes. 3) Chloride concentration was linearly related to specific conductance. Therefore chloride concentration profiles were calculated from specific conductance, and integrated with depth (using the lake bathymetry) to give the total chloride and the total NaCl content of a lake at every measurement date. There was clearly a seasonal pattern in all eight lakes, i.e. an accumulation of NaCl during the winter months and a decrease in total salt content during the summer. 4) Chloride concentrations near the lake bottom in Brownie Lake and Ryan Lake exceeded the chronic MPCA chloride standard of 230 mg/L during the entire 14 month measurement period. Chloride concentrations exceeded the chronic standard near the lake bottom in McCarrons Lake during late spring in 2004 and Medicine Lake during late spring in 2005. 5) The most plausible explanation for the observed behavior is the transport of road salt consisting mainly of NaCl through urban surface drainage systems (stormsewers, ditches etc.) from the urban lakeshed into the lakes. Using an average value of 14.1 and 11.8 metric tons of salt per mile of road a total salt load was determined for each lakeshed. The estimated lakeshed NaCl load for each lake was found to be much greater than the amount of NaCl passed through each lake. Even if the lakeshed NaCl load estimates are too large, the total lakeshed load is more than enough to explain the flux of salt passing through the lakes. At this time the large difference cannot be explained. 6) We also calculated the density profiles from the temperature and conductance measurements. Most density increases due to salinity were less than 0.0001kg/m3 and made only a weak contribution to the density stratification. The largest density increase due to salinity was calculated for the bottom of Brownie Lake (∆ρ=0.00135kg/m3), a meromitic lake. 7) The effect of vertical density gradients on the vertical dispersion coefficient was calculated with and without the effect of salinity. Vertical dispersion changed by less than a factor of 1.25 in six of the eight lakes. In two lakes the vertical diffusivity changed by less than a factor of 1.8. In six of the eight lakes studied, the density profile and the vertical mixing coefficient are much more strongly affected by temperature than by the salinity. Salinity affects the density profile and the vertical mixing coefficient to a relatively larger degree in Brownie Lake and Ryan Lake; the two lakes with the highest salinity. 8) The overall conclusion is that lakes are in the hydrologic pathway of roadway deicing salts. They act as a sink and provide temporary storage of dissolved salt in the winter

15 months when road salts are applied, and they act as a source of salinity in the summer months. One of the eight lakes studied has provided permanent storage for salt, but it is still unknown under what conditions the other lakes would act likewise. A more detailed, long term study may shed some light on this question.

References

American Public Health Association (APHA), American Water Works Association (AWWA), Water Environment Federation (WEF), 1992. Standard Methods, 18th Edition, A.E. Greenberg, L.S.Clesceri and A.D. Eaton, Ed., APHA Publication Office, Washington D.C.

Arnebeck, Rick. February 22, 2006. Personal Interview.

Bubeck et. al. (1971). Runoff of Deicing Salt: Effect on Irondequoit Bay, Rochester, New York. Science 172(3988), 1128 – 1132.

Crowley, D.P. (1968). A global numerical ocean model: Part 1. J. Comp. Phys. 3, 111- 147.

Ellis, C.R, Champlin, J, and Stefan, H.G. (1997) Density current intrusions in an ice- covered urban lake. J. of the American Water Resources Association 33(6), 1363 – 1373.

Environment Canada. (2001) Priority Substances List Assessment Report: Road Salt. Ottawa, Canada.

Godwin, K.S., Hafner, S.D., and Buff, M.F. (2003). Long-term trends in sodium and chloride in the Mohawk River, New York: the effect of fifty years of road-salt application. Environmental Pollution 124, 273 – 281.

Gorham and Boyce, 1989. Influence of lake surface area and depth upon thermal stratification and the depth of the summer thermocline. J. Great Lakes Res. 15 (1989), pp. 233–245.

Henderson-Sellers, B. (1984). Engineering Limnology. Pitman Press, Bath, Avon, 356 pp.

Hondzo, M. and H.G. Stefan (1993). Lake water temperature simulation model. J. Hydraulic Engineering, ASCE, 119(11): 1251-1273.

McCutcheon et. al (1993). Water Quality. In Maidment, DR (Ed.) Handbook of Hydrology. New York, New York: McGraw-Hill. 11.3.

16 Minnesota Department of Transportation, Office of Communications (2004). Snow and Ice Fact Sheet. http://www.dot.state.mn.us/workzone/winterfacts.html

Minnesota Pollution Control Agency (2004). MPCA 2004 303(d) List: List of Impaired Waters. http://www.pca.state.mn.us/publications/reports/tmdl-list-2004.pdf

Mullaney, J. March 30, 2005. Preliminary Analysis of the Effects of Deicing Chemicals on Ground and Surface-Water Quality in Glaciated Parts of the United States. Lecture at the 2005 Road Salt Symposium. St. Cloud, Minnesota.

Rosenberry et. al. (1999). Movement of road salt to a small New Hampshire lake. Water, Air, and Soil Pollution 109, 179-206.

Swain, E. B. 1984. The paucity of blue-green algae in meromictic brownie lake: iron limitation or heavy metal toxicity? Ph.D. dissertation, University of Minnesota, Minneapolis, MN.

Thomann, R.V. and J.A. Mueller (1987). Principles of Surface Water Quality Modeling and Control. Harper Collins Publishers, New York, NY, 644 pp.

Thunqvist, E.L. (2003). Increased chloride concentration in a lake due to deicing salt application. Water Science and Technology 48(9), 51-59.

Thunqvist, E.L. (2004). Regional increase of mean chloride concentration in water due to the application of deicing salt. Science of the Total Environment 325, 29-37.

U.S. Environmental Protection Agency (1973). Water Quality Criteria 1972. Ecological Research Series, EPA-R3-73-033, march 1973, 594 pp.

Wenck Associates, Inc. (2004). Chloride TMDL. Prepared for: Shingle Creek Water Management Commission and the Minnesota Pollution Control Agency, St. Paul, MN

YSI Model 63 Operations Manual (1999). YSI, Inc.

Table 1. Lake Characteristics

Lake Watershed Length of roads Maximum Predominant Lake surface area area* in watershed depth (m) land use (m2) (km2) (miles)

Bass 9.4 828,880 4.7 Residential 32.7

Brownie 14.3 43,140 2.2 Urban/Residential 19.6

Cedar 15.5 733,860 1.7 Urban/Residential 17.4

Isles 9.5 584,450 2.3 Urban/Residential 26.8

Johanna 13.1 855,100 1.6 Residential 13.9

McCarrons 17.4 297,120 4.0 Residential 35.0 Medicine 14.9 84,420 - Residential - (SW Bay)

Ryan 10.0 70,460 1.3 Urban/Residential 15.0

* Does not include lake surface area

Table 2. Specific conductivity (µS/cm @ 25 ºC ) calibration data

SC (Feb.4, 2005) Sample SCk SCm (Feb.18, 2004) m

De-ionized 0 4.6 5.5 Water Lab Standard 450 451 461.4 " 4500 4460 4321

18 Table 3. Calibration equations

Standard Error Date Equation R2 (µS/cm @ 25º C) SC = 0.99 (SC ) + 5.0 Feb.18, 2004 m 0.6 0.99 (1) SC = 0.96 (SC ) + 17.5 Feb.4, 2005 m 18.1 0.99 (2)

Table 4. Dates of field measurements

Measurement Date

Lake t1 t2 t3 t4 t5 t6 t7 t8 t9

Bass 2.15.04 3.2.04 4.21.04 6.11.04 8.6.04 10.11.04 2.9.05 3.5.05 4.9.05

Brownie 2.18.04 2.29.04 4.9.04 6.11.04 8.6.04 10.5.04 2.11.05 3.6.05 4.10.05

Cedar 2.18.04 2.29.04 4.9.04 6.11.04 8.6.04 10.5.04 2.11.05 3.6.05 4.10.05

Isles 2.18.04 2.29.04 4.9.04 6.11.04 8.6.04 10.5.04 2.11.05 3.6.05 4.10.05

Johanna 2.18.04 2.29.04 4.21.04 6.11.04 8.6.04 10.14.04 2.9.05 3.5.05 4.13.05

McCarron 2.18.04 3.2.04 4.9.04 6.11.04 8.6.04 10.14.04 2.9.05 3.5.05 4.13.05

Medicine 2.18.04 2.29.04 4.9.04 6.11.04 8.6.04 10.14.04 2.9.05 3.5.05 4.13.05

Ryan 2.18.04 2.29.04 4.9.04 6.11.04 8.6.04 10.5.04 2.11.05 3.5.05 4.13.05

19 Table 5. Ionic composition of five water samples

Sample Concentration (mg/L) Brownie Ion Brownie (2m) Brownie (10m) Ryan (2m) Ryan (10m) (10 m) 50% dilution Al+3 < 0.179 < 0.179 < 0.179 < 0.179 < 0.179

B+3 < 0.023 0.053 < 0.023 < 0.023 < 0.023

Ca+2 19.171 57.228 34.365 52.854 29.255

Cd+2 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006

Cl- 118.56 680.16 81.66 155.98 330.45

Cr+3 < 0.014 < 0.014 < 0.014 < 0.014 < 0.014

Cu+/Cu+2 < 0.026 < 0.026 < 0.026 < 0.026 < 0.026

Fe+2/Fe+3 0.058 46.547 0.039 < 0.017 22.743

K+ 2.223 12.095 2.823 4.270 6.145

Mg+2 3.838 11.759 6.142 7.278 6.044

Mn+2 0.061 3.686 0.046 2.180 1.871

Na+ 73.185 403.850 50.709 96.260 201.81

Ni+2 < 0.022 < 0.022 < 0.022 < 0.022 < 0.022

P+ < 0.035 3.052 < 0.035 1.351 1.469

Pb+2 < 0.084 < 0.084 < 0.084 < 0.084 < 0.084

Zn+2 < 0.007 0.011 0.008 < 0.007 0.009

Table 6. Total estimated chloride (Cl-) mass (metric tons)

Measurement Date

Lake t1 t2 t3 t4 t5 t6 t7 t8 t9

Bass 237 251 202 209 188 - 283 289 220

Brownie 78 87 89 66 68 65 78 84 86

Cedar 538 550 513 471 436 467 584 620 562

Isles 171 168 141 - 102 110 140 140 120

Johanna 595 603 576 - 566 561 541 648 577

McCarron 328 335 291 277 284 271 335 342 324 Medicine 65 68 69 64 65 62 78 83 80 (SW Bay) Ryan 51 56 46 40 45 45 57 58 48

Table 7. Total estimated sodium chloride (NaCl) mass (metric tons)

Measurement Date

Lake t1 t2 t3 t4 t5 t6 t7 t8 t9

Bass 391 414 333 345 310 - 467 476 363

Brownie 129 143 147 109 112 107 129 138 142

Cedar 887 907 846 776 719 770 963 1022 926

Isles 282 277 232 - 168 181 231 231 198

Johanna 981 994 950 - 933 925 892 1068 951

McCarron 541 552 480 457 468 447 552 564 534

Medicine 107 112 114 106 107 102 129 137 132 (SW Bay)

Ryan 84 92 76 66 74 74 94 96 79

22 Table 8. Increase in total lake chloride (Cl-) mass from late summer to late winter (metric tons)

Medicine Bass Brownie Cedar Johanna Isles McCarrons Ryan (SWBay)

Increase in 24.0 82.3 29.6 7.9 56.4 5.2 23.1 44.0 2003/04

Increase in 42.4 76.7 44.0 17.9 21.5 32.5 26.0 29.6 2004/05

23 Table 9. Annual road salt application rates for Mn/DOT and various cities

Road salt applied Year City/Agency (tons (metric tons)/mile)

2004/05 Mn/DOT 13.0 (11.8) 2003/04 “ 15.5 (14.1)

1999/00 Minneapolis 14.0 (12.7)

1999/00 Robbinsdale 1.9 (1.7) 1998/99 “ 6.6 (6.0) 1997/98 “ 7.1 (6.4) 1996/97 “ 7.2 (6.5) 1995/96 “ 7.4 (6.7)

1999/00 Plymouth 8.3 (7.5) 1998/99 “ 7.7 (7.0) 1997/98 “ 9.0 (8.2) 1996/97 “ 11.6 (10.5) 1995/96 “ 12.8 (11.6)

1999/00 Brooklyn Center 12.5 (11.3) 1998/99 “ 12.1 (11.0) 1997/98 “ 9.9 (9.0) 1996/97 “ 12.5 (11.3)

24 Table 10. Salt loading from lakeshed and increase in lake salt (NaCl) content from late summer to late winter for 2003/04 and 2004/05

Estimated seasonal NaCl increase Estimated seasonal NaCl load from from late summer to late winter lakeshed (metric tons) Lake (metric tons)

2003/04 2004/05 2003/04 2004/05

Bass 458 386 40 70

Brownie 274 231 136 127

Cedar 244 205 49 73

Isles 375 316 13 30

Johanna 195 164 93 35

McCarrons 490 413 9 54

Ryan 210 177 73 49

Figure 1. Lake Locations

Figure 2.a Bass Lake bathymetric map

Figure 2.b Brownie Lake bathymetric map

Figure 2.c Cedar Lake bathymetric map

Figure 2.d Lake of the Isles bathymetric map

Figure 2.e Lake Johanna bathymetric map

Figure 2.f Medicine Lake (West Bay) bathymetric map

Figure 2.g Lake McCarrons bathymetric map

Figure 2.h Ryan Lake bathymetric map

34 Area (acres) 0 25 50 75 100 125 150 175 200 225 0

Depth (ft) Depth 20

Figure 3.a Bass Lake area vs. depth curve

Area (acres) 024681012 0

25 Depth (ft) 30

Figure 3.b Brownie Lake area vs. depth curve

35 Area (acres) 0 25 50 75 100 125 150 175 200 0

25 Depth (ft) Depth 30

Figure 3.c Cedar Lake area vs. depth curve

Area (acres) 0 25 50 75 100 125 150 175 200 225 0

Depth (ft) 25

Figure 3.d Johanna Lake area vs. depth curve

36 Area (acres) 0 25 50 75 100 125 0

Depth (ft) Depth 20

Figure 3.e Lake of the Isles area vs. depth curve

Area (acres) 0 10203040506070 0

30 Depth (ft)

Figure 3.f Lake McCarrons area vs. depth curve

37 Area (acres) 0 5 10 15 20 25 0

25 Depth (ft) Depth 30

Figure 3.g Medicine Lake (SW Bay) area vs. depth curve

Area (acres) 0 5 10 15 20 0

Depth (ft) 20

Figure 3.h Ryan Lake area vs. depth curve

38 Time Averaged Specific Conductance (µ S/cm) 500 1000 1500 2000 2500 3000 3500 1

Bass 5 Brownie Cedar Isles 7 Johanna McCarron Medicine Depth (m) Depth 9 Ryan

Figure 4. Average specific conductance profiles in 8 lakes (Feb.15, 2004 to April 18, 2005)

750

700

) 650 2.15.04 S/cm 3.2.04 (µ 600 4.21.04 6.11.04 8.6.04 2.11.05 550 3.5.5 4.9.05

SpecificConductance 500

450

400 012345678 Depth (m)

Figure 5.a Bass Lake - Specific conductance profiles

4000

3500

3000 )

2.18.04 S/cm

(µ 2500 2.29.04 4.9.04 6.11.04 2000 8.6.04 10.5.04 2.11.05 1500 3.6.05 4.10.05 Specific Conductance 1000

500

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Depth (m)

Figure 5.b Brownie Lake - Specific conductance profiles

40 750

700

2.18.04 ) 2.29.04 650 S/cm 4.9.04 (µ 6.11.04 8.6.04 600 10.5.04 2.11.05 3.6.05 550 4.10.05 Specific Conductance Conductance Specific

500

450 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Depth (m)

Figure 5.c Cedar Lake - Specific conductance profiles

800

750

700 )

S/cm 2.18.04

(µ 650 2.29.04 4.9.04 6.11.04 600 8.6.04 10.5.04 2.11.05 550 3.6.05 4.10.05 Specific Conductance Conductance Specific 500

450

400 012345678910 Depth (m)

Figure 5.d Lake of the Isles - Specific conductance profiles

41 850

800 ) 2.18.04 750 S/cm 2.29.04 (µ 4.21.04 6.11.04 8.6.04 700 10.14.04 2.9.05 3.5.05 650 4.13.05 Specific Conductance Conductance Specific

600

550 0123456789101112 Depth (m)

Figure 5.e Lake Johanna - Specific conductance profiles

1200

1100

1000 )

S/cm 2.18.04

(µ 900 3.2.04 4.9.04 6.11.04 800 8.6.04 10.14.04 2.9.05 700 3.5.05 4.13.05 Specific Conductance Conductance Specific 600

500

400 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Depth (m)

Figure 5.f. Lake McCarrons - Specific conductance profiles

42 1200

1100 2.18.04

) 2.29.04 1000

S/cm 4.9.04 (µ 6.11.04 900 8.6.04

800 10.14.04

2.9.05 700 3.5.05 Specific Conductance Conductance Specific

4.13.05 600

500 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Depth (m)

Figure 5.g Medicine Lake (SW Bay) - Specific conductance profiles

1800

1600

) 2.18.04 1400 2.29.04 S/cm

(µ 4.9.04 6.11.04 1200 8.6.04 10.5.04 1000 2.11.05 3.5.05 4.13.05 800 Specific Conductance

600

400 0123456789101112 Depth (m)

Figure 5.h. Ryan Lake - Specific conductance profiles

2.15.04 15 3.2.04 4.21.04 6.11.04 10 8.6.04

Temperature (°C) Temperature 2.11.05 3.5.05 4.9.05 5

0 012345678 Depth (m)

Figure 6.a Bass Lake – Temperature profiles

2.18.04 15 2.29.04 4.9.04 6.11.04 8.6.04 10 10.5.04

Temperature (°C) Temperature 2.11.05 3.6.05 4.10.05 5

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Depth (m)

Figure 6.b Brownie Lake – Temperature profiles

44 25

2.18.04 2.29.04 15 4.9.04 6.11.04 8.6.04 10.15.04 10 2.11.05 Temperature (°C) Temperature 4.10.05

0 0123456789101112 Depth (m)

Figure 6.c Cedar Lake – Temperature profiles

2.18.04 2.29.04 15 4.9.04 6.11.04 8.6.04 10.5.04 10 2.11.05 3.6.05 Temperature (°C) Temperature 4.10.05

0 012345678910 Depth (m)

Figure 6.d Lake of the Isles – Temperature profiles

45 25

2.18.04 2.29.04 15 4.21.04 6.11.04 8.6.04 10.14.04 10 2.9.05

Temperature (°C) Temperature 3.5.05 4.13.05

0 012345678910111213 Depth (m)

Figure 6.e Lake Johanna – Temperature profiles

2.18.04 3.2.04 15 4.9.04 6.11.04 8.6.04 10.14.04 10 2.9.05

Temperature (°C) Temperature 3.5.05 4.13.05

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Depth (m)

Figure 6.f Lake McCarrons– Temperature profiles

46 25

2.18.04 2.29.04 15 4.9.04 6.11.04 8.6.04 10.14.04 2.9.05 10 3.5.05 Temperature (°C) Temperature 4.13.05

0 012345678910111213 Depth (m)

Figure 6.g Medicine Lake (SW Bay) – Temperature profiles

2.18.04 2.29.04 4.9.04 15 6.11.04 8.6.04 10.5.04 2.11.05 10 3.5.05 Temperature (°C) Temperature 4.13.05

0 0123456789101112 Depth (m)

Figure 6.h Ryan Lake – Temperature profiles

47 800

700

600

500

400 [Cl-] (mg/L) 300

200

100

0 0 500 1000 1500 2000 2500 3000 Specific Conductance (µ S/cm)

Figure 7. Laboratory analyzed chloride concentration [Cl-] vs. in situ field measurements of specific conductance for selected lake samples

700

600 Bass

500 Brownie

Cedar

400 Isles

Johanna

300 McCarron

Medicine

200 Ryan Total Chloride Mass (Metric Tons) Mass (Metric Chloride Total 100

0 12/25/03 2/23/04 4/23/04 6/22/04 8/21/04 10/20/04 12/19/04 2/17/05 4/18/05 6/17/05 Time (Date)

Figure 8. Estimated total chloride (Cl-) mass in each lake vs. time

120

Winter '04 Winter '05 100

60 % Increase %

0 Bass Brownie Cedar Isles Johanna Medicine McCarron Ryan Lake Figure 9. Increase in lake chloride (Cl-) mass from late summer to late winter. The reference is the lake chloride mass in late summer (100%)

49 500

450

400

350 Bass Brownie 300 Cedar

250 Isles Johanna 200 McCarron

150 Medicine Ryan 100 Average Chloride Concentration (mg/L) Concentration Chloride Average 50

0 12/25/03 2/23/04 4/23/04 6/22/04 8/21/04 10/20/04 12/19/04 2/17/05 4/18/05 6/17/05 Time (Date) Figure 10. Average chloride (Cl-) concentration in each lake vs. time

200

180

Bass

160 Brownie

Cedar 140 Isles

Johanna 120 McCarron

Medicine 100 Ryan

80 Chloride Concentration Measured @ 1m (mg/L) Measured Concentration Chloride

60 12/25/03 2/23/04 4/23/04 6/22/04 8/21/04 10/20/04 12/19/04 2/17/05 4/18/05 6/17/05 Time (Date) Figure 11. Chloride (Cl-) concentration at 1m depth in each lake vs. time

50 1200

1000

Bass

800 Brownie Cedar

Isles 600 Johanna McCarron 400 Medicine

Ryan

200 Deepest Measured Chloride Concentration (mg/L) Concentration Chloride Measured Deepest 0 12/25/03 2/23/04 4/23/04 6/22/04 8/21/04 10/20/04 12/19/04 2/17/05 4/18/05 6/17/05 Time (Date) Figure 12. Chloride (Cl-) concentration at the deepest point of each lake vs. time

ρ (g/cm3) 0.998 0.9985 0.999 0.9995 1 1.0005 0

3 8.6.04 S = 0 8.6.04 4 3.5.05 S = 0

Depth (m) 3.5.05

Figure 13.a Bass Lake - Density profiles

ρ (g/cm3) 0.998 0.9985 0.999 0.9995 1 1.0005 1.001 1.0015 0

8.6.04 S = 0 8.6.04 6 3.6.05 S = 0 3.6.05

Depth (m) Depth 8

Figure 13.b Brownie Lake - Density profiles

52 ρ (g/cm3) 0.998 0.9985 0.999 0.9995 1 1.0005 0

8.6.04 S = 0 8.6.04 6 3.6.05 S = 0

Depth (m) Depth 3.6.05

Figure 13.c. Cedar Lake - Density profiles

ρ (g/cm3) 0.998 0.9985 0.999 0.9995 1 1.0005 0

8.6.04 S = 0 4 8.6.04 3.6.05 S = 0 3.6.05

Depth (m) Depth 5

Figure 13.d Lake of the Isles - Density profiles

53 ρ (g/cm3) 0.9975 0.998 0.9985 0.999 0.9995 1 1.0005 0

8.6.04 S = 0 8.6.04 6 3.5.05 S = 0 3.5.05 Depth (m)

12 Figure 13.e Lake Johanna - Density profile

ρ (g/cm3) 0.9975 0.998 0.9985 0.999 0.9995 1 1.0005 0

6 8.6.04 S = 0 8.6.04 3.5.05 S = 0 8

Depth (m) Depth 3.5.05

Figure 13.f Lake McCarrons - Density profiles

54 ρ ( g/cm3 ) 0.9975 0.998 0.9985 0.999 0.9995 1 1.0005 1.001 0

8.6.04 S = 0 8.6.04 6 3.5.05 S = 0 3.5.05

Depth (m) 8

Figure 13.g Medicine Lake (SW Bay) – Density profiles

ρ (g/cm3) 0.998 0.9985 0.999 0.9995 1 1.0005 1.001 0

8.6.04 S = 0 6 8.6.04 3.5.05 S = 0

Depth (m) Depth 3.5.05

Figure 13.h Ryan Lake - Density profile

55 Diffusivity Ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04 4 3.5.05 Depth (m)

Figure 14.a Bass Lake – Diffusivity ratio

- 0.4 (Equ.10)DR = Dz(S) / Dz(S=0) = ((dρ/dz)(S) / (dρ/dz)(S=0))

Diffusivity Ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04 3.5.06 Depth (m) 8

Figure 14.b Brownie Lake – Diffusivity ratio

56 Diffusivity Ratio

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04 6 3.6.05 Depth

Figure 14.c Cedar Lake – Diffusivity ratio

Diffusivity Ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

4 8.6.04 3.6.04 Depth (m) Depth

Figure 14.d Lake of the Isles – Diffusivity ratio

57 Diffusivity Ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04

6 3.5.05 Depth (m) Depth

Figure 14.e Lake Johanna – Diffusivity ratio

Diffusivity Ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04 8 3.5.05 Depth (m)Depth

Figure 14.f Lake McCarrons – Diffusivity ratio

58 Diffusivity Ratio

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04 6 3.5.05 Depth (m) Depth

Figure 14.g Medicine Lake (SW Bay) – Diffusivity ratio

Diffusivity Ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0

8.6.04 6 Depth (m) Depth 3.5.05

Figure 14.h Ryan Lake – Diffusivity ratio

59 600

2003/04 Total Lakeshed NaCl Load

2003/04 NaCl Increase From Late Summer to 500 Late Winter 2004/05 Total Lakeshed NaCl Load

2004/05 NaCl Increase From Late Summer to Late Winter 400

300 1 NaCl (MetricNaCl Tons)

200

100

0 Bass Brownie Cedar Isles Johanna McCarrons Ryan

Figure 15. Comparison of increase in total lake salt (NaCl) mass from late summer to late winter (metric tons) and total lakeshed salt loading for 2003/04 and 2004/05

Figure 16.a Bass lakeshed delineation

Figure 16.b Brownie, Cedar, and Isles lakeshed delineations

Figure 16.c Johanna lakeshed delineation

Figure 16.d McCarrons lakeshed delineation

Figure 16.e Ryan lakeshed delineation