Physical Processes Controlling Taste and Odor Episodes in Lake Ontario Drinking Water

J. Great Lakes Res. 29(1):70–78 Internat. Assoc. Great Lakes Res., 2003

Physical Processes Controlling Taste and Odor Episodes in Lake Ontario Drinking Water Yerubandi R. Rao1,*, Michael G. Skafel1, Todd Howell2, and Raj C. Murthy1 1National

Water Research Institute 867 Lakeshore Road Burlington, Ontario L7R 4A6

2Environmental

Monitoring and Reporting Branch Ontario Ministry of the Environment 125 Resources Road Toronto, Ontario M9P 3V6

ABSTRACT. Circulation and thermal structure of the coastal waters were studied as a part of an interdisciplinary program to investigate the taste and odor problem in drinking water along the north and western shores of Lake Ontario. The currents and temperature variations were found to be strongly linked to winds, with winds from the west causing upwelling and eastward flowing currents, and winds from the east inducing downwelling and warm westward flowing currents. The downwelling along the north shore during late August and early September of 2000 was associated with a pulse in concentration of the taste and odor causing compound geosmin. This study indicates that during this episode the onshore directed mean currents and cross-shore fluxes in the surface layer transported geosmin to the coastal waters of the north shore. INDEX WORDS: cline, currents.

Lake Ontario, taste and odor, physical processes, upwelling, downwelling, thermo-

INTRODUCTION Lake Ontario represents an important source of drinking water for millions of consumers. During the late summer months drinking water from Lake Ontario is susceptible to earthy taste and odor (T/O), undesirable properties in drinking water. The occurrence of objectionable taste and odor in drinking water is caused by both anthropogenic and naturally produced chemicals (Ridal et al. 2000). The most commonly identified biological causes of taste and odor events are two moderately volatile metabolites of certain micro-organisms, geosmin and 2-methylisoborneol (MIB). These metabolites can be produced by cyanobacteria and/or actinomycetes in diverse aquatic and terrestrial habitat zones. Only certain taxa within these groups produce odor, which varies with growth and environment. Because both geosmin and MIB are discernable at extremely low threshold levels, and

*Corresponding

resist oxidation, they are difficult to remove with typical drinking water treatment (Young et al. 1996). In the Great Lakes, both production and transport of these metabolites are influenced by large scale meteorological forcing, watershed, basin, diffuse/point-source loading, and hydrological processes. In response to the severe T/O episodes in 1998 and 1999 in western Lake Ontario, a multidisciplinary research team (Watson et al. 2002) was established to identify the biological sources and environmental triggers of these events, and to develop predictive and remedial tools. Early work identified an abrupt increase in geosmin concentration coinciding with T/O problems of drinking water along the north-western shores of Lake Ontario. Geosmin production is observed to be indigenous, peaks annually, but only periodically at nuisance levels, and is hypothesized to originate from offshore planktonic cyanobacteria. Historical data from water treatment plants showed that the higher geosmin levels coincided with higher water

author. E-mail: [email protected]

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FIG. 1. Winds at Toronto Island airport, observed temperature, and geosmin concentration at RL Clark (Toronto) water intakes during the summers of 1998 and 1999. temperatures in late summer. The prevailing winds at Toronto Island station and water temperature and geosmin concentrations from Toronto water treatment plant are depicted in Figure 1 during the summers of 1998 and 1999. The winds were rotated 80° in clockwise direction to conform the alongshore direction. The westward directed winds from 3 August 1998 (Day 216) reversed the prevailing upwelling, and temperatures rose to over 20°C. Although winds reversed briefly later, the downwelling persisted, and the peak in geosmin concentrations was observed on 17 August 1998 (Day 230). Similarly during the summer of 1999, the abrupt increase in geosmin levels coincided with increased temperatures due to downwelling forced by westward winds from 18 August (Day 230) to 7 September 1999 (Day 250) (Fig. 1b). Based on this evidence, and similar observations at other water intakes along the northshore of Lake Ontario, it was hypothesized that strong downwelling and associated shoreward currents may favor the transport of geosmin produced at offshore locations to nearshore areas causing the T/O problem. The thermal structure and circulation in the Great Lakes generally depends on the season because of the large annual variation of surface fluxes (Boyce et al. 1989). The lakes are stratified in summer and fall; the thickness of the epilimnion may reach 30 m in the deeper basins of the lower lakes. During this period of stratification, significant wind events will cause upwelling and downwelling of the thermocline along the shore. The scale of the offshore distance over which these events takes place depends on the wind stress and nearshore bathymetry, and is

typically of the of the order of 5 to 10 km, hence within the coastal boundary layer. The coastal boundary layer along the northshore of Lake Ontario is generally 8 to 10 km wide, and consists of a zone of 3 km from the shore that is influenced by bottom and shore friction, and beyond that inertial oscillations adjust to the shore parallel flow (Rao and Murthy 2001a). During the summer stratified season, the temperature variations along the northwest shore of Lake Ontario were found to be linked to the wind, with winds from westerly direction causing upwelling and cooling, and easterly winds inducing downwelling and warming. Previous studies have shown that the flow and structure within the coastal boundary layer along the north shore of Lake Ontario is complex during upwelling and downwelling episodes. The upwelling events are characterized by relatively weaker eastward flow, and downwelling events with strong westward currents, sometimes associated with the propagation of internal Kelvin waves due to the thermocline oscillations (Simons and Schertzer 1989, Rao and Murthy 2001b). In 2000 an intensive field investigation in the western end of Lake Ontario was undertaken to gain new information about the source and distribution of geosmin in the coastal waters. As part of the investigation, current meters and temperature sensors were deployed in the vicinity of several water treatment plant intakes as well as other locations. The purpose of this paper is to describe the circulation observed with the instrument array and to explain the probable physical mechanism that resulted in the high concentrations of geosmin detected in

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the raw water collected at Lake Ontario water treatment plants during summer of 2000. EXPERIMENTAL DATA During the summer of 2000, several instrumented moorings were deployed within the coastal boundary layer of Lake Ontario (Fig. 2). The main component of the field experiment was an array of bottom mounted Acoustic Doppler Current Profilers (ADCP) at five locations, and four Aandera current meter (RCM7 and 9) moorings each consisting of at least two levels of measurements. The long-term accuracy of velocity profiles obtained from broadband ADCPs are of the order of ± 2%. The currents from RCMs are accurate to the order of 1 cm/s. The temperature data were obtained from fixed temperature profilers (FTP) at two locations as shown in Figure 2, and also from the temperature probe on RCMs. The temperature data are accurate to the order of 0.1°C. Each current meter and FTP recorded both currents and temperature at time intervals of 20 minutes or 1 hour. The data were resampled at each hour from 1 August (Day 214) to 30 September (Day 274) to have a common interval for all the moorings. The east and north velocities are resolved into alongshore (U) and cross-shore (V) components. Table 1 shows the details of data availability and the angle of rotation from north to obtain shore-parallel and shore-perpendicular components of currents. The FTP data were supple-

TABLE 1.

FIG. 2. Map of north and western shores of Lake Ontario showing locations of current meters, and temperature profiles.

mented by shipboard observations during a downwelling event. Wind records were obtained from routine weather observations at Toronto Island Airport. The wind stress at the water surface was computed by the quadratic law given as τ = ρaCdWW, where ρa = 1.2 kg/m 3 is the air density, W is wind velocity (m/s). In general, the drag coefficient Cd increases with the wind speed and estimated as Cd = (0.8 +

Deployment data, and angle of rotation from north to shoreline.

Instrument type ADCP—1200 kHz (bottom mounted) vertical profiles of horizontal currents and temperature at bottom RCM—Current and Temperature

Station/measurement depths (m) 1/ 2.5–12.5 2/ 2.5–14.5 3/ 2.5–9.5 5/ 2–21.0 7/2.5–10.5 4/ 6 6/ 5, 29 8/ 5, 29

Fixed temperature profiler (FTP)

Toronto (10–40) Lakeview (5–15)

Winds

Toronto Island Airport (+10 m)

Distance offshore (km) 2.9 2.8 2.1 2.8 1.8

Data period 1 Aug. to 30 Sept. 22 Aug. to 30 Sept. 4 Aug. to Sept. 28 1 Aug. to 30 Sept. 1 Aug. to 30 Sept.

Angle of rotation from the north (deg.) 81 68 73 52 327

0.8 3.5 2.8

1 Aug. to 30 Sept. 1 Aug. to 30 Sept. 1 Aug. to 30 Sept.

316 34 64

2.3 2.7

1 Aug. to 30 Sept. 1 Aug. to 30 Sept. Aug. 1 to Sept. 30

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FIG. 3. Time series of hourly values of (a) wind stress components at Toronto Island station, and (b) temperature profiles at Toronto FTP (Positive values of wind stress components corresponds to eastward or onshore components). 0.065 W) × 10 –3 for W >1 m/s (Wu 1980). The stresses were also decomposed into alongshore and cross-shore components with alongshore direction being aligned with general orientation of north shore of Lake Ontario (80° from north). Positive values are toward the east and onshore, respectively. Geosmin concentrations in raw water collected at five water treatment plants drawing source water from Lake Ontario were measured over the summer and fall of 2000. The sampling interval over August and September was approximately weekly. Water samples were analyzed for geosmin by high-resolution mass spectrometry using Ontario Ministry of the Environment standard method for taste and odor compounds (Palmentier et al. 1998).

RESULTS AND DISCUSSION Summer (1 August to 30 September) Samples taken from several shipboard surveys have shown that the level of geosmin in Lake Ontario is below detection limits (< 1 ng/L) until late July (Ridal et al. 2000). Therefore observations of currents and thermal characteristics were analyzed from 1 August to 30 September 2000. The positions of the 10° to 13°C isotherms were used to define upwelling and downwelling events in Lake Ontario (Rao and Murthy 2001a). The position of the 10°C isotherm was chosen as the depth of the thermocline to identify upwelling and downwelling episodes. Figures 3a and 3b show the time series of hourly values of wind stress and temperature pro-

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files at Toronto Island. The alongshore component of the wind stress is higher than the cross-shore component indicating that the winds during this period are roughly oriented in east-west direction. However, on a few occasions winds blew in the offshore direction. The time series of temperature data reveal that the water column is stratified, and thermocline moves up (upwelling) and down (downwelling). The temperature profiles show oscillations at near-inertial period (~17 hours) throughout the observational period, which is common during the summer stratified season. Comparison of wind stress and temperature data shows that the variability of thermal structure is associated with prevailing winds. Upwelling events are caused by winds from the west, and downwelling events are caused by winds from the east. Figure 4 shows the time series of low-pass filtered (> 24 h) currents at 5 m below the surface at five selected stations from 4 August to 28 September. The alongshore currents were comparatively stronger than cross-shore currents all along the northshore. However, at Port Dalhousie, which is located on the southwestern shore of the lake, both components were more or less of equal magnitude. The alongshore currents show that the low-frequency oscillations (> 3 days) are dominant and are related to alongshore wind stress. The mean alongshore currents (3 to 5 cm/s) were toward the west along the north shore and toward the east at Port Dalhousie. This is consistent with earlier observations that circulation in large lakes is cyclonic, i.e., counterclockwise (Emery and Csanady 1973, Schwab et al. 1995). Along the northshore downwelling events were characterized by westward flowing currents. As seen from Figures 3 and 4, from 19 to 26 August 2000 winds were relatively calm. During this period, the previously upwelled thermocline relaxed. The eastward flowing currents were replaced by warm westward flowing currents, and the thermocline downwelled to a depth of 20 to 25 m. As a result, the warm waters advanced farther and farther along the shore and eventually reached the western end of the lake. A rough calculation indicates that the warm currents from Cobourg to Hamilton propagated at around 20 to 30 km/day. This is more or less in accordance with the propagation speeds of internal Kelvin waves along the north shore of Lake Ontario (Simons and Schertzer 1989). From the temperature profiles it may be observed that the most conspicuous downwelling event occurred from 27 August (Day 240) to 6 September (Day 250). During this period, concentrations of geosmin increased and

reached maximum concentrations in raw water collected at water treatment plants. In the remainder of this paper the characteristics of this particular downwelling event are discussed. Downwelling Episode (27 August to 6 September) Beginning at about noon on 27 August, strong winds (~1.5 dynes/cm2) blew from the northeast for a day (Fig. 3a). Prior to this event winds were weak and the thermocline was downwelling due to the westward propagating currents along the northshore. This was further strengthened by the downwelling favorable winds. During this period along the south shore (Port Dalhousie), the easterly winds generated upwelling and surface currents show significant offshore transport (Fig. 4). Although winds relaxed from 28 August, the downwelling of the thermocline still persisted due to westward flowing currents along the north shore. The thermocline intersected the bottom at Toronto Island (Fig. 3b) and remained there until the end of this episode. From the evening of 3 September, moderate but persistent winds from the northeast brought warm currents to the west. The strong onshore directed winds resulted in very warm (> 20°C) waters penetrating to lower levels. Figure 5 shows the thermal structure off the Lakeview station on 29 August. As observed in the time series plot, this plot also shows that during this period the thermocline shifted to the bottom, thus increasing the depth of surface mixed layer to 30 to 40 m. The downwelling of the thermocline is confined to within 5 to 6 km of the shore. Figures 6a and 6b show the variation of mean values of cross-shore and alongshore components of the currents for this downwelling event. Along the north shore from Cobourg to the east of Hamilton (0 to 150 km), cross-shore velocity shows onshore flow in the surface layer and offshore flow in the bottom. The mean alongshore currents are primarily oriented toward the west along the north shore. Although bottom currents are relatively small, they still show a similar structure. As discussed earlier, on the southwestern shore the currents are toward the offshore due to upwelling caused by easterly winds during the same period. The vertical structure of currents is analyzed further at all ADCP stations. Figure 7 shows an example of mean currents at the Lakeview station. Here the summer means are calculated from 1 August to 30 September, and mean values during the down-

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FIG. 4. Time series of low-pass filtered (> 24 h) alongshore wind stress at Toronto Island Airport, and currents at five selected stations. Here the cross-shore values are filled. (Positive values correspond to eastward or onshore components.)

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FIG. 5. Cross-shore distribution of temperature off Mississauga (Lakeview) station 29 August 2000. The contour interval is 2°C. welling event from 27 August to 6 September are provided for comparison purposes. The mean alongshore currents (~2 cm/s) during the summer are toward the west and do not show any zerocrossings. The cross-shore currents are relatively small and show zero crossings over the depth of the water column. During the downwelling event, the mean westward currents increased significantly with considerable vertical shear. The cross-shore currents show two-layer structure with strong onshore currents in the upper mixed layer and offshore currents below it. This is because during the downwelling the westward and onshore directed wind stress forces the surface circulation toward the coast and return offshore flow develops in the bottom. In order to estimate the alongshore and crossshore heat fluxes, temperature and current fluctuations (u′, v′, T′) were calculated by subtracting the mean values from the hourly time series of currents and temperature. The net alongshore and cross-shore fluxes during the downwelling episode were plotted against depth at Lakeview station (Figure 8). The alongshore fluxes are high in the surface mixed layer and fall rapidly below it. As observed in the mean currents the alongshore fluxes are oriented in the westward direction. The crossshore fluxes also show a two layer structure with onshore directed fluxes in the upper 10 m and offshore fluxes below it. This clearly indicates that during the downwelling both mean and turbulent currents transport warm waters in the surface layer to the coastal region and the alongshore currents advect these waters in the westward direction.

FIG. 6. The net cross-shore and alongshore components of currents during the downwelling event plotted against alongshore distance from Cobourg (0 km) to Port Dalhousie (200 km).

Geosmin Concentrations (28 August to 6 September) Figure 9 shows the time series of geosmin concentrations obtained in water collected at drinking water intakes along the north and western shores of Lake Ontario. Geosmin concentrations clearly show a sharp increase during the week of 28 August and a peak on 5 September. Although geosmin levels were low compared to severe episodes in 1998 and 1999, patterns of occurrence with similarity to these years were detected and suggested a similar mode of production. The easterly winds during this period generated downwind currents along the shore and return flow in the offshore. This caused upwelling of cold waters along the south shore, and down-

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FIG. 8. The cross-shore and alongshore heat fluxes at Mississauga (Lakeview) station (Positive values correspond to eastward or onshore components.) CONCLUSIONS The currents and temperature measurements in 2000 along the north and western shores of Lake Ontario show upwelling and downwelling of the thermocline. Upwelling was caused by winds from

FIG. 7. The mean alongshore and cross-shore velocity components as a function of depth at Mississauga (Lakeview) station. (Positive values correspond to eastward or onshore components.) welling and warm currents along the north shore. The peak concentrations on 5 September clearly show that the warm onshore currents during the downwelling have brought geosmin into the nearshore areas. Geosmin concentrations are significantly less in the western basin (Grimsby) because this region experienced upwelling and offshore transport during this period.

FIG. 9. Observed geosmin concentrations at drinking water intakes during summer, 2000.

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the west generating eastward and offshore flowing currents, downwelling by winds from the east with strong westward currents. Occasionally downwelling was also caused by the penetration of warm currents due to the propagation of internal Kelvin waves. The downwelling event from 27 August to 6 September correlated with a rise in geosmin concentrations at the water treatment plants along the north shore. During this event the alongshore currents flowed in the westward direction, and crossshore currents exhibited onshore flow in the surface layer and offshore flow in the bottom layer. This study indicates that this wind-driven downwelling circulation carried warm offshore surface waters into the vicinity of the water treatment plants from Cobourg to Hamilton. The analysis supports the hypothesis that the taste and odor problems experienced along the northern coast of Lake Ontario results from advection of geosmin originating in the warm offshore waters in the days and weeks preceding the events. ACKNOWLEDGMENTS We thank Murray Charlton and Susan Watson for discussions and making available to us their shipboard temperature transect data. We thank Barry Lesht and the reviewers for suggestions to improve the manuscript. Wayne Burchat, Ontario Power Generation, provided data from their Darlington and Pickering ADCPs. REFERENCES Boyce, F.M., Donelan, M.A., Hamblin, P.F., Murthy, C.R., and Simons, T.J. 1989. Thermal structure and circulation in the Great Lakes. Atmos. Ocean. 27(4): 607–642. Emery, K.O., and Csanady, G.T. 1973. Surface circula-

tion of lake and nearly land locked seas. Proc. Natl. Acad. Sci., USA, 70:93–97. Palmentier, F.P., Taguchi, V.Y., Jenkins, W.D., Wang, D.T., Ngo, K.P., and Robinson, D. 1998. The determination of geosmin and 2-methylisoborneol in water using isotope dilution high resolution mass spectrometry. Wat. Res. 32:287–294. Rao, Y.R., and Murthy, C.R. 2001a. Coastal boundary layer characteristics during summer stratification in Lake Ontario. J. Phys. Oceanogr. 31:1088–1104. ——— , and Murthy, C.R. 2001b. Nearshore currents and turbulent exchange processes during upwelling and downwelling events in Lake Ontario. J. Geophys. Res. 106 C(2):2667–2678. Ridal, J.F., Brownlee, B., and Lean, D.R.S. 2000. Is Lake Ontario the source of taste and odor compounds to the Upper St. Lawrence river? J. Great Lakes Res. 26:315–322. Schwab, D.J., O’Connor, W.P., and Mellor, G. 1995. On the net cyclonic circulation in large stratified lakes. J. Phys. Oceanogr. 25:1516-1520. Simons, T.J., and Schertzer, W.M. 1989. The circulation of Lake Ontario during summer of 1982 and the winter of 1982/83. Scientific series, 171, National Water Research Institute, CCIW, Burlington. Watson, S.B., Charlton, M., Brownlee, B., Skafel, M.G., Howell, T., Moore, L., Ridal, J., and Zaitlin, B. 2002. Aquatic odor in Lake Ontario: tracing origins through an environmental maze. 37th Central Canadian Symposium on Water Pollution Research, Burlington, February 4–5, 2002. Wu, J. 1980. Wind-stress coefficients over sea surface near neutral conditions—A revisit. J. Phys. Oceanogr. 10:727–740. Young, W.F., Horth, H., Crane, R., Ogden, T., and Arnot, M. 1996. Taste and odor threshold concentrations of potential potable water contaminants. Wat. Res. 30:331–340. Submitted: 20 March 2002 Accepted: 25 September 2002 Editorial handling: Barry M. Lesht