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Residual chlorine decay in a broad, shallow river
War. Res. Vol. 27, No. 6, pp. 993-1001, 1993 Printed in Great Britain.All fightsrmerved
0043-13M/93 $6.00+ 0.00 Copyright © 1993 Pergamon Press Ltd
RESIDUAL CHLORINE DECAY IN A BROAD, SHALLOW RIVER G. D. M[LNEO, S. J. STANLEYand D. W. SmTH Environmental Science and Engineering, Department of Civil Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2(37 (First received February 1992; accepted in revised form December 1992) Almraet--Field investigations into the mixing and decay of discharge plumes containing total residual chlorine (TRC) concentrations of 2.0 mg/l were conducted in the North Saskatchewan River downstream from two water treatment plants in the City of Edmonton, Canada. Environmental concentrations of TRC within the receiving body were found to be a function of the hydraulic mixing characteristics of the receiving stream and of the chemical decay of residual chlorine. In-stream TRC concentration profiles resulting from steady-state discharges were monitored at downstream locations. Dilution of the plume by the receiving stream was quantified by calibrating a two-dimensional mixing model to the mixing characteri~'cs of the North Saskatchewan River using results obtained from steady-state dye-tracer studies using Rhodamine WT. First-order decay coefficients for TRC within the North Saskatchewan River were obtained by modelling the chlorinated plume mixing and fitting a first-order decay coefficient to the observed field data. Gross first-order d__.c~ycc~-flicients attributed to all chlorine sinks except dilution and adjusted to a standard temperature of 20°C were found to be, at two different locations, 20 and 28d -t. Key words--residual chlorine, cldoramines, field investigations, pollutant decay, Rhodamine WT, mixing, modelling
NOMENCLATURE C = c~entratiun (ms/l) C' ffidimenmoniem concentration C~ = completely-mixed pollutant concentration (mg/l) Co = initial concentration (ms/l) E~ = transverse mixing coe~icient (m3/s) h = local stream ~ p t h (m) k = first-order d_-c~__ycoemcient, base e (d -t) q = cumulative streamflow (m3/s) Q : stream discharge (m3/s) t =time (d) T = absolute temperature (K) u ffi local velocity component in the x direction (m/s) u. - bed shear velocity VOC ffivolatile organic carbon (rag/l) W - ~ream width (m) x = longitudinal distance (m) z =transverse distance (m) --norm.li~d ~tmulative di~harge 0 = v'ant Hoff-Arrhenins temperature correction factor
INTRODUCTION The widespread application of chlorine for disinfection purposes by water and wastewater treatment industries inevitably results in discharges of residual chlorine species into the environment, Drinking water treatment plants discharge residual chlorine in filter backwash and other process wustewaters, wastewater treatment plants occamonally p r a n c e effluent chlorination, and cooling water discharges from thermal power plants often contain chlorine residuals. The
biocidal and chemical properties of chlorine may result in adverse impacts to the receiving environment. The impact of these discharges is determined by the species and concentration of chlorine present, the duration of exposure and other environmental factors. Many regulatory agencies have established guidelines or limits which restrict the allowable concentration of total residual chlorine (TRC) within receiving water bodies. F o r the protection of aquatic life, a value of 0.002 mg/1 T R C has been adopted by Environment Canada as the maximum allowable concentration in receiving waters (Environment Canada, 1987). Similar standards have been adopted by the U.S. EPA and European agencies (Environment Canada, 1987). In large rivers, some agencies allow the use o f a mixing zone, or limited-nse zone, to provide an initial dilution of the effluent before imposing the above guidelines. These mixing zones, which are typically no more than 33% o f the stream width, are an area in which the specified standards may be exceeded. Beyond this zone, the pollutant concentration must be less than the applicable standard. Proper design of the outfall and impact assessment necessitates that in-stream concentrations at the boundaries of the mixing zone be predicted under various discharge, hydraulic and environmental conditions. The assessment and prediction of environmental chlorine concentrations necessitates knowledge of the chemical behaviour of chlorine within receiving streams.
993
994
G.D. MILNEet al.
The field of chlorine chemistry, especially when applied to water treatment applications, has been the subject of intensive study. Concerns about the health effects of chlorination by-products and engineering requirements to maximize the efficient use of chlorine have resulted in the compilation of a substantial volume of literature on chlorine chemistry. However, the focus of most of this research has been on chlorine reactions as applied to disinfection processes. While knowledge of chlorine disinfection chemistry within the relatively well-controlled environment of laboratory and water treatment applications may be approaching a satisfactory level, a comparable level of understanding within the natural environment has not yet been achieved. The high oxidation potential of chlorine, which makes it such a valuable tool in water treatment, severely complicates prediction of its behaviour within the highly diverse natural environment. Residual chlorine, which may be present as one or several chlorinated species, behaves as a non-conservative pollutant within natural waters, and residual concentrations are reduced through several chemical and physical pathways. Reactions with organic and inorganic compounds in the water column, volatilization, photo-degradation, adsorption and interactions with aquatic biota and the benthos may all contribute to the in-stream decay of chlorine residuals. Current knowledge of environmental rates of TRC decay, and of the dominant decay mechanisms within freshwaters remains at a very rudimentary level. This paper describes the results of research into the environmental rate of decay of residual chlorine in the North Saskatchewan River at Edmonton, Alberta, Canada. Instream concentration profiles resulting from steady-state discharges of potable water containing a total residual chlorine concentration of approx. 2.0 mg/i were monitored downstream from bank ouffalls at two water treatment plants. The large size of the river prevented complete mixing of the discharge with the receiving body, even at large distances downstream, resulting in TRC concentrations within the plume being a function of both turbulent mixing with the ambient water and of the rate of TRC decay. Physical mixing of the discharge with the receiving stream through diffusion and differential advection resulted in dilution of the effluent plume. Simultaneously, chemical decay of TRC was occurring. A two-dimensional finite-difference river mixing and pollutant decay model was used to analyse plume mixing under hydraulic conditions present at the time of the study. This model was calibrated using results obtained from steadystate dye tracer studies. The first-order rate coefficients for TRC decay downstream from the two water treatment plants were then determined by matching model predictions and the field data.
BACKGROUND
When discharged into freshwaters, chlorine may exist in molecular form, or it may combine with organic and inorganic compounds present within the water matrix. The chemical and toxicological properties of total chlorine residuals are a function of the concentration and speciation of chlorine present. Total chlorine residuals are composed of the free chlorine species, HOC! and OCI-, plus the combined chlorine residual, composed of the chloramines NH2CI, NHCi2 and NCI3 and organic chioramines. Chlorine speciation within wastewaters and freshwaters is determined primarily by aqueous chlorine and ammonia concentrations, temperature and solution pH. In waters with a sufficient chlorine:ammonia ratio (3:1 by mass), the total chlorine residual will essentially be composed of combined chlorine, primarily monochloramine (AWWA, 1990). The rate of formation of monochloramine from HOCI is rapid, with the maximum rate occurring at pH 8.4, where the forward rate constant is 3.8× 10~l/mol/s (AWWA, 1990). The rate of formation of the higher molecular mass chloramines is much slower, and they only appear in significant concentrations when the chlorine: ammonia ratio exceeds 5:1. Chlorine decay kinetics in natural waters are usually described as a first-order process following (Gowda, 1978; Reckhow et al., 1990; Jolley and Carpenter, 1983): c = Coe -k'
(1)
where C is the concentration at time t, Co is the initial concentration and k is the first-order reaction-rate coefficient. Limited information on the magnitude of the rate coefficient k is available for TRC decay within natural streams. Several researchers have attempted to quantify the environmental rate of decay of various chlorine species through laboratory studies (Heinemaun et al., 1981; Abdel-Gawad and Bewtra, 1988), but adequate simulation of the numerous physical and chemical mechanisms by which residual chlorine is depleted in a stream is difficult. The mechanisms influencing the rate of decay of chloramines are complex and site-specific, and laboratory tests may not accurately reproduce the chemical and physical processes occurring in the field. Reaction mechanisms such as water column demand may be adequately simulated in bottle studies, but decay pathways such as benthic demand and volatilization generally cannot be simulated properly. Research by Heinemann et al. (1981), Abdel-Gawad and Bewtra (1988) and Reckhow et aL (1990) suggests that water column demand may not be the predominant decay mechanism in natural streams. Benthic demand and volatilization are recognized as major s/nks for numerous aquatic contaminants, and may be important in the environmental decay of chlorine residuals. As the proportion of the overall decay rate that each pathway contributes is not yet well defined, it is
Residual chlorine decay in a broad, shallow fiver
995
Table 1. Chlorinedecayrates observedin fieldinvntiptlom of wastewatertreatmentplant discharges Chlorine k Depth species TRC TRC TRC TRC TRC
(d -I, 20~C, base e)
(In)
Study site
Reference
35.3-119.6 18.7-41.85 4.9-14.7 144 I00
0.35 0.20 0.22 0.25 Not specified
Boyne River, Ontario Aurora C3reek, Ontario (summer) Aurora Creek, Ontario (winter) Lamlmon Brook, Massachusetts Deep Brook, Connecticut
Gowda (1978) Gowda (1978) Gowda (1978) Reckhow e! ol. (1990) U.S. EPA (1984)
prudent that none of these possible mechanisms be neglected. Study of chlorine decay kinetics is further complicated by inadequacies in the available analytical methods. Practical analytical limitations currently enable measurements of total chlorine residuals to levels of approx. 0.01 mg/l ( A P H A et al., 1989) and no reliable method has been developed that will allow quantification at the low microgram per litre levels of the estabfished standards. The decay pathways and kinetics of hypochlorous acid, hypochlorite ion, and mono-, di- and trichloramine vary greatly, and it is difficult to reliably measure specific decay rates for individual species at concentrations of environmental concern.
A search of the literature indicates that only a small number of field investigations on the rate of decay of total chlorine species have been conducted. Gowda (1978, 1981), the U.S. EPA (1984) and Reckhow el al. (1990) conducted in-stream measurements of residual chlorine in wastewater treatment plant discharges. These studies yielded decay rates for total residual chlorine varying over several orders of magnitude; often in waters of seemingly similar qualities. Firstorder decay rate coefficients from these studies are summarized in Table 1. A discrepancy between laboratory and field results was also reported by the U.S. EPA (1984), which noted an apparent order-of-magnitude difference in TRC decay rate coefficients between laboratory and field studies. The wide variation in decay rates within the literature suggests that understanding of the various mechanisms governing TRC decay should be improved, and that site-specific field investigations are desirable to obtain rate coefficients in the specific environment of concern. The rate coefficient describing chlorine decay is temperature dependent, and is usually reported at a standard temperature of 20°C. This dependence is frequently represented by the v'ant Hoff-Arrhenius equation, simplified as: k2o "~ kr 0(2°- r)
(2)
where 0 is the v'ant Hoff-Arrhenius temperature correction factor and T is temperature in °C. Gowda (1978) reported 0 values for TRC ranging from 1.025 to 1.031 over a temperature range of 4-25°C and a pH range of 7.0-8.5. These values are typi~d of natural waters, and a value of 0 ffi 1.03 was recommended for most practical applications. All ratecoefficients determined in this study were converted
to standard temperature of 20°C using a value of 0 = 1.03. SITE CHAItACTERISTICS
The North Saskatchewan River at Edmonton is a major river in the Saskatchewan-Nelson River system. Water quality in the North Saskatchewan River is generally rated as good relative to Alberta and Canadian surface water quality guidelines (Hrudey, 1986). A large portion of the upstream watershed is uninhabited mountain and forest areas, with little municipal or industrial development. The major source of upstream pollution is runoff from forested and agricultural lands. Only one known point source of chlorine exists upstream of the study area. This is from a small secondary wastewater treatment plant at the town of Devon, 20 km upstream. With respect to this study, chlorine concentrations downstream from this plant are insignificant, and no background chlorine was detected during the field studies. General water quality parameters within the North Saskatchewan River at the time of the field studies are summarized in Table 2. Chlorinated waters for the studies were discharged from the City of Edmonton's Rossdale and E. L. Smith water treatment plants. The E. L. Smith plant is located in the west end of the city, and Rossdale is 17 km downstream in the city centre. These plants, each with a design capacity of 200 MI/d, provide drinking water for the City of Edmonton. Chlorine dioxide is used as the primary disinfectant, with chloramines added to maintain an effective residual within the distribution network. No detectable CIO2 residual is present within the discharged waters. The North Saskatchewan River at Edmonton is an entrenched single channel with a few mid-channel gravel bars. The channel meanders irregularly, with point and side bars and several bends undercutting high cliffs of weathered sandstone. The reach averaged sinuosity is approx 1.40, with an average Table 2. Water quality parameters in the North Saskatchewan River during field studies Parameter Temperature (°C) pH Turbidity (NTU) u.v. Aheorbanee (@200 nm) VOC (rag/I)
Colour (TCU)
Total hardness (mg/I M CaCO~) Total alkalinity (m$/I as CaCO3)
Rossdale 17 8.4 14.0 6.8 0.017
"/
157 134
E.L. Smith 14.2 8.3 5.0 4.4 <0.001
4
155 126
G. D. Mn.NE et aL
996
wavelength of 3.3 km (Kellerhals et al., 1972). Bed materials are primarily quartzite gravels, with a median particle size of 0.24 nun and a m o d a l size o f 16 mm (Anderson et al., 1986). Table 3 summarizes the m e a n hydraulic conditions---based u p o n the mean of data obtained at six cross sections---downstream from each of the two water treatment plants. Further details can be obtained in Milne (1991). Mean monthly discharge in the N o r t h Saskatchewan River ranges from 113 m3/s in January to 339 m3/s in August. The expected low flow, taken as the lowest 7 day flow with a 10 year return period (TQI0), is approx. 60 m3/s. The m e a n discharge over the period o f the tracer and chlorine plume tests, as measured at a Water Survey o f C a n a d a stream gauging station, was 196.6 and 196.8 m3/s respectively, at the Rossdale plant. Similarly, the discharges at the E. L. Smith plant were 145.6 and 146.8 m3/s. METHODS
(1) River mixing The large size of the North Saskatchewan River precludes complete mixing of the eltiucnt for up to 70 km downstream. Complete vertical mixing is generally assumed to occur within 100 ~nnnel depths downstream of the outfall, which would suggest that no vertical concentration gradients would exist within the plume beyond 150 m downstream of the ouffall. However, large transverse concentration gradients were expected throughout the domain of chlorine persistence. Through the combined action of dilution and decay, TRC residuals within the river were expected to drop below the analytical detection limit less than 5 km downstream of the ouffall. The reduction in TRC concentration within the plume resulting from dilution of the discharge waters by the main stream flow was quantified through the use of the steedy-state fiver mixing and pollutant transport model TRSMIX, which is briefly described below. The transverse growth of a conservative, neutrally bouyant pollutant which is completely mixed in the vertical is described by h OC + uh OC = 0 (hE, clC'~
(3)
where x represents lonsitudinai distance, z transverse distance, u and h streamwise mean velocity and depth, C the pollutant concentration and E: the transverse mixing coeificient, which includes both the effects of turbulent transverse diffusiun and transverse dispersion due to the presence of any mean secondary currents. Equation O) is applicable only to prismatic channels. To account for the effects of local variations in stream geometry, velocity and secondary currents which exist in natural channels, Yotsukura and Cobb (1972) introduced the concept of a dimensionless transverse coordinate system defmed as
q(z) =
uhdz
(4)
Table 3. Mean hydraulic properties of North Saskatchewan River at E. L. Smith and Roadale water treatment plants Parameter E.L. Smith WTP Roudale WTP Wklth (m) 145.0 158.2 Depth (m) 1.38 1.63 Velocity (m/s) 0.80 0.828 Slope 5..58 x 10-4 2.6 x 10-4 u. (m/s) 0.059 0.064
where q represents the cumulative strcamflow, z : 0 represents the left bank (looking downstream) and u and h respectively, denote the depth and local depth averaged velocity. At the right bank, z ffi IV, the total stream width, and q = Q, the total discharge. Introducing the above transformation into equation (3), assuming steady-state conditions, and with concentration (C') and transverse location (7) expressed in dimensionless forms, the two-dimensiunal, depth-averaged mixing equation reduces to
oc" = ± O (.h E Oc% o= Q=o~\ , ~]
(5)
where q ffi q /Q and C" = C /C=, where C® ffi the completelymixed pollutant concentration. Equation (5) can be modified to account for the non-conservative behaviour of a pollutant such as chlorine by inclusion of a first-order decay term as per equation (1).
OC" 1/ -~-x ~ u h
a C ' \ kC' E.-~--)- u
2
(6)
TRSMIX uses an implicit finite-difference solution of the two-dimensional, steady-state, advectiun-diffnsion equation [equation (6)] to predict transverse concentration profiles downstream of a pollutant discharge (Putz, 1984; Putz et al., 1984). It requires detailed field data on channel geometry and streamflow properties. This includes channel geometry and velocity profiles at a series of representative cross sections; a mixing coefficient at each cross section; a decay coefficient; and an initial input concentration distribution. Channel geometry downstream from both water treatment plants was determined from sonic sounding surveys conducted at several cross sections. These cross sections were selected to provide a representative description of the hydrauiic properties of the river over the expected domain of chlorine persistence. Transverse velocity profiles, channel slopes and other streamflow characteristics were measured at the time of the field studies. These data were used as controlling input for the model simulations. All necessary data for application of the TRSMIX model were known or measured, with the exception of local values of the transverse mixing coefficient E,. As accurate determination of E: can only be obtained by direct field measurement, steadystate dye tracer studies were conducted downstream from both water treatment plants to obtain these coefficients. The conservative dye tracer Rhodamine WT was selected for use in this study. The tracer study provided information on the local stream mixing characteristics downstream from both treatment plants, and served as a conservative tracer from which chlorine decay rates could be assessed. A 2% solution of Rhodamine WT was prepared in the laboratory and transported to the injection site. It was stored on-site in a covered 1001. polyethylene reservoir along the riverbank. Dye was injected immediately downstream of bank outfalls at each plant using a positive-displacement pump, at a flowrate calculated to provide a tracer concentration of 0.10#8/1 when fully-mixed with the river. The injection flowrate was measured periodically throughout the duration of the test, with no observed variation. Collection of tracer samples commenced once steady-state conditions were reached in the plume. Using a boat, samples were collected at various distances off the bank at each cross section. Transverse distance across the channel was measured using a Topofil®, a surveying instrument which uses a thin string to measure distances. Triplicate samples were collected at each point in 125 mi polyethylene bottles from a depth of approx. 150 nun. Each sample was collected individually to reduce the effects of local concentration variations due to turbulence. The samples were tagged with premarked labels and immediately placed in covered coolers to shield them from u.v. light. A series of samples was also obtained upstream from the injection point to quantify
997
Residual chlorine decay in a broad, shallow river background fluorescence levels at the test section. All tracer samples were analysed in the laboratory using a Fluorometer. Repficate samples were averaged to obtain a mean concentration at each sampling point.
then obtained by running the calibrated model using different decay-rate t o e . d e n t s and selecting the value that gave the best fit to the field data. The first-order rate coefficient describing the decay rate of (2) In-stream chlorine distributhm chloramines was assumed constant throughout the The determination of in-stream chlorine residuals within plume. This coefficient is primarily a function of the the North Saskatchewan River presented several practical water quality and hydraulic properties of the stream. problems. Through the combined effects of decay and dilution, residuals were anticipated to be very low. This, Within the relatively small study region described combined with the expected rapid decay of chlorine necessi- herein, the river composition is essentially uniform, tated that residual chlorine levelsbe measured immediately and the overall decay rate would be expected to be following sample collection. Therefore a mobile laboratory constant. The assumption of first-order chlorine decontaining all necessary equipment was established in one boat. The "low-level amperometric titration method for cay was supported by several laboratory studies, residual chlorine" (Standard Method 4500-C1 E) (AWNVA which indicated that residual chlorine was decaying et al., 1989) was selected to allow accurate residual measureaccording to equation (1). The results from tests ment at the low concentrations expected. This method is conducted downstream of the Rossdale and E. L. similar to the standard amperometric titration method, but Smith water treatment plants are presented below. uses a more dilute titrant and a graphical procedure to determine the endpoint. The low-level amperometric titration method is recommended for detection and quantifi- (1) Rossdale water treatment plant cation of chlorine residuals below 0.2 mg/l and is reported A steady-state dye tracer study using Rhodamine to allow measurement of chlorine concentrations as low as 10 #g/l (AWWA et ai., 1989). It is tittle affected by common WT was conducted downstream of the Rossdale oxidizing agents, turbidity, colour or temperature vari- water treatment plant in early September 1990. ations. Results are reported as a total chlorine residual, and Samples were collected from six cross sections, exit is not possible to differentiate between the various species tending over a distance of 4500 m downstream from of free and combined chlorine. Samples were analysed using a pair of Wallace and Tiernan Series A-790 amperometric the water treatment plant. Using measured velocity profiles and channel geometry data, a mass balance titrators with platinum electrodes. The method was modified slightly from that described in of Rhodamine WT flux, performed over the length o f Standard Method~. Sodium thiosulphate was substituted for the study reach, indicated the tracer did behave as a phenylarsine oxide as the titrant. These two chemicals have been reported by Aieta et al. (1984) as interchangeable in the conservative substance. Complete mixing over the amperometric titration method. The substitution was made depth of the channel was obtained within 20 m as past experience indicated that better performance at low downstream of the injection point. However, distinct chlorine levels could be obtained using sodium thiosulphate. transverse concentration profiles were observed over Using the above procedure, the in-field detection limit for the length of the study area. Transverse mixing total residual chlorine was approx. 0.01 mg/1. A similar method to that used for dye sample collection coefficients between cross sections were found to was used for collection of residual chlorine samples. To range between 0.016 and 0.893 m2/s. In the test region eliminate chlorine consumption in the sample containers, the North Saskatchewan River is quite sinuous, with samples were collected headspace free in 250 ml chlorine- four large changes in thalweg orientation. These demand-free glass bottles prepared according to Standard Method 4500-C1 C (AWWA et al., 1989). Immediately rapid changes in channel geometry, and the presence following collection, the samples were analysed for total of strong secondary currents could be the reason for residual chlorine in the portable laboratory. One boat was the wide range of observed mixing coefficients. dedicated to sample collection, and another was used for The tracer distribution predicted by T R S M I X at residual determination. Sample analysis was immediate, cross-section 3, 1400 m downstream of the outfall is with typical times from sample collection to completion of plotted in Fig. 1, along with the observed dye profile. the titration being in the order of I-3 rnin. The data are presented in a dimensionless form, where C" is the dimensionless concentration with RESULTS
The TRSMIX model was calibrated to the sitespecific mixing characteristics of the North Saskatchewan River using data obtained from the dye-tracer studies. All necessary data describing the geometric and flow properties of the River were available, with the exception of local values for the transverse mixing coefficient E~. As downstream tracer concentration profiles were available at each cross section, the transverse mixing coefficients were evaluated by visually selecting the value for which the model output most accurately simulated the field data. The mixing coefficient was allowed to vary between the cross sections to account for the variable flow characteristics typically encountered with bank discharges. The rate coefficient for TRC decay was
45 - o 40 ~35
o Observed
30 2 ' \
--.--
Predicted
2, 20 15lO 5 0
\ \ 0.02
'No 0.04
0.06
0.08 0.10
0.12
0.14
11 Fig. 1. Observod and pmticmi g h o d u ~ e WT tracer profile 1400m downstream of the RosMale water tr~tnumt plant.
G . D . M I L ~ et ai.
998
C' ffi 1.0 representing the fully-mixed concentration, and ¢ represents the cumulative discharge from the left bank normalized to the total stream discharge. This cross section is presented because the chlorine decay rate was determined using data at this location. Numerical restrictions within the model did not allow for the use of data collected upstream of this cross section because the plume width was too small relative to that of the river. The model predicted the transverse tracer plume profile at cross-section 3 reasonably well. At this point, the tracer plume was quite narrow and closely followed the contours of the left bank. Detectable tracer concentrations existed for less than 15 m off the hank; a distance that represented less than 10% of the total flow area. The day following the dye study, field investigations of chlorine persistence were conducted downstream from the plant. Little change in the river discharge occurred between the time of the tracer and the chlorine studies. A flowrate 0.376 m3/s of treated water was discharged from an outfall along the north bank at the Rossdale water treatment plant. The discharge waters contained a mean total chlorine residual of 2.12 mg/1 with monitored residuals ranging from 2.05 to 2.16 mg/l. The chlorine residual was comprised entirely of combined chlorine, primarily monochloramine, with no detectable free chlorine. Although no chlorine speciation was possible within the field, monochloramine probably remained the dominant species comprising the residual. Following the establishment of a steady-state plume, chlorine distributions downstream from the outfall were measured at each cross section, with the results summarized in Fig. 2. The plume stayed very close to the left bank, and detectable chlorine concentrations were found only within 25 m of the bank, representing less than 5% of the total flow area. Rapid dilution of the discharge waters with the river water and the chemical decay of TRC quickly reduced residual levels to below the analytical detection limit. Crosssection 3, 1400 m downstream, was the last section at which a measurable TRC concentration was observed. Samples collected further downstream exhibited no detectable TRC concentration. 0.9 0.8 0.7 "" 0.6
o Cross o Cross /~ Cross 0 Cross
•-~ 0.5 ~ 0.4 0.3
section section section section
1 ( I I 0 m) IA (380 m) 2 (640 m) 3 (1400 m)
0.2 o ~', 0.1
35 30 ~ , , , ~ I
~
o Observed -- k = 00.0 d -!
~ 2o 15 10 5 0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
11 Fig. 3. Predicted dimensionless chlorine concentration profile 1400m downstream of the Rossdale water treatment plant. First-order decay coefficients of 0.0 and 26.0d -I (k --0.0 and 28.0d-I at 20°C). Following analysis of the tracer data, the mixing behaviour of the plume was found to be reasonably well defined, and a conservative datum from which to assess chlorine decay rates could be established. The difference between the observed chlorine concentration and that predicted through the conservative mixing analysis might be attributed to chlorine decay within the stream. Proceeding on this basis, several simulations were made using the TRSMIX model. Iteratively, values of the first-order decay coefficient were selected until a best-fit to the field data was achieved. Numerical restrictions in the model did not permit concentration predictions immediately downstream of the discharge point and the results were compared at cross-section 3, 1400m downstream. Figure 3 shows the predicted chlorine concentration distributions and field data 1400m downstream of the outfall for various values of the decay coefficient. Where tripficate samples were available, the 90% confidence interval is provided for the field samples. A 90% level of significance was selected due to the limited sample size and resultant low degrees of freedom available. Interpretation of the data is hindered somewhat by the wide confidence intervals associated with some of the data points. The wide variance observed in some of the replicates is indicative of the turbulent nature of the stream, the close proximity of the plume to the stream boundaries, and the presence of localized swirling eddies. The decayrate coefficient providing the best-fit to the field data was found to be 26.0 d - k Correcting the observed value to standard temperature using equation (2), the observed rate of decay of TRC downstream from the Rossdale water treatment plant was found to be 28.0 d- i. (2) E. L. Smith water treatment plant
5
10
15
20
25
30
T r a n s v e r s e d i s t a n c e f r o m left b a n k ( m )
Fig. 2. Measured TRC concemration la'Ofiles 110, 380, 640 and 1400m downstream of the Ro~lale water treatment plant.
A similar series of field tests was conducted at the E. L. Smith water treatment plant in late September 1990. Hydraulic and geometric data at six cross sections over a 4 km distance downstream from the plant were gathered. Rhodamine WT was injected immediately downstream of the north bank plant
999
Residual chlorine decay in a broad, shallow river outfall. Results from the steady-state tracer study and subsequent TRSMIX predictions at cross-section 3, 1270m downstream, are presented in Fig. 4. Transverse mixing coefficients downstream form the E. L. Smith plant were found to range between 0.0173 and 0.0847 m2/s over the 3100 m section of channel studied. The channel downstream from the discharge point is horseshoe shaped, and did not have the highly variable profile characteristic of the Rossdale site. As a result, section-to-section variation in the mixing coefficients was not as great as that observed downstream of the Rossdale plant. The rate of decay of residual chlorine downstream from the E. L. Smith plant was similarly determined by discharging chlorinated potable water from a bank outfall at a rate of 0.376 m3/s. Mean TRC concentration in the discharge waters was 2.06 mg/l, and varied between 2.04 and 2.10mg/l. Similar to the Rossdale tests, this residual was composed entirely of chioramines. Downstream concentration profiles of TRC were measured at cross sections 125, 465, 1270 and 1750m downstream from the outfall, with the observed results summarized in Fig. 5. A rapid decrease in TRC concentration was also observed at E. L. Smith. At a distance of 1750m downstream from the ouffail, TRC levels were very near the detection limit of 0.01 mg/l. Cross-section 3, located 1270 m downstream from the discharge point, was selected to assess the rate of TRC decay. This section was chosen as agreement between the model tracer predictions and the field mixing results was good and the transverse concentration profile of the chlorine plume was well defined. Upstream of this point, the plume occupied too small a proportion of the total flow to enable accurate numerical simulation using TRSMIX. The first-order decay coefficient which provided the best fit to the observed data was determined in the same iterative manner, with k - - 1 7 . 0 d -~ yielding the best result (Fig. 6). Adjusting this value to a standard temperature of 20°C, the first-order decay coefficient for TRC downstream of the E. L. Smith water treatment plant was found to be 20 d-L
30
_
~ ~
~
•" u
n....o'" "' '.
'"'"q .""
o Cross section (125 m) \ n Cross section (465 m) \ /, Cross section \ (1270 m) ~ O Cross section ~(1750 m)
1 2 3 4
~0.4
0.2
~ = ~ = =_~_: ~ _ ~ , ~ . ~ _ . ~ , _ _ , , j
0
5
10
15
20
A
25
30
T r a n s v e r s e d i s t a n c e f r o m left b a n k (m) Fig. 5. Measured TRC concentration profiles 125, 465, 1270 and 1750 m downstream o f the E. L. Smith water treatment plant.
DISCUSSION
Chlorine concentrations reported herein are total residual chlorine, with no attempt to identify individual chlorine species. This restriction arose as the low-level amperometric titration method used for sample analysis could not differentiate between the various species which may have been present. Practically, this did not represent a large limitation, as the discharged waters were known to contain no free chlorine. This is a situation common to many chlorine discharges, because given a sufficient quantity of ammonia-nitrogen within the receiving body and at pH levels found in most natural waters, any available free chlorine will rapidly react to form monochloramine and higher chloramines. As the discharged waters were composed almost exclusively of monochloramine, the decay rates observed within the North Saskatchewan River represent the decay of monochloramine. However, as only the total chlorine residual was measured, this discussion will continue to refer to the results as TRC decay. Environmental rates of decay of total chlorine residuals resulting from discharges of chlorinated potable waters were determined in two independent field studies. The observed decay coefficients of 20 and 28 d-t represent the gross rate of reduction of TRC in the stream attributed to all mechanisms
30
o Observed '\o
---
20
"\
15
\
o\
10
5 0
~ / o ~
35F "
35 ~ 25 -
2.2 --~ 2.0 1.8 1.6 o 1.4 "~o 1.2 1.0 "~ 0.80.6
o Observed
25 I \
Predicted
,-O.0d-'
~ 20
. . . . .
-
10
',,,o ~.
I
1
0.02
0.04
5 I "~"-II-O----I,-.--.L
0.06 0.08
0.10
_.1
I
0.12 0.140.16
Fig. 4. Observed and predicted Rhodamine WT tracer profile 1270m downstream of the E.L. Smith water treatmerit plant.
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.06 0.09 0.10 TI
Fig. 6. Dimensionleu T R C concentration profile predicted 1270m downstream o f the E. L. Smith water treatment
plant outfall vs observed field data. Decay ~ t s of 0.0 and 17.0d -t (k --0.0 and 20.0d -I at 20°C).
1000
G.D. MILNEet al.
except dilution. Values of the decay-rate coefficient observed in the two tests fall within the broad range of values reported by other researchers (Table 1). Direct comparison of the results obtained in this study with others is difficult due to a lack of data on the environmental behaviour of chlorinated water treatment plant discharges. However, chlorinated discharge plumes from wastewater treatment plants have received some study, and limited field data do exist. The relatively large amount of organic matter present in these effluents suggests that more chloroorganic species may be present. However, monochloramine would probably still be the predominant species, possibly with relatively small concentrations of dichloramine and nitrogen trichloride, and the difference in decay rates between water treatment and wastewater treatment plant effluents would not be expected to be great. Water quality and hydrodynamic differences within the receiving stream would probably have a greater impact on the rate of decay than the small difference in residual chlorine speciation which would be expected between the two effluent types. In summer field tests conducted downstream of wastewater treatment plants on Aurora Creek, Ontario, Gowda (1978) reported first-order decay rates for TRC of 18.7-41.9d -m. These two rates represented adjacent river sections, and a value of 18.7 d -m was recommended as being representative of the reach as a whole. Winter studies at the same location yielded decay rates of 4.9-14.7d -m. Field studies on the Boyne River, Ontario, resulted in decay rates of 35.3-119.6 d -m. The decay observed by Gowda was that of chloramine species, with no free chlorine residual present. TRC decay rates observed in the North Saskatchewan River were found to fall within the range of those reported by Gowda. Field studies conducted by the Connecticut Department of Environmental Protection and reported by the U.S. EPA (1984) downstream of a secondary treatment plant exhibited first-order decay of residual chlorine of 100d -t. A more recent study downstream from a municipal wastewater treatment plant was reported by Reckhow et al. (1990). They measured TRC concentration profiles over a distance of 120 m below the ouffall, and found a gross first-order decay coefficient of 144 d-i. In all cases, monochloramine was found to be the dominant species present, and was usually the only species detected. Both the study reported by the U.S. EPA and that of Reckhow et al. were performed in small streams, where the effluents were assumed to be completely mixed with the receiving waters. This assumption may not have been entirely valid, and may have led to a slight overestimation of the decay rates. Comparison of the rates of TRC observed in this study with those reported by previous investigators suggests that TRC decay rates are generally not as rapid in the North Saskatchewan River as in other streams studied. Variations in water quality between
the different streams may account for some of the observed difference, but differences in the hydraulic regime and channel composition of the North Saskatchewan may also have been a significant factor influencing the decay rates. In a natural stream, several mechanisms may be responsible for the reduction in concentration of non-conservative pollutants. Volatilization, adsorption, photodegradation and direct chemical reaction with other compounds present within the system all contribute to the reduction of residual chlorine concentrations in natural water bodies. However, no field data are available to identify the dominant processes. While the physical characteristics of the stream are important, water quality, stream depth, the presence of an ice-cover, sediment composition, turbidity and many other factors may also greatly affect the rates of residual depletion through the above-mentioned pathways. CONCLUSION The small number of TRC decay-rate coefficients derived from field data indicate that rates vary widely, and that they appear to be dependent upon site-specific environmental and hydraulic conditions. TRC decay rates of 20 and 28 d- I ~ obserNed in the North Saskatchewan River. Laboratory studies may provide acceptable indications of chlorine decay rates attributable to direct chemical reactions, hut they do not adequately simulate the natural conditions of a stream, and may neglect important decay mechanisms. Until the specific environmental parametera governing TRC decay are more adequately identified, field estimates of chlorine decay rates will remain the only reliable method of selecting decay coefficients. Acknowledgements--Thh reseaw~ was funded by a research
contract from Water Plants Engineering, Publk Works Department, The City of Edmonton. The authors would also fike to thank Karen Erode, HongdeZhou and Sheldon Lovell of the Department of Civil Engineering, University of Alberta and Dave Andres of the Alberta Research Council for their technical and material assistance.
REFERENCES
Abdei-Gawad S. T. and Bewtra J. K. (1988) Decay of chlorine in diluted municipal effluents. Can. J. c/v. Enbn~g 15, 948-954. Aieta E. M., Roberts P. V. and Heraandez M. (1984) Determination of chlorine dioxide, chlorine, chlorite, and chlorate in water. J. Am. War. Wks Ass. 76, 64-70.
APHA, AWWA and WPCF (1989) Standard Methods for the Examination of Water and Wastewator, 17th edition. Amerkan Public Health Association, Alnerican Water Worka Aamciation, Water Pollution Control Federation, Washington, D.C. Amerk:an Water Works A~m¢iation (1990) Water Quality amt Treatm~t. MvGraw-Hill, New York. Anderson R. S., AndersunA. M., Ake~ A. M., Livingstone J. S., Maenda A., Mitchell P. A., Reynoldmn T. B., T~.w D. O. and Vukadinovic M. (1986) North Saskatchewan
Residual chlorine decay in a broad, shallow river River: Characterization of Water Quality in the Vicinity of Edmonton (1982-1983) Part L Water Quality Control Branch, Pollution Control Division, Environmental Protection Services, Alberta Environment, Edmonton, Alberta. Environment Canada (1987) Ca~_~dLon Water Quality Guidelines. Canadian Council of Resource and Environment Ministers, Water Quality Branch, Inland Waters D/rectorate, Environment Canada, Ottawa. Gowda T. P. H. (1978) Prediction of chlorine residuals in streams receiving sewage effluent. Water Resources Paper No. I0, Ontario Ministry of the Environment, Water Resources Branch, Toronto. Gowda T. P. H. (1981) Critical concentrations of toxic ammonia and chlorine in mixing zones of rivers. Brat. Pollut. Res. J. Can. 15, 255-270. Heinemann T. J., Lee, G. F., Jones R. A. and Newbry B. W, (1981) Summary of studies on modellin8 persistence of domestic wastewater chlorine in Colorado Front Range Rivers. In Water Chlorination: Environmental Impact and Health Effects ~,dited by Jolley R. L., Brungs W. A., Cotruvo J. A., Ctmnning R. B., Mattice J. S. and Jacobs V. A.), Vol. 4, pp.97-112. Ann Arbor Science, Ann Arbor, Mich. Hrudey S. E. (1986) A CriticalA.zsessmentof Drinking Water in Edmonton. Hmdey, Edmonton, Alberta. Jolley R. L. and Carpenter J. H. (1983) A review of the chemistry and environmental fate of reactive oxidant species in chlorinated water. In Water Chlorination: Environmental Impact and Health Effects (Edited by Jolley R. L., Brunge W. A., Cotruvo J. A., Cumming R. B.,
w127/¢t.--E
1001
Mattice J. S. and Jacobs V. A.), VoL 4, pp. 3-47. Ann Arbor Science, Ann Arbor, Mich. Kellerhals R., Neill C. R. and Bray D. I. (1972) Hydraulic and seomorphic characteristics of rivers in Alberta. Research Council of Alberta River Engineerin8 and Surface Hydrology Report 72-1, Alberta Research Council, Edmonton, Alberta. Milne G. D. (1991) Chlorine decay in a large river. M.Sc. thesis, University of Alberta, Edmonton, Alberta. Putz G. (1984) TRSMIX (Transverse Mixing Computer Model) Users Manua/, Environmental Engineering Technical Report 84-2, Department of Civil Engineering, University of Alberta, Edmonton, Alberta. Putz G., Smith D. W. and Gerard R. (1984) Application of a transverse mixing computer program to the prediction of effluent plume configuration. China Canada Workshop on Computer Solution Techniques, pp. 180-199. Canadian Society for Civil Engineering, Montreal, Quebec. Reckhow D. A., Ostendorf D. W. and Billa M. E. (1990) Fate and transport of combined residual chlorine in a small stream. J. envir. Engng Dio., Am. Soc. cir. F~q~s 116, (EE6) 1125-1142. U.S. EPA (1984) Technical Guidance Manual for Performing Wastelond Allocatimts, Book II, Chap. 3. EPA-440-4-S& 024, U.S. EPA O/rice of Water Regulations and Standards, Washington, D.C. Yotsukura N. and Cobb E. D. (1972) Transverse difus/on of solutes in natural streams. United States Geological Survey Professional Paper 582C, USGS, Washington, D.C.
0043-13M/93 $6.00+ 0.00 Copyright © 1993 Pergamon Press Ltd
RESIDUAL CHLORINE DECAY IN A BROAD, SHALLOW RIVER G. D. M[LNEO, S. J. STANLEYand D. W. SmTH Environmental Science and Engineering, Department of Civil Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2(37 (First received February 1992; accepted in revised form December 1992) Almraet--Field investigations into the mixing and decay of discharge plumes containing total residual chlorine (TRC) concentrations of 2.0 mg/l were conducted in the North Saskatchewan River downstream from two water treatment plants in the City of Edmonton, Canada. Environmental concentrations of TRC within the receiving body were found to be a function of the hydraulic mixing characteristics of the receiving stream and of the chemical decay of residual chlorine. In-stream TRC concentration profiles resulting from steady-state discharges were monitored at downstream locations. Dilution of the plume by the receiving stream was quantified by calibrating a two-dimensional mixing model to the mixing characteri~'cs of the North Saskatchewan River using results obtained from steady-state dye-tracer studies using Rhodamine WT. First-order decay coefficients for TRC within the North Saskatchewan River were obtained by modelling the chlorinated plume mixing and fitting a first-order decay coefficient to the observed field data. Gross first-order d__.c~ycc~-flicients attributed to all chlorine sinks except dilution and adjusted to a standard temperature of 20°C were found to be, at two different locations, 20 and 28d -t. Key words--residual chlorine, cldoramines, field investigations, pollutant decay, Rhodamine WT, mixing, modelling
NOMENCLATURE C = c~entratiun (ms/l) C' ffidimenmoniem concentration C~ = completely-mixed pollutant concentration (mg/l) Co = initial concentration (ms/l) E~ = transverse mixing coe~icient (m3/s) h = local stream ~ p t h (m) k = first-order d_-c~__ycoemcient, base e (d -t) q = cumulative streamflow (m3/s) Q : stream discharge (m3/s) t =time (d) T = absolute temperature (K) u ffi local velocity component in the x direction (m/s) u. - bed shear velocity VOC ffivolatile organic carbon (rag/l) W - ~ream width (m) x = longitudinal distance (m) z =transverse distance (m) --norm.li~d ~tmulative di~harge 0 = v'ant Hoff-Arrhenins temperature correction factor
INTRODUCTION The widespread application of chlorine for disinfection purposes by water and wastewater treatment industries inevitably results in discharges of residual chlorine species into the environment, Drinking water treatment plants discharge residual chlorine in filter backwash and other process wustewaters, wastewater treatment plants occamonally p r a n c e effluent chlorination, and cooling water discharges from thermal power plants often contain chlorine residuals. The
biocidal and chemical properties of chlorine may result in adverse impacts to the receiving environment. The impact of these discharges is determined by the species and concentration of chlorine present, the duration of exposure and other environmental factors. Many regulatory agencies have established guidelines or limits which restrict the allowable concentration of total residual chlorine (TRC) within receiving water bodies. F o r the protection of aquatic life, a value of 0.002 mg/1 T R C has been adopted by Environment Canada as the maximum allowable concentration in receiving waters (Environment Canada, 1987). Similar standards have been adopted by the U.S. EPA and European agencies (Environment Canada, 1987). In large rivers, some agencies allow the use o f a mixing zone, or limited-nse zone, to provide an initial dilution of the effluent before imposing the above guidelines. These mixing zones, which are typically no more than 33% o f the stream width, are an area in which the specified standards may be exceeded. Beyond this zone, the pollutant concentration must be less than the applicable standard. Proper design of the outfall and impact assessment necessitates that in-stream concentrations at the boundaries of the mixing zone be predicted under various discharge, hydraulic and environmental conditions. The assessment and prediction of environmental chlorine concentrations necessitates knowledge of the chemical behaviour of chlorine within receiving streams.
993
994
G.D. MILNEet al.
The field of chlorine chemistry, especially when applied to water treatment applications, has been the subject of intensive study. Concerns about the health effects of chlorination by-products and engineering requirements to maximize the efficient use of chlorine have resulted in the compilation of a substantial volume of literature on chlorine chemistry. However, the focus of most of this research has been on chlorine reactions as applied to disinfection processes. While knowledge of chlorine disinfection chemistry within the relatively well-controlled environment of laboratory and water treatment applications may be approaching a satisfactory level, a comparable level of understanding within the natural environment has not yet been achieved. The high oxidation potential of chlorine, which makes it such a valuable tool in water treatment, severely complicates prediction of its behaviour within the highly diverse natural environment. Residual chlorine, which may be present as one or several chlorinated species, behaves as a non-conservative pollutant within natural waters, and residual concentrations are reduced through several chemical and physical pathways. Reactions with organic and inorganic compounds in the water column, volatilization, photo-degradation, adsorption and interactions with aquatic biota and the benthos may all contribute to the in-stream decay of chlorine residuals. Current knowledge of environmental rates of TRC decay, and of the dominant decay mechanisms within freshwaters remains at a very rudimentary level. This paper describes the results of research into the environmental rate of decay of residual chlorine in the North Saskatchewan River at Edmonton, Alberta, Canada. Instream concentration profiles resulting from steady-state discharges of potable water containing a total residual chlorine concentration of approx. 2.0 mg/i were monitored downstream from bank ouffalls at two water treatment plants. The large size of the river prevented complete mixing of the discharge with the receiving body, even at large distances downstream, resulting in TRC concentrations within the plume being a function of both turbulent mixing with the ambient water and of the rate of TRC decay. Physical mixing of the discharge with the receiving stream through diffusion and differential advection resulted in dilution of the effluent plume. Simultaneously, chemical decay of TRC was occurring. A two-dimensional finite-difference river mixing and pollutant decay model was used to analyse plume mixing under hydraulic conditions present at the time of the study. This model was calibrated using results obtained from steadystate dye tracer studies. The first-order rate coefficients for TRC decay downstream from the two water treatment plants were then determined by matching model predictions and the field data.
BACKGROUND
When discharged into freshwaters, chlorine may exist in molecular form, or it may combine with organic and inorganic compounds present within the water matrix. The chemical and toxicological properties of total chlorine residuals are a function of the concentration and speciation of chlorine present. Total chlorine residuals are composed of the free chlorine species, HOC! and OCI-, plus the combined chlorine residual, composed of the chloramines NH2CI, NHCi2 and NCI3 and organic chioramines. Chlorine speciation within wastewaters and freshwaters is determined primarily by aqueous chlorine and ammonia concentrations, temperature and solution pH. In waters with a sufficient chlorine:ammonia ratio (3:1 by mass), the total chlorine residual will essentially be composed of combined chlorine, primarily monochloramine (AWWA, 1990). The rate of formation of monochloramine from HOCI is rapid, with the maximum rate occurring at pH 8.4, where the forward rate constant is 3.8× 10~l/mol/s (AWWA, 1990). The rate of formation of the higher molecular mass chloramines is much slower, and they only appear in significant concentrations when the chlorine: ammonia ratio exceeds 5:1. Chlorine decay kinetics in natural waters are usually described as a first-order process following (Gowda, 1978; Reckhow et al., 1990; Jolley and Carpenter, 1983): c = Coe -k'
(1)
where C is the concentration at time t, Co is the initial concentration and k is the first-order reaction-rate coefficient. Limited information on the magnitude of the rate coefficient k is available for TRC decay within natural streams. Several researchers have attempted to quantify the environmental rate of decay of various chlorine species through laboratory studies (Heinemaun et al., 1981; Abdel-Gawad and Bewtra, 1988), but adequate simulation of the numerous physical and chemical mechanisms by which residual chlorine is depleted in a stream is difficult. The mechanisms influencing the rate of decay of chloramines are complex and site-specific, and laboratory tests may not accurately reproduce the chemical and physical processes occurring in the field. Reaction mechanisms such as water column demand may be adequately simulated in bottle studies, but decay pathways such as benthic demand and volatilization generally cannot be simulated properly. Research by Heinemann et al. (1981), Abdel-Gawad and Bewtra (1988) and Reckhow et aL (1990) suggests that water column demand may not be the predominant decay mechanism in natural streams. Benthic demand and volatilization are recognized as major s/nks for numerous aquatic contaminants, and may be important in the environmental decay of chlorine residuals. As the proportion of the overall decay rate that each pathway contributes is not yet well defined, it is
Residual chlorine decay in a broad, shallow fiver
995
Table 1. Chlorinedecayrates observedin fieldinvntiptlom of wastewatertreatmentplant discharges Chlorine k Depth species TRC TRC TRC TRC TRC
(d -I, 20~C, base e)
(In)
Study site
Reference
35.3-119.6 18.7-41.85 4.9-14.7 144 I00
0.35 0.20 0.22 0.25 Not specified
Boyne River, Ontario Aurora C3reek, Ontario (summer) Aurora Creek, Ontario (winter) Lamlmon Brook, Massachusetts Deep Brook, Connecticut
Gowda (1978) Gowda (1978) Gowda (1978) Reckhow e! ol. (1990) U.S. EPA (1984)
prudent that none of these possible mechanisms be neglected. Study of chlorine decay kinetics is further complicated by inadequacies in the available analytical methods. Practical analytical limitations currently enable measurements of total chlorine residuals to levels of approx. 0.01 mg/l ( A P H A et al., 1989) and no reliable method has been developed that will allow quantification at the low microgram per litre levels of the estabfished standards. The decay pathways and kinetics of hypochlorous acid, hypochlorite ion, and mono-, di- and trichloramine vary greatly, and it is difficult to reliably measure specific decay rates for individual species at concentrations of environmental concern.
A search of the literature indicates that only a small number of field investigations on the rate of decay of total chlorine species have been conducted. Gowda (1978, 1981), the U.S. EPA (1984) and Reckhow el al. (1990) conducted in-stream measurements of residual chlorine in wastewater treatment plant discharges. These studies yielded decay rates for total residual chlorine varying over several orders of magnitude; often in waters of seemingly similar qualities. Firstorder decay rate coefficients from these studies are summarized in Table 1. A discrepancy between laboratory and field results was also reported by the U.S. EPA (1984), which noted an apparent order-of-magnitude difference in TRC decay rate coefficients between laboratory and field studies. The wide variation in decay rates within the literature suggests that understanding of the various mechanisms governing TRC decay should be improved, and that site-specific field investigations are desirable to obtain rate coefficients in the specific environment of concern. The rate coefficient describing chlorine decay is temperature dependent, and is usually reported at a standard temperature of 20°C. This dependence is frequently represented by the v'ant Hoff-Arrhenius equation, simplified as: k2o "~ kr 0(2°- r)
(2)
where 0 is the v'ant Hoff-Arrhenius temperature correction factor and T is temperature in °C. Gowda (1978) reported 0 values for TRC ranging from 1.025 to 1.031 over a temperature range of 4-25°C and a pH range of 7.0-8.5. These values are typi~d of natural waters, and a value of 0 ffi 1.03 was recommended for most practical applications. All ratecoefficients determined in this study were converted
to standard temperature of 20°C using a value of 0 = 1.03. SITE CHAItACTERISTICS
The North Saskatchewan River at Edmonton is a major river in the Saskatchewan-Nelson River system. Water quality in the North Saskatchewan River is generally rated as good relative to Alberta and Canadian surface water quality guidelines (Hrudey, 1986). A large portion of the upstream watershed is uninhabited mountain and forest areas, with little municipal or industrial development. The major source of upstream pollution is runoff from forested and agricultural lands. Only one known point source of chlorine exists upstream of the study area. This is from a small secondary wastewater treatment plant at the town of Devon, 20 km upstream. With respect to this study, chlorine concentrations downstream from this plant are insignificant, and no background chlorine was detected during the field studies. General water quality parameters within the North Saskatchewan River at the time of the field studies are summarized in Table 2. Chlorinated waters for the studies were discharged from the City of Edmonton's Rossdale and E. L. Smith water treatment plants. The E. L. Smith plant is located in the west end of the city, and Rossdale is 17 km downstream in the city centre. These plants, each with a design capacity of 200 MI/d, provide drinking water for the City of Edmonton. Chlorine dioxide is used as the primary disinfectant, with chloramines added to maintain an effective residual within the distribution network. No detectable CIO2 residual is present within the discharged waters. The North Saskatchewan River at Edmonton is an entrenched single channel with a few mid-channel gravel bars. The channel meanders irregularly, with point and side bars and several bends undercutting high cliffs of weathered sandstone. The reach averaged sinuosity is approx 1.40, with an average Table 2. Water quality parameters in the North Saskatchewan River during field studies Parameter Temperature (°C) pH Turbidity (NTU) u.v. Aheorbanee (@200 nm) VOC (rag/I)
Colour (TCU)
Total hardness (mg/I M CaCO~) Total alkalinity (m$/I as CaCO3)
Rossdale 17 8.4 14.0 6.8 0.017
"/
157 134
E.L. Smith 14.2 8.3 5.0 4.4 <0.001
4
155 126
G. D. Mn.NE et aL
996
wavelength of 3.3 km (Kellerhals et al., 1972). Bed materials are primarily quartzite gravels, with a median particle size of 0.24 nun and a m o d a l size o f 16 mm (Anderson et al., 1986). Table 3 summarizes the m e a n hydraulic conditions---based u p o n the mean of data obtained at six cross sections---downstream from each of the two water treatment plants. Further details can be obtained in Milne (1991). Mean monthly discharge in the N o r t h Saskatchewan River ranges from 113 m3/s in January to 339 m3/s in August. The expected low flow, taken as the lowest 7 day flow with a 10 year return period (TQI0), is approx. 60 m3/s. The m e a n discharge over the period o f the tracer and chlorine plume tests, as measured at a Water Survey o f C a n a d a stream gauging station, was 196.6 and 196.8 m3/s respectively, at the Rossdale plant. Similarly, the discharges at the E. L. Smith plant were 145.6 and 146.8 m3/s. METHODS
(1) River mixing The large size of the North Saskatchewan River precludes complete mixing of the eltiucnt for up to 70 km downstream. Complete vertical mixing is generally assumed to occur within 100 ~nnnel depths downstream of the outfall, which would suggest that no vertical concentration gradients would exist within the plume beyond 150 m downstream of the ouffall. However, large transverse concentration gradients were expected throughout the domain of chlorine persistence. Through the combined action of dilution and decay, TRC residuals within the river were expected to drop below the analytical detection limit less than 5 km downstream of the ouffall. The reduction in TRC concentration within the plume resulting from dilution of the discharge waters by the main stream flow was quantified through the use of the steedy-state fiver mixing and pollutant transport model TRSMIX, which is briefly described below. The transverse growth of a conservative, neutrally bouyant pollutant which is completely mixed in the vertical is described by h OC + uh OC = 0 (hE, clC'~
(3)
where x represents lonsitudinai distance, z transverse distance, u and h streamwise mean velocity and depth, C the pollutant concentration and E: the transverse mixing coeificient, which includes both the effects of turbulent transverse diffusiun and transverse dispersion due to the presence of any mean secondary currents. Equation O) is applicable only to prismatic channels. To account for the effects of local variations in stream geometry, velocity and secondary currents which exist in natural channels, Yotsukura and Cobb (1972) introduced the concept of a dimensionless transverse coordinate system defmed as
q(z) =
uhdz
(4)
Table 3. Mean hydraulic properties of North Saskatchewan River at E. L. Smith and Roadale water treatment plants Parameter E.L. Smith WTP Roudale WTP Wklth (m) 145.0 158.2 Depth (m) 1.38 1.63 Velocity (m/s) 0.80 0.828 Slope 5..58 x 10-4 2.6 x 10-4 u. (m/s) 0.059 0.064
where q represents the cumulative strcamflow, z : 0 represents the left bank (looking downstream) and u and h respectively, denote the depth and local depth averaged velocity. At the right bank, z ffi IV, the total stream width, and q = Q, the total discharge. Introducing the above transformation into equation (3), assuming steady-state conditions, and with concentration (C') and transverse location (7) expressed in dimensionless forms, the two-dimensiunal, depth-averaged mixing equation reduces to
oc" = ± O (.h E Oc% o= Q=o~\ , ~]
(5)
where q ffi q /Q and C" = C /C=, where C® ffi the completelymixed pollutant concentration. Equation (5) can be modified to account for the non-conservative behaviour of a pollutant such as chlorine by inclusion of a first-order decay term as per equation (1).
OC" 1/ -~-x ~ u h
a C ' \ kC' E.-~--)- u
2
(6)
TRSMIX uses an implicit finite-difference solution of the two-dimensional, steady-state, advectiun-diffnsion equation [equation (6)] to predict transverse concentration profiles downstream of a pollutant discharge (Putz, 1984; Putz et al., 1984). It requires detailed field data on channel geometry and streamflow properties. This includes channel geometry and velocity profiles at a series of representative cross sections; a mixing coefficient at each cross section; a decay coefficient; and an initial input concentration distribution. Channel geometry downstream from both water treatment plants was determined from sonic sounding surveys conducted at several cross sections. These cross sections were selected to provide a representative description of the hydrauiic properties of the river over the expected domain of chlorine persistence. Transverse velocity profiles, channel slopes and other streamflow characteristics were measured at the time of the field studies. These data were used as controlling input for the model simulations. All necessary data for application of the TRSMIX model were known or measured, with the exception of local values of the transverse mixing coefficient E,. As accurate determination of E: can only be obtained by direct field measurement, steadystate dye tracer studies were conducted downstream from both water treatment plants to obtain these coefficients. The conservative dye tracer Rhodamine WT was selected for use in this study. The tracer study provided information on the local stream mixing characteristics downstream from both treatment plants, and served as a conservative tracer from which chlorine decay rates could be assessed. A 2% solution of Rhodamine WT was prepared in the laboratory and transported to the injection site. It was stored on-site in a covered 1001. polyethylene reservoir along the riverbank. Dye was injected immediately downstream of bank outfalls at each plant using a positive-displacement pump, at a flowrate calculated to provide a tracer concentration of 0.10#8/1 when fully-mixed with the river. The injection flowrate was measured periodically throughout the duration of the test, with no observed variation. Collection of tracer samples commenced once steady-state conditions were reached in the plume. Using a boat, samples were collected at various distances off the bank at each cross section. Transverse distance across the channel was measured using a Topofil®, a surveying instrument which uses a thin string to measure distances. Triplicate samples were collected at each point in 125 mi polyethylene bottles from a depth of approx. 150 nun. Each sample was collected individually to reduce the effects of local concentration variations due to turbulence. The samples were tagged with premarked labels and immediately placed in covered coolers to shield them from u.v. light. A series of samples was also obtained upstream from the injection point to quantify
997
Residual chlorine decay in a broad, shallow river background fluorescence levels at the test section. All tracer samples were analysed in the laboratory using a Fluorometer. Repficate samples were averaged to obtain a mean concentration at each sampling point.
then obtained by running the calibrated model using different decay-rate t o e . d e n t s and selecting the value that gave the best fit to the field data. The first-order rate coefficient describing the decay rate of (2) In-stream chlorine distributhm chloramines was assumed constant throughout the The determination of in-stream chlorine residuals within plume. This coefficient is primarily a function of the the North Saskatchewan River presented several practical water quality and hydraulic properties of the stream. problems. Through the combined effects of decay and dilution, residuals were anticipated to be very low. This, Within the relatively small study region described combined with the expected rapid decay of chlorine necessi- herein, the river composition is essentially uniform, tated that residual chlorine levelsbe measured immediately and the overall decay rate would be expected to be following sample collection. Therefore a mobile laboratory constant. The assumption of first-order chlorine decontaining all necessary equipment was established in one boat. The "low-level amperometric titration method for cay was supported by several laboratory studies, residual chlorine" (Standard Method 4500-C1 E) (AWNVA which indicated that residual chlorine was decaying et al., 1989) was selected to allow accurate residual measureaccording to equation (1). The results from tests ment at the low concentrations expected. This method is conducted downstream of the Rossdale and E. L. similar to the standard amperometric titration method, but Smith water treatment plants are presented below. uses a more dilute titrant and a graphical procedure to determine the endpoint. The low-level amperometric titration method is recommended for detection and quantifi- (1) Rossdale water treatment plant cation of chlorine residuals below 0.2 mg/l and is reported A steady-state dye tracer study using Rhodamine to allow measurement of chlorine concentrations as low as 10 #g/l (AWWA et ai., 1989). It is tittle affected by common WT was conducted downstream of the Rossdale oxidizing agents, turbidity, colour or temperature vari- water treatment plant in early September 1990. ations. Results are reported as a total chlorine residual, and Samples were collected from six cross sections, exit is not possible to differentiate between the various species tending over a distance of 4500 m downstream from of free and combined chlorine. Samples were analysed using a pair of Wallace and Tiernan Series A-790 amperometric the water treatment plant. Using measured velocity profiles and channel geometry data, a mass balance titrators with platinum electrodes. The method was modified slightly from that described in of Rhodamine WT flux, performed over the length o f Standard Method~. Sodium thiosulphate was substituted for the study reach, indicated the tracer did behave as a phenylarsine oxide as the titrant. These two chemicals have been reported by Aieta et al. (1984) as interchangeable in the conservative substance. Complete mixing over the amperometric titration method. The substitution was made depth of the channel was obtained within 20 m as past experience indicated that better performance at low downstream of the injection point. However, distinct chlorine levels could be obtained using sodium thiosulphate. transverse concentration profiles were observed over Using the above procedure, the in-field detection limit for the length of the study area. Transverse mixing total residual chlorine was approx. 0.01 mg/1. A similar method to that used for dye sample collection coefficients between cross sections were found to was used for collection of residual chlorine samples. To range between 0.016 and 0.893 m2/s. In the test region eliminate chlorine consumption in the sample containers, the North Saskatchewan River is quite sinuous, with samples were collected headspace free in 250 ml chlorine- four large changes in thalweg orientation. These demand-free glass bottles prepared according to Standard Method 4500-C1 C (AWWA et al., 1989). Immediately rapid changes in channel geometry, and the presence following collection, the samples were analysed for total of strong secondary currents could be the reason for residual chlorine in the portable laboratory. One boat was the wide range of observed mixing coefficients. dedicated to sample collection, and another was used for The tracer distribution predicted by T R S M I X at residual determination. Sample analysis was immediate, cross-section 3, 1400 m downstream of the outfall is with typical times from sample collection to completion of plotted in Fig. 1, along with the observed dye profile. the titration being in the order of I-3 rnin. The data are presented in a dimensionless form, where C" is the dimensionless concentration with RESULTS
The TRSMIX model was calibrated to the sitespecific mixing characteristics of the North Saskatchewan River using data obtained from the dye-tracer studies. All necessary data describing the geometric and flow properties of the River were available, with the exception of local values for the transverse mixing coefficient E~. As downstream tracer concentration profiles were available at each cross section, the transverse mixing coefficients were evaluated by visually selecting the value for which the model output most accurately simulated the field data. The mixing coefficient was allowed to vary between the cross sections to account for the variable flow characteristics typically encountered with bank discharges. The rate coefficient for TRC decay was
45 - o 40 ~35
o Observed
30 2 ' \
--.--
Predicted
2, 20 15lO 5 0
\ \ 0.02
'No 0.04
0.06
0.08 0.10
0.12
0.14
11 Fig. 1. Observod and pmticmi g h o d u ~ e WT tracer profile 1400m downstream of the RosMale water tr~tnumt plant.
G . D . M I L ~ et ai.
998
C' ffi 1.0 representing the fully-mixed concentration, and ¢ represents the cumulative discharge from the left bank normalized to the total stream discharge. This cross section is presented because the chlorine decay rate was determined using data at this location. Numerical restrictions within the model did not allow for the use of data collected upstream of this cross section because the plume width was too small relative to that of the river. The model predicted the transverse tracer plume profile at cross-section 3 reasonably well. At this point, the tracer plume was quite narrow and closely followed the contours of the left bank. Detectable tracer concentrations existed for less than 15 m off the hank; a distance that represented less than 10% of the total flow area. The day following the dye study, field investigations of chlorine persistence were conducted downstream from the plant. Little change in the river discharge occurred between the time of the tracer and the chlorine studies. A flowrate 0.376 m3/s of treated water was discharged from an outfall along the north bank at the Rossdale water treatment plant. The discharge waters contained a mean total chlorine residual of 2.12 mg/1 with monitored residuals ranging from 2.05 to 2.16 mg/l. The chlorine residual was comprised entirely of combined chlorine, primarily monochloramine, with no detectable free chlorine. Although no chlorine speciation was possible within the field, monochloramine probably remained the dominant species comprising the residual. Following the establishment of a steady-state plume, chlorine distributions downstream from the outfall were measured at each cross section, with the results summarized in Fig. 2. The plume stayed very close to the left bank, and detectable chlorine concentrations were found only within 25 m of the bank, representing less than 5% of the total flow area. Rapid dilution of the discharge waters with the river water and the chemical decay of TRC quickly reduced residual levels to below the analytical detection limit. Crosssection 3, 1400 m downstream, was the last section at which a measurable TRC concentration was observed. Samples collected further downstream exhibited no detectable TRC concentration. 0.9 0.8 0.7 "" 0.6
o Cross o Cross /~ Cross 0 Cross
•-~ 0.5 ~ 0.4 0.3
section section section section
1 ( I I 0 m) IA (380 m) 2 (640 m) 3 (1400 m)
0.2 o ~', 0.1
35 30 ~ , , , ~ I
~
o Observed -- k = 00.0 d -!
~ 2o 15 10 5 0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
11 Fig. 3. Predicted dimensionless chlorine concentration profile 1400m downstream of the Rossdale water treatment plant. First-order decay coefficients of 0.0 and 26.0d -I (k --0.0 and 28.0d-I at 20°C). Following analysis of the tracer data, the mixing behaviour of the plume was found to be reasonably well defined, and a conservative datum from which to assess chlorine decay rates could be established. The difference between the observed chlorine concentration and that predicted through the conservative mixing analysis might be attributed to chlorine decay within the stream. Proceeding on this basis, several simulations were made using the TRSMIX model. Iteratively, values of the first-order decay coefficient were selected until a best-fit to the field data was achieved. Numerical restrictions in the model did not permit concentration predictions immediately downstream of the discharge point and the results were compared at cross-section 3, 1400m downstream. Figure 3 shows the predicted chlorine concentration distributions and field data 1400m downstream of the outfall for various values of the decay coefficient. Where tripficate samples were available, the 90% confidence interval is provided for the field samples. A 90% level of significance was selected due to the limited sample size and resultant low degrees of freedom available. Interpretation of the data is hindered somewhat by the wide confidence intervals associated with some of the data points. The wide variance observed in some of the replicates is indicative of the turbulent nature of the stream, the close proximity of the plume to the stream boundaries, and the presence of localized swirling eddies. The decayrate coefficient providing the best-fit to the field data was found to be 26.0 d - k Correcting the observed value to standard temperature using equation (2), the observed rate of decay of TRC downstream from the Rossdale water treatment plant was found to be 28.0 d- i. (2) E. L. Smith water treatment plant
5
10
15
20
25
30
T r a n s v e r s e d i s t a n c e f r o m left b a n k ( m )
Fig. 2. Measured TRC concemration la'Ofiles 110, 380, 640 and 1400m downstream of the Ro~lale water treatment plant.
A similar series of field tests was conducted at the E. L. Smith water treatment plant in late September 1990. Hydraulic and geometric data at six cross sections over a 4 km distance downstream from the plant were gathered. Rhodamine WT was injected immediately downstream of the north bank plant
999
Residual chlorine decay in a broad, shallow river outfall. Results from the steady-state tracer study and subsequent TRSMIX predictions at cross-section 3, 1270m downstream, are presented in Fig. 4. Transverse mixing coefficients downstream form the E. L. Smith plant were found to range between 0.0173 and 0.0847 m2/s over the 3100 m section of channel studied. The channel downstream from the discharge point is horseshoe shaped, and did not have the highly variable profile characteristic of the Rossdale site. As a result, section-to-section variation in the mixing coefficients was not as great as that observed downstream of the Rossdale plant. The rate of decay of residual chlorine downstream from the E. L. Smith plant was similarly determined by discharging chlorinated potable water from a bank outfall at a rate of 0.376 m3/s. Mean TRC concentration in the discharge waters was 2.06 mg/l, and varied between 2.04 and 2.10mg/l. Similar to the Rossdale tests, this residual was composed entirely of chioramines. Downstream concentration profiles of TRC were measured at cross sections 125, 465, 1270 and 1750m downstream from the outfall, with the observed results summarized in Fig. 5. A rapid decrease in TRC concentration was also observed at E. L. Smith. At a distance of 1750m downstream from the ouffail, TRC levels were very near the detection limit of 0.01 mg/l. Cross-section 3, located 1270 m downstream from the discharge point, was selected to assess the rate of TRC decay. This section was chosen as agreement between the model tracer predictions and the field mixing results was good and the transverse concentration profile of the chlorine plume was well defined. Upstream of this point, the plume occupied too small a proportion of the total flow to enable accurate numerical simulation using TRSMIX. The first-order decay coefficient which provided the best fit to the observed data was determined in the same iterative manner, with k - - 1 7 . 0 d -~ yielding the best result (Fig. 6). Adjusting this value to a standard temperature of 20°C, the first-order decay coefficient for TRC downstream of the E. L. Smith water treatment plant was found to be 20 d-L
30
_
~ ~
~
•" u
n....o'" "' '.
'"'"q .""
o Cross section (125 m) \ n Cross section (465 m) \ /, Cross section \ (1270 m) ~ O Cross section ~(1750 m)
1 2 3 4
~0.4
0.2
~ = ~ = =_~_: ~ _ ~ , ~ . ~ _ . ~ , _ _ , , j
0
5
10
15
20
A
25
30
T r a n s v e r s e d i s t a n c e f r o m left b a n k (m) Fig. 5. Measured TRC concentration profiles 125, 465, 1270 and 1750 m downstream o f the E. L. Smith water treatment plant.
DISCUSSION
Chlorine concentrations reported herein are total residual chlorine, with no attempt to identify individual chlorine species. This restriction arose as the low-level amperometric titration method used for sample analysis could not differentiate between the various species which may have been present. Practically, this did not represent a large limitation, as the discharged waters were known to contain no free chlorine. This is a situation common to many chlorine discharges, because given a sufficient quantity of ammonia-nitrogen within the receiving body and at pH levels found in most natural waters, any available free chlorine will rapidly react to form monochloramine and higher chloramines. As the discharged waters were composed almost exclusively of monochloramine, the decay rates observed within the North Saskatchewan River represent the decay of monochloramine. However, as only the total chlorine residual was measured, this discussion will continue to refer to the results as TRC decay. Environmental rates of decay of total chlorine residuals resulting from discharges of chlorinated potable waters were determined in two independent field studies. The observed decay coefficients of 20 and 28 d-t represent the gross rate of reduction of TRC in the stream attributed to all mechanisms
30
o Observed '\o
---
20
"\
15
\
o\
10
5 0
~ / o ~
35F "
35 ~ 25 -
2.2 --~ 2.0 1.8 1.6 o 1.4 "~o 1.2 1.0 "~ 0.80.6
o Observed
25 I \
Predicted
,-O.0d-'
~ 20
. . . . .
-
10
',,,o ~.
I
1
0.02
0.04
5 I "~"-II-O----I,-.--.L
0.06 0.08
0.10
_.1
I
0.12 0.140.16
Fig. 4. Observed and predicted Rhodamine WT tracer profile 1270m downstream of the E.L. Smith water treatmerit plant.
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.06 0.09 0.10 TI
Fig. 6. Dimensionleu T R C concentration profile predicted 1270m downstream o f the E. L. Smith water treatment
plant outfall vs observed field data. Decay ~ t s of 0.0 and 17.0d -t (k --0.0 and 20.0d -I at 20°C).
1000
G.D. MILNEet al.
except dilution. Values of the decay-rate coefficient observed in the two tests fall within the broad range of values reported by other researchers (Table 1). Direct comparison of the results obtained in this study with others is difficult due to a lack of data on the environmental behaviour of chlorinated water treatment plant discharges. However, chlorinated discharge plumes from wastewater treatment plants have received some study, and limited field data do exist. The relatively large amount of organic matter present in these effluents suggests that more chloroorganic species may be present. However, monochloramine would probably still be the predominant species, possibly with relatively small concentrations of dichloramine and nitrogen trichloride, and the difference in decay rates between water treatment and wastewater treatment plant effluents would not be expected to be great. Water quality and hydrodynamic differences within the receiving stream would probably have a greater impact on the rate of decay than the small difference in residual chlorine speciation which would be expected between the two effluent types. In summer field tests conducted downstream of wastewater treatment plants on Aurora Creek, Ontario, Gowda (1978) reported first-order decay rates for TRC of 18.7-41.9d -m. These two rates represented adjacent river sections, and a value of 18.7 d -m was recommended as being representative of the reach as a whole. Winter studies at the same location yielded decay rates of 4.9-14.7d -m. Field studies on the Boyne River, Ontario, resulted in decay rates of 35.3-119.6 d -m. The decay observed by Gowda was that of chloramine species, with no free chlorine residual present. TRC decay rates observed in the North Saskatchewan River were found to fall within the range of those reported by Gowda. Field studies conducted by the Connecticut Department of Environmental Protection and reported by the U.S. EPA (1984) downstream of a secondary treatment plant exhibited first-order decay of residual chlorine of 100d -t. A more recent study downstream from a municipal wastewater treatment plant was reported by Reckhow et al. (1990). They measured TRC concentration profiles over a distance of 120 m below the ouffall, and found a gross first-order decay coefficient of 144 d-i. In all cases, monochloramine was found to be the dominant species present, and was usually the only species detected. Both the study reported by the U.S. EPA and that of Reckhow et al. were performed in small streams, where the effluents were assumed to be completely mixed with the receiving waters. This assumption may not have been entirely valid, and may have led to a slight overestimation of the decay rates. Comparison of the rates of TRC observed in this study with those reported by previous investigators suggests that TRC decay rates are generally not as rapid in the North Saskatchewan River as in other streams studied. Variations in water quality between
the different streams may account for some of the observed difference, but differences in the hydraulic regime and channel composition of the North Saskatchewan may also have been a significant factor influencing the decay rates. In a natural stream, several mechanisms may be responsible for the reduction in concentration of non-conservative pollutants. Volatilization, adsorption, photodegradation and direct chemical reaction with other compounds present within the system all contribute to the reduction of residual chlorine concentrations in natural water bodies. However, no field data are available to identify the dominant processes. While the physical characteristics of the stream are important, water quality, stream depth, the presence of an ice-cover, sediment composition, turbidity and many other factors may also greatly affect the rates of residual depletion through the above-mentioned pathways. CONCLUSION The small number of TRC decay-rate coefficients derived from field data indicate that rates vary widely, and that they appear to be dependent upon site-specific environmental and hydraulic conditions. TRC decay rates of 20 and 28 d- I ~ obserNed in the North Saskatchewan River. Laboratory studies may provide acceptable indications of chlorine decay rates attributable to direct chemical reactions, hut they do not adequately simulate the natural conditions of a stream, and may neglect important decay mechanisms. Until the specific environmental parametera governing TRC decay are more adequately identified, field estimates of chlorine decay rates will remain the only reliable method of selecting decay coefficients. Acknowledgements--Thh reseaw~ was funded by a research
contract from Water Plants Engineering, Publk Works Department, The City of Edmonton. The authors would also fike to thank Karen Erode, HongdeZhou and Sheldon Lovell of the Department of Civil Engineering, University of Alberta and Dave Andres of the Alberta Research Council for their technical and material assistance.
REFERENCES
Abdei-Gawad S. T. and Bewtra J. K. (1988) Decay of chlorine in diluted municipal effluents. Can. J. c/v. Enbn~g 15, 948-954. Aieta E. M., Roberts P. V. and Heraandez M. (1984) Determination of chlorine dioxide, chlorine, chlorite, and chlorate in water. J. Am. War. Wks Ass. 76, 64-70.
APHA, AWWA and WPCF (1989) Standard Methods for the Examination of Water and Wastewator, 17th edition. Amerkan Public Health Association, Alnerican Water Worka Aamciation, Water Pollution Control Federation, Washington, D.C. Amerk:an Water Works A~m¢iation (1990) Water Quality amt Treatm~t. MvGraw-Hill, New York. Anderson R. S., AndersunA. M., Ake~ A. M., Livingstone J. S., Maenda A., Mitchell P. A., Reynoldmn T. B., T~.w D. O. and Vukadinovic M. (1986) North Saskatchewan
Residual chlorine decay in a broad, shallow river River: Characterization of Water Quality in the Vicinity of Edmonton (1982-1983) Part L Water Quality Control Branch, Pollution Control Division, Environmental Protection Services, Alberta Environment, Edmonton, Alberta. Environment Canada (1987) Ca~_~dLon Water Quality Guidelines. Canadian Council of Resource and Environment Ministers, Water Quality Branch, Inland Waters D/rectorate, Environment Canada, Ottawa. Gowda T. P. H. (1978) Prediction of chlorine residuals in streams receiving sewage effluent. Water Resources Paper No. I0, Ontario Ministry of the Environment, Water Resources Branch, Toronto. Gowda T. P. H. (1981) Critical concentrations of toxic ammonia and chlorine in mixing zones of rivers. Brat. Pollut. Res. J. Can. 15, 255-270. Heinemann T. J., Lee, G. F., Jones R. A. and Newbry B. W, (1981) Summary of studies on modellin8 persistence of domestic wastewater chlorine in Colorado Front Range Rivers. In Water Chlorination: Environmental Impact and Health Effects ~,dited by Jolley R. L., Brungs W. A., Cotruvo J. A., Ctmnning R. B., Mattice J. S. and Jacobs V. A.), Vol. 4, pp.97-112. Ann Arbor Science, Ann Arbor, Mich. Hrudey S. E. (1986) A CriticalA.zsessmentof Drinking Water in Edmonton. Hmdey, Edmonton, Alberta. Jolley R. L. and Carpenter J. H. (1983) A review of the chemistry and environmental fate of reactive oxidant species in chlorinated water. In Water Chlorination: Environmental Impact and Health Effects (Edited by Jolley R. L., Brunge W. A., Cotruvo J. A., Cumming R. B.,
w127/¢t.--E
1001
Mattice J. S. and Jacobs V. A.), VoL 4, pp. 3-47. Ann Arbor Science, Ann Arbor, Mich. Kellerhals R., Neill C. R. and Bray D. I. (1972) Hydraulic and seomorphic characteristics of rivers in Alberta. Research Council of Alberta River Engineerin8 and Surface Hydrology Report 72-1, Alberta Research Council, Edmonton, Alberta. Milne G. D. (1991) Chlorine decay in a large river. M.Sc. thesis, University of Alberta, Edmonton, Alberta. Putz G. (1984) TRSMIX (Transverse Mixing Computer Model) Users Manua/, Environmental Engineering Technical Report 84-2, Department of Civil Engineering, University of Alberta, Edmonton, Alberta. Putz G., Smith D. W. and Gerard R. (1984) Application of a transverse mixing computer program to the prediction of effluent plume configuration. China Canada Workshop on Computer Solution Techniques, pp. 180-199. Canadian Society for Civil Engineering, Montreal, Quebec. Reckhow D. A., Ostendorf D. W. and Billa M. E. (1990) Fate and transport of combined residual chlorine in a small stream. J. envir. Engng Dio., Am. Soc. cir. F~q~s 116, (EE6) 1125-1142. U.S. EPA (1984) Technical Guidance Manual for Performing Wastelond Allocatimts, Book II, Chap. 3. EPA-440-4-S& 024, U.S. EPA O/rice of Water Regulations and Standards, Washington, D.C. Yotsukura N. and Cobb E. D. (1972) Transverse difus/on of solutes in natural streams. United States Geological Survey Professional Paper 582C, USGS, Washington, D.C.