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A model for nitrate-nitrogen transport and denitrification in the river Thames
Water Research Vol. 13, pp. 855 to 863 © Pergamon Press Ltd 1979, Printed in Great Britain
0043-1354/79/0901-0855502.00/0
A M O D E L FOR NITRATE-NITROGEN TRANSPORT A N D DENITRIFICATION IN THE RIVER THAMES G. D. THOMSON Thames Conservancy Division, Thames Water Authority, Nugent House, Vastern Road, Reading, England (Received 1 March 1979)
Abstract--A mathematical model describing the mass flow, including denitrification, of nitrate in the main river Thames has been developed. A three year period, including severe drought and flooding, was used to test the model. Good agreement was found between the nitrate concentrations observed at several sites on the river Thames and those calculated by the model. The model may be used to establish the sensitivity of nitrate levels in the Thames above intakes to changes in tributary inputs which could be affected by various management strategies. INTRODUCTION
The ultimate aim of the work described here is to produce a mathematical model capable of predicting nitrate-nitrogen concentrations in the river Thames upstream of the major water intakes under various flow and climatic conditions and various management strategies. These management strategies may concern, for example, the positioning of sewage treatment works and the type of treatment to be given. The problem of constructing such a catchment model may be divided into three parts: (i) defining and quantifying the sources of nitrate throughout the catchment, (ii) quantifying the effect of climate and management strategies on these sources and (iii) establishing the fate of nitrate once it has entered the river system. Part (i) and, to a certain extent, part (ii) are relatively straightforward for point sources of nitrate. Nonpoint sources, that is surface run-off and groundwater accretion are more complex. It was decided to tackle part (iii) of the problem first and construct a mathematical model describing the mass flow of nitrate and denitrification occurring in the main river Thames. This model could be used to predict, for any given flow and temperature regime, nitrate concentrations at all points in the river Thames given the nitrate concentrations at tributary confluences and at the upper end of the river. This may seem trivial in the sense that it merely moves the problem of prediction from the abstraction points at the lower end of the river to the tributaries, but is an essential first step. The model may be used to establish which tributaries of the main river make significant contributions to the nitrate levels at water intakes and these tributaries may then be studied in more detail. This paper describes the construction and verification of the model and discusses the possible ways in which it may be used.
stream, a distance of about 155 km. This is shown schematically in Fig. 1. In this stretch of the river there are 33 locks and weirs. Several major tributaries join the main river throughout the stretch. There are abstractions of river water at Farmoor, Swinford,
. BuscoI
R.Windrush Farmoor intake Swinford intake R.Evenlode R.Cherwell North field brook Culham intake R.Ock Sutton Courtenay intake Days
R.Thome
R. Pang
R Kennel
Caversham
R.Loddon Marsh
R .Wye
The Cut -
-
Boveney AREA OF STUDY
The stretch of the River Thames covered by the model is from Buscot upstream to Boveney down855
Fig. 1. Schematic diagram of part of the River Thames showing major tributaries, intakes and sampling points.
856
G.D. THOMSON
Culham and Sutton Courtenay. The major abstractions are downstream of Boveney. Preliminary investigations had shown that nitrogen was lost from the river system. This was particularly evident during the exceptionally dry summer of 1976. How much nitrogen is lost and how the rate of loss is affected by temperature, flow and river characteristics was largely unknown.
flows without being so short as to give unreasonable computing times. The denitrification rate used was based on Toms e t al. (1975). Rate of change of NOs-N due to denitrification = -KIK2ACmg
where A = surface area of river mud contact (m 2) C = concentration of NOa-N in overlying water (rag 1- l) logx0(K2) = 0.6888 + 0.0282 T T = water temperature (°C)
MODEL CONSTRUCTION The peculiar nature of the River Thames with respect to the numerous locks and weirs controlling the flow, together with the need for a dynamic model capable of predicting nitrate concentrations over a wide range of flow patterns suggested a model which could be described as a cascading segment model. The River Thames is naturally divided into segments by the locks and weirs. The model works in discrete time increments and is illustrated in schematic form in Fig 2. During each time increment volumes of water, defined by the river flow rates, cascade from each segment to the next. Inputs to each segment also occur from tributaries and local run-off. Each segment is assumed to be uniformly mixed during the time increment and a simple mass balance is used to calculate the concentration in each segment after each time increment. The resulting concentrations are also adjusted for the denitrification taking place at the end of each time increment. It can be seen that this model, although not explicitly requiring information on retention times in each segment, implicitly allows for variable retention times. Under low flow conditions the volume of water input to a segment will be small and so the effect of a change in input concentration to a segment will take a number of time increments to reach subsequent segments. During high flow conditions the volume of water input to a segment relative to the volume of the segment will be much greater and therefore the response will be more rapid. The time increment chosen for operating the model was half a day. This was found to be sufficiently short to allow for low retention times in periods of high IN
N day - l
K t is a constant dependant upon the type of river bed substrate.
FLOW AND QUALITY DATA USED The period of time considered for establishing and verifying the model was from October 1973 until September 1976. October 1973 was towards the end of a long dry periocL river flows remaining at low levels until early January 1974 when heavy rain brought rapid run-off and very high river flows. The winter of 1974/5 was exceptionally wet with flooding occurring in the later autunm 1974 and in early spring 1975. The winter of 1975/6 was exceptionally dry and with little rainfall occurring river flows fell throughout spring and summer 1976 with severe drought conditions occurring. From daily gaugings of the river Thames at Eynsham, Days and Teddington a hydrological model Mander (1973), was used to estimate mean daily flows at all weirs between Buscot and Boveney, just upstream of Dachet the first major abstraction point. These estimates provide the input and output flow rates to each model segment. Daily flows of the major tributaries, in most cases near the confluence with the river Thames, were obtained from flow gauging records. Flow accretion across a segment not accounted for by a tributary input was regarded as local run-off or groundwater accretion. However, the
qi ql¢l
Segment i
v,.,
Segment i+l
is
°i Vi.2
Segment i + 2 Fig~ 2. Diagram of the cascading segment representation of the flow balance and/or mass flow balance used in the model. V~ represents the volume/mass of the ith segment, q~ represents the flow/mass flow into the ith segment via tributaries, runoff or groundwater accretion, h represents the flow/mass flow from the ith segment to the (i + i)th segment.
Nitrate-nitrogen transport and denitrification quality of such water was assumed to be represented by the quality of the tributary. Average volumes and depths of all segments were estimated from river profiles. The volumes are used directly in the model in the mass balance for each segment at each time increment. The volumes and depths are combined to estimate the river bed surface area needed for the denitrification rate equation. For this purpose a rectangular tank profile is assumed for the river. The volumes and depths used are estimates under average flow conditions and are assumed to be constant throughout the three year period modelled. There is obviously scope for greater sophistication in the model to allow for volumes and depths to vary with time under various flow conditions, and it is hoped to examine this aspect in the future. At Buscot, which is the starting point for the model, weekly quality samples were taken and analysed. Sampling of the major tributaries at or near their confluences with the Thames was less frequent, usually fortnightly or monthly. As the model requires daily data, at each sampling point Lagrange interpolation was used to interpolate daily values of concentration from the observed data (Celia, 1969l This is essentially a method of interpolating smoothly between data points. Linear interpolation would produce a jagged curve. The Lagrange interpolation method used fits a cubic curve, concentra: tion = at 3 + bt 2 + ct + d where t = time, to each set of four consecutive data points. This curve is then used to interpolate data between the two centre points. The result is a continuous smooth curve passing through all the data points from which the MOlL
THRME8 CHLORIDE MODEL
857
interpolated concentrations can be read for days when no data is available. This method can only produce a smooth curve between the observed data points, it cannot 'predict' fluctuations which may have occurred during periods when no samples were taken. Therefore if a peak of concentration occurred and one or more samples were taken during the time that the peak occurred then the interpolated data would include a similar peak. If, however, no samples were taken or, perhaps, only one sample was taken half-way up the peak, then the interpolated data would either remain approximately steady or rise to the level of the single sample and fall again.
RESULTS AND DISCUSSION In order to ensure that the model adequately described the flow balance of the river it was first applied to a conservative substance, chloride. To do this the chloride concentrations in the river at Buscot and in the tributaries were used as quality inputs to the model and the denitrification rate was set at zero. Results of this are shown graphically in Figs. 3-7, where the model output is plotted against concentrations observed at five Thames samphng sites. It should be noted that these observed data are not used to correct the model as it operates. The only quality data for the River Thames input to the model are those from Buscot, the starting point for the model. It can be seen that good agreement is found between observed and calculated concentrations of chloride.
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The model was then applied to nitrate-nitrogen. Initially only nitrate-nitrogen was included in the input data for the model. However some difficulty was encountered in matching observed and calculated concentrations. It was found that for a few tributaries with effluent discharges close to the Thames con-
fluences, particularly the Northfield Brook which acts mainly as a carrier for Oxford sewage Works efl]uenL ammoniacal nitrogen concentrations had a significant effect on the Thames nitrate concentrations. For most tributaries in-river nitrification ensures that little ammoniacal nitrogen remains at Thames confluences.
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When this was allowed for in the input to the model significant improvement was obtained, A value of 2.5 was used for the constant K t in the denitrification rate equation giving a rate of 32.3 mgN/m2d per mgN/1 (0.032 m d - t) at 15°C. Results for nitrate using this value are shown graphically in THRRES CHLORIDE
ROlL
IIODEL
Figs. 8-13 again showing good agreement between observed and calculated concentrations, The only major disagreements were at Days lock during the summer of 1976 (Fig. 9). This is thought to be due to the extremely low flows that occurred in the River Thames upstream of Days lock. For a
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Fig. 8. period of 4 weeks water was pumped back over five weirs to above Eynsham to make more water available for abstraction at the Farmoor and Swinford intakes. The assumption of complete mixing in each lock reach used in the model would be invalidated in these circumstances,
MODEL USES AND EXTENSION The model described here may now be used to establish the sensitivity of nitrate concentrations at water intakes, or other points of interest, to flow and temperature regimes and to various management
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strategies. Clearly the usefulness of the model depends critically on an understanding of the way that tributary nitrate concentrations vary with flow, rainfall, soil moisture deficit, temperature, sewage treatment practice, agricultural practice etc. For example, if the
MG/L
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Fig. 12. cal. The model described here may be used to establish which sub-catchments of the Thames have the most effect on nitrate concentrations at water intakes so that such sub-catchments may be picked out for closer examination. Having developed this model for nitrate it may also be extended for use with other quality parameters. llO/L
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-
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Any quality determined could be modelled provided that its chemical or biochemical kinetics in the river is adequately understood. SUMMARY A model has been developed to describe the mass flow, including denitrification, of nitrate in the main
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Nitrate-nitrogen transport and denitrification River Thames. It has been shown to adequately reproduce nitrate concentrations observed at several points in the River Thames over a 3 year period covering both flood and severe drought. The model may be used in its present form to establish the sensitivity of river nitrate concentrations to changes in tributary inputs. Acknowledgements--The author wishes to thank H. Fish, Esq., O.B.E., B.Sc., F.R.I.C., F.I.P.H.E., F.I.W.P.C., Chief Executive, Thames Water, for his kind permission to publish this paper and many colleagues for their helpful criti-
863
cism. The views expressed are the author's and do not necessarily reflect the views of the Authority.
REFERENCES
Celia C. A. (1969) Introduction to Numerical Analysis. McGraw-Hill, New York. Mander R. J. (1973) Discharge from compounded storage. Thames Conservancy internal report. Toms I. P., Mindenhall M. J. & Harman M. M. I. (1975) Factors affecting the removal of nitrate by sediments from rivers, lagoons and lakes. Water Research Centre Technical report TR14.
0043-1354/79/0901-0855502.00/0
A M O D E L FOR NITRATE-NITROGEN TRANSPORT A N D DENITRIFICATION IN THE RIVER THAMES G. D. THOMSON Thames Conservancy Division, Thames Water Authority, Nugent House, Vastern Road, Reading, England (Received 1 March 1979)
Abstract--A mathematical model describing the mass flow, including denitrification, of nitrate in the main river Thames has been developed. A three year period, including severe drought and flooding, was used to test the model. Good agreement was found between the nitrate concentrations observed at several sites on the river Thames and those calculated by the model. The model may be used to establish the sensitivity of nitrate levels in the Thames above intakes to changes in tributary inputs which could be affected by various management strategies. INTRODUCTION
The ultimate aim of the work described here is to produce a mathematical model capable of predicting nitrate-nitrogen concentrations in the river Thames upstream of the major water intakes under various flow and climatic conditions and various management strategies. These management strategies may concern, for example, the positioning of sewage treatment works and the type of treatment to be given. The problem of constructing such a catchment model may be divided into three parts: (i) defining and quantifying the sources of nitrate throughout the catchment, (ii) quantifying the effect of climate and management strategies on these sources and (iii) establishing the fate of nitrate once it has entered the river system. Part (i) and, to a certain extent, part (ii) are relatively straightforward for point sources of nitrate. Nonpoint sources, that is surface run-off and groundwater accretion are more complex. It was decided to tackle part (iii) of the problem first and construct a mathematical model describing the mass flow of nitrate and denitrification occurring in the main river Thames. This model could be used to predict, for any given flow and temperature regime, nitrate concentrations at all points in the river Thames given the nitrate concentrations at tributary confluences and at the upper end of the river. This may seem trivial in the sense that it merely moves the problem of prediction from the abstraction points at the lower end of the river to the tributaries, but is an essential first step. The model may be used to establish which tributaries of the main river make significant contributions to the nitrate levels at water intakes and these tributaries may then be studied in more detail. This paper describes the construction and verification of the model and discusses the possible ways in which it may be used.
stream, a distance of about 155 km. This is shown schematically in Fig. 1. In this stretch of the river there are 33 locks and weirs. Several major tributaries join the main river throughout the stretch. There are abstractions of river water at Farmoor, Swinford,
. BuscoI
R.Windrush Farmoor intake Swinford intake R.Evenlode R.Cherwell North field brook Culham intake R.Ock Sutton Courtenay intake Days
R.Thome
R. Pang
R Kennel
Caversham
R.Loddon Marsh
R .Wye
The Cut -
-
Boveney AREA OF STUDY
The stretch of the River Thames covered by the model is from Buscot upstream to Boveney down855
Fig. 1. Schematic diagram of part of the River Thames showing major tributaries, intakes and sampling points.
856
G.D. THOMSON
Culham and Sutton Courtenay. The major abstractions are downstream of Boveney. Preliminary investigations had shown that nitrogen was lost from the river system. This was particularly evident during the exceptionally dry summer of 1976. How much nitrogen is lost and how the rate of loss is affected by temperature, flow and river characteristics was largely unknown.
flows without being so short as to give unreasonable computing times. The denitrification rate used was based on Toms e t al. (1975). Rate of change of NOs-N due to denitrification = -KIK2ACmg
where A = surface area of river mud contact (m 2) C = concentration of NOa-N in overlying water (rag 1- l) logx0(K2) = 0.6888 + 0.0282 T T = water temperature (°C)
MODEL CONSTRUCTION The peculiar nature of the River Thames with respect to the numerous locks and weirs controlling the flow, together with the need for a dynamic model capable of predicting nitrate concentrations over a wide range of flow patterns suggested a model which could be described as a cascading segment model. The River Thames is naturally divided into segments by the locks and weirs. The model works in discrete time increments and is illustrated in schematic form in Fig 2. During each time increment volumes of water, defined by the river flow rates, cascade from each segment to the next. Inputs to each segment also occur from tributaries and local run-off. Each segment is assumed to be uniformly mixed during the time increment and a simple mass balance is used to calculate the concentration in each segment after each time increment. The resulting concentrations are also adjusted for the denitrification taking place at the end of each time increment. It can be seen that this model, although not explicitly requiring information on retention times in each segment, implicitly allows for variable retention times. Under low flow conditions the volume of water input to a segment will be small and so the effect of a change in input concentration to a segment will take a number of time increments to reach subsequent segments. During high flow conditions the volume of water input to a segment relative to the volume of the segment will be much greater and therefore the response will be more rapid. The time increment chosen for operating the model was half a day. This was found to be sufficiently short to allow for low retention times in periods of high IN
N day - l
K t is a constant dependant upon the type of river bed substrate.
FLOW AND QUALITY DATA USED The period of time considered for establishing and verifying the model was from October 1973 until September 1976. October 1973 was towards the end of a long dry periocL river flows remaining at low levels until early January 1974 when heavy rain brought rapid run-off and very high river flows. The winter of 1974/5 was exceptionally wet with flooding occurring in the later autunm 1974 and in early spring 1975. The winter of 1975/6 was exceptionally dry and with little rainfall occurring river flows fell throughout spring and summer 1976 with severe drought conditions occurring. From daily gaugings of the river Thames at Eynsham, Days and Teddington a hydrological model Mander (1973), was used to estimate mean daily flows at all weirs between Buscot and Boveney, just upstream of Dachet the first major abstraction point. These estimates provide the input and output flow rates to each model segment. Daily flows of the major tributaries, in most cases near the confluence with the river Thames, were obtained from flow gauging records. Flow accretion across a segment not accounted for by a tributary input was regarded as local run-off or groundwater accretion. However, the
qi ql¢l
Segment i
v,.,
Segment i+l
is
°i Vi.2
Segment i + 2 Fig~ 2. Diagram of the cascading segment representation of the flow balance and/or mass flow balance used in the model. V~ represents the volume/mass of the ith segment, q~ represents the flow/mass flow into the ith segment via tributaries, runoff or groundwater accretion, h represents the flow/mass flow from the ith segment to the (i + i)th segment.
Nitrate-nitrogen transport and denitrification quality of such water was assumed to be represented by the quality of the tributary. Average volumes and depths of all segments were estimated from river profiles. The volumes are used directly in the model in the mass balance for each segment at each time increment. The volumes and depths are combined to estimate the river bed surface area needed for the denitrification rate equation. For this purpose a rectangular tank profile is assumed for the river. The volumes and depths used are estimates under average flow conditions and are assumed to be constant throughout the three year period modelled. There is obviously scope for greater sophistication in the model to allow for volumes and depths to vary with time under various flow conditions, and it is hoped to examine this aspect in the future. At Buscot, which is the starting point for the model, weekly quality samples were taken and analysed. Sampling of the major tributaries at or near their confluences with the Thames was less frequent, usually fortnightly or monthly. As the model requires daily data, at each sampling point Lagrange interpolation was used to interpolate daily values of concentration from the observed data (Celia, 1969l This is essentially a method of interpolating smoothly between data points. Linear interpolation would produce a jagged curve. The Lagrange interpolation method used fits a cubic curve, concentra: tion = at 3 + bt 2 + ct + d where t = time, to each set of four consecutive data points. This curve is then used to interpolate data between the two centre points. The result is a continuous smooth curve passing through all the data points from which the MOlL
THRME8 CHLORIDE MODEL
857
interpolated concentrations can be read for days when no data is available. This method can only produce a smooth curve between the observed data points, it cannot 'predict' fluctuations which may have occurred during periods when no samples were taken. Therefore if a peak of concentration occurred and one or more samples were taken during the time that the peak occurred then the interpolated data would include a similar peak. If, however, no samples were taken or, perhaps, only one sample was taken half-way up the peak, then the interpolated data would either remain approximately steady or rise to the level of the single sample and fall again.
RESULTS AND DISCUSSION In order to ensure that the model adequately described the flow balance of the river it was first applied to a conservative substance, chloride. To do this the chloride concentrations in the river at Buscot and in the tributaries were used as quality inputs to the model and the denitrification rate was set at zero. Results of this are shown graphically in Figs. 3-7, where the model output is plotted against concentrations observed at five Thames samphng sites. It should be noted that these observed data are not used to correct the model as it operates. The only quality data for the River Thames input to the model are those from Buscot, the starting point for the model. It can be seen that good agreement is found between observed and calculated concentrations of chloride.
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,
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.
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"
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,,? 't.
8NINFORD
Fig. 3.
1976
j
R 6
858
G. D. THOM~N
MO/L
THRMES CHLORIDE
MODEL
1973-6 O OB6ERVED ~CRLCULRTED
120.0-
®
ItO.O8
lO0.O. 90.0 80.0
®e/
7O.O
~.
60,0
® ,_
|
.®of J
,
50 .O
®
•
,.,"
.A.I
v.
®
40.0 ®
•
Q
®
30 .O 20.0
tO.O O.O
O'N'O'J'F'M'A'M'J' J'R'8' O'N'D'J'F'M'R'M'J'J'A'8'O'N'O'J'F'R'R'M'J' 1974
1976
J'R'8'
19?6
SITE: DRYS
Fig. 4.
The model was then applied to nitrate-nitrogen. Initially only nitrate-nitrogen was included in the input data for the model. However some difficulty was encountered in matching observed and calculated concentrations. It was found that for a few tributaries with effluent discharges close to the Thames con-
fluences, particularly the Northfield Brook which acts mainly as a carrier for Oxford sewage Works efl]uenL ammoniacal nitrogen concentrations had a significant effect on the Thames nitrate concentrations. For most tributaries in-river nitrification ensures that little ammoniacal nitrogen remains at Thames confluences.
THAMES CHLORIDE MODEL 1973-6
MG/L
O OBSERVED --CRLCULRTEO
120.0 tlO.O 100.0 90 .O
80.0 70 .O QO
I
Q
60,0
•
e
•
=(k~m
el~Le
~
5S .0 4S .0 30.0 Q
•
20.0
• 0
10.0 0.0
O'N'O'
j'F'M'R'M'j'j'R'S'O'N'D'J'F'H'R'M'j'j'RVS'O'N'D'j'F'M'R'M'j' 19"/4 1975
SITE: CRVERSHRM
Fig. 5.
j'A'8 1976
'
Nitrate-nitrogen ~ansportand deni~ification THRHES CHLORIDE
HO/L
MODEL
859
1973-6 O
120.0 -
OBSERVED
--CRLCULRTEO
110.0100.0
-
90 .O 80.0 ?0.0 BO .O 0°
Q
50.0
®
®
® tp d".®o .(
40.0 QI
30.0
•
• •
20.0
•
•
8
10.0 0.0
O'N'O'J'F'B'A'H'j' 19"/4 SITE:
j'R'$'O'N'D'
j'F'B'R'R'j' 19"76
j'R'S'O'N'O'
j'F~R'A'R'j'j'R'S 19'76
'
HRRSH
Fig. 6.
When this was allowed for in the input to the model significant improvement was obtained, A value of 2.5 was used for the constant K t in the denitrification rate equation giving a rate of 32.3 mgN/m2d per mgN/1 (0.032 m d - t) at 15°C. Results for nitrate using this value are shown graphically in THRRES CHLORIDE
ROlL
IIODEL
Figs. 8-13 again showing good agreement between observed and calculated concentrations, The only major disagreements were at Days lock during the summer of 1976 (Fig. 9). This is thought to be due to the extremely low flows that occurred in the River Thames upstream of Days lock. For a
19'73-6 OBSERVEO --CRLCULRTEO
120.0 110.0100.0
-
90.0 90.0 "/0.0 60.0 50,0 40.0 30.0 ZO .0 lO.O 0.0
O'N'O'J'F'B'R'fl'j' 1994
SITE:
j'R'8'O'N~O'
j'F'H'R'R'j~
1995
BOVENEY Fig. 7.
j'R'S'O'N'D'
j'F'RIR'M'j'
1996
j'R'S
'
860
G.D. THOMSON MO/L
THRMES NITRRTE + RMMONIR MODEL
1973-6 O OBSERVED --CRLCULRTED
20.0 18.0
16 .O 14.0 12.0 O
10.0 00
• Q
8.0
•
~0
. ~
~m
•
•
• •
• •
•
6,0 •
•
•
4.0 0
2.0 O.O
OIN ' D ~ J ' F ' M ' R ~ F ~ ' j ' 1974
j'R'8'OJN'D'
j'F'M'R'H'j'j'R'S'O'N'DBJ'F'M'R'It'j'j'R~S 1976
'
1976
S I T E : SNINFORD
Fig. 8. period of 4 weeks water was pumped back over five weirs to above Eynsham to make more water available for abstraction at the Farmoor and Swinford intakes. The assumption of complete mixing in each lock reach used in the model would be invalidated in these circumstances,
MODEL USES AND EXTENSION The model described here may now be used to establish the sensitivity of nitrate concentrations at water intakes, or other points of interest, to flow and temperature regimes and to various management
THRMES NITRRTE + RMMONIR MODEL
MB/L
1973-6 ® 08SERVED
20.0
--CRLCULRTED
18.0 t6.0
0
14.0 12.0 10.0
0 Q
•
8.0
•
0
•
0 0
6.0 4.0
•
O0
~
•
~ • ®
2.0
0.0
O'N'D'J'F~MJR'H'j'j'R'S'OJNiOWJ'F'MIR'pI'j'j'R~S'OJN'D'J'F)M~R~H'j'jwR' 1974 1975 1976
SITE: DRYS
Fig. 9.
$'
Nitrate-nitrogen transport and denitriflcation THRME5
RO/L 20.0
NITRRTE
+ RMMONIR
MODEL
861
19"/3-6 O OBSERVEO --CRLCULRTEO
(D
18.0 16.0 14.0 12.0 10.0 o
8.0
m
0,
•
ePe
6.0
•
•
0
4.0 2.0 0.0
O'NvO~J~F~M~R'M=j~ 1974
SITE:
j~R~SJO'N'O~J'F'M'R'M'j'j'R'S~O~N~D'J~F'M'RVH'j'j'RLST 1975 1976
CRVERSHRM Fig. 10.
strategies. Clearly the usefulness of the model depends critically on an understanding of the way that tributary nitrate concentrations vary with flow, rainfall, soil moisture deficit, temperature, sewage treatment practice, agricultural practice etc. For example, if the
MG/L
effect of denitrifying effluent at large sewage treatment works on nitrate concentrations at abstraction points is required, the effect on individual tributary levels must be established. Ideally all tributaries should be examined and modelled in detail but this is impracti-
THRME$ NITRRTE * RMMONIR MODEL
19'73-6 00BSERVEO --CRLCULRTED
20.0 lB.O 16.0 14.0
®
~9
12.0 10.0 8
8.0 6.0
0
¢1
4°0 2.0 0.0
O~NJO'J'F~M~R=M'j~j~RnS~O~N=O'JIFnMnRVM~j~j'R~S~OnN'O'J~F~M'R~M'j'j~R~ 1974 1975 1976
S I T E : MRRSH
Fig. 11.
S'
862
G.D. "I~oMsoN hg/L
THRMES NITRRTE" + RMMONIR MODEL
1973-6 O OB6ERVEO
20.0
~CRLCULRTEO
18.0 16.0 14.0
o
12.0 lO.O G,
8.0 •
•
6.0
®
• o
4.0
o
v
o
,
,,-o
2.0 0.0 O'N'O'
j'F'M'R'H'jr 1974
j'R'S'O'N'O'J'F'I,I'R'H=j' 1975
SITE:
j,R,9,0,N,O,j,F,M,R,tt,j, 1976
j,Rr~
BOVENEY
Fig. 12. cal. The model described here may be used to establish which sub-catchments of the Thames have the most effect on nitrate concentrations at water intakes so that such sub-catchments may be picked out for closer examination. Having developed this model for nitrate it may also be extended for use with other quality parameters. llO/L
THA'*ES "JITRAT,r
-
A:~rGNIA MODEL
Any quality determined could be modelled provided that its chemical or biochemical kinetics in the river is adequately understood. SUMMARY A model has been developed to describe the mass flow, including denitrification, of nitrate in the main
1973 .
20.0
0
0
DBSC~EO CALCULATED(INCLUDING~NITRIFICATION)
18.0 .....
CALC~ATCO(RASS BALANCE)
16.0 14.0
12.0 10.0
¢
i"~
8.0 6.0
4.0 2.0 0.0
,.j O'N'O'J~F'MWR'h'j'j'R'S'O'N'D'J'F'M'R'M~j'j'R~S'O~N'DWJ'F~flWR'h,j,j,R~S, 1974
1975
SITE=
CRVERSHRM
Fig. 13.
1976
Nitrate-nitrogen transport and denitrification River Thames. It has been shown to adequately reproduce nitrate concentrations observed at several points in the River Thames over a 3 year period covering both flood and severe drought. The model may be used in its present form to establish the sensitivity of river nitrate concentrations to changes in tributary inputs. Acknowledgements--The author wishes to thank H. Fish, Esq., O.B.E., B.Sc., F.R.I.C., F.I.P.H.E., F.I.W.P.C., Chief Executive, Thames Water, for his kind permission to publish this paper and many colleagues for their helpful criti-
863
cism. The views expressed are the author's and do not necessarily reflect the views of the Authority.
REFERENCES
Celia C. A. (1969) Introduction to Numerical Analysis. McGraw-Hill, New York. Mander R. J. (1973) Discharge from compounded storage. Thames Conservancy internal report. Toms I. P., Mindenhall M. J. & Harman M. M. I. (1975) Factors affecting the removal of nitrate by sediments from rivers, lagoons and lakes. Water Research Centre Technical report TR14.