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Water discolouration and the role of the reservoir
Phys. Chem. Earth, Vol. 20, No. 2, pp. 175-181, 1995. Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0079-1946/95 $9.50 + 0.00
Pergamon
0079-1946(95)00021-6
Water Discolouration and the Role of the Reservoir V. A. Pattinson, D. P. Butcher, J. C. Labadz and J. Shacklock
University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K.
ABSTRACT Little research to date has considered the role of reservoirs in the storage, transmission and release of water colour. Edwards (1987) and Yorkshire Water (1992) describe the reservoir as the second line of defence, after good catchment management, in the protection of water supplies in direct supply systems. This paper discusses whether the reservoir fulfils this role with respect to water colour. Results suggest that while the reservoir often acts as a buffer, reducing natural discolouration, under some conditions it is possible for the reservoir to act as a colour source increasing the cost and complexity of future treatment.
KEYWORDS Water colour, reservoir, catchment, water quality
INTRODUCTION Water discolouration, whilst not a significant cause for concern on a global scale, presents considerable difficulties to water companies in the UK who are bound by law to provide "an adequate supply of wholesome water at the consumers tap" (Yorkshire Water, 1992). It is a problem mainly confined to the upland peat catchments, particularly the Pennines. Water colour is generated in the catchment, when a moisture deficit is created in the peat during the warm summer months. Increased oxygen levels in the upper layers cause increases in the activity rates of bacterial organisms which are responsible for the breakdown of organic material within the peat (Mitchell, 1991: Tipping, 1988). The products of this oxidative degradation form the basis of water colour, consisting of a series of natural organic compounds, primarily humic andfulvic acids. Excess colour builds up in the catchment during summer but is not released until catchment soils are rehydrated during autumn and winter, producing a typical 'autumn flush' and a seasonal pattern of colour, which is low in spring and summer but high in antumn and winter. The incidence of colour causes a number of problems both in terms of treatment and supply, and may have health impacts on sensitive groups within society. 1. Water colour was, in the early 1990's, the single largest cause of complaints in the Yorkshire water region, and as such is a high priority to improve customer satisfaction.
175
176
V.A. Pattinsonet aL
2. Water colour is both difficult and expensive to treat. The cost offlocculants and rapid filtering is high and the treatment of high levels of colour is complicated by the effectiveness of the process. As colour levels exceed 60 Hazen ( Hazen = Ab.m"1 * 15; Yorkshire Water, 1990) the capacity to remove colour is reduced. 3. The use of large amounts of chemicals in the removal of colour also causes problems for industrial users of the water, particularly the dyeing industry. In addition the use of aluminium sulphate as a flocculant has been linked with adverse health effects. These include dialysis encephalopathy (dementia) in kidney dialysis patients (Davison et al., 1981) and Alzheimer's disease (Martyn et al., 1989). A possible relationship has also been suggested between water chlorination, organic substances and certain cancers (Anon, 1981). Whilst much is known on the production and cycles of the colour within the catchment, little research has looked at the fate of the colour when it has entered the reservoir or at the role that the reservoir plays in the modification of colour.
THE ROLE OF THE RESERVOIR IN THE COLOUR PROCESS When water flows into a reservoir, a number of processes act on the colour it contains. These processes effectively control the levels of colour in the water and the subsequent necessity and cost of treatment. The net colour level in the reservoir is dependent on the balance of natural amelioration and generation processes operating within the reservoir coupled with the quality of water entering from the catchment. A summary of reservoir colour processes can be seen in fig. 1, showing the basic actions and controls in the system.
~ S U N L I G H T ENERGY~ INPUTS
] o rP s..
......,.
.....RESERVOIR I ~--~ I ...................... v~[etsedmert
energy
-. enefgy
WIND ENERGY
~'
RESERVOIR ",.,,,,
\ '~
/ P~S
~
I AMEL,O TiON
[~OUTPUTS
IN '
I
GENERATION
I- dyr,m~c d~mrl~nce f ¢ l ~ o n
ii
CONTROLS
- ueamln~,ua~ogl~eetrad~cn aggregEed =eerme1~cn
of
' " """'l"',,,,,
I =or=a aeL==d==rid. ~ m = ~ ~ n ~ n
J ofnew¢aouronretervdrme.gin
"% '-,%
Fig~ 1 Summary of Reservoir Colour Processes.
Water Discolourationand the Role of the Reservoir
177
C o l o u r A m e l i o r a t i o n ~ Initial research suggested that the decolourisation of reservoir water was due solely to the bleaching of the water by the action of sunlight. It has long been established that, while colour reduction occurs throughout the reservoir, it is considerably more rapid in the upper layers (Stearns, 1915). This paints to a more rapid photolytic breakdown in the upper layers, whilst at greater depth a slower anaerobic microbial/bacterial breakdown takes place. A number of different authors have sought to determine the amount of bleaching at different depths and the subsequent vertical colour profile. It has been reported that the bleaching action at the surface of the reservoir is most significant, as high as 50% in a month (Whipple, 1933). The rapid rate of surface decolourisation is, however, limited to the penetration of sunlight (1-2m) and the availability of oxygen. Whilst the rate of decolourisation is certainly reduced below the initial 2m, subsequent research has suggested that eolour redaction still occurs (Golterman, 1975). C o l o u r G e n e r a t i o n a n d R e l e a s e ~ Various workers have discussed the generation of colour within reservoirs. Taylor (1987) and Howarth (1987) both suggest that there might be significant impoundment sources of colour, as reservoir sediments often contain high proportions of organic peaty material. The drying of these materials on reservoir draw-down will create a store of colour equivalent to those in the catchment, which is likely to be released on re-wetting or disturbance.
More recent research has suggested that reservoir colour levels can increase (Pattinson, 1994). This is especially the case during storm events where the wind is oriented along the long axis of the reservoir. It is thought that the shear forces acting on the basal sediments, caused either by wind disturbance or internal circulation currents, has the potential to resuspend sediment likely to contain stores of colour. The subsequent releases of colour can be attributed at least in part to the disturbance of the reservoir.
CASE STUDY OF THORNTON MOOR RESERVOIR Research has been undertaken at two field sites in the southern Pennines of UK. The initial work on the nature of reservoir colour began at Thornton Moor reservoir and subsequent research is taking place at nearby Lower Laithe reservoir. This paper will concentrate on the former site. Thornton Moor Reservoir (SE050330) is located approximately 20 km due west of Bradford. It has a characteristic upland peat catchment overlying Namurian Millstone Grit geology. It was initially selected for study because it has consistently high raw water colour levels, it periodically fails to fulfil EC water requirements (Yorkshire Water 1988), and is a site with the potential for the management of the catchwater in order to reduce the colour of inflowing water. In addition to other information, tracing analysis was used to consider flow patterns within the reservoir. Phage tracing experiments showed that the residence time varied from 8 - 26 hours (Pattinson, 1994). The predominant flow appears to follow the dam walls from outlet to inlet. The reservoir contains large amounts of peaty material in the areas of predominant flow representing potentially important sources of colour.
SPATIAL AND TEMI~RAL VARIATIONS IN WATER COLOUR The colour of inflow and outflow water from Thornton Moor was measured for a twelve month period from May 1991 to May 1992. In addition water colour was measured spatially across the reservoir over a period of nine months, in conjunction with investigations of reservoir sediments and colour disturbance. Results can be exemplified by two shorter periods, between 2nd September 1991 and the 20th September 1991 there was a distinct decrease in the colour leaving the reservoir. The colour in the inlet averaged 120 Hazen whilst the colour entering the treatment works was on average 50 I-Iazen lower at 70. By contrast, between 5th and 12th March 1992 it would appear that the level of colour was greater when the water reached the treatment works than when it entered the reservoir. On average the colour was only increased by 15 I-Iazen, from 35 I-Iazen in the reservoir inlet to 50 Hazen at the treatment works.
178
V.A. Pattinsonet aL
In addition to long term data for inlet/outlet relationships, a number of short term events were analysed to investigate spatial variations. Samples were taken across the reservoir surface and at a depth of 2 metres. The results of this spatial sampling showed that on six occasions the reservoir failed to act as a buffer, given that the colour in the outlet water was higher than that of the water entering the reservoir. This suggested that the reservoir has contributed to the level of colour entering the treatment works. It does need to be remembered that in these experiments the colour at the inlet is not directly comparable with the outlet water because of the time delay (approximately 6 hours) from inlet and outlet. Statistical analysis of the data was conducted, and a null hypothesis was formulated. The null hypothesis stated that colour altered either positively or negatively with increasing distance from the inlet. The results showed that in 11 out of 27 cases the null hypothesis could be rejected. On these occasions the reservoir remained well mixed and colour changes did not occur with distance from the inlet. Both negative and positive correlations were also recorded; a negative correlation indicating that, as distance from the inlet increases, so the colour decreases. In these cases the reservoir successfully acted as a buffer to the incoming colour. A positive correlation indicated that as the distance from the inlet increased the colour similarly rose. In this scenario the reservoir failed in its role as a buffering mechanism. An example of the failure of the reservoir to act as a buffer, can be seen in Fig. 2. This depicts the spatial distribution of colour at Thornton Moor for the 13th April 1992. In this instance it was not possible to sample the body of the reservoir, although this was normal practice. Here a significant positive relationship was found. This suggested that the reservoir had failed as a buffer, actively increasing the colour entering the treatment works. This increase was linked with a high index wind event, the wind index used incorporated both wind direction and wind speed3 (Yorkshire Windpower Ltd, 1990). The 'event' lasted 3 hours, approximately 9 hours before sampling took place. The time delay in this case can be accounted for by the travel time across the reservoir, as the peak colour release occurred approximately 2 hours after the disturbing event. In the diagram below the samples taken were predominantly taken from edge locations. Ideally samples would have been taken from all the reservoirs surface, however, due to the disturbing event experienced on the reservoir it was adjudged to be unsafe for boat sampling.
VALVE TOWER
4 .s
s .3
43.,
49.9
4 • 31.5 INLET-
51
e ~ $ 2
.9
46.1
52.$ •
54 •
57.0 49.5
$9.3
49.9
I
N
•
0 -
39.38 Hazen
i 0 0 metres _--
Fi& 2 True Colour Variations At Thornton Moor Reservoir, April 13th 1992.
The relationship between the colour and distance from the inlet is difficult to compare on the above diagram because of the nature of the sampling conducted. The results are displayed in Fig. 3 below, the scattergram more clearly shows the connection between increasing distance and higher levels of colour.
Water Discolourafion and the Role of the Reservoir 60
179 • Reservoir Samples
55 •
"E
50
•
• •
• •
•
•
IM
{0 3: w 0 "6 (5 o
2
I--
45 40 35 30 25
I
0
I
200
400
I
600
I
800
I
1000
Distance From The Inlet (Metres)
Fig. 3 Variations in Thornton Moor Reservoir Water Colour With Distance From The Inlet.
Analysis suggested that wind events have the potential to influence the colour levels within the reservoir (Pattinson, 1994). High winds appear to disturb the sediment, whilst colour is lost to the reservoir when a low wind index is recorded. The relationship between wind disturbance and colour is clearly oversimplified in this brief analysis. Other factors need to be considered, such as the effects of density currents, flow rates and sunlight duration and intensity and temperature which may also affect the reservoir colour levels.
COLOUR RELEASE FROM RESERVOIR SEDIMENTS Analysis of the relationship between colour and the area of sediment exposed by draw down of water level, demonstrated that there is a significant negative relationship (0.001 Confidence Level) between the difference between inlet and outlet colour, and the area of exposed sediment (Fig,. 4). This suggests that water colour declines within the reservoir between inlet and draw-off point when the reservoir water is low and the sediment is subsequently exposed. The key factor here is that draw-down is generally greatest during the summer months and therefore the reservoir water colour will also be subject to rapid photolytic breakdown of colour within the upper layers. In addition, the sediment containing 'stores' of colour will not be covered with water and will subsequently not be available for release. Colour release appears to occur when the reservoir water level is higher. It is believed that the majority of the colour release occurs as a result of disturbance of newly generated colour within reservoir sediments (Stearns, 1915). The infilling of the reservoir with peaty material will clearly allow the reservoir to produce colour, in a similar way to that which occurs in the catchment, when the reservoir is drawn down. This is certainly the case at Thornton Moor, which has accumulated 93,000m 3 of sediment with an average organic content of 27.1% (Butcher et al, 1992).
180
V. A. Pattinson et al. 100 80 60 40 20
0 -20 -40 -60 -80 -100 Jr
I
I
o")
o"3
--~---o')
I o-~
----~ o"3
~
~ m
0")
I
I
I
I
O3
o'~
o"3
~'~
I-
[)am
[
~
Colour Loss/Gain In The R e s e r v o i r (Hazen)
Fig. 4
•
E~posed R e s e r v o i r Sedirnent (100On2)
Colour Loss/Gain At Thornton Moor Reservoir Compared to Exposed Sediment 1979-1991 (N.B. * excellent examples of lag effects which show increases in colour due to formerly exposed sediment being rehydrated and are releasing colour).
Laboratory analysis of reservoir sediment samples taken 50 days apart showed that samples exposed for longer periods of time released greater amounts of colour. Simulated wind events acting on the sediments showed that maximum colour release occurred approximately 1 hour after disturbance ceased. The samples which released high levels of colour had a higher percentage moisture content, low dry bulk density and higher organic content than average. Significant statistical relationships were found between moisture content and colour release, and significant negative relationships were found between dry bulk density and colour release. The lack of a significant relationship between colour release and organic content indicates that the relationship is more complex and that colour availability is influenced by a number of factors such that an examination of content alone is insufficient.
CONCLUSION In many instances reservoirs provide a buffer for water quality problems by diluting, dispersing and delaying contaminant transmission to the abstraction point. In the case of water discolouration, however, the reservoir appears, on occasion, to contribute to this problem. The present investigation of the reservoir sediments suggests that they typically release a fraction of their available colour store. There is, therefore, the potential for future colour release. Having defined the role of the reservoir in the reduction and addition of water colour to the supply, the next stage of the research is to evaluate the key processes involved in the amelioration and generation of colour. A priority is an investigation into the mechanisms of natural decolourisation of water, to be carded out in conjunction with an examination of colour storage and the ability of the sediments to release stored colour. ACKNOWLEDGEMENTS We are grateful for the support and conribution of Yorkshire Water Services plc.
Water Discolouration and the Role of the Reservoir
181
REFERENCES Anon (1981) Cancer and Chlorinated Water. The Lancet. 1, 1142-1149. Butcher D. P, Claydon J, Labadz J. C, Pattinson V. A, Potter A. W. R, and White P (1992) Environmental Problems in the Peat Moorlands of the southern Pennines : Reservoir Sedimentation and Discolouration of Water Supplies. Journal of the Institute of Water and Environmental Management. _6, 4, 418-432. Davison A. M, Walker G. S, Sloan M, Oli H and Giles G. R (1981) Water SupplyAluminium Concentration and Dialysis. Department of Renal Medicine, St James Hospital, Leeds. pp.9. Edwards A. M. C (1987) Coloured Runoff in the Yorkshire Pennines and its consequences for water supply in: Edwards A.M.C, Martin D and Mitchell G.N (eds.), Colour in Upland Water Supplies. Proceedings of Yorkshire Water Workshop, Leeds. ppl-8 Howarth E (1987) Coloured Water the Pennine Problem. IWES Symposium on the Aesthetic Aspects of Water Quality. City Conference Centre, London. 7pp. Martyn C. L, Osmond C, Edwardson J. A, Barker D. J. P, Harris E. C and Flace R (1989) Geographical Relations between Alzheimer's and Aluminium in Drinking water. The Lancet. pp49-62 Mitchell G. N. (1991) Aspects of Solute Movement in the British Uplands. Unpublished Ph.D. Thesis. School of Geography. University of Leeds. Pattinson V. A. (1994) The Transfer, Storage and Release of Water Colour in a Reservoired Catchment. Unpublished Ph.D. Thesis. University of Huddersfield Saville C. M. (1929) Colour Reduction In Storage Reservoirs. Journal of New England Water Works Association. 43,416-443. Steams R. H. (1915) Decolourisation of Water by Storage. Journal of New England Water Works Association. 30, 20-34. Taylor P. C. (1987) Quality of the Raw Water Entering the Elan Treatment Works. Unpublished Severn Trent Water Authority Report. 4pp Tipping E, Hurley M. A. (1988) A Model of Solid Solution Interactions in Organic Soils, Based on the Complexation. properties of Humic Substances. Journal of Soil Science. 39, 302-311. Wetzel R. G. (1938) Light in Lakes in : Limnology. pp42-65. Saunders College Publishing. New York. Whipple G. C. (1993) The Microscopy of Drinking Water. pp157-178. John Wiley. London. Yorkshire Water Authority (1988) Water Quality Report. Unpublished Report. 213pp. Yorkshire Water Services (1992) Guidelines for the Protection of Direct Supply Reservoirs and their Gathering Grounds. Unpublished Report. 213pp Yorkshire Water Directive (1990) Personal Communication Concerning Water Conversion Units. Yorkshire Windpower Ltd (1990) Proposed Wind Turbine Generators, Ovenden Moor, West Yorkshire. Environmental Impact Assessment. 312pp.
Pergamon
0079-1946(95)00021-6
Water Discolouration and the Role of the Reservoir V. A. Pattinson, D. P. Butcher, J. C. Labadz and J. Shacklock
University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K.
ABSTRACT Little research to date has considered the role of reservoirs in the storage, transmission and release of water colour. Edwards (1987) and Yorkshire Water (1992) describe the reservoir as the second line of defence, after good catchment management, in the protection of water supplies in direct supply systems. This paper discusses whether the reservoir fulfils this role with respect to water colour. Results suggest that while the reservoir often acts as a buffer, reducing natural discolouration, under some conditions it is possible for the reservoir to act as a colour source increasing the cost and complexity of future treatment.
KEYWORDS Water colour, reservoir, catchment, water quality
INTRODUCTION Water discolouration, whilst not a significant cause for concern on a global scale, presents considerable difficulties to water companies in the UK who are bound by law to provide "an adequate supply of wholesome water at the consumers tap" (Yorkshire Water, 1992). It is a problem mainly confined to the upland peat catchments, particularly the Pennines. Water colour is generated in the catchment, when a moisture deficit is created in the peat during the warm summer months. Increased oxygen levels in the upper layers cause increases in the activity rates of bacterial organisms which are responsible for the breakdown of organic material within the peat (Mitchell, 1991: Tipping, 1988). The products of this oxidative degradation form the basis of water colour, consisting of a series of natural organic compounds, primarily humic andfulvic acids. Excess colour builds up in the catchment during summer but is not released until catchment soils are rehydrated during autumn and winter, producing a typical 'autumn flush' and a seasonal pattern of colour, which is low in spring and summer but high in antumn and winter. The incidence of colour causes a number of problems both in terms of treatment and supply, and may have health impacts on sensitive groups within society. 1. Water colour was, in the early 1990's, the single largest cause of complaints in the Yorkshire water region, and as such is a high priority to improve customer satisfaction.
175
176
V.A. Pattinsonet aL
2. Water colour is both difficult and expensive to treat. The cost offlocculants and rapid filtering is high and the treatment of high levels of colour is complicated by the effectiveness of the process. As colour levels exceed 60 Hazen ( Hazen = Ab.m"1 * 15; Yorkshire Water, 1990) the capacity to remove colour is reduced. 3. The use of large amounts of chemicals in the removal of colour also causes problems for industrial users of the water, particularly the dyeing industry. In addition the use of aluminium sulphate as a flocculant has been linked with adverse health effects. These include dialysis encephalopathy (dementia) in kidney dialysis patients (Davison et al., 1981) and Alzheimer's disease (Martyn et al., 1989). A possible relationship has also been suggested between water chlorination, organic substances and certain cancers (Anon, 1981). Whilst much is known on the production and cycles of the colour within the catchment, little research has looked at the fate of the colour when it has entered the reservoir or at the role that the reservoir plays in the modification of colour.
THE ROLE OF THE RESERVOIR IN THE COLOUR PROCESS When water flows into a reservoir, a number of processes act on the colour it contains. These processes effectively control the levels of colour in the water and the subsequent necessity and cost of treatment. The net colour level in the reservoir is dependent on the balance of natural amelioration and generation processes operating within the reservoir coupled with the quality of water entering from the catchment. A summary of reservoir colour processes can be seen in fig. 1, showing the basic actions and controls in the system.
~ S U N L I G H T ENERGY~ INPUTS
] o rP s..
......,.
.....RESERVOIR I ~--~ I ...................... v~[etsedmert
energy
-. enefgy
WIND ENERGY
~'
RESERVOIR ",.,,,,
\ '~
/ P~S
~
I AMEL,O TiON
[~OUTPUTS
IN '
I
GENERATION
I- dyr,m~c d~mrl~nce f ¢ l ~ o n
ii
CONTROLS
- ueamln~,ua~ogl~eetrad~cn aggregEed =eerme1~cn
of
' " """'l"',,,,,
I =or=a aeL==d==rid. ~ m = ~ ~ n ~ n
J ofnew¢aouronretervdrme.gin
"% '-,%
Fig~ 1 Summary of Reservoir Colour Processes.
Water Discolourationand the Role of the Reservoir
177
C o l o u r A m e l i o r a t i o n ~ Initial research suggested that the decolourisation of reservoir water was due solely to the bleaching of the water by the action of sunlight. It has long been established that, while colour reduction occurs throughout the reservoir, it is considerably more rapid in the upper layers (Stearns, 1915). This paints to a more rapid photolytic breakdown in the upper layers, whilst at greater depth a slower anaerobic microbial/bacterial breakdown takes place. A number of different authors have sought to determine the amount of bleaching at different depths and the subsequent vertical colour profile. It has been reported that the bleaching action at the surface of the reservoir is most significant, as high as 50% in a month (Whipple, 1933). The rapid rate of surface decolourisation is, however, limited to the penetration of sunlight (1-2m) and the availability of oxygen. Whilst the rate of decolourisation is certainly reduced below the initial 2m, subsequent research has suggested that eolour redaction still occurs (Golterman, 1975). C o l o u r G e n e r a t i o n a n d R e l e a s e ~ Various workers have discussed the generation of colour within reservoirs. Taylor (1987) and Howarth (1987) both suggest that there might be significant impoundment sources of colour, as reservoir sediments often contain high proportions of organic peaty material. The drying of these materials on reservoir draw-down will create a store of colour equivalent to those in the catchment, which is likely to be released on re-wetting or disturbance.
More recent research has suggested that reservoir colour levels can increase (Pattinson, 1994). This is especially the case during storm events where the wind is oriented along the long axis of the reservoir. It is thought that the shear forces acting on the basal sediments, caused either by wind disturbance or internal circulation currents, has the potential to resuspend sediment likely to contain stores of colour. The subsequent releases of colour can be attributed at least in part to the disturbance of the reservoir.
CASE STUDY OF THORNTON MOOR RESERVOIR Research has been undertaken at two field sites in the southern Pennines of UK. The initial work on the nature of reservoir colour began at Thornton Moor reservoir and subsequent research is taking place at nearby Lower Laithe reservoir. This paper will concentrate on the former site. Thornton Moor Reservoir (SE050330) is located approximately 20 km due west of Bradford. It has a characteristic upland peat catchment overlying Namurian Millstone Grit geology. It was initially selected for study because it has consistently high raw water colour levels, it periodically fails to fulfil EC water requirements (Yorkshire Water 1988), and is a site with the potential for the management of the catchwater in order to reduce the colour of inflowing water. In addition to other information, tracing analysis was used to consider flow patterns within the reservoir. Phage tracing experiments showed that the residence time varied from 8 - 26 hours (Pattinson, 1994). The predominant flow appears to follow the dam walls from outlet to inlet. The reservoir contains large amounts of peaty material in the areas of predominant flow representing potentially important sources of colour.
SPATIAL AND TEMI~RAL VARIATIONS IN WATER COLOUR The colour of inflow and outflow water from Thornton Moor was measured for a twelve month period from May 1991 to May 1992. In addition water colour was measured spatially across the reservoir over a period of nine months, in conjunction with investigations of reservoir sediments and colour disturbance. Results can be exemplified by two shorter periods, between 2nd September 1991 and the 20th September 1991 there was a distinct decrease in the colour leaving the reservoir. The colour in the inlet averaged 120 Hazen whilst the colour entering the treatment works was on average 50 I-Iazen lower at 70. By contrast, between 5th and 12th March 1992 it would appear that the level of colour was greater when the water reached the treatment works than when it entered the reservoir. On average the colour was only increased by 15 I-Iazen, from 35 I-Iazen in the reservoir inlet to 50 Hazen at the treatment works.
178
V.A. Pattinsonet aL
In addition to long term data for inlet/outlet relationships, a number of short term events were analysed to investigate spatial variations. Samples were taken across the reservoir surface and at a depth of 2 metres. The results of this spatial sampling showed that on six occasions the reservoir failed to act as a buffer, given that the colour in the outlet water was higher than that of the water entering the reservoir. This suggested that the reservoir has contributed to the level of colour entering the treatment works. It does need to be remembered that in these experiments the colour at the inlet is not directly comparable with the outlet water because of the time delay (approximately 6 hours) from inlet and outlet. Statistical analysis of the data was conducted, and a null hypothesis was formulated. The null hypothesis stated that colour altered either positively or negatively with increasing distance from the inlet. The results showed that in 11 out of 27 cases the null hypothesis could be rejected. On these occasions the reservoir remained well mixed and colour changes did not occur with distance from the inlet. Both negative and positive correlations were also recorded; a negative correlation indicating that, as distance from the inlet increases, so the colour decreases. In these cases the reservoir successfully acted as a buffer to the incoming colour. A positive correlation indicated that as the distance from the inlet increased the colour similarly rose. In this scenario the reservoir failed in its role as a buffering mechanism. An example of the failure of the reservoir to act as a buffer, can be seen in Fig. 2. This depicts the spatial distribution of colour at Thornton Moor for the 13th April 1992. In this instance it was not possible to sample the body of the reservoir, although this was normal practice. Here a significant positive relationship was found. This suggested that the reservoir had failed as a buffer, actively increasing the colour entering the treatment works. This increase was linked with a high index wind event, the wind index used incorporated both wind direction and wind speed3 (Yorkshire Windpower Ltd, 1990). The 'event' lasted 3 hours, approximately 9 hours before sampling took place. The time delay in this case can be accounted for by the travel time across the reservoir, as the peak colour release occurred approximately 2 hours after the disturbing event. In the diagram below the samples taken were predominantly taken from edge locations. Ideally samples would have been taken from all the reservoirs surface, however, due to the disturbing event experienced on the reservoir it was adjudged to be unsafe for boat sampling.
VALVE TOWER
4 .s
s .3
43.,
49.9
4 • 31.5 INLET-
51
e ~ $ 2
.9
46.1
52.$ •
54 •
57.0 49.5
$9.3
49.9
I
N
•
0 -
39.38 Hazen
i 0 0 metres _--
Fi& 2 True Colour Variations At Thornton Moor Reservoir, April 13th 1992.
The relationship between the colour and distance from the inlet is difficult to compare on the above diagram because of the nature of the sampling conducted. The results are displayed in Fig. 3 below, the scattergram more clearly shows the connection between increasing distance and higher levels of colour.
Water Discolourafion and the Role of the Reservoir 60
179 • Reservoir Samples
55 •
"E
50
•
• •
• •
•
•
IM
{0 3: w 0 "6 (5 o
2
I--
45 40 35 30 25
I
0
I
200
400
I
600
I
800
I
1000
Distance From The Inlet (Metres)
Fig. 3 Variations in Thornton Moor Reservoir Water Colour With Distance From The Inlet.
Analysis suggested that wind events have the potential to influence the colour levels within the reservoir (Pattinson, 1994). High winds appear to disturb the sediment, whilst colour is lost to the reservoir when a low wind index is recorded. The relationship between wind disturbance and colour is clearly oversimplified in this brief analysis. Other factors need to be considered, such as the effects of density currents, flow rates and sunlight duration and intensity and temperature which may also affect the reservoir colour levels.
COLOUR RELEASE FROM RESERVOIR SEDIMENTS Analysis of the relationship between colour and the area of sediment exposed by draw down of water level, demonstrated that there is a significant negative relationship (0.001 Confidence Level) between the difference between inlet and outlet colour, and the area of exposed sediment (Fig,. 4). This suggests that water colour declines within the reservoir between inlet and draw-off point when the reservoir water is low and the sediment is subsequently exposed. The key factor here is that draw-down is generally greatest during the summer months and therefore the reservoir water colour will also be subject to rapid photolytic breakdown of colour within the upper layers. In addition, the sediment containing 'stores' of colour will not be covered with water and will subsequently not be available for release. Colour release appears to occur when the reservoir water level is higher. It is believed that the majority of the colour release occurs as a result of disturbance of newly generated colour within reservoir sediments (Stearns, 1915). The infilling of the reservoir with peaty material will clearly allow the reservoir to produce colour, in a similar way to that which occurs in the catchment, when the reservoir is drawn down. This is certainly the case at Thornton Moor, which has accumulated 93,000m 3 of sediment with an average organic content of 27.1% (Butcher et al, 1992).
180
V. A. Pattinson et al. 100 80 60 40 20
0 -20 -40 -60 -80 -100 Jr
I
I
o")
o"3
--~---o')
I o-~
----~ o"3
~
~ m
0")
I
I
I
I
O3
o'~
o"3
~'~
I-
[)am
[
~
Colour Loss/Gain In The R e s e r v o i r (Hazen)
Fig. 4
•
E~posed R e s e r v o i r Sedirnent (100On2)
Colour Loss/Gain At Thornton Moor Reservoir Compared to Exposed Sediment 1979-1991 (N.B. * excellent examples of lag effects which show increases in colour due to formerly exposed sediment being rehydrated and are releasing colour).
Laboratory analysis of reservoir sediment samples taken 50 days apart showed that samples exposed for longer periods of time released greater amounts of colour. Simulated wind events acting on the sediments showed that maximum colour release occurred approximately 1 hour after disturbance ceased. The samples which released high levels of colour had a higher percentage moisture content, low dry bulk density and higher organic content than average. Significant statistical relationships were found between moisture content and colour release, and significant negative relationships were found between dry bulk density and colour release. The lack of a significant relationship between colour release and organic content indicates that the relationship is more complex and that colour availability is influenced by a number of factors such that an examination of content alone is insufficient.
CONCLUSION In many instances reservoirs provide a buffer for water quality problems by diluting, dispersing and delaying contaminant transmission to the abstraction point. In the case of water discolouration, however, the reservoir appears, on occasion, to contribute to this problem. The present investigation of the reservoir sediments suggests that they typically release a fraction of their available colour store. There is, therefore, the potential for future colour release. Having defined the role of the reservoir in the reduction and addition of water colour to the supply, the next stage of the research is to evaluate the key processes involved in the amelioration and generation of colour. A priority is an investigation into the mechanisms of natural decolourisation of water, to be carded out in conjunction with an examination of colour storage and the ability of the sediments to release stored colour. ACKNOWLEDGEMENTS We are grateful for the support and conribution of Yorkshire Water Services plc.
Water Discolouration and the Role of the Reservoir
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