Q U A L I T Y O F S T O R M W A T E R R U N O F F F R O M AN U R B A N I S E D
WATERSHED CHUI PENG CHEONG School of Civil and Structural Engineering, Nanyang Technological Institute, Republic of Singapore, Singapore 2263
(Received November 1990) Abstract. A field monitoring network was set up within the Stamford canal watershed in 1989to study both the quantitative and qualitative aspects of storm runoff from this urbanised catchment. The data acquisition equipment comprised a continuous recording rain gauge, a water level recorder and an automatic water sampler capable of sampling storm runoff at preset intervals during rainfall events. Water samples were collected after each storm and laboratory tests were carried out on the physicaland chemicalproperties of the storm water. Preliminaryfindingson the temporal variations of stormwater quality during singlestorms and the effects of antecedent dry weather period on the quality are presented. The average ranges of some of the significant quality parameters found in the storm runoff were also established. The quality of storm runoff from the catchment under study was found to be of an acceptableleveland could potentiallybe developedas a water catchment area.
Introduction Singapore is a small country with a total land area of approximately 640 square kilometers. A b o u t 43 percent of the land area has been developed as residential, commercial a n d industrial areas, a n d 15 percent is agricultural land. The remaining areas are either natural reserves or other n o n - b u i l t - u p areas. The average a n n u a l rainfall is 2400 m m (Meteorological Service Singapore, 1985), which is also the m a i n source of water supply. As water is a scarce a n d limited resource in Singapore, almost half of the total land area has been developed as water catchment areas for the collection of rainwater into the fourteen i m p o u n d i n g reservoirs. These catchment areas cover all the natural reserves and also include some of the heavily built-up residential, commercial a n d light industrial areas. The current daily water c o n s u m p t i o n is slightly less than 1 million cubic metres. But with c o n t i n u i n g increases in economic a n d population growth a n d also with increase in social affluence, the d e m a n d for water is expected to continue to rise in the future. Thus, the search for additional sources of water supply like abstracting water from unprotected u r b a n catchment areas or other u n c o n v e n t i o n a l sources must be explored. It is even more evident after the recent dry spell (February - April 1990), where rainfall over the whole island was much lower than the average values a n d with total reservoir stock falling below the 70 percent level. In 1989, a field m o n i t o r i n g system was installed within the Stamford canal watershed, comprising a rain gauge, a water level recorder a n d a water sampler. The system was implemented to continuously collect data on rainfall, a n d both the quantitative a n d qualitative characteristics of stream flow within the basin. A considerable a m o u n t of field Environmental Monitoring and Assessment 19: 449-456, 1991. 9 1991Kluwer Academic Publishers. Printed in the Netherlands.
450
CI-IUI PENG
CHEONG
data has since been collected and some preliminary studies on the temporal variation patterns of common water quality parameters have been carried out. The average ranges of these selected quality parameters were also established. Among other things, the results obtained could be used to assess the feasibility of abstracting storm runoff from unprotected urban catchment areas for water supply and could also be used as a baseline condition for future studies on the effects of urbanisation on storm runoff quality. This paper presents the main features of the field monitoring system and some preliminary results of the study.
Description of Study Area The study area is a sub-basin of the main Stamford canal catchment, which embraces some of the most prestigious shopping, commercial and residential areas in Singapore. The drainage network of the Stamford canal system comprises four major upstream tributaries (Tributaries I, II, III and IV) and the main canal downstream, which discharges into the Marina bay (Figure 1). The sub-basin is drained by Tributary I as shown Figure 2. It has a drainage area of about 110 hectares and is relatively undeveloped. Presently, about 20 percent of the basin (22 hectares) are impervious areas consisting mainly buildings, roads and vehicle parking areas. The remaining areas are turfed areas such as the Singapore Botanic Gardens, a nature park and a golf course. The basin has a steep terrain and the highest land altitude is about 40 m above the mean sea level. The mean slope is about 15 percent and the direction of slope is towards the main arterial road - Holland Road and Napier Road - which bisects the basin. The basin is fully served by a separate sewerage system where excess rainfall is discharged directly into storm drains while sewage is discharged to a separate waste sewerage pipe. Tributary I, which is the main channel in the basin, runs along one side of the arterial road. It is constructed of reinforced concrete with a length of 1.7 km and a cross section of 3.7 m wide by 2.3 m deep at the monitoring site. The other minor drains consist of standard V-shaped and larger U-shape concrete sections.
Sampling and Monitoring System The location of the water sampler, raingauge and water level recorder is shown in Figure 2. The sampling site has been selected at the downstream end of the Tributary I sub-basin so that representative samples of storm runoff from the whole catchment can be collected. The water level in the channel is continuously monitored by means of a float-type stage recorder which is hooked on to an on-site data logger. Water levels are recorded at one minute intervals and the data are retrieved fortnightly using a lap-top computer. The measured stage hydrographs are subsequently converted into discharge hydrographs using a simple computer program based on the Manning's equation. Rainfall in the catchment is measured by a tipping bucket raingauge located at the Botanic Gardens. Stormwater is sampled using an ISCO automatic sampler which could retrieve up to a
Fig. h
0
SCALE
0.5
1 KM
Map of location of Stamford Canal watershed.
N
MARINA
BAY
KEY PLAN
P.
UBIN
&
,
-
s S~J/]
e ,.? o~r
o~176 ~,...
-.,.,,
,~
Fig, 2.
N
~C::2
9
""-.-'~
Ii
/I
II
20
o
Layout of Tributary I sub-basin.
%
SCALE
0.2
~
I
0.4 KM
9
['
=---
I
STAGE RECORDER WATER SAMPLER
RAINGAUGE
BUILDINGS
&
IN M E T R E S
& CARPARKS
CONTOURS
ROADS
TRIBUTARY
LEGEND
STORMWATER
RUNOFF
FROM
AN URBANISED
WATERSHED
453
maximum of 24 • 500 mL samples at preset time intervals and quantities. The sampler consists of a battery operated pumping system, a suction hose attached with a strainer at the end, and the sampling bottles. The sampler is triggered off by means of a water level actuator which is attached to the strainer. Each time the water level in the channel rises above the strainer, which is about 250 mm above the invert of the drain, the pumping system is activated. By comparing this initial sampling level with the stage hydrograph, the starting time of sampling can be found. In the present study, samples are collected at 15 minute intervals during each storm event and each time, four sampling bottles are filled giving a total sample volume of 2 litres. The samples are then retrieved after the storm, transported to the laboratory, and preserved for quality analysis. The following laboratory analyses are conducted on the stormwater samples according to the 'Standard Methods (1980)': total suspended solids (TSS), total dissolved solids (TDS), total organic carbon (TOC), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total phosphates, pH, conductivity, alkalinity, turbidity, oil and grease, and heavy metals such as lead, manganese etc.
Preliminary Results and Discussion The most important sources of pollution in the basin are likely to come from traffic, vehicle washing, plant fertilisers, grasscuttings, dustfall, litter and leaves. The rainfall itself, is also contaminated by air pollution. When it reaches the ground, the water removes pollutants from the surface before discharging into the drain. Roads and vehicle parking areas can produce traffic induced substances such as lead, oil and polycyclic aromatic hydrocarbons. Plant fertilisers and decaying tree leaves can produce pollutants like phosphates and other nutrients, which can lead to excessive algal growth in the aquatic environment when the concentrations are high. The laboratory testing programme for the project is carried out in two phases as the volume of each sample collected is insufficient for a complete analysis of all quality parameters. The first phase which was carried out in the last quarter of 1989, involved the testing of such physical characteristics as pH, turbidity, TSS, TDS and oil and grease of the stormwater. In the current second phase of testing, the chemical properties such as nutrients and heavy metals concentrations of the stormwater are analysed. Data from four representative storm events are presented in this paper. Table I shows the variations in the stormwater quality with different sampling time. These results were derived based on the flow-weighted mean concentrations of the storm events i.e. by dividing the pollutant loads with runoffvolumes. The first flush effect, which refers to the significant concentration of pollutants during the initial stage of streamflows, was not observed in the study for most of the selected parameters except for oil and grease and conductivity. It was, however, observed that there was an initial increase in the turbidity, TSS, COD and TOC concentrations, up to a peak which almost coincided with the peak of the storm flow and thereafter, the concentrations decreased through time. Although heavy metals concentrations and total phosphates content remained fairly constant throughout runoffs, more pollutants may be removed from the surface at high
454
CHUI PENG CHEONG TABLE I
Variations of flow-weighted mean concentrations with sampling time and ranges of concentrations with permissible levels for potable abstractions (Concentrations in mg/L unless specified). Time intervals in minutes Parameters
pH Turbidity (NTU) Colour Conductivity (/~S/cm) Total Alkalinity TSS TDS Oil & Grease COD TOC Total Phosphates Ca Na K
Mg Zn Cu Fe Pb TKN
0-15
6.5
15-30 30--45 45-60
6.3
6.4
6.6
71 76
137 159
129 160
104 138
68 15 106 38 3.3 54 10
52 16 211 34 2.8 43 10
52 14 168 39 1.8 80 20
61 13 107 45 1.2 52 11
2.7 6.8 1.8 2.4 0.48 ND 0.06 0.10 0.06 ND
2.8 7.1 3.5 2.1 0.46 0.~ ND 0.10 ND ND
3.0 6.6 1.3 2.0 0.5 ND ND 0.06 0.06 ND
2.7 6.0 1.0 1.7 0.~ ND ND ND ND ND
Range 60-70
75-90
Max
6.7
6.6
7.1
87 126 75 12 87 50 0.9 43 8 2.6 5.4 1.0 1.7 0.4 ND 0.04 0.04 0.04 ND
Min
6.3
Mean
6.5
Permissible Level
5.5-9.0
67 90
154 198
56 74
109 136
54 12 67 37 1.1 23 6
112 17 223 74 4.6 86 29
45 10 48 30 0 23 3
59 1000 13.9 500 141 40 500 1.93 52 30 11.7
2.6 6.7 1.0 1.8 0.5 ND ND 0.I0 ND ND
3.2 8.4 4.8 3.3 0.6 0.1 0.1 0.1 0.1
2.5 5.3 0.8 1.5 0.4 0 0 0 0
2.8 6.5 1.8 2.0 0.~ 0.01 0.01 0.07 0.03
200
0.7 75 200 30 1.0 0.2 0.3 0.05 1.0
ND-NotDetectable
f l o w rates. H e a v y metals like m a n g a n e s e , c h r o m i u m , c a d m i u m a n d T K N were n o t f o u n d in all the s t o r m w a t e r samples tested. T h e t o t a l p o l l u t a n t loads for the f o u r s t o r m events are c o m p u t e d a n d g i v e n in T a b l e II. T h e d a t a p r e s e n t e d in the table gives a n i n d i c a t i o n o n the effect o f a n t e c e d e n t dry p e r i o d o n the t o t a l loads. A l o n g e r a n t e c e d e n t d r y p e r i o d p r i o r to the s t o r m e v e n t t e n d s to p r o d u c e higher p o l l u t a n t loadings. T h e relationship b e t w e e n oil a n d grease a n d a n t e c e d e n t d r y p e r i o d is p a r t i c u l a r l y strong. T h e total p o l l u t a n t l o a d for oil a n d grease after an a n t e c e d e n t d r y p e r i o d o f 47 h o u r s was f o u n d to be a l m o s t d o u b l e that after 19 hours. T h i s c o u l d be due to the fact t h a t oil a n d grease is n o t readily r e m o v a b l e b y n o r m a l street s w e e p i n g as c o m p a r e d with litter and leaves. T h e ranges a n d m e a n c o n c e n t r a t i o n s o f selected q u a l i t y p a r a m e t e r s are also s u m m a r i s e d in T a b l e I. F o r all the p a r a m e t e r s analysed, n o significant variations in the c o n c e n t r a t i o n s were observed. T h e permissible levels o f surface waters suitable for p o t a b l e a b s t r a c t i o n s ( T e b b u t t 1983, C a i r n c r o s s 1983, C h a n l e t t 1979) are also s h o w n in the s a m e table. A c o m p a r i s o n b e t w e e n the m e a s u r e d p a r a m e t e r s a n d the permissible values shows
STORMWATER
RUNOFF
FROM
AN URBANISED
455
WATERSHED
TABLE II Rainfall characteristics and total pollutant loads (in kg) for four storm events. Storm event
17 Nov 89
23 Dec 89
Total rainfall (ram) Average Intensity (mm/hr) Antecedent dry period (hr) TSS TDS Oil & Grease COD TOC Total phosphates Ca Na K Mg Zn Cu Fe Pb
20.6 41 47 571 172 11.3 * * * * * * * * * * *
24.5 12 19 546 152 6.4 * * * * * * * * * * *
29 Dec 89 21.8 3.4 18 * * * 75 8.3 4.0 8.4 1.8 2.3 0.6 0.03 0.02 0.08 0.02
4 Jan 90 15.2 11.2 75 * * * 97 30 5.1 12.9 4.2 4.2 0.9 0 0.02 0.12 0.06
* Not tested that the quality o f s t o r m w a t e r f r o m the T r i b u t a r y I basin is quite acceptable and could be utilised as a potential source o f water supply.
Conclusion Storm r u n o f f quality has been m o n i t o r e d in an upstream urbanised basin within the S t a m f o r d canal catchment. The quality o f stormwater f r o m the Tributary I basin was f o u n d to be generally o f an acceptable level, and suitable to be developed as a water catchment area. The first flush p h e n o m e n o n was only recognisable for such parameters as oil and grease and conductivity. F o r other quality parameters such as turbidity, TSS, T D S and C O D , it was observed that the occurrence o f their peak concentrations coincided with the peak o f the runoff. The a m o u n t o f pollutant loads a c c u m u l a t e d in the basin were also f o u n d to be related to the length o f the antecedent dry period. The present study will be extended to cover the remaining tributaries o f the S t a m f o r d canal system so as to allow areal variations o f the quality characteristics and the influence o f the percentage o f impervious surfaces, traffic activity, and rainfall characteristics on stormwater quality to be studied.
Acknowledgements The a u t h o r wishes to thank the D r a i n a g e D e p a r t m e n t , Ministry o f the E n v i r o n m e n t , for their financial assistance and c o o p e r a t i o n towards the project. Thanks are also due to all staff f r o m the E n v i r o n m e n t and Hydraulics Laboratory, N a n y a n g Technological
456
CHU1 PENG CHEONG
Institute, for their effort a n d s u p p o r t in the project.
References Cairncross, S. and Feachem, R. G.: 1983, EnvironmentalHealthEngineering in the Tropics, John Wiley & Sons Ltd. Chanlett, E.T.: 1979, EnvironmentalProtection, 2nd edition, McGraw Hill Book Co. Meteorological Service Singapore: 1985, Monthly Reports 1985-86, Changi Airport, Singapore. Standard Methods: 1980, Standard Methods for the Examination of Water and Wastewater, APHA-AWWAWPCF, 15th edition. Tebbutt, T. H. Y.: 1983, Principles of Water Quality Control, 3rd edition, Pergamon Press.
WATERSHED CHUI PENG CHEONG School of Civil and Structural Engineering, Nanyang Technological Institute, Republic of Singapore, Singapore 2263
(Received November 1990) Abstract. A field monitoring network was set up within the Stamford canal watershed in 1989to study both the quantitative and qualitative aspects of storm runoff from this urbanised catchment. The data acquisition equipment comprised a continuous recording rain gauge, a water level recorder and an automatic water sampler capable of sampling storm runoff at preset intervals during rainfall events. Water samples were collected after each storm and laboratory tests were carried out on the physicaland chemicalproperties of the storm water. Preliminaryfindingson the temporal variations of stormwater quality during singlestorms and the effects of antecedent dry weather period on the quality are presented. The average ranges of some of the significant quality parameters found in the storm runoff were also established. The quality of storm runoff from the catchment under study was found to be of an acceptableleveland could potentiallybe developedas a water catchment area.
Introduction Singapore is a small country with a total land area of approximately 640 square kilometers. A b o u t 43 percent of the land area has been developed as residential, commercial a n d industrial areas, a n d 15 percent is agricultural land. The remaining areas are either natural reserves or other n o n - b u i l t - u p areas. The average a n n u a l rainfall is 2400 m m (Meteorological Service Singapore, 1985), which is also the m a i n source of water supply. As water is a scarce a n d limited resource in Singapore, almost half of the total land area has been developed as water catchment areas for the collection of rainwater into the fourteen i m p o u n d i n g reservoirs. These catchment areas cover all the natural reserves and also include some of the heavily built-up residential, commercial a n d light industrial areas. The current daily water c o n s u m p t i o n is slightly less than 1 million cubic metres. But with c o n t i n u i n g increases in economic a n d population growth a n d also with increase in social affluence, the d e m a n d for water is expected to continue to rise in the future. Thus, the search for additional sources of water supply like abstracting water from unprotected u r b a n catchment areas or other u n c o n v e n t i o n a l sources must be explored. It is even more evident after the recent dry spell (February - April 1990), where rainfall over the whole island was much lower than the average values a n d with total reservoir stock falling below the 70 percent level. In 1989, a field m o n i t o r i n g system was installed within the Stamford canal watershed, comprising a rain gauge, a water level recorder a n d a water sampler. The system was implemented to continuously collect data on rainfall, a n d both the quantitative a n d qualitative characteristics of stream flow within the basin. A considerable a m o u n t of field Environmental Monitoring and Assessment 19: 449-456, 1991. 9 1991Kluwer Academic Publishers. Printed in the Netherlands.
450
CI-IUI PENG
CHEONG
data has since been collected and some preliminary studies on the temporal variation patterns of common water quality parameters have been carried out. The average ranges of these selected quality parameters were also established. Among other things, the results obtained could be used to assess the feasibility of abstracting storm runoff from unprotected urban catchment areas for water supply and could also be used as a baseline condition for future studies on the effects of urbanisation on storm runoff quality. This paper presents the main features of the field monitoring system and some preliminary results of the study.
Description of Study Area The study area is a sub-basin of the main Stamford canal catchment, which embraces some of the most prestigious shopping, commercial and residential areas in Singapore. The drainage network of the Stamford canal system comprises four major upstream tributaries (Tributaries I, II, III and IV) and the main canal downstream, which discharges into the Marina bay (Figure 1). The sub-basin is drained by Tributary I as shown Figure 2. It has a drainage area of about 110 hectares and is relatively undeveloped. Presently, about 20 percent of the basin (22 hectares) are impervious areas consisting mainly buildings, roads and vehicle parking areas. The remaining areas are turfed areas such as the Singapore Botanic Gardens, a nature park and a golf course. The basin has a steep terrain and the highest land altitude is about 40 m above the mean sea level. The mean slope is about 15 percent and the direction of slope is towards the main arterial road - Holland Road and Napier Road - which bisects the basin. The basin is fully served by a separate sewerage system where excess rainfall is discharged directly into storm drains while sewage is discharged to a separate waste sewerage pipe. Tributary I, which is the main channel in the basin, runs along one side of the arterial road. It is constructed of reinforced concrete with a length of 1.7 km and a cross section of 3.7 m wide by 2.3 m deep at the monitoring site. The other minor drains consist of standard V-shaped and larger U-shape concrete sections.
Sampling and Monitoring System The location of the water sampler, raingauge and water level recorder is shown in Figure 2. The sampling site has been selected at the downstream end of the Tributary I sub-basin so that representative samples of storm runoff from the whole catchment can be collected. The water level in the channel is continuously monitored by means of a float-type stage recorder which is hooked on to an on-site data logger. Water levels are recorded at one minute intervals and the data are retrieved fortnightly using a lap-top computer. The measured stage hydrographs are subsequently converted into discharge hydrographs using a simple computer program based on the Manning's equation. Rainfall in the catchment is measured by a tipping bucket raingauge located at the Botanic Gardens. Stormwater is sampled using an ISCO automatic sampler which could retrieve up to a
Fig. h
0
SCALE
0.5
1 KM
Map of location of Stamford Canal watershed.
N
MARINA
BAY
KEY PLAN
P.
UBIN
&
,
-
s S~J/]
e ,.? o~r
o~176 ~,...
-.,.,,
,~
Fig, 2.
N
~C::2
9
""-.-'~
Ii
/I
II
20
o
Layout of Tributary I sub-basin.
%
SCALE
0.2
~
I
0.4 KM
9
['
=---
I
STAGE RECORDER WATER SAMPLER
RAINGAUGE
BUILDINGS
&
IN M E T R E S
& CARPARKS
CONTOURS
ROADS
TRIBUTARY
LEGEND
STORMWATER
RUNOFF
FROM
AN URBANISED
WATERSHED
453
maximum of 24 • 500 mL samples at preset time intervals and quantities. The sampler consists of a battery operated pumping system, a suction hose attached with a strainer at the end, and the sampling bottles. The sampler is triggered off by means of a water level actuator which is attached to the strainer. Each time the water level in the channel rises above the strainer, which is about 250 mm above the invert of the drain, the pumping system is activated. By comparing this initial sampling level with the stage hydrograph, the starting time of sampling can be found. In the present study, samples are collected at 15 minute intervals during each storm event and each time, four sampling bottles are filled giving a total sample volume of 2 litres. The samples are then retrieved after the storm, transported to the laboratory, and preserved for quality analysis. The following laboratory analyses are conducted on the stormwater samples according to the 'Standard Methods (1980)': total suspended solids (TSS), total dissolved solids (TDS), total organic carbon (TOC), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total phosphates, pH, conductivity, alkalinity, turbidity, oil and grease, and heavy metals such as lead, manganese etc.
Preliminary Results and Discussion The most important sources of pollution in the basin are likely to come from traffic, vehicle washing, plant fertilisers, grasscuttings, dustfall, litter and leaves. The rainfall itself, is also contaminated by air pollution. When it reaches the ground, the water removes pollutants from the surface before discharging into the drain. Roads and vehicle parking areas can produce traffic induced substances such as lead, oil and polycyclic aromatic hydrocarbons. Plant fertilisers and decaying tree leaves can produce pollutants like phosphates and other nutrients, which can lead to excessive algal growth in the aquatic environment when the concentrations are high. The laboratory testing programme for the project is carried out in two phases as the volume of each sample collected is insufficient for a complete analysis of all quality parameters. The first phase which was carried out in the last quarter of 1989, involved the testing of such physical characteristics as pH, turbidity, TSS, TDS and oil and grease of the stormwater. In the current second phase of testing, the chemical properties such as nutrients and heavy metals concentrations of the stormwater are analysed. Data from four representative storm events are presented in this paper. Table I shows the variations in the stormwater quality with different sampling time. These results were derived based on the flow-weighted mean concentrations of the storm events i.e. by dividing the pollutant loads with runoffvolumes. The first flush effect, which refers to the significant concentration of pollutants during the initial stage of streamflows, was not observed in the study for most of the selected parameters except for oil and grease and conductivity. It was, however, observed that there was an initial increase in the turbidity, TSS, COD and TOC concentrations, up to a peak which almost coincided with the peak of the storm flow and thereafter, the concentrations decreased through time. Although heavy metals concentrations and total phosphates content remained fairly constant throughout runoffs, more pollutants may be removed from the surface at high
454
CHUI PENG CHEONG TABLE I
Variations of flow-weighted mean concentrations with sampling time and ranges of concentrations with permissible levels for potable abstractions (Concentrations in mg/L unless specified). Time intervals in minutes Parameters
pH Turbidity (NTU) Colour Conductivity (/~S/cm) Total Alkalinity TSS TDS Oil & Grease COD TOC Total Phosphates Ca Na K
Mg Zn Cu Fe Pb TKN
0-15
6.5
15-30 30--45 45-60
6.3
6.4
6.6
71 76
137 159
129 160
104 138
68 15 106 38 3.3 54 10
52 16 211 34 2.8 43 10
52 14 168 39 1.8 80 20
61 13 107 45 1.2 52 11
2.7 6.8 1.8 2.4 0.48 ND 0.06 0.10 0.06 ND
2.8 7.1 3.5 2.1 0.46 0.~ ND 0.10 ND ND
3.0 6.6 1.3 2.0 0.5 ND ND 0.06 0.06 ND
2.7 6.0 1.0 1.7 0.~ ND ND ND ND ND
Range 60-70
75-90
Max
6.7
6.6
7.1
87 126 75 12 87 50 0.9 43 8 2.6 5.4 1.0 1.7 0.4 ND 0.04 0.04 0.04 ND
Min
6.3
Mean
6.5
Permissible Level
5.5-9.0
67 90
154 198
56 74
109 136
54 12 67 37 1.1 23 6
112 17 223 74 4.6 86 29
45 10 48 30 0 23 3
59 1000 13.9 500 141 40 500 1.93 52 30 11.7
2.6 6.7 1.0 1.8 0.5 ND ND 0.I0 ND ND
3.2 8.4 4.8 3.3 0.6 0.1 0.1 0.1 0.1
2.5 5.3 0.8 1.5 0.4 0 0 0 0
2.8 6.5 1.8 2.0 0.~ 0.01 0.01 0.07 0.03
200
0.7 75 200 30 1.0 0.2 0.3 0.05 1.0
ND-NotDetectable
f l o w rates. H e a v y metals like m a n g a n e s e , c h r o m i u m , c a d m i u m a n d T K N were n o t f o u n d in all the s t o r m w a t e r samples tested. T h e t o t a l p o l l u t a n t loads for the f o u r s t o r m events are c o m p u t e d a n d g i v e n in T a b l e II. T h e d a t a p r e s e n t e d in the table gives a n i n d i c a t i o n o n the effect o f a n t e c e d e n t dry p e r i o d o n the t o t a l loads. A l o n g e r a n t e c e d e n t d r y p e r i o d p r i o r to the s t o r m e v e n t t e n d s to p r o d u c e higher p o l l u t a n t loadings. T h e relationship b e t w e e n oil a n d grease a n d a n t e c e d e n t d r y p e r i o d is p a r t i c u l a r l y strong. T h e total p o l l u t a n t l o a d for oil a n d grease after an a n t e c e d e n t d r y p e r i o d o f 47 h o u r s was f o u n d to be a l m o s t d o u b l e that after 19 hours. T h i s c o u l d be due to the fact t h a t oil a n d grease is n o t readily r e m o v a b l e b y n o r m a l street s w e e p i n g as c o m p a r e d with litter and leaves. T h e ranges a n d m e a n c o n c e n t r a t i o n s o f selected q u a l i t y p a r a m e t e r s are also s u m m a r i s e d in T a b l e I. F o r all the p a r a m e t e r s analysed, n o significant variations in the c o n c e n t r a t i o n s were observed. T h e permissible levels o f surface waters suitable for p o t a b l e a b s t r a c t i o n s ( T e b b u t t 1983, C a i r n c r o s s 1983, C h a n l e t t 1979) are also s h o w n in the s a m e table. A c o m p a r i s o n b e t w e e n the m e a s u r e d p a r a m e t e r s a n d the permissible values shows
STORMWATER
RUNOFF
FROM
AN URBANISED
455
WATERSHED
TABLE II Rainfall characteristics and total pollutant loads (in kg) for four storm events. Storm event
17 Nov 89
23 Dec 89
Total rainfall (ram) Average Intensity (mm/hr) Antecedent dry period (hr) TSS TDS Oil & Grease COD TOC Total phosphates Ca Na K Mg Zn Cu Fe Pb
20.6 41 47 571 172 11.3 * * * * * * * * * * *
24.5 12 19 546 152 6.4 * * * * * * * * * * *
29 Dec 89 21.8 3.4 18 * * * 75 8.3 4.0 8.4 1.8 2.3 0.6 0.03 0.02 0.08 0.02
4 Jan 90 15.2 11.2 75 * * * 97 30 5.1 12.9 4.2 4.2 0.9 0 0.02 0.12 0.06
* Not tested that the quality o f s t o r m w a t e r f r o m the T r i b u t a r y I basin is quite acceptable and could be utilised as a potential source o f water supply.
Conclusion Storm r u n o f f quality has been m o n i t o r e d in an upstream urbanised basin within the S t a m f o r d canal catchment. The quality o f stormwater f r o m the Tributary I basin was f o u n d to be generally o f an acceptable level, and suitable to be developed as a water catchment area. The first flush p h e n o m e n o n was only recognisable for such parameters as oil and grease and conductivity. F o r other quality parameters such as turbidity, TSS, T D S and C O D , it was observed that the occurrence o f their peak concentrations coincided with the peak o f the runoff. The a m o u n t o f pollutant loads a c c u m u l a t e d in the basin were also f o u n d to be related to the length o f the antecedent dry period. The present study will be extended to cover the remaining tributaries o f the S t a m f o r d canal system so as to allow areal variations o f the quality characteristics and the influence o f the percentage o f impervious surfaces, traffic activity, and rainfall characteristics on stormwater quality to be studied.
Acknowledgements The a u t h o r wishes to thank the D r a i n a g e D e p a r t m e n t , Ministry o f the E n v i r o n m e n t , for their financial assistance and c o o p e r a t i o n towards the project. Thanks are also due to all staff f r o m the E n v i r o n m e n t and Hydraulics Laboratory, N a n y a n g Technological
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Institute, for their effort a n d s u p p o r t in the project.
References Cairncross, S. and Feachem, R. G.: 1983, EnvironmentalHealthEngineering in the Tropics, John Wiley & Sons Ltd. Chanlett, E.T.: 1979, EnvironmentalProtection, 2nd edition, McGraw Hill Book Co. Meteorological Service Singapore: 1985, Monthly Reports 1985-86, Changi Airport, Singapore. Standard Methods: 1980, Standard Methods for the Examination of Water and Wastewater, APHA-AWWAWPCF, 15th edition. Tebbutt, T. H. Y.: 1983, Principles of Water Quality Control, 3rd edition, Pergamon Press.
