20 Years of Marine Water Quality Monitoring in Hong Kong

| Director's Message | Introduction | Background of the EPD’s river water quality monitoring programme | The scientific basis of the EPD’s river water quality monitoring programme | River water sampling procedures: testing, analysis, and publication of results | Eastern New Territories | Northwestern New Territories | Lantau Island | Southwestern New Territories & Kowloon | Summary | Appendices | Acknowledgements | Disclaimer |

The scientific basis of the EPD’s river water quality monitoring programme

Brief historical background

The general background to the setting up of the EPD’s river monitoring programme was sketched in Chapter 1. This chapter will describe the programme in more detail, and look at the scientific basis that underpins the collection of water samples and the testing strategies used.


On being established in 1986, the EPD took on the responsibility of monitoring river water quality. A number of clear objectives lay behind the establishment of the river water quality monitoring programme. Part of the programme’s role was to provide ongoing snapshots of the condition of Hong Kong’s most important rivers. As time went on, these snapshots would build up into a cumulative picture of long-term changes in water quality. The information thus provided would constitute the data against which the Government could assess the effectiveness of any pollution abatement measures that were introduced, as well as check compliance with the statutory Water Quality Objectives (as they were drawn up for each Water Control Zone). Another important role envisioned for the data provided by the programme was as a basis for planning future strategies for controlling pollution.


[Photo of A river monitoring station in Tai Chung Hau Stream set up in 1986]

The river water quality monitoring programme is simple in concept. It consists first of setting up a number of monitoring stations in rivers and streams across Hong Kong. These monitoring stations are not pieces of fixed equipment or buildings, but simply points on rivers that are accessible and from which EPD staff can take representative water samples. Once these stations have been selected, EPD personnel make regular monthly visits to carry out in situ water quality measurements and to collect water samples for laboratory analysis later on.



Water quality parameters

The EPD’s approach to testing the water of rivers in the programme is a comprehensive and scientific one. Each water sample is tested for numerous physical, chemical and microbiological water quality parameters. When the programme first started in 1986, water testing included 37 different parameters, and since then another 11 parameters have been added to make a total of 48.


By a ‘parameter’ is meant one particular characteristic or property of water. Groups of related parameters show important general characteristics of the river from which the sample is taken. For example, the EPD uses six parameters that are important for getting a general picture of the condition of a river. They describe the basic physical chemical properties of the water, namely flow, water temperature, conductivity, dissolved oxygen, pH, and turbidity. 


Three parameters determine the amount of insoluble materials in the water: total solids, total volatile solids, and suspended solids.


Organic pollution of river water is measured by three parameters, namely 5-day biochemical oxygen demand, chemical oxygen demand, and total organic carbon.


[Photo of Nutrient parameters being analysed at the Government laboratory, using a flow injection analyzer (photograph provided by the Government Laboratory)]

Eutrophication is another potential form of river water pollution. It refers to the process whereby rivers receive excess nutrients (e.g. from sewage, livestock waste or fertilisers) which stimulate excessive plant growth in the form of algae or weeds. This in turn reduces oxygen in the water and affects aquatic life. The potential for river eutrophication was originally measured by seven nutrient parameters: ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen, total Kjeldahl nitrogen, ortho-phosphate, total phosphorus, and silica. More recently, in 2001, two further parameters relating to eutrophication were added. These are the plant pigment parameters of chlorophyll-a and Pheo-pigment.


Industrial pollution has been a problem in certain areas of Hong Kong, and 12 parameters relating to metals and metalloids were initially used in 1986 to measure the degree of such pollution. These parameters are aluminium, arsenic, boron, cadmium, chromium, copper, iron, mercury, manganese, nickel, lead, and zinc. In 1997, another seven were added (antimony, barium, beryllium, molybdenum, silver, thallium, and vanadium), making a total of 19. In addition, another four parameters closely connected with industrial and commercial pollution are used, measuring total cyanide, fluoride, oil and grease, and detergent.


Water pollution can bring with it unpleasant smells which are often due to oxygen depletion, and two parameters measure the chemicals that produce the odour associated with pollution. They are sulphide and free hydrogen sulphide.


From 1989, selected monitoring stations also monitored E. coli and faecal coliforms, which indicate faecal contamination from domestic sewage and animal waste. These two bacterial parameters were extended to all stations from 1999.


Data relating to faecal coliforms and E. coli in this report is presented in the form of annual geometric mean figures, while data on the other 46 water quality parameters is presented in the form of annual median figures.



The Water Quality Index (WQI)

Each of the 48 parameters discussed above is important and has its own story to tell. Together they provide a detailed picture of river water quality, but this can be somewhat complex for the general public to understand. To make it easier to get a picture of general river water quality, the EPD has also adopted a simple Water Quality Index (WQI). Developed by the Ministry of Transport and Public Works in The Netherlands, the WQI uses just three key parameters to create a simple and clear way of measuring a river’s level of organic pollution and its ability to support aquatic life.


How the Water Quality Index is calculated

No. of points awarded





91 – 110

< 3

< 0.5


71 – 90
111 – 120

3.1 – 6.0

0.5 – 1.0


51 – 70
121 – 130

6.1 – 9.0

1.1 – 2.0


31 – 50

9.1 – 15.0

2.1 – 5.0


< 30 or > 130

> 15.0

> 5.0


These three parameters are dissolved oxygen (DO), 5-day biochemical oxygen demand (BOD5), and ammonia-nitrogen (NH3-N) content. Points are awarded for each parameter as can be seen from the accompanying table: the better the results for each parameter, the fewer the points awarded. For example, a river that had a DO of 91% saturation or above, a BOD5 reading of less than 3, and less than 0.5 mg per litre of NH3-N, would receive three points. A river at the other end of the scale would receive 15 points.


How the Water Quality Index is graded

Water Quality Index

Water quality condition

3.0 – 4.5


4.6 – 7.5


7.6 – 10.5


10.6 – 13.5


13.6 – 15.0

Very Bad


The WQI of a particular river monitoring station can be expressed as both a monthly and an annual figure. The monthly figure is simply obtained by adding up the points gained from the three parameters after the monthly sampling and testing has taken place. The annual WQI is obtained by averaging out all the monthly WQI for the year. In both cases the points total is then given an evaluative label, as the accompanying chart shows, which may range from ‘Excellent’ to ‘Very Bad’.



Compliance with the Water Quality Objectives (WQOs)

Another way in which the EPD analyses and presents data from its water monitoring programme is to express the level of compliance with the statutory Water Quality Objectives (WQOs). These WQOs are specific objectives laid out in the Water Pollution Control Ordinance (see Chapter 1). They specify the long-term water quality goals that the Government is to achieve and maintain for individual rivers in Hong Kong. Each river has a slightly different set of WQOs, specific to its particular characteristics and beneficial uses. 


This report presents the annual average compliance with key WQOs of each individual river. The parameters used for calculating WQO compliance include pH, suspended solids, dissolved oxygen, 5-day biochemical oxygen demand, and chemical oxygen demand. In addition to the compliance data for individual rivers, the EPD also calculates an overall compliance figure for all of Hong Kong.


Compliance with the WQOs is expressed as a percentage, with 100% indicating full compliance. These annual compliance percentages are based on results of all samples taken from an individual monitoring station throughout the year. For example, if all samples from a monitoring station taken through the year meet the WQO for dissolved oxygen, then its WQO compliance rate will be 100%. If only half meet that WQO, its compliance rate will be 50%.


Because some rivers have more than one sampling station, WQO compliance for these rivers is expressed as an average of the annual compliance rate of all its individual sampling stations. In the same way, the overall compliance figure for the whole of Hong Kong is averaged out from all of the sampling stations in the territory.



Long-term water quality trends

A further scientific approach to sampling work taken by the EPD relates to the analysis of long-term water quality trends. The EPD uses certain proven statistical tools to highlight and assess trends in water quality over longer periods. The non-parametric Seasonal Kendall Test used by the EPD is based on ten or more years of monitoring data. When correctly applied, it gives an accurate indication of whether river water quality parameters at each monitoring station show statistically significant increases or decreases, or whether no trends have developed.



Pollution load reduction

A final method used by the EPD to assess improvements in water quality is known as ‘pollution load reduction’. This method measures the reduction over time in the overall organic pollution load in a given river catchment area, expressed in terms of kilograms of biochemical oxygen demand per day (kg BOD/day).



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