Analysing water samples

[Photo of EPD Laboratory: testing for E. coli bacteria using the membrane filtration method]

After collection, the water samples are sent to either the EPD laboratories or the Government Laboratory for further analysis.

These laboratories use a number of advanced analytical technologies, including Flow Injection Analysis (for nutrients), Inductively Coupled Plasma - Atomic Emission Spectroscopy and Inductively Coupled Plasma - Mass Spectrometry (for metals), and Gas Chromatography - Mass Spectrometry and a UV detector coupled with a Fluorescence detector (for trace organics).

 

In 1986, the collected water samples were tested for a total of 23 different physical, chemical, microbiological and other parameters, while sediment samples were tested for 18 physical and chemical parameters. The number of parameters measured in water monitoring has remained largely stable over the intervening years, but in the 1990s the sediment monitoring programme was expanded to include a number of metals and organic pollutants. In 2005, 61 physical and chemical parameters were measured and analysed in sediments. These included particle size, electrochemical potential (as highly anoxic sediment with negative potential is related to organic pollution), chemical oxygen demand (which indicates organic pollutants), total sulphide (inorganic constituents, source of the unpleasant-smelling gas hydrogen sulphide), 15 metals and metalloids (aluminium, arsenic, barium, boron, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, silver, vanadium and zinc), and trace toxic organics (PAHs and PCBs - 16 compounds and 18 congeners respectively).

 

The two tables below give a summary of parameters for water and sediment samples tested by the EPD and Government Laboratory. For full details of the methods used for testing water and sediment samples, please refer to Appendix A-13 and A-14.

Parameters tested for in water samples
Physical and Aggregate Properties	Aggregate Organic Constituents	Nutrients and Inorganic Constituents	Biological and Microbiological Examination
- Temperature - Salinity - Dissolved Oxygen - Turbidity - pH - Secchi Disc Depth - Suspended Solids - Volatile Suspended Solids	- 5-day Biochemical Oxygen Demand (BOD5)	- Ammonia Nitrogen - Unionised Ammonia - Nitrite Nitrogen - Nitrate Nitrogen - Total Inorganic Nitrogen - Total Kjeldahl Nitrogen - Total Nitrogen - Orthophosphate Phosphorus - Total Phosphorus - Silica (as SiO2)	- Chlorophyll-a - Escherichia coli (E. coli) - Faecal Coliforms - Phytoplankton

 

 

Parameters tested for in sediment samples
Physical and Aggregate Properties	Aggregate Organic Constituents	Nutrients and Inorganic Constituents	Metals and Metalloids	Trace Organics Compounds
- Particle Size - Electrochemical Potential - Total Solids - Total Volatile Solids - Dry Wet Ratio	- Chemical Oxygen Demand (COD) - Total Carbon	- Ammonia Nitrogen - Total Kjeldahl Nitrogen - Total Phosphorus - Total Sulphide - Total Cyanide	- Aluminium (Al) - Arsenic (As) - Barium (Ba) - Boron (B) - Cadmium (Cd) - Chromium (Cr) - Copper (Cu) - Iron (Fe) - Lead (Pb) - Manganese (Mn) - Mercury (Hg) - Nickel (Ni) - Silver (Ag) - Vanadium (V) - Zinc (Zn)	- Polychlorinated Biphenyls (PCBs) - Polyaromatic Hydrocarbons (PAHs)

 

Tools for data analysis and presentation

As the data collected builds up over time, the EPD uses various proven statistical means to analyse water quality trends and patterns. The first of these is the Seasonal Kendall Test, which enables accurate analysis of long-term trends. The Seasonal Kendall Test overcomes a number of problems that can commonly skew the results of long-term studies, such as non-normal data, missing values, seasonality and serial dependence (where data is dependent on other data).

 

Another common tool used by the EPD is the Wilcoxon-Mann-Whitney Test, which tracks significant differences in two sets of comparable data. This test is particularly useful for examining year-to-year changes in water monitoring results.

 

Apart from using traditional graphs showing changes in water quality over time, the EPD also uses colour contour maps as a powerful presentation tool. A series of contour maps showing the consecutive monthly values of a certain parameter is a useful way of illustrating gradual seasonal water quality changes in different water bodies.
[Photo of Typical contour maps showing monthly changes in dissolved oxygen in Hong Kong water]

 

Published reports and data

[Photo of The EPD's annual marine reports (1986-2004)]

All the information that the EPD collects from its marine water monitoring programme is made available to the public by various methods. Since 1988 it has compiled and published annual reports that summarise the data collected for each year. Originally these reports were produced in hard copy and displayed at the EPD's resource centres, placed on sale at Government Publication Sales Centres, and kept at public and university libraries. From 1998 the EPD began also producing the reports in electronic format. The reports were uploaded onto the EPD's website, where they could be viewed online or freely downloaded. From 2002 the EPD discontinued its hard copy editions of the annual reports as a paper-saving measure, and from that year produced only a CD-ROM version in addition to its web-mounted version. The raw water quality data collected during monitoring is also made available to the public online.

 

Besides these annual reports, information is uploaded onto the EPD website monthly which provides recent marine water quality information covering the two key parameters of dissolved oxygen (DO) and E. coli bacteria, based on data from one representative marine monitoring station located in each of the ten Water Control Zones.

 

The reports and other data made public by the EPD are widely used within the Government in its efforts to protect and manage marine water quality for Hong Kong. Besides Government policymakers and planners, the data is important to legislators, academics, researchers, environmental consultants, media workers and the public, and is used extensively in activities such as teaching, environmental research, impact assessment and public consultation for major developments and project