Monitoring and Assessment of Chemical Quality
Session Objectives
· To highlight the
relative priority for microbiological and chemical water quality monitoring and
emphasis the need for a rational, health-based approach to monitoring of water
quality.
· To outline the key
characteristics of monitoring programmes which may be implemented for chemical
water quality.
· To describe the analytical
ranges commonly employed in chemical water quality monitoring, highlight key
constraints in chemical analysis and stress the need for quality control.
· To emphasis the value of risk
assessment as a key supporting activity of chemical analysis and in planning
monitoring programmes.
Introduction
Chemical testing is generally not undertaken as frequently as
microbiological analysis because, in general, the health risks posed by
chemicals are chronic rather than acute and because changes in water chemistry
tend to be longer-term unless a specific pollution event has occurred. It should
be stressed that monitoring the microbiological quality of water is much more
important than monitoring of chemical quality and chemical testing should
generally be a lower priority.
However, where resources permit, routine testing of the chemical
quality of water should be undertaken. Priority should be given to those
substances which are known to be of importance to health and which are known to
be present in significant concentrations in drinking-water. For instance, the
monitoring of nitrate is recommended in many water supplies and in particular
those which are located in rural areas, or where recharge occurs in an
agricultural area. In these circumstances, regular monitoring is recommended to
ensure that early warning of increases is noted or when nitrate releases are
highly seasonal in nature.
An assessment of the chemical quality of water should be
undertaken during source selection and this should relate to known activities
within the catchment of the source and possible natural pollutants. This should
be as comprehensive as possible and cover a wide range of pollutants.
In areas where toxic chemicals are released into the aquatic
environment, routine monitoring should be undertaken and closely linked with an
emergencies warning procedure which should function to alert water suppliers,
surveillance agencies and health bodies of any accidental releases of substances
into water sources.
Types of monitoring programme
As with any form of monitoring, it is important that clear
objectives are set before the start of data collection activities and that
sample sites and frequency of analysis are determined to meet the objectives and
not vice versa. In the past, some water quality assessments have worked from the
other way round and monitoring programmes have been designed to fit existing
infrastructure. The problem with this approach is that it very often results in
a failure to address the most pressing problems and also a failure to provide a
full picture of the problem being monitored.
In general, monitoring of the chemical quality of water may be
undertaken in two ways.
1. Routine monitoring of known problem
substances: this type of programme is designed to keep a continuous watch on
substances which are known to have a health impact or compromise treatment
efficiency and which are known or suspected to be in the water supply to be
monitored. It is important that substances whose concentration is likely to
change are monitored more regularly than those where concentrations are
essentially stable. This is largely determined by the source of the
contamination. Contaminants from essentially natural sources, such as fluoride,
are unlikely to vary significantly over time and therefore do not require
frequent analysis. Although there may be exceptions to this such as the raised
arsenic levels in some groundwaters in West Bengal, India. Contaminants deriving
from anthropogenic sources of pollution may require more frequent analysis, for
instance heavy metals in water sources downstream of tannery waste discharges.
Equally, where treatment is employed to remove or control specific substances
(e.g. nitrate or phosphorous), these should be routinely monitored at the plant
to ensure that treatment is effective.
2. Periodic quality assessment: this type of monitoring
is either routine or non-routine assessment of water quality done on a
relatively infrequent basis (annual or greater). Such assessments will certainly
be done during the source selection procedure and may involve periodic
evaluations of trends in water quality over time. Such assessments are likely to
include a wider analytical range and be used to provide regular comprehensive
assessments of water quality to assist in long-term water source and supply
management and for long-term trend analysis.
Both approaches will concentrate on water quality in the source
and as it leaves the treatment works or borehole, with a limited number of
samples taken from within the distribution system, unless the materials used in
the distribution system are suspected of providing a significant proportion of a
harmful substance. In these circumstances it is usually more effective to
monitor and control the quality of materials and chemicals used in water
treatment during their production and prior to their use. However, where
materials or chemicals have been used without quality control during
manufacture, some monitoring of specific chemicals may be required by the public
health agency. For instance, where lead pipes or lead-based solders are used,
regular monitoring of lead may be recommended.
Selection of variables for monitoring and
assessment
As mentioned above, during source selection, a comprehensive
assessment should be made of water quality to ensure that any likely risks to
health are identified and appropriate action taken with regard to source
protection, treatment requirements and blending of water. Thus analysis of the
major ions and nutrients should be done on all water supplies as well as any
other substances deemed likely to be present on the basis of land-use within the
catchment of the source. However, whilst it is preferable to have a complete and
comprehensive description of water quality before a water supply is
commissioned, there are a number of constraints in trying to achieve this.
Many analyses are expensive to carry out, both in terms of the
equipment required to perform the analysis and in terms of the consumable
required. This means that if analysis is required for a particular analyte which
uses sophisticated equipment, this may only be done occasionally when the
laboratory has enough samples to make it economic to start up the equipment and
run the analysis. It is never economic to start equipment such HPLC or a flame
photometer to carry out a single analysis. Therefore appropriate storage
facilities are required for the sample and appropriate preservatives must be
available to prevent sample deterioration. This will further increase the costs
of analysis.
Thus for some parameters, there may be a considerable time to
wait before the results of analysis are known. However, the delay in opening the
water supply, particularly in drought-prone areas, may be unacceptable.
Therefore during source selection, parameters should be divided into essential
and desirable. This should be done based on the risk to health, potential to
cause consumer rejection, likelihood of causing operational problems, cost and
ease of analysis, likelihood of presence in drinking-water.
The net result is likely to be a range of parameters which are
analysed rapidly and perhaps on-site and before the source is commissioned (for
instance, nitrate, fluoride, iron etc.) and those which will be done, but
possibly after the source has been commissioned.
There are a number of parameters which, when used in conjunction
with a pollution source assessment, provide a good overall indication of
chemical water quality and others whose impact on human health or the
environment are great and should be included in initial testing. The presence at
high levels of these parameters in the source water may indicate that other
analyses are required. These include: nitrate, pH, Eh, fluoride, dissolved
oxygen and chloride.
The presence of elevated levels of nitrate in water indicates
pollution of the source and it is important that the type and source of the
pollution is identified. Nitrate pollution may occur from agricultural source,
sewage disposal and urban runoff. Agricultural sources may indicate that there
will also be a problem with other agricultural pollutants such as pesticides. It
is important that a survey is carried out to identify whether there is use of
pesticides in the area and to find out application rates and time of
application. Pesticide analysis is difficult and expensive, indeed there are a
number of pesticides for which no analytical methods exists for detection in
water, therefore routine analysis of pesticides will not be carried out at the
start of a programme and is rarely fully developed. Nitrate contamination which
can be linked to a sewage outfall may also indicate unacceptably high levels of
microbiological contamination which should be addressed as a matter of priority.
For routine analysis, both the monitoring agency and the
supplier should aim to concentrate on those chemical parameters which are of
greatest health significance or provide a general description of water quality
and for which analysis is inexpensive, quick and may be done on-site. There are
variables such as pH and Eh (redox potential) which should, by preference, be
done on-site as the sample may deteriorate during transport.
Risk assessment
As with microbiological monitoring, it is important that
monitoring of chemical water quality is linked to a process of hazard
identification and risk assessment. Thus when designing a monitoring programme,
an inventory of likely sources of pollution and the likely vulnerability of a
water source or distribution system to contamination should be made. This means
that information will be required on the following:
· geographical
features, including topography, relief, lithology, climate, land-use, hydrology;
· other water uses from the
source; and
· pollution sources, treatment
of wastes and discharge consents in operation.
Risk assessment should be a dynamic process which is conducted
or updated routinely by suppliers and surveillance bodies to ensure that no new
risks are developing for which remedial or preventative action is required.
Thus, for each new activity established within the catchment of the water source
used for drinking purposes, a detailed description of likely pollutants that may
be discharged, wastewater treatment arrangements, recycling and discharge
consents must be obtained. These should be used to allow water suppliers and
surveillance bodies to object to developments which will compromise public
health through likely discharges and to establish monitoring programmes which
are focused on health-based risk assessment.
Chemical testing of drinking-water supplies is often linked to
source quality monitoring and thus it is very important that hydrological data
are collected at the same time as quality data as this has a profound influence
on water quality. Flows in rivers will determine the concentration of pollutants
in the aquatic environment. For instance, in the UK at low flows, up to 95 per
cent of the flow of many rivers which pass through urban areas is municipal
effluent, whilst at high flows this percentage will be greatly decreased and
effluent may only account for 30% of the flow.
The status of rivers with regard to groundwater is also
important as this will influence water quality. Hydrogeological data are also
important as water level, flow patterns and water movement rates will all affect
water quality. For example, changes in water level may significantly alter water
quality as pollutants removed from infiltrating water in the unsaturated zone by
sorption may be eluted (de-sorbed) if the groundwater level later rises.
Quality control
It is important that data generated in chemical testing
programmes in different regions are comparable and that time series of data are
also comparable. Therefore: standard operating procedures are required for
sampling, field testing and data reporting; AQC schemes should be carried out
for all laboratories carrying out analysis; field equipment should be regularly
checked and calibrated; staff should be adequately trained and supervised.
Provided the same analytical techniques are used over the time
period to be studied and the above are implemented, data time series should be
comparable. However, as analytical techniques are continually improving and
changing, it is common to find that techniques for analysing particular
variables change and that the results produced are not directly comparable to
previous methods. When this happens, it is important that both the new and old
technique are used to analyse samples for a hand-over period to allow a
conversion graph to be prepared to allow comparison of the results of both
methods.
Where there are a number of laboratories involved in water
quality analysis, there should, preferably be some form of inter-laboratory
comparison. This may take the form of a reference laboratory provided spiked
samples to laboratories in which the concentrations of chemical constituents is
not known by the participating laboratories. Alternatively, laboratories can
rotate quality assurance sample preparation. The purpose of such procedures is
to improve the overall reliability of the data produced in water quality
analysis.
Conclusion
Chemical monitoring is a lower priority than microbiological
monitoring. As monitoring of chemical water quality is developed, a clear
priority should be given to substances of known health impact and which are
known or suspected to be in the water supply. Monitoring may be carried out
routinely for some chemicals whose presence in water is likely to change over
time, for which treatment is applied, or which have highly seasonal profiles.
For most chemicals, and for all contaminants which have a natural source in the
environment, monitoring may be done through periodic assessment of water
quality.
Monitoring of chemical water quality should incorporate hazard
identification and risk assessment as a key tool for managing risks. Thus, water
suppliers and surveillance bodies should be aware of all potentially polluting
activities within the catchment of a water source and use this information to
help design monitoring programmes. Where activities involving the use or
production of toxic chemicals, adequate emergency warning procedures must be
established which will ensure that water suppliers and surveillance bodies are
kept informed of any accidental spill into water sources.
References
Chapman, D. (ed) (1996) Water Quality Assessments. 2nd edition.
Chapman and Hall.
Anon. (1993) WHO Guidelines for Drinking-water Quality, Volume
1. WHO, Geneva.
Howard, G and Simonds, A, (1995) Where there is no training -
pollution risk assessment for field staff. Waterlines, Vol 14, No. 1. July 1995.
Presentation Plan
|
Section |
Key points |
OHP |
|
Introduction |
· chemicals evaluated during the
preparation of the 2nd edition of the Guidelines |
1,2,3 |
|
· many chemicals, such as
nitrate, lead and arsenic, can be toxic to humans and may come from natural and
anthropogenic sources |
|
|
· chemical testing not
undertaken as often as microbiological testing because most health risks are
chronic not acute |
|
|
· changes in water chemistry
also tend to be long-term unless specific pollution event occurs |
|
|
· where possible do routine
monitoring of chemicals of health concern and known to be in drinking-water -
e.g. nitrate |
|
|
· comprehensive assessment of
chemical water quality should be done during source selection |
|
|
· early warning procedures
essential and should link resource managers, water suppliers, surveillance
agency and health bodies |
|
|
Types of monitoring programme |
· need to have clear objectives
before data collection starts and monitoring network should be designed to match
objectives not vice versa |
4,5 |
|
· where objectives are set to
match monitoring programmes may fail to meet most pressing needs |
|
|
· two key approaches to chemical
monitoring |
|
|
|
1 routine monitoring of known problem substances |
|
|
|
- designed for continuous surveillance of substances of health
concern and which are in water supply |
|
|
|
- only routinely monitor substances where concentration likely
to change because of a pollution event or treatment failure |
|
|
|
2 periodic quality assessment |
|
|
|
- either routine or non-routine assessment of water quality on
relatively infrequent basis |
|
|
|
- assessment certainly done during source selection and
subsequent occasional evaluations |
|
|
|
- such assessments likely to have a broader analytical range
than routine monitoring |
|
|
· both types of monitoring tend
to focus on sources and where water leaves pumping station or treatment plant
unless distribution system suspected of leaching substances into the water |
|
|
· where substances in water are
derived from chemicals and materials used to treat and distribute water, it is
often better to monitor and control manufacture than in drinking-water |
|
|
Variable selection |
· preferable to have a complete
description of quality of a water supply prior to commissioning, but may be
problems in achieving this |
6 |
|
· many analytes expensive to
analyse for and only economic for analysis of a limited number of samples,
therefore may delay analysis |
|
|
· this has implications for
source commission as unacceptable in many circumstances to wait until results
available for commissioning |
|
|
· therefore need to identify a
restricted range of analytes of health concern which can be used to indicate
broader problems and which are relatively easy to analyse,e.g. nitrate and pH
|
|
|
· nitrate is of particular
concern and in many circumstances is routinely monitored and source identified
as this may indicate other pollution |
|
|
· for routine analysis
concentrate on chemicals of known health concern and can be easily monitored
|
|
|
· some parameters should be done
on-site to prevent sample deterioration |
|
|
Risk assessment |
· monitoring chemical quality
should also be linked to risk assessment and hazard identification |
7 |
|
· when assessing vulnerability
make sure collect information on geographical/geological features that may
increase vulnerability |
|
|
· during risk assessment
identify all likely sources of pollution |
|
|
· risk assessment is dynamic and
should be routinely undertaken |
|
|
· both supplier and surveillance
agency should be aware of new activities within the catchment to predict likely
impacts on water supply |
|
|
· need to collect
hydrological/hydrogeological data as well as quality data |
|
|
Quality control |
· need to be able to compare
data from different regions and time series of data |
|
|
· therefore quality control is
essential and standard operating procedures are required for sampling, analysis
and reporting |
|
|
· all equipment, including field
kits, require calibration and staff trained |
|
|
· where techniques change over
time, ensure that new techniques is calibrated against old technique to ensure
comparability |
|
|
· inter-laboratory comparison is
important for improving and maintaining analytical quality |
|
|
Conclusions |
· chemical monitoring is a lower
priority than microbiological monitoring |
|
|
· priority should be given to
those parameters of known health concern |
|
|
· routine monitoring should be
done for parameters whose concentration is likely to vary, for which treatment
is carried out or which have seasonal profiles |
|
|
· hazard identification and risk
assessment should also be carried out and an early warning system implemented
|
|
