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WELL FACTSHEET Go BackThe Microbiological Contamination of Water Supplies Author: Steven Sugden, April 2006 Quality Assurance: Sandy Cairncross The contamination of boreholes and shallow wells from
on-site latrines is an issue that is
generally poorly understood and irrationally
assessed by organisations implementing water
supply and sanitation programmes. This
should not be the case as the health risks
are often lower than popularly anticipated.
The method of risk assessment outlined in
this fact sheet is within the technical
capacity of a competent engineer and should
be regarded as being the first step in
gaining a better understanding of the
problem. This fact sheet provides background
information on the factors that lead to
microbiological contamination, the basic
principles of risk assessment, and points
those requiring more guidance in the right
direction. It does not contain any
information about assessing nitrate or
chemical contamination from latrines, which
can be a problem in some areas. Pathogens characteristics and water point contamination The majority of disease organisms (pathogens) lack the
capacity to propel themselves through the
environment in which they live, and those that can
are not capable of travelling very great distances.
Instead, pathogens are carried from one point to
another within the medium in which they live and in
the case of water point contamination from latrines,
this is in the liquid that accumulates within the
pit. Pathogens, therefore do not travel further or
faster than the water in which they are suspended
and this is an important fact to remember when
trying to understand water point contamination. There are two other important attributes of pathogens that
effect their ability to contaminate a water point;
their size and their die-off rate. Size
Helminth (worm) eggs and Protozoa are relatively large and are efficiently removed through the physical filtration process in the soil (Lewis, Foster et al 1980). Bacteria and viruses are much smaller and are much more able to travel unrestricted through the subsoil. The bacteria and viruses in the table below are some of the greatest causes of concern,
Faecal micro-organisms, like all life forms, have a limited
life span in the environment and die off
exponentially at rates which vary enormously from a
few hours to several months. In ground water, some
viruses are known to survive for up to 150 days. In
the case of E. coli indicator bacteria, an estimated
half life (ie the time taken for 50% reduction in
numbers) in temperate ground water has been noted as
being as high 10 to 12 days, with survival of high
numbers up to 32 days. Some salmonella species have
been shown to persist for up to 42 days (ARGOSS). If the time taken for pathogens to be transferred to the
water point is large, the pathogens will have died
off and the water will no longer present a threat to
public health. The figure below shows six different factors which can effect pathogen transmission from a latrine to a nearby water point. These are discussed in turn.
1.
Amount
of liquid in the pit
Any liquid in the pit is certain to be grossly contaminated. The amount of this liquid depends on the type of latrine and the method of anal cleansing. If the pit (or tank) is full of liquid a large static head is created within the pit and the liquid forced under pressure into the unsaturated zone of the subsoil (i.e. the zone above the ground water table, which is not saturated with water). If the pit is dry, there is no liquid to create a static head, no pressure is exerted, and there is no flow into the unsaturated zone. With dry latrine systems the pathogens remain within the pit and water point contamination does not occur. This puts the dry systems used in ecological sanitation among the safest options from the perspective of ground water contamination. General rule: The smaller the amount of liquid in the pit, the lower the risk of water point contamination.
2.
Nature
of the unsaturated zone The spaces between the grains in some types of sub-soil are
so small that they physically prevent the passage of
a pathogen. In effect the sub-soil acts as a filter.
This filtering process is enhanced in established
latrines when an organic film of micro-organisms
develops on the surface of the soil particles (as in
a slow sand filter) and this effectively further
restricts the passage of the pathogen.
Some clay soils also have the capacity to absorb viruses and prevent their passage to the saturated zone. General rule: The smaller
the sediment grain size the lower the risk of
contamination
3.
Distance
between the base of the pit and water table The further water containing the pathogen has to travel to
the water table, the more tortuous its route and the
longer it is retained. This additional time allows
for greater numbers of pathogens to die off
naturally. Care
is needed when assessing this factor to consider the
higher water table level in the wet season and not
just the dry season water levels. General rule: The greater the distance between the base of the pit and the water table, the lower the risk of contamination
4.
Nature
of the saturated zone (aquifer)
The ease at which water can flow through a rock is known as its permeability (measured in metres per day (m/d)) and is dependent on both the size of the spaces (or pores) and how well they are connected with each other. Sands and gravels have large well connected pore spaces between their grains and allow water to flow relatively easily. As a result they have permeability ranging between 10 to 100 m/d. Clays have a high porosity, but are poorly connected and water has difficulty in passing through them easily; as a result clay has permeability ranging from only 0.01 to 0.1 m/d.
The ability of an aquifer to store water depends on the
volume of the spaces (or pores) between the grains.
Sands can have a porosity of 0.3 (i.e. 30% of their
volume is air space), whilst consolidated rock
porosities rarely exceed 0.01. General rule: The greater an aquifer’s
permeability, the higher the risk of water point
contamination
5.
Horizontal
distance between latrine and water point The further the horizontal distance the pathogen has to
travel from the point of entry into the water table
to the water point, the longer it is retained and
the more likely the pathogen is to die.
General rule: The
greater the distance between the latrine and the
water point, the lower the risk of contamination
6.
Direction
and velocity of the groundwater flow The rule that water flows downhill holds true for the vast
majority of ground water, although there are
exceptions. It would be more accurate to say that
water always travels down a hydraulic gradient from
areas of high water pressure to areas of low
pressure. Groundwater will generally follow the
slope of a hill and flow towards a river, sea or
lake. The steeper the hydraulic gradient the faster
the groundwater (and the pathogen it contains) will
travel towards the water point. If the latrine is located physically lower than the water
point it is highly unlikely that contamination from
the latrine will be a problem. However, many rural
villages, and the latrines they contain, are sited
on the highest points in an area whilst water points
are usually found in the valleys where it is easier
to find and access ground water.
General rule: The
greater the hydraulic gradient towards the water
point, the higher the risk of water point
contamination
With an understanding of these 6 general rules it is possible to undertake a rudimentary risk assessment. Assessing the risk of water point contamination Assessing the risk of water point contamination from
latrines is based on gaining an understanding of the
amount of time it would take the water, and the
pathogens it contains, to travel from the pit to the
water point. The longer it takes, the greater the
reduction in the number of pathogens through natural
die-off. The overall aim in either siting a latrine
or water point is to ensure that the pathogen
die-off has been sufficient to reduce the risk to a
level where it is not a public health concern.
The time taken can be used as a proxy indicator for risk of
contamination. The Guidelines for Assessing the Risk
to Groundwater from On-Site Sanitation (ARGOSS)
produced by the British Geological Survey (BGS)
states that the following times are applicable to
assessing risk from microbiological contaminates.
(BGS - ARGOSS 2001) AGROSS takes care to stress that the ‘low risk’
category should provide confidence, but no
guarantees, that the travel time would result in
levels of micro-organisms which are unlikely to
represent a major risk to health.
The ‘very low risk’ category provides a
further margin of safety and therefore greater
confidence that the water will meet WHO guidelines
and that the more persistent pathogens will have
been removed. Assessment
stage one – Is the unsaturated zone sufficiently
reducing the pathogen levels?
Because of the very low velocities of unsaturated flow, the
unsaturated zone is the most important line of
defence against faecal pollution of the aquifers
(Cave & Kolsky 1999). If the rate of
transmission to the aquifer is slow, by the time the
water from the pit reaches the aquifer, the
pathogens in it will have died off and the risk to
public health will be minimal. The capacity of the latrine design and the unsaturated zone
to reduce the risk of contamination can be estimated
by using a combination of the following tables.
Reduction through latrine design
Reduction
in the unsaturated zone
If these do not sufficiently reduced
the pathogens to ‘low risk’ levels,
it will be necessary to estimate the effect
of the aquifer has on pathogen reduction. Assessment
step two – The effect of the saturated zone on
pathogen levels
This is based on the number of days
the pathogen remains in the aquifer before it enters
the water point. It is calculated using the
following formula,
ARGOSS provides the following table to
act as a guide when the exact figures are not known.
It also suggests using a hydraulic gradient of 1/100
(0.01).
Example 1: In a clean sand aquifer where the latrine is situated 20m from a water point the number of days taken for a pathogen to travel to the water point is:
Example 2: In a fine silty sand aquifer where the latrine is situated 20m from a water point the number of days taken for pathogen to travel to the water point is: If the actual figures for porosity and permeability are not
known, it is worthwhile placing figures from the
top, bottom and mid-way of the given ranges into the
calculations for a specific situation This will
provide a guide as to the maximum and minimum
ability of the saturated zone to reduce the
pathogens to a safe level and allow the designer to
make a more considered assessment. Other factors to consider
If the assessment clearly shows that the risk of
contamination is very low, then no other action is
necessary other than to monitor the situation to
ensure distances are adhered too and the designs and
quality of construction remain high. If the assessment shows a low risk of contamination it may
be worthwhile to confirm the result with a series of
water quality checks of a representative sample of
water points. If the results confirm some form
contamination, it will be necessary to verify that
the latrines are in fact the cause of the problem.
Water point contamination can occur from many
causes, including faulty or substandard water point
construction which allows surface runoff to enter. Be careful not to jump to conclusions. The methodology
described is conservative and makes a number of
assumptions based on approximate categories of soil
type, conductivities, gradients etc. If the results
show borderline risks it may be worthwhile employing
the services of a hydrogeologist to undertake a more
exact assessment. It is important to keep the community informed and to
discuss with them the implications of your findings.
With community-owned water points the users should
have the ultimate decision as to what action to
take. Your role may be only to ensure that they base
their decisions on sound knowledge and an awareness
of the different options. Methods of reducing the risk of contamination
Other issues to consider before taking action
Some
final points to ponder? “Groundwater contamination is
thus a matter of degree, and rather than basing
all decisions on absolute water quality targets or
guidelines, it may be more helpful to strive for
the best practicable water quality which may be
achieved with economic, financial, technical, and
social constraints. Such an approach will vary
with locally available alternatives of water
supply” “If however, one reviews
the epidemiological evidence concerning the
relationship between dose and response in drinking
water, the evidence for the most commonly used
indictor, (E coli), appears significant at doses
greater than 1000 E.coli / 100ml… It would this
appear unwise to forego the health benefits of
affordable and sustainable sanitation to eliminate
the risk of groundwater contamination of less than
1000 E.coli / 100ml” Cave
and Kolsky, Groundwater, latrines and health, WELL
Task 163 1999.
The Guidelines for Assessing the Risk to Groundwater from On-Site Sanitation (ARGOSS), British Geological Survey (BGS) 1991. Groundwater, latrines and health, WELL Task 163, Ben Cave and Pete Kolsky 1999. Home > Resources > Fact sheets > Public Private Partnerships and the poor |
