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WELL FACTSHEET Go BackOn-site sanitation in areas with a high groundwater table Author: Sarah Parry-Jones, 1999 Reviewed and Updated: Rebecca Scott, September 2005 Quality assurance: Sandy Cairncross In
areas that experience a seasonally high groundwater
table or that are prone to flooding, constructing
affordable on-site sanitation facilities can be very
problematic. It is a challenge that affects many
countries worldwide. This technical brief provides
practical guidance on some sanitation options in
such conditions. More details on each option
outlined can be found in the references in the
further reading section. A
preventative approach includes land-use planning, to
avoid building homes in areas with a high
ground-water table and examining the causes of the
high groundwater table and flooding, which may be
due to poor drainage provision. As this is not
always possible, other options may need to be
considered. Where
there is a seasonally high water table, a raised
latrine may be the most appropriate option for
on-site sanitation. The pit should be dug at the end
of the dry season, to maximise the available depth
of unsaturated soil that can be excavated. In areas
with a recurring high groundwater table, this may be
as little as 1 to 3 metres. It is worth trying to
dig the pit below the water table if possible:
firstly this increases the available volume and
secondly there is evidence that wet pits take longer
to fill, since the digestion processes in wet pits
described by Mara and Sinnatamby (1986) are more
efficient. The
pit should be lined with appropriate, locally
available materials such as fired clay bricks,
blockwork, porous concrete, large stones or pieces
of rock, pre-cast concrete rings or ferrocement. It
is also possible to use a 200 litre oil drum as a
lining if these are readily available; this however
makes a pit with a very low volume and therefore a
short life. The lining is extended above ground
level to provide the required pit volume, as shown
in Figure 1. Excavated material can be used to build
up a mound or embankment around the latrine. This mound can be used for liquid infiltration from the pit if it is: a. formed with permeable soil b. well compacted with a stable side slope of 1:1.5, and c. thick enough to ensure that filtrate does not seep out of the sides of the mound The slab should be constructed at least half a metre above the highest water level.
Figure 1: Types of raised pit latrine The Latrine Project in Mozambique (Brandberg, 1985) promoted the use of wide diameter elevated pit latrines in areas of high groundwater table, as shown in Figure 2. The lining was made with soil-cement blocks with the joints mortared above ground level. The pits were covered by a 1.5 metre diameter re-usable unreinforced dome slab.
Figure 2: A wide diameter elevated pit latrine
Social Impact of raised latrines Raised
latrines may not be socially acceptable
if people feel 'exposed' and
uncomfortable about being seen going in
and out of the super-structure.
They may also restrict access for
disabled people, the elderly, pregnant
women and others. If
space permits, a household may choose to dig a
shallow, unlined pit latrine above the water table
that will have a short life. When this pit becomes
full, the household can simply abandon it and dig a
new one. For this approach to be cost-effective, the
slab and superstructure must be designed to be
reusable and moveable. The
Arbour-loo Some
households in Zimbabwe use a shallow pit for about
one month, after which time the slab and
super-structure are moved to a freshly dug pit and a
tree is planted in the used pit (see Morgan, 2004).
Fruits like papaya thrive in this way. Although this
sounds like a simple and easy technical solution,
there are many social and health implications
associated with this type of system. For example,
people may be reluctant or unable to move the slab
and superstructure so frequently or there may be
resistance to eating fruit grown from a tree
fertilized by human excreta. Such issues must be
given serious consideration, as the approach will
only be appropriate in certain environments and
situations. The simplest form of pour-flush latrine is the installation of a pan with a water-seal in the defecating hole over a pit. Pour-flush latrines are most appropriate for people who use water for anal cleansing and squat to defecate. Where the use of solid cleansing materials is common there may be a risk of blockage; however, pour-flush latrines with seats have also been used successfully in the Caribbean, where water is not normally used for anal cleansing.
Figure 3: Varieties of pour flush latrines
Figure 4: Offset pour-flush latrine with two external pits
Figure 5: A raised latrine constructed on stilts Low-cost, offset pour-flush latrines have been successfully used in marshy areas of rural Myanmar. The pour-flush latrines (as shown in the photograph in Figure 5) are built a metre above ground level on bamboo stilts with plastic pour-flush pans set into wooden floors. The pits, offset from the superstructure with the pipe sloping at 45 degrees, are covered only with a simple bamboo trellis and matting. This makes the pit cheap enough to be abandoned if it fills or becomes silted up. In this case a new pit can be dug and the pipe moved to connect to a new pit. Composting
latrines consist of a single or double vault
construction, usually with a system to ensure that
urine is kept separate from faeces. The urine is an
effective fertiliser (when diluted with 3-6 parts
water), while the faeces contains most of the
disease-causing micro-organisms. Faeces collect in
the vault and are regularly mixed with earth, wood
ash or other organic waste material to deodorize it
and control the moisture content. The accumulated
waste must be left for at least a year, to ensure
that all pathogenic organisms have died off, before
it is applied to land or disposed of safely The
collecting vault is often constructed above ground,
as this improves access for removing the composted
waste (it can, however, make the construction more
costly). In theory then, the system is
suitable for regions with a high groundwater water. Composting
latrines continue to raise much debate among
international sanitation experts, with strong
arguments both for and against using the system. The
health risks associated with poorly managed
composting latrines must be considered, together
with the low level of user acceptance in many
countries and cultures. Composting
latrines should only be constructed where there is a
proven track record of operation and acceptance in
the region and the existing practice of re-using
human excreta for agricultural purposes. An
extensive hygiene education programme should precede
and accompany the introduction of composting
latrines.
Figure 6: A raised composting latrine
Contamination of drinking water supplies Where the source of drinking water is an aquifer with a high groundwater table, the risk of contamination from pit latrines needs to be considered. As a general rule,
abstraction of groundwater should be at least 10
metres from the latrine
The
risk of pollution through sub-surface movement of
bacteria and viruses depends on a number of factors,
such as: soil composition, hydraulic gradient, the
soil’s pH and organic content, and rainfall.
Therefore the risk of pollution needs to be assessed
for each individual case. It
has been found that the linear travel of pollution
is governed primarily by the groundwater flow
velocity and the viability of the organisms (Lewis
et al, 1980). A useful and widely accepted guideline
based on this research is that the maximum distance
faecal pathogens will move through unfissured soil
(including sand) is as far as the groundwater moves
in ten days. In low-lying flat areas, with a high
groundwater table, the groundwater flow is almost
certain to be less than one metre/day, so a distance
of 10 metres from latrine to source is adequate. If
there is considered to be a real risk of pollution
of groundwater from a pit latrine, the risk can be
reduced by constructing an artificial sand barrier
around the pit to create a filter effect. This is an
expensive solution and it may often be more
practical to develop alternative drinking water
sources, at a safe distance from the on-site
sanitation facilities. In
densely populated urban areas with very high
groundwater tables, the groundwater quality is
likely to be poor. The provision of off-site water
(water piped in from elsewhere) is likely to be a
lot cheaper than the provision of off-site
sanitation facilities such as sewerage. See
Cave and Kolsky (1999), for further information. Shallow
cess pits (an enclosed single tank with no outlet)
could be considered, if protecting drinking water
supplies from pollution is critical. If cess pits
are used, a reliable means of emptying them is
essential to their functioning and sustainability.
At the planning stage, a supply of tankers for
emptying must be both reliably available and
affordable to users (especially once any external
funding and support is removed).
Cave, B and Kolsky, P. (1999) Groundwater, latrines and health, WELL, Loughborough University http://.www.lboro.ac.uk/well/resources/well-studies/full-reports/task0163.pdf
(Oct 05) http://www.who.int/water_sanitation_health/hygiene/envsan/onsitesan/en/
(Oct 05). Also available in Spanish and French.
Morgan,
P. (2004), The Arborloo Book,
Morgan & SEI (Stockholm Environment
Institute), Zimbabwe http://aquamor.tripod.com/Arbook.htm
(Oct 05)
WELL (1998) Guidance manual on water supply and sanitation programmes, DFID, London www.lboro.ac.uk/well/resources/Publications/guidance-manual/guidance-manual.htm (Oct
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