How to predict effluent flow path in a seepage pit for a village
I'm designing a village school toilet for about 500 students
using a VIP seepage pit design. The toilet will be built on a hillside, which brings up my
question. Is there a means of predicting the underground effluent flow path? Specifically, will
the effluent surface at some level below the seepage pit? If so, where and
EDIT: in response to both of the comments received, here’s a little more siting information.
There are 2 possible toilet sites: one on the edge of a steep slope (~40-50%) with the foundation being within 1 meter of the beginning of the slope (top of the hill). There’s no well within several hundred meters of this site, but a streambed at the bottom of the slope, maybe 15 meters below the toilet site. The soil is a loam with some clay mixed in places.
The second site is also on a slope, but much more gradual (~10-15%) and in the middle of the sloping hillside rather than on the top. This site also has no well within several hundred meters, but has farm fields beginning at about 30 yards downhill of the building site. There are no visible springs in the areas below either sites (other than the flowing stream below site #1). The soil here is also a loam/clay mix.
I’m concerned that the toilet subsoil effluent might surface in either the stream in site #1 or the farm fields below site #2. It would appear that from one response received that the effluent, if mostly flowing downward due to gravity, might surface at the stream level below site #1, or might not surface at all in site #2. I’m also concerned that the subsurface soil in site #1 might become saturated due to the volume of liquid (which I’m calculating to be about 1.5 liter/person/day or about 750 liter/day with a dry pit, no flush, VIP toilet over a 30 square meter pit area) and become unstable. What affect would that volume have on the subsoil stability?
I hope this provides the additional information needed.
The short answer is predicting the precise flow of underground flow of effluent would be very difficult if not impossible. It would depend on the steepness of the slope, the rock type and water table level.
If the rock is porous such as sandstone then water moves in the spaces between the grains and will flow fairly uniformly down through the rock until it reaches the water table where the spaces are already saturated. It will then flow laterally with the rest of the water. Broadly speaking the surface of the water table mirrors the ground topography so underground water flows from hills towards valleys just as surface water does. Where the water table intersects the surface of the ground you will find a spring line or seepage into a lake, river or wetland. It is here that the effluent will come out.
In fractured rocks such as limestone and granite the bulk of groundwater flow is along the fractures. Again water will go predominantly downwards under gravity until it reaches the saturated zone but if your seepage pit happens to intersect a fracture that leads to the surface nearby it could pop out anywhere.
Mapping such fractures may be possible with expensive and complicated geophysics but that is not realistic. Here in Zimbabwe they simply make a blanket recommendation that domestic latrines should be more than 30m from wells or other water sources. With the school latrine that you are talking about the volumes will be much greater so a greater separation would be advisable. With the design that you are using does the liquid seepage have to be under the toilets or could it be piped downhill as far as possible towards a safe place which will minimise the risk of contamination?
I agree with Martin's comments, but would add that the
rule-of-thumb distances between toilets and water sources/points tend to assume a worst case
scenario (and can lead to expensive solutions and/or unnecessary costs ... all in the name of
keeping things simple). In addition, you should be aware that pathogens need a transport medium
(usually water), can be filtered out by the soil (depending on their size), and have a limited
life span outside the body (which means that some die off before reaching a water
The amount of water entering the seepage pit is a critical parameter. Dry (non-flush) VIP latrines tend to have a relatively small static head to force water into the unsaturated zone around the pit, which means that most of the pathogens remain within the pit. But the depth to the water table is also important.
The BGS Guidelines for Assessing the Risk of Groundwater from On-Site Sanitation (ARGOSS) suggest that you calculate the "number of travel days" = soil porosity x horizontal distance/(permeability x hydraulic gradient).
The information you provided would suggest a "very low risk" or microbiological contamination, largely because the soil porosity and permeability in a loam/clay soil are low (resulting in a long travel time for the pathogens). VIP latrines are generally considered low risk because of the relatively low static head likely in the pit ... but this all depends on the pit sizing, amount of water entering the pit, and whether (as Martin notes above) there are any local soil characteristics that encourage faster groundwater flows (e.g. layers of sand, rock fractures, voids etc).
I will forward on a useful WELL factsheet on Microbiological Contamination of Water Suppies (Steve Sugden, 2006) which outlines the main issues to consider, and explains how to do some of the simple calculations, and hope that this provides you with some appropriate guidance? But it sounds to me that your second site is preferable (as flatter and further away from water sources), and that it would be worth doing some sort of simple infiltration test at this site to check the soil permeability (and to help size the leach pit)?
Finally, I wouldn't worry too much about contaminating the farm fields, as - if the effluent reaches that far - the nutrients in the excreta (particularly the urine) should be beneficial?
Good luck - andy
Sorry for the delay in response.
The simple answer to the question is that shit, and effluents derived from it, will flow down hill. The rate at which this will happen, and hence the level of risk to the stream or to surrounding water sources will depend on the nature of the soil. As far as I am aware all the theories regarding the reduction of pathogen numbers in the effluent are dependent on the speed of the flow though the porous soil media. Provided that the effluents are about a metre above the water table travel through the unsaturated zone should be slow and hence a high degree of die off should occur as the head to drive the flow in the unsaturated zone will be derived slowly from the depth of the effluent in the pit. If the effluent forms a hydraulic connection with the water table then the effluent will be transported at the same rate as the flow as that of the groundwater. It is not rally feasible to predict the concentrations that might reach the stream as the concentrations gradient will be affected by dispersion, dilution and natural remediation. The impact of the stream will depend obviously on the volume flow rate of the stream. The higher the flow rate the lower the risk of significant contamination as concentrations are reduced through dilution and the natural attenuation processes of the stream will occur quite quickly. Given the description of the site I would say that it is quite likely that the stream is hydraulically connected to the groundwater system in the area.
An added complication is the presence of macro-pores or soil pipes that are often formed in the subsurface on hill slopes. If these are present then flow will be governed by the open channel flow regime rather than Darcy’s law and zero remediation can be expected. I mention this because I am aware of at least one situation where pit latrines on hill slopes were implicated as the most likely source of contamination of a spring at the bottom of a hill slope, located quite a significant distance from the pits. This contamination was responsible for an outbreak of typhoid amongst the community using the spring (and the pit latrines).
From the description given I would say that there is a low to medium risk of the effluent reaching the stream, provided that the water table in the area is well below the bottom of the pit. This risk is non-negligible, and the potential associated impacts depend on whether or not the water in the stream is used for bathing or for consumption without treatment. Perhaps the best approach would be to consider using a septic tank/aqua privy system connected to a system of French drains to disperse the resulting effluent? I would be relatively unconcerned about the potential impacts on the farmland. I think the chances of seepage from the pit emerging at the surface in a volume that would ... (more)
The answers are revealing, and illustrate the complex situation. I would first suggest you consider a different type of toilet, such as UDDT.
If you must have a pit latrine, dig a deep square hole, line it with stone, or a thick layer of clayish soil, so the seepage is deeper; put a berm around it. Then use a telltale indicator, easier to detect than e.coli or coliform.
Just an idea...
This thread is public, all members of KnowledgePoint can read this page.