Sanitary seals around self-supply tube-wells
Dear RedR Experts
We have received a query from Michael, a graduate student at the University of South Florida (Tampa, Florida, USA) working/living in Madagascar and studying the self-supply manually-drilled well, pitcher pump market there
He is interested in sanitary seals and sanitary aprons around self-supply tube-wells. His project involves the design of low-cost well head protection for unsubsidized self-supply pitcher pumps in Madagascar and is looking for research information on the effects of sanitary aprons on water quality. To date, his literature search has uncovered minimal relevant documentation on sufficient/low-cost wellhead protection for household/self-supply tubewells.
His question, then, is "Are there any studies, grey literature, or other documents that give justification for minimal sizes, depths, and other design parameters of sanitary aprons or sanitary seals on tube-wells in any context?"
Most textbooks, documents, and experts on the subject of sanitary aprons for wells in a development aid context recommend sanitary aprons around 2m in diameter. Is there justification for this choice (e.g. tests of water quality from wells of aprons of different sizes/designs or engineering analysis of the design)? Is there a reason to choose a 2m sanitary apron over a 1m or 1/2 m sanitary apron? Or is this choice based off an engineering practice with no specific justification besides experience in knowing that an approximately 2m well apron size is sufficient?
Michael is looking particularly for justification and not recommendations.
If you can help, please post your answer on the RedR-Knowledge Point website or eMail firstname.lastname@example.org
Thanks and regards
Leigh Jones RedR TSS-Knowledge Point
I think it's going to be hard to find the 'scientific' justification that Michael is looking for (from the question it looks as though he wants technical data to back up the need for an apron). The answers given already are all sensible, but in terms of contamination problems, you don't necessarily need anything much except a good seal at the top of the annulus around the pipe. This is reflected in standard practice for monitoring boreholes (for which it is also important to avoid contamination from the surface). These emphasise sealing the borehole annulus with bentonite clay and cement grout, and if they mention a surface 'apron' it is small and designed just to shed water away from the borehole pipe (e.g. 3ft square, USEPA "Design and installation of monitoring wells", available on line).
As has already been said, the main reasons for pumped boreholes having large aprons are practical ones: avoiding erosion produced by people and animals walking on wet earth; avoiding erosion from waste water; anchoring the borehole pipe and pump; and providing a firm and clean platform for people to stand on while using the pump. The standard designs for handpump platforms are of course generally for community wells, and if you are working with self-supply then presumably the traffic at and use of the well is much less, there is less waste water and it is also easier to control behaviour (e.g. keeping animals out). Depends how many users you expect...
The other variable in terms of protecting the groundwater quality is of course soil and geology; if you have a sandy soil to the surface, as in many coastal areas of Madagascar such as Maroantsetra area or Fort Dauphin, and if your water table is shallow, then it's more important to take care than if you have clayey soils near the surface.
Sorry - i'm only giving you recommendations here, Michael, but i would say that the principle of self-supply is to provide as good a water source as you can for the resources you have; and for water quality the depth of the well screen, some kind of seal to prevent surface water filtering down the borehole annulus, and a means of draining waste water away are the most important elements. The seal could just be well-placed clay within the annulus; but for durability would ideally include a pad to shed water away from the pipe.
Whist I know that Leigh say that he is not looking for guidance, some of you may be interested in the publication : https://wedc-knowledge.lboro.ac.uk//d...
Distance Learning Programme Director
My understanding of the justification for a well head is that it needs to achieve the following
- The platform is raised slightly above the surrounding land so surface water drains away from the well, prevents erosion and muddy depressions forming around the wellhead and provides a good sanitary seal to prevent water flowing directly into the well
- It provides good access to all users including the old, young and disabled
- It protects well head from damage by cattle.
- It allows access to the well for maintenance or deepening
So provided the platform achieves this, there is no rule that says it needs to be circular or a certain size. The dimensions and the design should, I suggest, be driven by the needs of users. A 2m diameter platform provides a 1 metre space around the pump which is probably assumed to be the space required to collect water on.
In addition to Ian's comment. The sanitary seal is also to stop faecal contaminants draining either straight into the well or straight into the groundwater source close to the well. The latter is relevant as the action of pumping water from the well, even at a low rate, draws groundwater from the surrounding strata back into the well. So, the shorter the distance from the well, the greater the chance of faecal contaminants surviving the journey and being pumped back up the well. Therefore, if the well is tapping an unconfined groundwater source (it sounds as if this is the case), it is important to inhibit the deposition of faecal matter around the well. This means that the bigger the apron, the better the protection because the longer will be the time for the faecal material to reach the groundwater and therefore the greater the chance that harmful elements will have died off. Even if you only have a small apron, it is important to ensure that water is channelled away and cannot pool around the well head or around the edge of the apron because that will attract animals, who may well deposit faecal matter while they stop for a drink.
I would start by reiterating the need for drainage to ensure that contamination is minimised. This is particularly important where livestock watering or clothes washing is encouraged or permitted at the well. Often some suitable crop, such as banana is planted at the termination of the drain
Having consulted a hydrogeologist colleague it would seem that the usual recommendation is to provide a 5 m deep seal, provided that the productive aquifer is deeper than this. One useful document is: http://schneiderwater.com/pdf/Hydroph...
Other useful stuff can be found on RWSN: http://www.rural-water-supply.net/en/
Great answers here and I agree with most. Here is a link to an article which again does not answer this directly but shows some significant relevance. This is based on work that we did to improve water quality monitoring in the Niassa province in Mozambique. Embedding regular water quality testing for wells and boreholes was proving difficult and so we looked at whether or not a standard sanitary survey could give us a relative risk score and if we could correlate this with the actual water quality. The findings clearly demonstrated a positive link between the stored water quality and the sanitary risk (and seasonal variations), particularly the general care and cleanliness (for which and apron helps significantly. I have not re-read the article, but I think you will find a lot of additional useful data and insights. http://www.iwaponline.com/jwh/004/033...
Once again, not answering the original question, but probably of interest to many of you is the potential effect on PVCu casing of the heat of hydration of thick areas of cement grout (e.g. washouts) when it is curing around the pipe. Apparently this has been know to cause the pipe to deform (see page 28 in the excellent booklet Water Supply Guidelines for use in developing Countries). The same document provides good advice on sealant materials for the annular space around the permanent casing and diagrams of typical details.
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