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RedR TSS gravatar image

Before I comment on the technical aspects of this I have a public health orientated question, and this is not a criticism. Can you guarantee that the water will not be used for drinking, say for example by kids when they are collecting water or watering their animals? Or if your piped supply breaks down for a couple of days? If you can’t then there is a public health risk posed by the contamination. And if you can then I want to know where you are working because it is far more straightforward that anywhere I have ever been.

Faecal coliform are used as indicators of faecal contamination as they are ubiquitous in the guts of humans and animals. According to the WHO counts of by 0 per 100 mil represent no risk. Counts of 1-10 are considered to indicate low risk and counts of 11-100 are considered high risk. Anything above that is considered Russian Roulette.

The acceptable use question is kind of hard to answer. The problem is that the risk of getting sick is different to the risk of contamination. The former depends on the bacteria that are doing the contaminating. For example a low dosage of bacillary dysentery is relatively harmless while a single giardiasis occyst may make you very ill. What constitutes a low disk depends on the body mass of the individual and the infectiousness of the bug. So I can’t say objectively for what purposes the water could be used for. I personally would not use it for preparing food, bathing or washing utensils unless it had been boiled. I would definitely not let young kids near badly contaminated water unless it had been boiled.

The primary purpose of an apron around the well is to provide a sanitary seal to prevent contamination entering the well. Prevention of standing water is secondary, except when the standing water could re-enter the well and contaminate the water. A major concern in your case is that a sandy soil allows rapid drainage back into the well. If this water is contaminated with faeces then there will be insufficient contact time for the soil matrix to purify the water. As the water travels through the soil, bacteria are removed by adsorption onto the soil matrix and by predation. So if there are large amounts of faecal contamination around the well, as there quite often are, the water that is not being collected and is seeping back into the well could be the primary source of your contamination. Environmental Health Engineering in the Tropics, by Cairncross and Feachem, quotes 15-20 m as a minimum distance between a source of faecal contamination and water well. At least I think it does, I don’t have a copy handy, but it is an excellent and accessible resource, and you should get a copy if you are going to be dealing with these sorts of questions.

If the septic tanks are working properly there should be large amounts of digestion and the heat in the tank will actually be killing most of the coliforms. Actually the WHO guidelines specify thermotolerant faecal coliform in their guide (surviving above 44 degrees Celsius). There should also be minimal seepage from the bottom of the tank due to sludge build up. To pose your question in terms of a risk assessment Source-Pathway-Receptor framework; 1. How far is the bottom of the tank from the water table and far is the tank from the well? 2. If the groundwater is being contaminated by the sceptic tank, which way is the water flowing. Bacteria will be advected with the water so if the groundwater is flowing away from your well it will not be contaminated by the septic tank. 3. How much faecal matter/rubbish is around the well heads.

So much for me being ‘a know it all’ scientist. As a practical suggestion if I were in your position I would try the following in the first instance: 1. Organise a cleanup around the wells with the contamination for about 15 m radius- a few people with shovels to scrape up any crap that is around. 2. At the same time as the clean up dig a drain around the radius to prevent surface run off getting into the well. 3. Fence the area around the wells to stop animals from fouling the area, it may also be worth your while to do a quick survey to see what facilities are available to people who get caught short while waiting to collect water. You should also check that the water sampled came directly from the well and was not taken from peoples storage containers. These can get quite dirty when they old and could be a source of contamination. I am not trying to be smart here, just thorough. I have seen a perfectly good cistern closed because some old dear hadn’t washed her water bucket properly.

Having done this you could then close a well for a couple of days and chlorinate the water. If after a few days you still have contamination then it is likely that you have a constant source up gradient of your well. If there is no contamination, or much reduced contamination then you should probably consider installing an apron with proper drainage and animal troughs at the well. Actually you should do this anyway; it should be an absolute minimum standard. If you do find wells that are badly contaminated from the septic tanks then you need to discuss it with the well users. Try and get them marked or fenced off in some way, tell kids stories about the monster that lives in the bottom, anything that will reduce contact. In an ideal world you would take the pump off it, close the hole and dig another. However we don’t live in an ideal world, so probably the only thing you can do is let people know what the risks are and try to help them identify and implement their solutions.