In the flat center of Sanadougou where I live, the water table lies almost perfectly uniformly between 7.5 meters below ground level at the peak of hot season and 4 meters below ground level after the groundwater has been recharged by 4 months of rainy season. In the center of Sanadougou where about 3,800 of the 4,400 permanent residents live, development-minded villagers have been digging soak pits 1 meter deep and varying in diameter (usually about 1 to 1.5 meters) depending on the number of people in their households and the volume of wastewater generated by their respective nyegens. Since the pathogens originating in wastewater can usually seep up to half a meter through hard-packed soil and sedimentary rock, the water table should never come closer than 2.5 meters to the sewage generated by these modern latrines and thus the groundwater consumed through wells and pumps should be adequately protected from direct contamination by human fecal matter.
However, even within the demarcated borders of Sanadougou there are some places where the soak pit is an inappropriate technology. Namely, there is an outlying neighborhood called Filablena which is significantly lower in elevation than the rest of the town and sits around a couple of large seasonal ponds. Here the water table varies between 5 meters below ground level at the peak of rainy season and 1.5 meters below ground level after rainy season.
The wells here are so shallow, and with less rock they are cut into nothing but soft soil which is much more permeable and conducive to groundwater flows. The pressure in a well is somewhat less than within the soil, so the water levels of wells are slightly higher than the water table; in Filablena during rainy season, the well water surface is only slightly less than a meter below ground level.
If we built 1 meter deep soak pits here like we have in the rest of Sanadougou, soak pits would in fact exacerbate the water sanitation problem by directly polluting the groundwater with raw sewage. That contaminated groundwater would then eventually make its way to people’s wells from which they get the bulk of their drinking water. The absolute worst-case scenario would be that contaminated water makes its way into the seasonal ponds and – though it probably wouldn’t be as obvious as the ones which form behind "traditional latrines" – render them into gigantic seasonal cesspools.
In Filablena we are just beginning to introduce a specialized technology: the infiltration trench. An infiltration trench serves the same function as a soak pit in that it contains the wastewater emitted from “traditional latrines” underground so that it cannot serve as a fertile breeding environment for filth flies and mosquitoes and cockroaches and a vector for all sorts of disease. It has to be able to store roughly the same volume of wastewater as a soak pit, but in an environment where the water table is prohibitively high an infiltration trench must be dug at a much smaller depth. In truth, the volume of a soak pit is only really important so long as it briefly stores wastewater before in seeps into the surrounding soil and rock; what is much more important is the surface area which determines the rate of discharge into the ground where it is safe and isolated from human water and food supplies. Where there has not been a lot of room to maneuver, we have solved this problem by simply reducing the depth of our soak pits and increasing the diameter accordingly.
Infiltration trenches take that ideal of minimal depth and maximum surface area even further. First I found a group of Filablenakaw interested in rebuilding their nyegens, measured their dimensions and assigned them lengths of plastic piping between 4 and 6 meters in length. Then we took an afternoon and pierced holes in them; we took a dozen large nails and placed their ends in the fire until they became red hot, and with protective work gloves we held pliers to hold the hot nails and melted lines of holes down the length of the pipes. With these hole-ridden pipes, wastewater should flow out over a more evenly distributed area and facilitate more rapid and less concentrated wastewater seepage into the soil.
Instead of small circular pits, Filablenakaw have been digging 4- to 6-meter long trenches which begin about 20 centimeters and eventually expand to a maximum depth of no more than 50 centimeters. We fill them in with rocks in such a way that the plastic pipe is on a gradual incline downwards and wastewater flows all the way down. Then we fill them with more rocks to keep the pipe stable, and cover the end of the pipe with a large flat rock to protect it from closing up.
Eventually we’re going to cover the trenches all up with plastic sheeting and cover them with the dirt that was dug up in the first place so that the sewage is contained underground, people and animals can walk over them without falling in, and every year or so homeowners can open up their infiltration trenches to inspect them and clean them as necessary.
However, there are some negative aspects of this process which make the construction of an infiltration trench an unattractive option. The biggest down point of this technology is that burning holes in the plastic pipes produces noxious fumes and is fairly harmful to anyone who isn’t wearing a respirator – I covered my face with soaking wet handkerchiefs while doing this work, and even then I came down with really bad headaches. Infiltration trenches also require more than 6 times as much plastic piping and sheeting than your average soak pit – but the plastic materials are so cheap compared to the cement that goes into the nyegens that the cost of an infiltration trench cannot be prohibitively expensive to anyone who is building or revamping an entire nyegen. Nevertheless, in communities sitting atop extremely high water tables, infiltration trenches are the most practical, cost-effective technology available for sound wastewater management. It is unlikely that we will be able to completely sanitize Filablena's latrines with such infiltration trenches, but my hope is that the few models which we are building will serve as an example for the entire community to one day safely contain their waste underground and away from the rest of the water supply - insh'allah.
