Prospecting for safe (low fluoride) groundwater in the Eastern African Rift: the Arumeru District (Northern Tanzania)

A multidisciplinary research effort, including geological,
hydrogeological, hydro-chemical, geophysical and
hydrological investigations, was aimed at locating a source of
safe groundwater for a district of northern Tanzania, within
the western branch of the East Africa Rift Valley, where water
shortage is common and much of the surface water carries
unacceptable levels of dissolved fluoride. The 440 km2
study area lies in the northern part of Arumeru district and is
dominated by Mt. Meru (4565m a.s.l.). The local climate
is semi-arid, with distinct wet and dry seasons. Four hydrogeological
complexes were identified, occurring within
different volcanic formations, either alone or superimposed
upon one another. The groundwater flow system was interpreted
from the spatial distribution of the springs, combined
with a lithology- and geometry-based reconstruction of the
aquifers. The dominant pattern consists of a multi-directional
flow from the higher elevations in the south towards the lower
areas in the north, but this is complicated by structures such
as grabens, faults, lava domes and tholoids. After the identification
of the major fluoride source, an interference pattern
between groundwater and high fluoride surface water was
drawn. Finally, vertical electrical soundings were performed
to define the location of aquifers in regions where release of
fluoride was prevented. The methodological approach for the
prospecting of safe water in a semi-arid, fluoride polluted region
was validated by the drilling of a 60m deep well capable
of supplying at least 3.8 l/s of low fluoride, drinkable water.