Survival strategy of Macrotermes subhyalinus on hard rock and fracture porosity aquifers
This work has been presented in February 2010 as a poster at the 6th International Dyke Conference, Varanasi.
The geology of termite mounds has become famous in ore
exploration, especially for retrieving Ag, Au, Cr, Cu, Mn, Mo, Ni, Pb,
and Zn. In contrast, despite the strong dependency of termites on moisture
conditions (termite mounds maintain a humidity of 95%), the relationships
between termite settlement and groundwater have been scarcely inverstigated.
It is known, however, that some species are able to drill galleries tens
of meters deep to find moisture. This requires, however, that the soil
is thick enough, or the geological formation at surface is soft enough
because termites cannot drill into hard rock.
Field work carried out in the Qwara district of in January
2008 along one of these dykes has shown that it displays mounds only if
the dyke topography does not exceed 2 m. It has also revealed that the
mounds are periodically spaced along the dyke (Figure 1). Field work carried
out along other dykes in October 2009 has confirmed the latter observation.
1: Termite mound distribution along a dyke near the village of
Farshewa, Qwara district, northwestern Ethiopia. Many mounds are helpful
in the identification of dykes that would not be identified by other means
due to poor exposure. The mounds are periodically spaced, on average every
63 m (R2=0.995).
A hydrologic circuit has been proposed to account for the preferential location of termite mounds along the dykes and the dependence of their distribution on dyke topography (Fig. 2).
Figure 2: Hydrogeological circuit in the study area. During the rainy season, water percolates through and accumulates within the dyke fractures above the main water table (bottom, light blue). During the dry season, water is gradually released to the main water table (dark blue). Dykes having a topography higher than 2 m are surrounded by a porous debris slope that evacuates dyke moisture laterally and prevents termite settlement.
Figure 3: Migration of termite mounds with time in response to clay stress. Clay shortage triggers foundation of a new colony. The distance between two adjacent colonies is determined by the extent of the dimensions of the working area around each mound. The termites are blind and rely upon trail pheromone emissions to orientate and go back home, there is therefore a minimum distance required between adjacent mounds for each population to live without disturbance by the adjacent populations.
D. Mège and T. Rango, 2009, Permanent groundwater storage in basaltic dyke fractures and termite mound viability. Journal of African Earth Sciences, doi:10.1016/j.jafrearsci.2009.07.014.