Abstract:
Groundwater is globally important for human consumption, and changes in quality can have serious consequences.
The study area is within a coastal aquifer where groundwater quality is influenced by various
potential sources of salinity that determine the composition of water extracted from wells. Groundwater
chemistry data from the aquifer have been acquired to determine the geochemical conditions and processes
that occur in this area and assess their implications for aquifer susceptibility. Analysis of groundwater
samples shows that the dominant watertype is mostly NaCl with pH < 7 in both aquifers (i.e. upper
and lower) except for the shallow wells where CaHCO3 prevails with pHP7, and boreholes located near
the Indian Ocean, where coral reef limestone deposits are located and the watertype evolves towards
CaHCO3. In the lower aquifer, Cl is higher than in the upper aquifer. The origin of salinity in the area
is strongly influenced by groundwater ascending from deep marine Miocene Spatangid Shales through
faults, seawater incursion on the border of the Indian Ocean, and throughout, there is some salinity
within the Quaternary aquifer, especially in intercalated deltaic clays in the fluviatile deposits, showing
some marine influences. The seawater intrusion is linked to the strongly increasing groundwater exploitation
since 1997. Another process that plays a major role to the concentration of major ions in the
groundwater is calcite dissolution. Next to geogenic salinity and seawater intrusion, anthropogenic pollution
as well is affecting groundwater quality in the aquifer. An important result of this study is the
observation of high nitrate concentrations, that call for improved sanitation in the area, where domestic
sewage with on-site sanitation (mainly pit latrines) also threatens the groundwater resource
