Hydrogeological Investigations of Deep Coastal Aquifers, Tanzania

Abstract

ABSTRACT Borehole geophysical logging is viable method of determining the water quality and hydrogeological parameters of aquifers. A recent study was conducted to delineate the distribution of formation water salinity profile down to 600 meters below land surface of the Kimbiji coastal aquifer located 40 km south of the commercial capital, Dar es Salaam coastal plain in Tanzania. A hydrogeologic seismic cross section was also constructed. The borehole geophysical methods included short and long normal resistivity, natural gamma, spontaneous potential, temperature and caliper.

The logs show high permeability and the presence of formation fresh water as indicated by the separation of short/long normal resistivity readings down to 600m. The potential aquifers were determined based on the natural gamma values less than 23 counts per second to map the sandy intervals. The temperature log shows increasing values with depth from the surface up to 450 meters. This indicates that there is no vertical flow of the water in the borehole along this depth interval and it reflects the normal geothermal gradient. From 460 to 520 and 560 to 612 meters below land surface the log shows a constant or nearby constant temperature with depth which indicates the natural movement of water along the borehole axis (zones of vertical borehole-fluid movement). A clearly curved temperature signature was observed between 540 to 440 meters and this was interpreted as an effect of groundwater flow. A caliper log was used to identify fractures and possible water-producing openings and correct other geophysical logs for changes in borehole diameter. Major fractures and most interesting, occurred at 505 to 515 and 560 to 612 meters below land surface which coincides directly with the change in temperature gradient. These are interpreted as the most important water producing zones as indicated by both temperature log and caliper log.

The classification method used in this thesis defines freshwater as having a total dissolved solids concentration of less than 1100 mg/l; waters with a total dissolved solids concentration greater than 1100 mg/l are considered to be saltwater (or saline). For the aquifer formation the pronounced peak values of salinity occur at depth 330.5 to 331 meters below the land surface with pronounced peak value of 1590 mg/l at 330.7 meter. The lowest value of salinity along the profile is 526 mg/l. The salinity values show decrease trend with depth with more freshwater from 480 down to 612 meters. The average salinity for the whole potential aquifer was 850 mg/l. The calculated salinity profile in the Kimbiji aquifer is interpreted to be residual in which most parts of the aquifer have been completely flushed out by meteoric water.

The depth to the top of Eocene seismic reflector which is the base of Kimbiji aquifer is tectonically controlled as indicated by the seismic cross section.

The virtually untapped Kimbiji coastal aquifer system is considered to be an important supplemental source of water for public use in the highly populated coastal area of Dar es Salaam in Tanzania.