Water chemistry in the Bahi-Manyoni basin in Tanzania

Abstract

In the Bahi-Manyoni basin in Central Tanzania the geology and climate of the area are the main factors influencing the water chemistry and quality.

Most water samples are saturated with respect to quarts, dolomite, calcite, talc and sepiolite.

There is an excess amount of sodium compared to chloride. Cluster analysis showed that sodium was correlated with alkalinty and together with the activity diagrams weathering of albite was found to be the most likely source. Potassium is weakly correlated with sodium so other factors like biological uptake may also affect the amount of potassium. The activity diagrams indicated that the system was in equilibrium with respect to K-feldspar and muscovite, so weathering of these minerals where also limited. Both cations were upconcentrated with increasing EC.

The water samples are saturated with respect to dolomite, calcite and talc and precipitation of these minerals, together with dissolution of gypsum, controls the concentration of calcium, magnesium and sulphate in the shallow wells. While in the deeper wells upconcentration seem to occur.

The concentration of arsenic in the water samples seems to be mainly governed by increasing amount of arsenic with increasing EC. Arsenic can also be sorbed onto iron-hydroxides and then desorbed if an increase of pH occur. The large amount of arsenic found in the water sample from Chipanga B deep well i smost likely due to desorption because if increase in pH and anarobe conditions facilitating dissolution of iron-hydroxides.

Uranium is weathered from the granitic basement in the area and transported by water, probably sorbed onto apatite or clay minerals suspended in the water. When the speed of the water decreases enough or the ephemeral rivers disappears underground, deposition of the colloids can occur. The enrichments are clearly seen in radiometric maps over the area, with little uranium found in the main river channels, but onto the plains before Bahi Swamp local, surficial deposits are seen. Aeromagnetic maps have indicated a correlation placement of deep wells with high uranium content and uranium elevated dykes in the area.