A study on the impact of vermiculite mining and processing operations on groundwater quality in Namekhala area, Manafwa District

Abstract

Vermiculite is being mined in the village of Namekhala, Bugobero sub-county, Manafwa district. Elements within vermiculite (e.g. Fe and Al) and trace elements (e.g. Cu, Pb and Zn) which have a high affinity for vermiculite when re-mobilised into groundwater are likely to render it unsuitable for human consumption. This is therefore the first comprehensive study to investigate the impact of vermiculite mining on the groundwater chemical quality. The area is found within the Bukusu carbonatite complex which is in the older sub-volcanic group (26 ± 2.5 Ma). Groundwater in Namekhala is indicated to occur in confined aquifers within weathered carbonatite that is mixed with pyroxenite and trace vermiculite.

The pH of groundwater in the study area vary from 6.8 to 8.2 for both dry and wet seasons indicating that the waters are slightly alkaline, with the corresponding electrical conductivity ranging from 458 to 702 µS/cm. These waters satisfy the ranges and limits of desirable pH and EC for drinking water prescribed by NEMA (1996).

Groundwater chemistry in Namekhala study area is divided into three hydrochemical facies i.e. Ca-Mg-HCO3 - , Ca-HCO3 and Ca-Na-HCO3 while groundwater chemistry outside the mining area has six hydrochemical facies that include: Ca-Na-HCO3, Ca-Mg- Na-HCO3, Ca-Na-HCO3, Ca-HCO3, Na-Ca-HCO3 and Na-HCO3., which is indicative of alkali and recently recharged groundwater (Chadha, 1999). There is an increase in concentrations of elements analyzed in the wet season as compared to the dry season thus resulting into a change in the hydrochemical facies in the wet season from Ca-HCO3 and Ca-Na-HCO3 to Ca-Mg-HCO3. The major cations in the studied groundwater are in decreasing order of prevalence, Ca2+ > Mg2+ >Na+> K+, while the major anions include HCO3 – > SO4 2+ > Cl–. The high concentration of calcium is controlled by rock weathering of the carbonatite and possibly from other processes such as cation exchange.

The concentrations of the major, minor and trace elements analyzed generally increase in the wet season with the nearest monitoring well showing increased concentrations although they did not exceed NEMA (1996) guideline values. However, iron has its concentrations above the required NEMA (1996) guideline values around the mine pit during both the wet and dry seasons. Vermiculite mining in Namekhala does not adversely affect the chemical quality of groundwater since ion concentrations do not significantly deteriorate to exceed the prescribed NEMA guidelines (1996) during both the wet and dry seasons. However, a routine water quality monitoring program should be implemented since one sampling season i.e. one dry and wet season is not representative enough to determine the effects of vermiculite mining on the groundwater quality in Namekhala study area and more monitoring wells should also be set up especially to the east of the open mine pit. This will aid in determining whether there are any variations in the groundwater chemistry in relation to groundwater contamination resulting from the mining.