Investigation of hydrochemistry and uranium radioactivity in the groundwater of Namaqualand, Northern Cape, South Africa

Abstract

The demand for groundwater in South Africa has increased over the years, mainly in the arid, semi-arid and rural areas of the country. Groundwater is generally considered a safe and viable option to untreated surface water. Groundwater samples analyzed were limited to inorganic chemicals, stable isotopes and radiological constituents. Dissolved inorganic constituents and parameters analyzed included major anions and cations, trace metals, and physico-chemical parameters. Inorganic constituents were analyzed to provide basic information pertaining to water chemistry. The hydrochemistry of the area indicates that groundwater evolution is taking place and dissolution, ion exchange and mixing processes are the driving processes. Br-/Cl- ratio show that the salinity in the area is derived from seawater mixing, halite dissolution and atmospheric deposition/sea aerosol spray. Under excessive evaporative regimes, as experience in the study area some of the groundwater is heading towards hypersaline waters. Isotopes analyses disclosed two groundwater flow types, deep and shallow groundwater flow. The data further revealed groundwater mixing within the aquifer where there is no direct infiltration (the samples have undergone fractionation due to evaporation before infiltration), recharge in the area results from regional flow from the neighboring catchments. . The age of the groundwater calculated from average tritium of the local rainfall of 28.32 TU, ranges from 87.01 to 174.02 years. Gross alpha and beta emitters were detected in the water samples and are likely attributed to naturally occurring uranium isotopes present in the water. The radioactivity in the area is background activity, i.e. it is very low and is from the rock material.

Uranium occurs in high concentrations in the groundwater of the study area, ranges from 1 μg/l to 146 μg/l, which is above the WHO drinking water standard. Data analysis show that uranium occur as UO2 +(c), UO2OH+, and U(OH)5 - complexes in the absence of other complexing agents. In the presents of carbonates uranium is found in solution as UO2 +, UO2CO3 and UO2(CO3)2 2- and UO2(CO3)3 4- complexes.