The hydrochemical genesis of groundwater brines of the Eastern arm of the Makgadikgadi basin.

Hydrochemistry of groundwater brines of the eastern part of the large playa deposit in the Makgadikgadi basin in northern Botswana has been analyzed. Brine samples were collected from 37 production and monitoring wells in this area. Brine samples for analysis were filtered to 0.45 and analyzed for major and minor anions and cations as well as trace species. The results of the hydrochemical analysis revealed that the major element chemistry of these samples from the area is dominated by Na and Cl with minor components of K, CO3, HCO3, and SO4, and depleted in Ca and Mg, which is typical of seawater or coastal water. The brine type is Na-Cl type. However, the exact mechanism of the genesis of the brines is still ambiguous, hence comparison curves of Na/C1 against seawater concentration factor (SCF) and Ca/Mg against (SCF) in order to ascertain the brine genesis geochemically were employed. The relationship between the current results to previous seawater freezing and evaporation experiments by other researchers indicated that the brines were formed by seawater evaporation. Observed variations in hydrogeochemistry and salinity with depth support the results of previous studies indicating downward infiltration of brackish waters and evaporative and/or mixing processes. With respect to minor and trace element analysis, A comparison of measured concentrations of trace elements to their concentration in seawater when normalized against the concentration of chloride, it can be seen that the saline groundwater brines in the area are enriched in a number of trace elements including W, Th, Se, Pb while depleted in Sr. Enrichments in all of these elements which would be expected to exhibit conservative behavior in the brines suggest that the origin of the brine is not restricted to the simple evaporation of seawater or but to a combination of end members enriched in these elements such as riverine and groundwater inputs.