Over the years, research community has been in search for a valid phenomenon to explain the occurrences of saline groundwater and brines within the evaporate-deficit but shale rich sedimentary units of the Benue Trough, Nigeria (BTN). Thus, this study is an attempt at unraveling the origin and possible diagenetic reworking of the primary saline source as precursor of saline groundwater occurrences in the BTN, using Na-Ca-Cl and Na-Cl-Br systematics.
Due to the prominent surface occurrences in the Lower and Middle Benue Trough, thirty-eight (38) saline groundwaters and brines samples were collected alongside in-situ determination of physical parameters using a portable hand-held multi-parameter meter while laboratory analyses of major cations and anions were undertaken using Inductively Couple Plasma Atomic Emission Spectrometry (ICP-AES) and Ion Chromatography (IC) methods respectively at the ACME Laboratories, Vancouver, Canada.
The average electrical conductivity (EC) values are 55,929μS/cm and 31,965μS/cm and total dissolved solids (TDS) values are 36,354mg/l and 20,777mg/l in the Lower and Middle regions respectively. These variations in EC and TDS indicate variable dilution or mixing event in the two regions as laboratory analyses and interpretation revealed Na-Cl dominated type of fluids. Hydrochemical evaluation of the chemical elements indicates significant enrichment in Ca2+, Sr2+, and Cl- and depletion of Na+, Mg2+, K+, HCO-3, and SO42- relative to seawater values, attributed to subsurface intereactions with geological materials. Furthermore, rCl/Br and rNa/Br inferred two origin to the salinity as; halite precipitation and halite dissolution sources. However, the excess-deficit plot (Na-Ca-Cl) revealed seawater origin and modification in the original chemistry of the fluids in the order of 2 Na for 1 Ca exchange reaction, responsible for the enrichment in Ca2+ with a corresponding observed depletions of Na+; this enrichment-depletion patterns accompanying the salinity increases were mostly attributed to albitization of plagioclase feldspars as the major diagenetic reaction rather than dolomitization.
In summary, these saline groundwater and brines in the Benue Trough were actually seawater and dissolved halite originated; however, possible reworking / modification abound via interaction with the geologic materials and environments as a result of compaction of compressible sediments in the Trough during the tectonic episodes that affected the Trough.