Chemistry of groundwater from mud volcanoes in parts of Upper Benue Trough, northeastern Nigeria

Mud volcanoes grouped into 14 fields are found in southern Gombe, part of the Cretaceous Upper Benue Trough filled with Early Cretaceous continental deposits and Late Cretaceous marine deposits, having a history of magmatism dating from Albian to Pleistocene. They are mostly of small sizes, having cones not exceeding one and half metres in height and diameters ranging from 0.5 to 8 m at base, as well as pools with muddy water and mudflows were encountered. Results of the study show that the cationic and anionic concentration varies as follows: Ca²⁺ (8.26–46.4 mg/l), Mg²⁺ (1.16–20 mg/l), Na⁺(18.6–4789 mg/l), K⁺ (9.4–134 mg/l), Fe (0.02–5.74 mg/l), HC ₃ ⁻ (126–3111 mg/l), Cl– (5–2625 mg/l), SO ₄ ²– (12.5–16.9 mg/l), CO ₃ ²– (3–2040 mg/l) and SiO₂ (7–45.6 mg/l). The study also reveals that the water issuing from the mud volcanoes, in most cases, is highly alkaline (pH ranges from 7.94 to 8.99), saline (TDS varies from 140 to 1620 mg/l), very hard (TH varies from 102 to 1180 mg/l) and belongs to five hydrochemical facies, namely: Na–HCO₃, Na–HCO₃–Cl, Na–Ca–HCO₃, Na–Cl–CO₃–HCO₃ and Ca–Na–HCO₃. The dominant hydrochemical facies is Na–HCO₃ while Ca–Na–HCO₃ is the minor water type. Statistical correlation reveals positive correlation between most of the parameters. With the exception of Ca²⁺, Mg²⁺ and SO ₄ ²⁻ , all other parameters are far above the World Health Organisation standards for drinking water. These waters are mineralised and are not considered to be suitable for drinking and domestic purposes, but may be considered for therapeutic purposes.