Geochemical Characterisation of Wetland Soil Waters of Mpu, Enugu State, Southeastern Nigeria

Abstract

Mpu of Egugu State Southeastern Nigeria are underlain by Awgu and Mpu Formations. Awgu Formation consists mainly of black, micaceous, fissile carbonaceous shales that contain traces of pyrites and carbonate minerals while Mpu Formation is made up of conglomerates, lignites, sandstones and ironstones. The three wetlands used for this study are situated in Awgu Formation. Six pits, each measuring 1m in depth were dug in the wetlands, two per wetland. Waters that drained into each of the pits were sampled for major element chemical characterization. Results from the analysis revealed that the waters are alkaline. Only one sample showed a pH value less than 7 while the range for the rest is 7.2-7.7. Total dissolved solids ranges from 187.2mg/L to 404.95mg/L. Calcium (Ca²⁺)ranges from 0.03mg/L to 0.47mg/L. Potassium (K⁺) ranges from 0.93mg/L to 10.21mg/L while sodium (Na⁺) which is the dominant cation ranges from 6.06mg/L to 26.54mg/L. Chloride (CL^-) ranges from 108mg/L to 162mg/L while bicarbonate (HCO₃^-) varies from15.25mg/L to 61mg/L. Sulphates (SO₄^2-) varies from 13.66mg/L  to 114.5mg/L. The samples all have high concentrations of nitrates(NO₃^-). The range is 72.54 mg/L to 393.24mg/L. Iron(Fe²⁺) is also in excess. The range is 2.66mg/L to 20.02mg/L. Analysis for heavy metals like lead and cadmium turned up 0.00mg/L. The soil water chemistry of the wetlands have shown that the wetlands have strong buffering processes that change the acidic soils to alkaline soil water.  Pervious  studies of natural water from the Awgu Formation indicate that they have high concentration of lead and cadmium but at the wetlands, these heavy metals have been effectively removed from solution.  The low solute concentrations are also an indication of the effectiveness of the wetlands in positively modifying the composition of polluted water but the excess nitrates are due to fertilizer application on the wetlands.

Keywords: water, Mpu; wetlands; heavy metals; buffering