The use of anion geochemistry in mapping groundwater facies in the Port Harcourt area of the Niger delta, Nigeria.

Abstract

The groundwater system of the Port Harcourt area is home to three anion facies, the Cl – SO₄, the Cl – SO₄ – HCO₃ and the HCO₃ – Cl – SO₄. The first two types exist in both shallow – and deep – groundwater environments while the third is restricted to the deep environment. Although there are natural intermittent and, in some cases, continuous interactions between shallow and deep groundwaters, this paper establishes that the quality and facies types of the groundwaters are not exactly alike. 28 groundwater samples (12 from hand-dug wells; 16 from boreholes) were analysed for Cl, SO₄, HCO₃ and CO₃. The concentration levels of SO₄^2- were analysed using the HACH Spectrophotometer equipment, model No DR/2000 while those of Cl-, CO₃^2- and HCO₃- were by titrimetric method, using the Titro Process Dosimat 665 equipment. The results indicate that, for the hand-dug wells, SO₄^2- levels ranged from 0.00mg/l to 67.0mg/l with an average of 16.5mg/l; HCO₃- from 12.2 to 36.6 with an average of 15.25, while Cl-ranged from 11.332 to 121.905 with an average of 41.22mg/l. No CO₃^2- was detected in all the samples. For the borehole samples, the respective figures for SO₄^2-, HCO₃ - and Cl- are 0.00 to 33.0 with an average of 2.13; 12.2 to 24.4 with an average of 12.96, and 5.490 to 72.306 with an average of 17.037 mg/l. Again, no CO₃ ^2- was detected.
The total absence of CO₃^2- conforms with the relatively high acidity of the groundwater environment of the Port Harcourt Area; and the diminutive level of SO₄^2-, is associated with the effect of the bacteria catalysed reduction of the ion. This absence of some ions and the low levels in others do affect the number and type of mappable facies in groundwater systems. With the results obtained, the paper notes that:
(1) The shallow groundwater environment embedded less number of mappable facies than the deep groundwater environment;
(2) Facies sequences mapped are tending towards the composition of seawater; and concludes that the existence of different groundwater facies is an evidence that groundwater encounters strata of different mineralogical compositions along its flow path.

KEY WORDS: Anion facies, Hand-dug well, Mappable, Acidity, Sulphate reduction.