Hydraulic, textural and geochemical characteristics of the Ajali Formation, Anambra Basin, Nigeria: implication for groundwater quality

Abstract This study highlights the distribution of

hydraulic conductivity (K) in the regional aquiferous Ajali

Formation of SE-Nigeria on one hand and assesses the

possible influences of textural and geochemical characteristics

on the hydraulic conductivity on the other hand. The

investigation approach involved field sampling and collection

of 12 sandstone samples from different outcrop

locations, followed by laboratory studies such as grain-size

analysis (GSA), constant head permeameter test and geochemical

analysis of major and trace elements using X-ray

fluorescence method. GSA and textural studies show that

the sandstones range from fine to medium sands, constituting

about =–99% sand fraction, with graphic mean

grain size of 0.23–0.53 mm. Other parameters such as

coefficient of uniformity (Cu) range from 1.58 to 5.25 (av.

2.75), while standard deviation (sorting) values of 0.56Ø–

1.24Ø imply moderately well sorted materials. In addition,

the order of the estimated K values is Kpermeameter[

KBeyer[KHazen[KKozeny-Carmen[KFair-Hatch with average

values of 1.4 9 10-3, 4.4 9 10-4, 3.8 9 10-4, 2.2 9 10-4

and 8.1 9 10-5 m/s, respectively. These values fall within

the range of 10-5 and 10-3 m/s for fine to medium sands.

However, multivariate factor analysis of the data revealed

significant positive dependence of the empirically determined

K values on graphic mean grain size and percentage

sand content and much less dependence on sorting and total

porosity. Geochemical profiles of the fresh samples are

dominated by quartz with corresponding SiO2 content of

76.1–98.2% (av. 89.7%) while other major oxides are

generally below 1.0 wt.% in the fresh samples. However,

the ferruginized samples exhibited elevated concentrations

of Al2O3 (3.50–11.60 wt.%) and Fe2O3 (1.80–3.60 wt.%),

which are clear indications of weathering/ferruginization

processes with attendant trace metal release/enrichment

(2.5 mg/l Cu, 7.5 mg/l Pb, 6.5 mg/l Zn, 3.9 mg/l Ni and

19.6 mg/l Cr) call for concern in respect of the chemical

quality of the groundwater system. The associated

groundwater is generally soft, slightly acidic, and with low

dissolved solids (EC = 14–134 ls/cm) dominated by silica;

implying water from clean sandy aquifer devoid of

labile and weatherable minerals. Nonetheless, most of the

metals (with exception of Si, Fe and Mn) exhibited higher

degree of mobility (2–12 folds), which can be attributed to

reduction of Fe-/Mn-oxyhydroxides as sinks/hosts for trace

metals. Consequently, infiltration-induced geochemical

reactions (redox, ferruginization and leaching processes)

signify potential environmental impact in terms of

groundwater quality as well as borehole/aquifer management,

especially under humid tropical environment of the

study area.