Hydrodynamic Implications of Aquifer Quality Index (AQI) and Flow Zone Indicator (FZI) in groundwater abstraction: a case study of coastal hydro-lithofacies in South-eastern Nigeria

We integrated the surface geologically constrained 1-D geoelectric attributes and laboratory analysis of water and hydrogeological cored samples collected from the wells in the coastal area to estimate the effective porosity, permeability and their relations with aquifer quality index (AQI), flow zone indicator (FZI) and normalised porosity index, the ingredients of aquifer dynamics. The cored samples were derived from the economic hydrogeological units and the water associated constants such as density, dynamic viscosity and acceleration due to gravity were utilised to obtain some of the geohydraulic quantities as required in the empirical relations employed. Our main preoccupation is to appraise the hydrodynamic properties which control the pore water abstracted into the wells. The estimated magnitudes of hydrodynamic properties enabled the estimation of two unique hydraulic units called the graded gravelly sands with little or no fines and well graded sands with little or no fines. The area seemed to be predominantly covered by well graded gravelly sand hydraulic unit as about 87% of the groundwater repositories investigated reflects this unit. A specific hydraulic unit has been found to conform to specific flow zone, which are important in groundwater study concerning freshwater-saltwater intrusion. The concept of stratigraphic boundaries in characterising the groundwater repositories do not account for the intra-formational variation of hydrodynamic properties in the study area as evidenced in certain ranges of permeabilities belonging to a unit flow zone indicator. The considered formations, which were all sandy and gravelly in nature, give permeability ranges that conform to the documented ranges in literature and this attests to the workability of the method. The results equally show that highly permeable sands/gravels have low tortuosity and low specific surface area of unit grain volume, which give rise to easy abstraction of water from the geo-pores. The relations between the measured and estimated parameters and the graphic details of hydrodynamic properties can be utilised in contaminant modelling of the hydrogeologic units. Management of coastal groundwater can easily be handled effectively in the single hydraulic unit than geologic units as groundwater flow in hydraulic units can furnish unique and reliable information rather than guessable information about the prime cause of contaminations in a given hydraulic unit. The classification of the hydrolithofacies is significant in the conservation and management of hydrogeologic units in the coastal zone of the study area, which is vulnerable to contaminations.