FLOW PATTERNS AND CHEMICAL CHARACTERISTICS OF GROUNDWATER IN SHALLOW COASTAL AQUIFER IN BADAGRY AREA, SOUTHWESTERN NIGERIA

Abstract

Groundwater, more often than not,provides lifeline in the coastal regions for domestic, irrigational and industrial water needs of residents. In this study, groundwater chemistry and flow patterns in aquifers along the coastal zone of Badagry area were investigated to determine type and distribution of hydrochemicalfacies present.Sixty-nine hand-dug wells were randomly selected to cover the entire study area for groundwater flow and water table elevation determination. Watersamples were collected from thirty-nine of these shallow wells for chemical analyses and field determination of physicochemical parameters(pH, Temperature, Electrical Conductivity and Total Dissolved Solids). The physicochemical parameters were determined on the field using HANNACOMBO meter HI 98129.Dissolvedcation concentrations (Na+, K+, Ca2+, Mg2+, Cu2+, Pb2+, Fe2+) were determined using Atomic Absorption Spectrophotometerwhile the UVSpectrophotometer was used for NO3 -, and SO4 2-ionsand titrimetric method was employed for Cl- & HCO3 - concentrations.Results obtained gave average water table elevation of 4.199m above mean sea level with groundwater flowing in generally southward direction. Eight local discharge zones and nine local recharge zones were identifiedin the study area. The average values of pH, temperature, EC,and TDS obtained were 6.09, 28.04˚C, 340.58μS/cm and 164.67, respectively.Chemical analyses resultsshowed order of abundance of dissolved ionic concentrations as Na+> Ca2+> Mg2+> K+for cationsand HCO3 - >Cl-> SO4 2-> NO3for anions. The dissolvedFe2+, Pb2+ and Cu2+concentrations ranged from0.0133 to 0.7445mg/l, 0.0000 to 0.9181mg/l, and 0.0394 to 1.6559mg/l, respectively.Ca (HCO3)2facies constitutes the dominant groundwater type. The regional southward flow direction of groundwater in the study area is physiographically - controlled as the flow along coast is expected to be seaward with decline in terrain elevation toward the sea. However, the local discharge and recharge zones found within the study area were influenced by anthropogenic activities which can be attributed largely to the uneven rates of water abstraction from wells in the different localities.Chemically, dissolved concentration values of all the major cations and anions fell within the World Health Organisation (WHO) recommended limits forpotable water. Concentrations of Fe2+ and Pb2+ were above the WHO maximum recommended limits and can therefore be said to have reached pollution levels in many of the groundwater samples.These trace elements Fe2+, Pb2+ pollution found are attributable to human activities within the study area.The Ca (HCO3)2faciestype which dominates the groundwater chemistrysuggests groundwater of continental origin with minimal marine influence.