Geostatistical and geochemical model-assisted hydrogeochemical pattern recognition along the groundwater flow paths in Coimbatore district, South India

The deterioration of groundwater quality in Coimbatore district is principally attributed to the geological formations, application of agrochemicals and discharge of untreated industrial effluents. Groundwater samples were collected and analysed to obtain the hydrochemical characteristics for delineating the groundwater recharge and discharge zones. Alkaline water is present in the district during all seasons. Groundwater facies were evaluated and the majority of them were CaHCO3 and Ca-Na-HCO3 types. Generally, HCO3 was used to identify the recharge area and Cl the discharge area. Dominance of HCO3 in both water types helps in identifying the recharge zones. The TDS and major ions in conjunction with HCO3/Cl, SO4/HCO3, HCO3+CO3/Ca+Mg, HCO3+CO3/T.anions and Cl/T.anions were used in the delineation of recharge-discharge areas. Results of geochemical modelling using PHREEQC were in agreement with the analysis of ionic ratios. Carbonate minerals are saturated in the recharge zone and become undersaturated as the flow progresses. HCO3 to Cl types for the recharge to discharge zones were confirmed by the aqueous speciation modelling. Gradual increase of NO3 along the flow path also supported the increasing anthropogenic influences towards the discharge zone. The spatial distribution diagrams drawn for each of these ratios suggested that a major part of the study area is covered by recharge zones. Mettupalayam and Pollachi taluks were found to be discharge zones. The results of the study showed that the water quality in the discharge zones is largely controlled by anthropogenic activities. The study gains importance since part of the water supply to the Coimbatore corporation area is from a source in the neighbouring state and the major surface water source of this area comes under an interstate water dispute.