Investigation of the Source of Groundwater Contamination Near a Farmland Using the VLF-EM and Magnetic Methods

Abstract

Groundwater pollution continues to be a major problem in society where there is high level of urbanization, industrialization and conventional/inorganic farming. A detailed ground magnetic survey and VLF-EM was carried out on a site just beside a farmland to investigate the possible causes of the groundwater contamination in the area which lies within the Precambrian Basement Complex of Southwestern Nigeria. The survey was carried out using a proton precision magnetometer kept at a constant height during the survey, accompanied by Very Low Frequency - Electromagnetic Method (VLF-EM). Data for the two methods were acquired on five (5) traverses with the length of 100 m each. The distance between each traverse (inter traverse spacing) is 5 m. The VLF-EM data were interpreted qualitatively, using the Fraser and the Karous–Hjelt filters. The Fraser filtered data and relative current density pseudosections from the VLF-EM data indicate the presence of shallow and deep conductive zones that cross the farmland which are indicative of fractures. The quantitative interpretation of the data was done with a 2-D code that performs the inversion of the data. Magnetic maps were generated using Oasis Montaj, and profiles were plotted using Microsoft Excel software. From the results obtained, some major geological features were identified while linear features suspected to be fault/fractured zones, weathered zones, and bedrock depressions were delineated. Low magnetic intensity indicating presence of weak and incompetent zones were discovered. Earlier chemical analysis of both surface and groundwater collected at some depth on the farmland confirms the presence of water contamination. This knowledge supported the interpretation of the anomalies detected by VLF-EM and magnetic data as a possible cause of the contaminated groundwater flowing in connected fractures. The study concluded that the groundwater contamination originates from the farmland where some samples of the soil were found to contain nitrate chemicals resulting from fertilizer and pesticide used to enhance plant growth.

Keywords:Nitrate contamination, groundwater pollution, multiple fractures, contaminant transport, fractured zone, inorganic farming

DOI: 10.7176/JEES/12-6-04

Publication date:June 30^th 2022