GEOELECTRIC IMAGING OF GROUNDWATER POLLUMTION IN DELTA CASE STUDY: PTI DUMPSITE

GEOELECTRIC IMAGING OF GROUNDWATER POLLUMTION IN DELTA CASE STUDY: PTI DUMPSITE

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CHAPTER ONE

Introduction

1.0 BACKGROUND OF STUDY

Groundwater is commonly understood to mean water occupying all the voids within a geologic stratum. Groundwater is one of the nation’s most valuable natural resources; it is the source of about 40 percent of the water used for all purposes exclusive of hydropower generation and electric power plant cooling. Surprisingly for a resource that is so widely used and so important to health and to the economy of the country, the occurrence of ground water is not only poorly understood but is also, in fact , the subject of many widespread misconceptions. Common misconception includes the belief that ground water occurs in underground rivers resembling surface streams whose presence can be detected by certain individuals. These misconceptions and others have hampered the development and conservations of ground water and have adversely affected the protection of its quality. Groundwater occurs everywhere but sometimes its availability in economic quantity depends solely on the distribution of the subsurface geomaterials that are referred to as the aquifers. This implies that where groundwater is not potentially endowed enough, there may be either complete lack or inadequacy due to increasing industrial and domestic needs.

Pollution occurs when the concentration of various chemical or biological constituents exceed a level at which a negative impact on amenities, the ecosystem, resources and human health can occur. Pollution results primarily from human activities. There are different sources of pollution. When they are chemical or biological constituents creating pollution they are known as contaminants. Contaminants degrade the natural quality of a substance or medium. It can either be organic or inorganic.

Surface resistivity methods have been employed successfully for detecting and mapping ground-water contamination under a variety of conditions. The method is based on the fact that formation resistivity depends on the conductivity of the pore fluid as well as the properties of the porous medium. Under favorable conditions, contrasts in resistivity may be attributed to mineralized groundwater with a higher than normal specific conductance originating at a contamination source. Success with surface resistivity methods depends to a large extent on a good knowledge of subsurface conditions. Conditions favorable for delineating zones of contamination include uniform subsurface conditions, a shallow groundwater table, and good electrical contrast between mineralized and natural water.

One of the primary problems in field investigations of groundwater pollution is locating the contaminant plume. In most cases, the goal is to positively locate the pollutant and its movement by test holes and direct monitoring. In the interest of efficiency the investigative areas should be as focused as possible. In many cases a general knowledge of local hydrogeology allows a reasonable initial estimate of pollutant direction; in other instances even this may be lacking. Drilling of sampling holes on a hit-or-miss basis is both time-consuming and expensive. It can also be destructive to the property involved. Under certain subsurface conditions, surface geoelectrical profiling can quickly and cheaply locate the general location of the plume and identify areas most feasible for sampling and monitoring.

Numerous investigations have established the usefulness of surface electrical resistivity as a tool in the detection of ground water contamination.

1.1 Definition and Causes of groundwater pollution

Pollution has been found to be much more widespread than we had believed only a few years ago. Polluted ground water may pose a serious threat to health. Pollution of ground water refers to any deterioration in quality of the water resulting from the activities of man. Most pollution of ground water results from the disposal of wastes on the land surface, in shallow excavations including septic tanks, use of fertilizers, leak in sewers and pipelines. The magnitude of any pollution problem depends on the size of the area affected and the amount of the pollutant involved, the solubility, toxicity, and density of the pollutant, the mineral composition and the hydraulic characteristics of the soils and rocks through which the pollutant moves, and the effect or potential effect on ground-water use.

1.2STATEMENT OF PROBLEM

Here this study focuses mainly on the impact of Groundwater pollution in a dump site area and how it can be evaluated using resistivity method. But first I will like to discuss briefly about the impact of pollution on Groundwater before giving reasons of using resistivity method on ground water pollution in a dump site.

1.3 AIM AND OBJECTIVES

The aim of this work is to detect, delineate and denominate the extent of contaminant intrusion on ground water in an area, with the following objectives in mind:

Ø To study the geo electrical properties of the sub surface to depth in other to estimate contamination degree.

Ø To uncover the direction of pollutant flow relative to the ground water flow.

Ø To assess and map the vertical and lateral extent of contaminated groundwater into sub surface and how much ground water area it covered.

Ø To distinguish between polluted and non - polluted zones with respect to the groundwater contamination.

1.4 SIGNIFICANCE OF STUDY

The significance of the above study is important in following ways:

v It will provide useful information on the condition of ground water at dump site areas which can serve as a useful tool in environmental impact assessment (EIA), of that area.

v Information about water flow direction will assist in the design of efficient and cost-effective monitoring networks and remediation strategies of ground water pollution.

v Geoelectric details of the subsurface gotten from this study will give sound knowledge of the sub surface geology including infiltration and percolation process is prerequisite for managing contaminant transport in the saturated or aquifer zone.

1.5 SCOPE OF STUDY

In this present study, VES data were collected from the dumpsite area of PTI, Effurun Delta State. This geoelectric data of the subsurface were then used to detect the source of the pollution, estimate the degree of contamination, lateral and vertical extent covered, and map out zones of anomalies and estimate the spread rate.

1.6 LIMITATIONS TO STUDY:

Electrical resistivity profiling is simple in concept but has a number of significant practical limitations.

Ø Equipment range: The extreme limit for spread of the current electrodes, and consequently the depth of penetration of the current generator and the resistivity characteristics of the soil being measured. Highly resistive layers such as thick unsaturated zone require considerable current before the underlying saturated material can be sensed.

Ø Physical obstructions: In many situations, it is difficult to establish a long continuous electrode spread or profile line because of physical obstructions. These may include rocks, trees, bulidings, paved areas and the like.

Ø Electrical interferences: A careful check must be made of the area to be surveyed for any electrical inducing or conductive features. These include overhead and buried power lines, metal fences, above-ground and buried water lines, railroad tracks, and conductive pipes of all kinds. As a general rule one must be at least as far away from such interference as the "A" spacing.

Ø Topographic variations: The model assumes that the resistivity layers are uniform in thickness and infinite in extent. In hilly or rugged terrain, it becomes impossible to determine whether the observed change is due to subsurface variation in hydrogeology or to topographic changes.

Ø Hydrogeologic variations: Changes in soil type zone can mask the effects of pore-water resistivity change. The presence of silt and particularly clay will lower the apparent resistivity substantially and can easily be mistaken for a change in pore-water resistivity. Accordingly where such materials may occur, electrical interpretations should be made with reservation.

1.7 PROCEDURES INVOLVED IN THE STUDY

The following procedure is recommended for surface resistivity profiling:

Develop a hydrogeologic concept for the area to be investigated. Available geological and ground-water studies should be reviewed. If available, boring logs and water quality data should be obtained. From these, the pattern of ground-water flow and general resistivity model can be ascertained.

Ø Make a field survey of the area.

Ø Determine Profile Location: Based on the results of items 1 and 2, the selection of profile locations can be made. The profile line should cross the anticipated plume location, beginning and ending clearly on either side of the probable contaminated zone.

Ø Make Field Preparations: The line should be cleared and the electrode positions clearly marked in advance. Much time during the actual profiling can be saved by good site preparation. Equipment, especially condition of batteries and integrity of electrical wire, should be checked carefully before proceeding to the field.

Ø Make Vertical electrical sounding: Atleast one electrical sounding and preferably more should be made at the site to ascertain the most appropriate "A" spacing(s). Ideally these soundings should be made in an uncontaminated zone. The soundings should confirm the hydrogeologic model developed from item 1.

Ø Run Profiles: The profiles are then run at the selected A spacings and at a station separation no more than one-third the estimated plume width. Preliminary calculations of apparent resistivity should be made in the field; this allows for additional readings to be taken if results seem unusual or a region of electrical anomaly is encountered.


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