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GEOELECTRIC IMAGING OF GROUNDWATER POLLUTION
A resistivity survey was made in some part of PTI dumpsite in order to determine the quality of the ground water in that area. The survey consisted of 5 electrical soundings which were carried out using the Schlumberger array configuration with a current electrode separation of 126m. The data was interpreted by computer aided iteration techniques using the Resistivity modeling Software Application. The result of the interpretation shows four to five distinct geoelectric layers with resistivity ranging from 112.7Ωm to 426.8Ωm. Difference in apparent resistivity was assumed to be due only to differences in specific conductance of groundwater in the saturated zone. The result of the survey has shown that the aquifer in the study area has not yet being contaminated.
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.
Ø Perform analysis of Data: Analysis is made from plots of the calculated apparent resistivity against profile stations. The contaminated region should then appear as an anomalous low in the profile plot. If such a low does not appear, the sounding curve, available information and field conditions should be reexamined for a more sensitive electrode spacing, possible electrical interferences, or infeasibility of the method due to lack of sufficient pore-water contrast.
1.8 BASIC TERMS IN GROUNDWATER STUDY.
1.8.1 AQUIFER: An aquifer is a ground-water reservoir composed of geologic units that are saturated with water and sufficiently permeable to yield water in a usable quantity to wells and springs. Sand and gravel deposits, sandstone, limestone, and fractured crystalline rocks are examples of geological units that form aquifers. Aquifers provide two important functions:
(1) They transmit ground water from areas of recharge to areas of discharge, and
(2)They provide a storage medium for useable quantities of groundwater. The amount of water a material can hold depends upon its porosity. The size and degree of interconnection of those openings (permeability) determine the materials’ ability to transmit fluid.
An unconfined aquifer is one in which a water table varies in undulating form and in slope, depending on areas of recharge and discharge, pumpage from wells, and permeability. Rises and falls in the water table correspond to changes in the volume of water in storage within an aquifer.
Figure 1 shows an idealized section through an unconfined aquifer; the upper aquifer in is also unconfined. Contour maps and profiles of the water table can be prepared from elevations of water in wells that tap the aquifer to determine the quantities of water available and their distribution and movement. A special case of an unconfined aquifer involves perched water bodies (Figure 1). This occurs wherever a groundwater body is separated from the main groundwater by a relatively impermeable stratum of small areal extent and by the zone of aeration above the main body of groundwater. Clay lenses in sedimentary deposits often have shallow perched water bodies overlying them. Wells tapping these sources yield only temporary or small quantities of water.
Confined aquifers, also known as artesian or pressure aquifers, occur where groundwater is confined under pressure greater than atmospheric by overlying relatively impermeable strata. In a well penetrating such an aquifer, the water level will rise above the bottom of the confining bed, as shown by the artesian and flowing wells. Water enters a confined aquifer in an area where the confining bed rises to the surface; where the confining bed ends underground, the aquifer becomes unconfined. A region supplying water to a confined area is known as a recharge area; water may also enter by leakage through a confining bed. Rises and falls of water in wells penetrating confined aquifers result primarily from changes in pressure rather than changes in storage volumes. Hence, confined aquifers display only small changes in storage and serve primarily as conduits for conveying water from recharge areas to locations of natural or artificial discharge.
Figure 1: Schematic Cross-sections of Aquifer Types (Modified after Hartan et al, 1989)
Aquifers that are completely confined or unconfined occur less frequently than do leaky, or semi-confined, aquifers. These are a common feature in alluvial valleys, plains, or former lake basins where a permeable stratum is overlain or underlain by a semi-pervious aquitard or semi-confining layer. Pumping from a well in a leaky aquifer removes water in two ways: by horizontal flow within the aquifer and by vertical flow through the aquitard into the aquifer.
An aquitard is a partly permeable geologic formation. It transmits water at such a slow rate that the yield is insufficient. Pumping by wells is not possible. For example, sand lenses in a clay formation will form an aquitard.
An aquiclude is composed of rock or sediment that acts as a barrier to groundwater flow. Aquicludes are made up of low porosity and low permeability rock/sediment such as shale or clay. Aquicludes have normally good storage capacity but low transmitting capacity.
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