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GEOELECTRIC
IMAGING OF GROUNDWATER POLLUTION
ABSTRACT
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.
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.
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.
UNCONFINED
AQUIFERS
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
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)
LEAKY
AQUIFER
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.
1.8.2 AQUITARD
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.
1.8.3 AQUICLUDE
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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