2.1 Geophysical Investigations• Geophysical investigations involve simple methods of study madeon the surface with the aim of ascertaining subsurface detail. This isachieved by measuring certain physical properties and interpreting themmainly in terms of subsurface geology.

Importance of Geophysical Investigations• Geophysical methods are gaining importance very rapidly becauseof their success in solving a vast variety of problems.

• These investigations are carried out quickly. This means large areacan be investigated in a reasonable short period and hence time issaved.

• The geophysical instruments used in the field are simple, portableand can be operated easily. This means fieldwork is not laborious.

• Since the work is carried out quickly and only physicalobservations are made. Without the use of consumables (likeChemicals), it is economical too.

• Different interferences to suit different purposes can be drawnfrom the same field data, for example electric resistivity data canbe interpreted for knowing subsurface of rock types, geologicalstructures, groundwater conditions, ore deposits depth to the bedrock, etc. Hence the investigations are multipurpose.

Applications of Geophysical Investigations• Geophysical explorations are numerous, important and widely

varied.• Investigations aimed in solving problems of regional geology.• Investigations aimed at locating and estimating economically

important mineral deposits.• Investigations aimed at locating and assessing groundwater

potential and its quality• Investigations aimed at solving problems connected with geology.

Classification of Geophysical MethodsThere are many kinds of geophysical methods of investigation.These method are

• Gravimetric method• Magnetic method• Electrical method• Seismic method• Radiometric method• Geothermal method

Electrical Method• Among the methods different geophysical Methods electrical

method are numerous and more versatile, They are more popularbecause they are successful in dealing with a variety of problemslike groundwater studies, subsurface structure, and many others.

Controlling Properties• In electromagnetic methods, electrical conductivity, magmatic

permeability and dielectric constant of subsurface bodies are therelevant properties.Principle

• Electric methods are based on the fact that the subsurfaceformation, structures, ore deposits, etc. possess different

electrical properties. These differences are investigated suitablyand exploited to draw the necessary conclusion.

• Electrical resistivity methods, electromagnetic methods, self-potential methods and induce polarization methods are the veryimportant categories of electrical methods.

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Electrical Resistivity MethodPrinciple

• The electrical resistivity's of subsurface formation vary from oneanother if they are inhomogeneous and are studied with the helpof resistivity method. In the case of a resistive subsurface body,current lines move away from it and in the case of a conductivesubsurface body, the current lines move towards it.

• Profiling and Sounding are two types of resistivity investigations.Profiling is done to detect lateral changes in resistivity. Thisthrows light on the change in the subsurface lithology or structurefrom place to place.

• Sounding is done to determine the vertical changes in resistivity.In other words, this study reveals changes in lithology, etc. at aparticular place with increasing depth.

• All geological formations have a property called electricalresistivity which determines the ease with which electric currentflows through them. This resistivity is expressed in the units of Ώmohms meter and is indicated by the symbol ᵖ

Electrical Resistivity Measurements

Resistivity method and measurement of Resistivity• For the principle of the electrical resistivity method of exploration

and for measurement of resistivity. A high resistive overburden isa disadvantage for resistivity studies. This is so because very littlecurrent penetrates the ground which means that the investigationof deeper layer is not possible.

Classification of Resistivity Methods• The resistivity method are classified as profiling type, sounding

type, and potential type of methods.• Profiling method is used for measurement of resistivity in lateral

direction. Sounding type in which measurement are made invertical direction. Potential methods are used in ore prospectingand are of not of engineering relevance.

Seismic Methods• Controlling Properties• Elastic property differences in rocks is the controlling property.

Principle• Seismic method of study is based on the principle that subsurface

rock formations bear different elastic properties. Because of this,the velocities of propagation of seismic waves through thesubsurface layers of earth, suffer reflection or critical reflectionarrive at the surface of the earth where they are detected bygeophones. From the time taken by the waves to travel throughthe subsurface formation and from the seismic wave velocities ofthe media. It is possible to determine the depth of various elasticboundaries.

• With the help of geophones fixed at suitable intervals on theground, the different seismic waves reaching the surface arerecorded and from the times of their arrival, time –distance curvesare constructed. The direct waves are the first to reach thegeophones placed between point and the distance beyond thepoint is called the critical distance.

• Depending upon whether reflected waves or refracted waves areused in the investigation, there are two types of methods, namely,seismic reflection method and seismic refraction method.

• A geophone an amplifier and a galvanometer are the basic unitsrequired for reflected or refracted wave registrations.

• Seismic refraction studies are effective for depths more than100m but are not suitable for shallow exploration

• Refraction methods are employed for investigating depths fromclose to the surface to several kilometer deep. These methods arealso followed for the investigation of deeper crust under seismicstudies.

• Shallow seismic refraction have found effective application in

investigating the suitability of foundation sites for civil engineeringstructures.

• Seismic Refraction: the signal returns to the surface by refractionat subsurface interfaces, and is recorded at distances muchgreater than depth of investigation

• Seismic Reflection: the seismic signal is reflected back to thesurface at layer interfaces, and is

• recorded at distances less than depth of investigation

2.2 Ground water quality- Factors affecting ground water quality

• Sediment and natural organic materials• Nutrients• Bacteria• Toxic substances

Sediment and Natural Organic Materials• Sediment is defined as particles derived from soil, rock, or organic

matter that have been, or are being, transported by water orwind. Natural organic materials include plant debris, and humanand animal wastes. The erosion of land surfaces and stream banksproduces sediment. Erosion occurs naturally, but human activities,like farming, logging, or road construction can increase sedimenttransport to and within streams. Sediment deposited in streamscan restrict navigation. Sediment can also increase the potentialfor floods by decreasing reservoir storage and stream-channelcapacity. Suspended sediments contribute to the reduction ofwater clarity and quality. Fine sediments can severely alter aquaticcommunities by clogging fish gills and suffocating fish eggs andaquatic insect larvae. Harmful materials such as heavy metals andtoxic chemicals can attach to sediments and move with themdown the stream system. Sediment is a major water quality issuein most places.

Nutrients• Nutrients are any organic or inorganic compound needed to

sustain life. Examples include carbon, nitrogen, phosphorus, andpotassium. Nutrients are contributed to waters from theatmosphere, agricultural lands, golf courses, lawns, septicsystems, wastewater treatment plants, and factories. An excessamount of nutrients in water can result in a disproportionateamount of aquatic vegetation. The decomposition of this excessvegetation can remove oxygen from water and cause fish andother aquatic life to die. An overabundance of aquatic vegetationcan also interfere with recreation. High nitrate or ammoniaconcentrations can impact drinking water or kill fish. Nitrate andammonia are forms of nitrogen.

Bacteria• Some bacteria are disease-causing organisms that may be

delivered to surface water and groundwater by sewer overflows,leaking septic tanks, and runoff from animal feedlots orpastures. Some bacteria are a threat to humans, and indicatororganisms such as fecal coliform are used to determine theirpresence. Indicator bacteria are found in great numbers in theintestines of humans and other warmblooded animals. Whenwater tests confirm the presence of the indicator bacteria, thewater body may be contaminated by untreated sewage and othermore dangerous organisms may be present.

Toxic Substances• In sufficient quantities, toxic substances, such as cleaning solvents,

pesticides, and certain metals, can cause sickness, geneticdisorders, and even kill organisms. Toxic chemicals can enterwaters through direct discharge from industry or by improperdisposal of industrial, mining, farm, and household wastes.Contaminants contributed from industrial uses of water includetoxic substances produced from cleaning solvents, acids, andalkalis. The over application of pesticides can result in the excessentering waters through runoff to surface water and infiltrationinto groundwaters.

• Even extremely low concentrations of some chemicals arehazardous to humans and aquatic life. Toxic substances also canaffect an organism’s growth, metabolism, reproduction, orbehavior. The potential dangers of many toxic substances are onlynow being recognized. Assessing the environmental dangers ofthese substances has been enhanced as our ability to detectsmaller concentrations has improved and our understanding oftheir effects on the environment has increased.

2.3 Water quality requirements• Water quality criteria for aquaculture systems have typically

considered parameters such as temperature, dissolved oxygen,total gas pressure, ammonia, and nitrite. Many of the publishedcriteria are derived for environmental protection of a wide rangeof species and life stages. These criteria may not be appropriatefor a single species and life stage, especially in commercialapplications. The value of a given water quality criterion maydepend strongly on the species, size, and culture objectives. Inwater reuse systems, fine solids, refractory organics, surface-active compounds, metals, and nitrate may become important.The limiting factors in very high intensity reuse systems are notentirely understood at this time. Development of more relevantwater quality criteria for reuse systems will require production-scale trials. Separate water quality criteria for bio filter operationare also needed.

• The following are important requirements of water for domesticuse :

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2.4 Groundwater quality degradation• Groundwater pollution occurs when pollutants are released to the

ground and make their way down into groundwater. It can alsooccur naturally due to the presence of a minor and unwantedconstituent, contaminant or impurity in the groundwater, in whichcase it is more likely referred to as contamination rather thanpollution.

• The pollutant creates a contaminant plume within an aquifer.Movement of water and dispersion within the aquifer spreads thepollutant over a wider area. Its advancing boundary, often called aplume edge, can intersect with groundwater wells or daylight intosurface water such as seeps and spring, making the water suppliesunsafe for humans and wildlife. The movement of the plume,called a plume front, may be analyzed through a hydrologicaltransport model or groundwater model. Analysis of groundwaterpollution may focus on soil characteristics and site geology,hydrogeology, hydrology, and the nature of the contaminants.

• Pollution can occur from on-site sanitation systems, landfills,effluent from wastewater treatment plants, leaking sewers, petrolfilling stations or from over application of fertilizers in agriculture.Pollution (or contamination) can also occur from naturallyoccurring contaminants, such as arsenic or fluoride. Using pollutedgroundwater causes hazards to public health through poisoning orthe spread of disease.

• Different mechanisms have influence on the transport ofpollutants, e.g. diffusion, adsorption, precipitation, decay, in thegroundwater. The interaction of groundwater contamination withsurface waters is analyzed by use of hydrology transport models.

2.5 Reasons of groundwater quality degradation

Factors affecting ground water quality Principal sources and causes of pollution with regard to their

occurrence:-a. Municipal sources and causesb. Industrial sources and causesc. Agricultural sources and causesd. Miscellaneous sources and causes

1. Municipal sources and causesa) Sewer leakage Sources: Poor workmanship, defective sewer pipe, breakage by

tree roots, ruptures from heavy loads, earthquakes, loss offoundation support etc.

Results: Introduce high concentrations of BOD,COD, nitrate,organic chemicals, bacteria and heavy metals into groundwater

b) Liquid wastes Sources: Domestic wastes, Disposal wells industries, storm, runoff

etc. Results: Introduce bacteria, viruses, trace elements and heavy

metals, inorganic and organic chemicals etc.

c) Solid wastes Sources: Landfills Results: Leachate from landfills can pollute groundwater Leachate include iron manganese, nitrate, trace elements etc.

2. Industrial sources and causesa) Liquid wastes Sources: Industrial waste water discharged into pits, ponds,

lagoons etc. Results: Introduction of hazardous and toxic industrial wastes into

the groundwater

b) Tank and pipeline leakage Sources: Gasoline stations, fuel oil tanks, petroleum and

petroleum products from industrial pipelines and tanks Results: Immiscible liquids like oil and petroleum, liquid

radioactive wastes etc. reaches the water table and pollutes thegroundwater

c) Mining activities Sources:- Coal, phosphate and uranium mines Stone, sand and gravel quarries Results: Low pH, increase in iron, aluminium and sulphate content

in the soil

d) Oil-field brines Sources:- Substantial discharges of wastewater in the form of brine Constituents of brine include sodium, calcium, ammonia, boron,

chloride, trace metals and high total dissolved solids Results: Groundwater become saline

3. Agricultural sources and causesa) Irrigation Return Flows Sources: Irrigation return flow drains to surface channels or joins

the underlying water Results:- Increases salinity of groundwater Increases the amount of bicarbonate, sulphate, chlorides, nitrates

etc. in the groundwater

b) Animal wastes Sources: Wastes from slaughter houses Results:- The natural assimilative capacity of the soil become overtaxed Salts, organic loads and bacteria are transported into the soil Nitrate-nitrogen is the most important persistent pollutant that

may reach the water table

c) Fertilizers and Soil Amendments Sources: Leachate of phosphate and potassium fertilizers Leachate of soil amendments like lime, gypsum and sulphur Results: Increases salinity of soil

d) Pesticides, insecticides and herbicides Sources: Leachate of pesticides, insecticides and herbicides used

in agricultural fields Results: Causes serious consequences in relation to the portability

of water

4. Miscellaneous sources and causesa) Urbanisation

Groundwater pollution can occur both in rural as well as urbanareas and is affected by differences in chemical composition,biological and chemical reactions, density and distance fromdischarge areas

b) Spills and Surface Discharges Causal activities includes leaks from pipes and valves, uncontrolled

waste disposal, intermittent dumping of fluids on ground, flushinghazardous and flammable liquids into water etc.

Washing aircraft with solvents and spills of fuel at airports canform a layer of hydrocarbons floating on the water table.

c) Stockpiles Solid materials are frequently stockpiled near industrial plants,

construction site etc. Precipitation falling on unsheltered stockpiles causes leaching of

heavy metals, salts and other pollutants into the groundwater

d) Septic tanks and Cesspools Septic tank: A watertight basin intended to decompose the

domestic sewage and toDischarge this into the biologically active zoneOf the soil mantle through a subsurface percolation system

Cesspools: large buried chamber with porous walls designed toreceive and percolate raw sewage

e) Saline water intrusion Salt water may invade freshwater aquifers to create point or

diffuse pollution sources

g) Surface water Polluted surface water bodies that contribute to groundwater

recharge become sources of groundwater pollution