GROUNDWATERGEOCHEMISTRY GL 376

Lecturer: Dr. Asare Asante-AnnorSecond Semester 2014

UNIVERSITY OF MINES AND TECHNOLOGY (UMAT), TARKWAFaculty of Mineral Resources Technology

Department of Geological Engineering

Organisational Aspects

Room: Geological EngineeringDepartment Email: aasante-annor@umat.edu.gh Office hours: Tue - Wed 12-16hrs Assessment

◦ Class attendance [ 10 marks ]◦ Continues Assessment: Assignments + Class tests [ 30 marks ]◦ End of semester exams [ 60 marks ]

others

Course Outline

Basic Concepts Sampling and Analysis of Groundwater Chemical Reactions and Processes that

Affect Groundwater Quality Geochemical Equilibrium Modelling Geochemical Environments

References1. Appelo, C.A.J., and Postma, D. (2005). Geochemistry, Goundwater

and Pollution. Second Edition, Taylor and Francis, UK, TheNetherlands, 672 pp.

2. Freeze, R. A and Cherry, J. A. (1979). Groundwater. Prentice Hall,Inc., Englewood Cliffs, New Jersey, 604p.

3. Hounslow, A. W. (1995). Water Quality Data; Analysis andInterpretation, Lewis Publishers, New York, 397p.

4. Deutsch, W. J. (1997). Groundwater Geochemistry: Fundamentalsand Applications to Contamination, CRC Lewis, London, 221p.

5. Todd, D. K. and Mays, L (2008). Groundwater Hydrology. ThirdEdition, John Wiley and Sons, New York, USA, 656 pp.

6. Schwartz, F. W. and Zhang, H. (2008). Fundamentals ofGroundwater. John Wiley and Sons, NY, USA, 592p.

7. Adelana, S. M. A. and MacDonald, A. M. (2008) AppliedGroundwater Studies in Africa, IAH Selected Papers, CRC LewisPress, Oxon, UK, 507 pp.

IntroductionHydrogeology is the study of ‘the occurrence and movement of

aqueous solutions through porous andpermeable rock media, regardless of thephase or concentration of the solutions.

the modifications caused to the chemicalcomposition of the solutions, of othersubsurface fluids and of the rock media bytheir mutual interaction’.

Physical Hydrogeology

Chemical Hydrogeology

Definition: Groundwater Geochemistry

The chemistry of water in the subsurfaceenvironment. The chemical composition of

groundwater is the combined result ofthe composition of water that enters thegroundwater reservoir and reactions withminerals present in the rock that maymodify the water composition

Why study GW GeochemistryPractical reasons for studying GW geochemistry Groundwater is mainly used in the home,

irrigating lands, or general industrial purposes Although drinking water can be manufactured in

a chemical plant as for example in desalinizationplants, this is very expensive Preservation of good groundwater resources is

of a high priority for environmental authorities Geological conditions influencing GW quality

Composition and Suitabilityo The chemical and biochemical constituents in

GW or its quality determines if it is suitable forconsumption, industry or agriculture; orwhether it may cause sickness, adversely affectsoil properties, or precipitate mineral scales.

The problems may be caused by theconcentration levels of only one or two solutesand the processes controlling these levels mustbe understood to make optimal use of thegroundwater resource

Why study GW Geochemistry

Will concentrations increase or remainstable during exploitation of the aquifer?What happens when air enters the

saturated zone due to the drawdown ofpiezometric levels by production from theaquifer?What are the changes in water

composition due to up-coning ofsaltwater in an over-exploited reservoir?

Why study GW Geochemistry

Local and Global Perspective

Apart from natural processes as controllingfactors on the groundwater quality, pollutionof aquifers has also become an importantissue globally. Locally and regionally, groundwater quality

has been impaired by the release ofhazardous chemicals. The restoration of these systems will

require the creative integration of chemical,biological, and hydrologic understanding.

Sources of Pollution

< Diffuse > < Localised > Examples:

◦ Groundwater acidification by acid rain andnitrate pollution by the use of fertilizers bothresult from diffuse sources of pollution.

◦ Landfills and chemical waste deposits fromwhich organic substances and heavy metalsmay leach.

Diffuse Source

Point Source

Sources of Pollution contd.

The important question in both cases iswhether pollutants are transported asconservative components, or to whatextent they will be retarded bygeochemical processes within the aquifer. Due to the long residence time of

groundwater in the invisible subsurfaceenvironment the effects of pollution mayfirst become apparent tens to hundredsof years afterwards.

Expectations

The essential point in the present courseis to demonstrate how water obtains itscomposition and to describe theprocesses, which modify its composition.With a good understanding of the

processes, the observed water qualitiescan be interpreted hydrologically in termsof the origin and flowpath of water.

BASIC CONCEPTSCHAPTER ONE

Atomic structure

An element cannot be changed intoanything simpler by ordinary chemicalprocesses An atom is the smallest particle of an

element that possesses the properties ofthat element. It consists of a positivelycharged central nucleus with negativelycharged electrons surrounding thisnucleus

Atomic structure cont’d The atomic nucleus consists of a varying number of

positively charged protons and neutrons. The number of protons is called the atomic number The total number of protons and neutrons is called

the mass number Most elements consist of mixtures of atoms of

different masses, that is, all have the same number ofprotons, but different numbers of neutrons. Theseare called isotopes The average weight of the isotopes contained in a

naturally occurring element is its atomic weight

Bonding of atoms

Most matter consists of groups of atomsjoined by chemical bonds where only theouter part of the atoms is in contact.

These are called the valence electrons

Types of bonding

Ionic bonds occur where one or moreelectrons are transferred from thevalence shell of one element to thevalence shell of another. Attraction takes place between ions of

opposite charge – example NaCl and LiF◦ one atom losing electron(s), yielding a cation◦ and the other atom gaining electron(s),

yielding an anion

Types of bonding cont’d

Covalent bonds result from the equalsharing of electrons. An example is two atoms of the same

element such as Cl2, H2. Covalent compounds are true molecules

held together by strong intra-molecularforces

Types of bonding cont’d

Polar covalent bonds lie between theabove extremes. The electrons are sharedunequally by the adjoining atoms. The resultis a molecule with positive and negativeends, called a dipole. Van der Waals bonds are intermolecular

forces that cause interactions betweenpermanent dipoles. These bonds are similarin some ways to polar covalent bondsexcept that they are much weaker.

Types of bonding cont’d

Electronegativity is a measure of theability of an atom to attract sharedelectrons in a structure. The electronegativity difference between

two bonded atoms, is used to assess thedegree of polarity in the bond. The greater the difference the more ionic

(polar) the bond becomes.◦ Metals have low electronegativities and non-

metals have high electronegativities.