GROUNDWATER CHEMISTRY AND CONTAMINATION

 

 Groundwater chemistry was initially studied in relation to the respective suitability of groundwater for different usages. However, man’s impact upon groundwater systems has created many environmental problems so that hydrochemical studies in respect if groundwater contamination/pollution has become very important. Although the Chemists may be providing excellent data and reliable contamination/pollution control, it is probable that much of the information made available is not fully appreciated by water Engineers and Hydrogeologists. Also it may well be that the chemists have problems in viewing their data in a hydrogeological context. Hence integration of hydrochemical data into the overall aquifer assessment is essential and worthwhile for adequate hydrochemical assessment of groundwater.

General Concept / Approach

The ultimate goal, rarely achieved at present, is to predict the quality of groundwater changes in space and time and the rates of these changes. As in any other project, in a groundwater quality project, the desired objectives must be clearly formulated.

·         for research, these objectives are the testing, verification or refuting of one or more hypotheses;

·         for applied studies they are identification of the problem, and of the controlling parameters and an evaluation of alternative ways of solving or coping with the problem.

Hydrogeologists are frequently required to assess the quality of groundwater on a regional basis. This requires a sampling and monitoring programme, which is significantly different spatially and temporally from a site-specific study. When hydrogeological and geochemical principles are applied appropriately, the result is efficient, meaningful and therefore successful groundwater assessments.

However, to be able to identify the controlling hydrogeochemical reactions under such groundwater assessment, one must know the present and past geological, hydrological, biological, soil chemical, meteorological, and human factors that affect the chemistry of water as highlighted in the figure below:

 

In the unsaturated zone, the input of chemical or biological species of the recharge area is largely controlled by:

•         atmospheric deposition,

•         land use (including vegetation and soil cover),

•         the frequency, amount and duration of precipitation and irrigation,

•         the mineralogy of the soil,

•         the air temperature,

•         the soil gas/air exchange rate, and transport properties.

Therefore, the quality of infiltrating surface water is controlled by:

•         the composition of the surface water together with the temperature,

•         the composition of the associated bottom sediments and

•         the residence time within these bottom sediments.

In the saturated zone, factors that affect chemistry of groundwater are:

•         chemical reaction rate,

•         residence time within the saturated zone, and

•         mineralogy of the rock matrix.

Here, the residence time and flow path are determined by factors like aquifer thickness, permeability, porosity and amount of recharge.

·         The phenomena of mixing of water from different areas, aquifers or confining beds, from seawater intrusion or trapped saline water, or contaminants impose a hydrological control on the chemical character of groundwater.

·         Superimposed on all these primarily natural factors are anthropogenic effects leading to chemical and physical stresses on the hydrogeological system that may be dominant in some areas.

Therefore, to be able to formulate and carry out regional studies, the following scientific and technical aspects associated with hydrogeology must be understood:

•         the basic concepts of organic and inorganic chemistry;

•         the use of environmental isotopes and geochemical modeling;

•         principles of advective/dispersive transport and the coupling with reactions the consequences of surface water/groundwater interaction or soil moisture/groundwater interaction on groundwater quality;

•         the effects of land use on water quality; and application of geochemical principles to regional aquifer systems;

•         salt-water intrusion as well as specific contamination problems in karst aquifers.

Most of the above topics are critical to better understanding of water and environmental geosciences as an emerging aspect of modem hydrogeology. Consequently, within the framework of this course, the focus will be on concepts and principles of hydrogeochemical assessment of groundwater system with emphasis on the following subject matters:

•         Definition of Chemical Terms and Concepts

•         Water as a chemical substance and unique Properties of Water

•         Measurement / Concentration Units

•         Principles and Processes Controlling Composition of Natural Groundwater Systems

•         Chemical activities and ionic Species and/or Component

•         Ionization of Water and weak Acids and Carbonate equilibrium

•         Common Types of Chemical Reactions in Water

•         Evolution of Groundwater Chemistry

•         Water quality and Treatment

•         Sources of Water Contamination / Pollution

•         Acid Precipitation

•         Measurement of Water Quality

•         Water Quality Standards

Definition of Chemical Terms and Concepts

A pollutant is a substance present in greater than natural concentration as a result of human activity and having a net detrimental effect upon its environment.

The hydrosphere includes all water in the earth’s crust.

The lithosphere includes the outer portion of the earth’s mantle and the earth’s crust in geology; however, in environmental sciences, the term is used to refer to the minerals, organic matter, water, and air, which make up the surface of the solid earth, in particular the soil.

The atmosphere is the envelope of gases surrounding the Earth. The biosphere includes all living organisms and their surrounding environment.

A phase is simply a homogenous substance with uniform composition and properties, e.g., the atmosphere, seawater, a mineral grain. The concept of a phase becomes less clear on a microscopic scale near phase boundaries, because there is always a variation in composition near phase interfaces.

A solution is generally a liquid phase composed predominantly of one component, the solvent (e.g., water) and less abundant components, the solutes (e.g., dissolved salts). A solution can also be a solid or a gas phase, e.g., a solid solution (e.g., halite, the solvent, with minor impurities such as sodium bromide, a solute) or a mixture of gas components (e.g., the atmosphere). The term fl can refer to either a liquid or gas phase.

Anions - negatively charged ions or complexes in aqueous solutions. An anion forms by reduction through acceptance of electrons. The major anions and their charges are OH^-, Bf^-, Cr^-, F^-, HCO₃^-, HS^-, N0₃^-, CO₃^2-, SO₄^2-, PO₄^3-.

Cations - positively charged ions or complexes in aqueous solutions. A cation forms by oxidation through donation of electrons. The major cations and their charges are H⁺, K⁺, Na⁺, Ca²⁺, Fe²⁺, Mg^2+, Al³⁺, Fe³⁺ The important class of pollutants known as heavy metals generally exists in solution as cations.

Complexes - molecules that can be anions, cations, or neutral in charge. For example, SiO₂ combines with 2H₂O in aqueous solution as a neutral aqueous molecule in the form of H₄SiO4. Another example is Al(OH)₄^-, a negatively charged complex consisting of an Al³⁺ cation and 4(OH)_- anions joined together.

Electrical Balance - In a solution, the charges contributed by the cations have to balance those contributed by the anions. Elemental analyses are frequently done which list the concentrations of the major components without determining if they exist in solution by themselves or as part of different complexes. For example, an analysis will only determine total Na concentration and not the proportion that actually exists in solution as Na⁺, NaCl^-, NaHCO₃^-, NaCO₃^-, etc.

However, the overall electrical balance can still be estimated by summing the electrical charges in the elemental analyses using the dominant charge for each of the elemental components. For example all of the sodium is given a +1, all of the chloride is given a -1, the entire sulfate is given a -2, etc. The overall electrical balance gives an idea of the accuracy of the solution analyses.

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