The following is a list of activities that happen in groundwater that alters its chemical make up.

These reactions involve the interaction between ions and molecules that are already in solution in the ground water.

Acid-Base reactions involve the transfer of a proton [H⁺] from one molecule or ion to another. The molecule or ion losing the proton is called the acid while the one acquiring the proton is called the base.

There are three especially important acid-base reactions commonly occurring in ground water. First is the dissociation of water into hydrogen ions [H⁺] and hydroxide ions [OH^-].

H₂O + H₂O= H⁺ + OH^-

Second are the reactions involved in the solution of CO₂ gas into water.

CO₂(g) + H₂O = H₂CO₃

H₂CO₃ = HCO₃^- + H⁺

HCO₃^- = CO₃^2-+ H⁺

Third are the reactions that involve the solution of solid silica into water.

SiO₂(s) + H₂O = H₂SiO₃

H₂SiO₃ = HSiO₃^- + H⁺

HSiO₃^- = SiO^2- + H⁺

The first reactions are measured by pH while the second is measured by alkalinity.

pH

Acid-base reactions can result in increases or decreases of protons (i.e., hydrogen ions [H⁺]). The concentration of hydrogen ions is measured by pH.

pH = -log [H⁺]

Acid-base reactions that provide high concentrations of hydrogen ions result in low pH values (i.e. pH <7.0) in solutions. These solutions are said to be acidic. Acid-base reactions that provide low concentrations of hydrogen ions result in high pH values (pH > 7.0) in solutions. These solutions are said to be basic. The pH of ground water controls which cations, anions, gases and solids dissolve into ground water (i.e., go into solution) and which exit from groundwater (i.e., precipitate or volatilize).

Alkalinity

Alkalinity is defined as the net concentration of strong base in excess of strong acid with a pure CO₂ - water system as a point of reference. It is controlled by pH and the concentrations of strong bases such as carbonate and silicate ions.

Strong acids are not common in natural ground water. Their occurrence represents contamination from human activity. The solution of silicate and carbonate minerals does provide strong bases in solution in natural situations. Consequently, as ground water flows through an aquifer, it dissolves more carbonate and silicate minerals thereby increasing the alkalinity and the pH.

Water, being an excellent solvent, can dissolve gases, liquids and solids and thereby increase concentrations of solutes in ground water. Moreover, these dissolved solutes and be lost to ground water by volatilization, exsolutions and precipitation.

A. Gas Solution and Exsolution: Relationship Between Gas and Liquid Phases

Gas enters (i.e., is dissolved) ground water in large concentrations in the aeration zone through the soil atmosphere (i.e., the part of the intergranular spaces not filled by water). The greater the concentration of the gases in the soil atmosphere, the more gas dissolved into the capillary films. Photosynthesis and atmospheric diffusion add O₂ into the soil atmosphere. Moreover, respiration and decomposition of organic matter provides additional CO₂ to that diffused from the atmosphere. In addition, decomposition and bacterial metabolic processes also provide gaseous H₂S, CH₄, and NH₃ into the soil atmosphere. These, in turn, may oxidize in the soil atmosphere, forming CO₂, CO, SO₂, NO₂, and N_2. All these gases are diffused into capillary films and are then dissolved. The concentration depends on the concentration in the soil atmosphere and the solubility of the individual gases.

These gases are generally held in solution as hydrostatic pressure increases down the ground water flow path. But once approaching ground water discharge, hydrostatic pressure decreases, thereby stimulating exsolution (i.e., the formation of gas bubbles) and volitilization (i.e., the return of these gases from the liquid environment of ground water into the gaseous atmosphere). Consequently, the concentration of dissolved gases increased with recharge and decreases with discharge.

B. Solution of Non-Aqueous Liquids: Relationship Between Liquid Phases

Organic compounds make up the vast majority of dissolved liquids. (These are referred to as Non-Aqueous Phase, Liquids or NAPLs. If they are denser than water, they are referred to as DNAPLs of if less dense than water LNAPL's.) The ability of these liquids to dissolve in water is controlled by the molecular size. The smaller the molecule, the greater the solubility. If the organic molecule is charged or contains oxygen or nitrogen it will be relatively soluble. In addition, the position of the functional group in the organic molecule can significantly alter its solubility. The functional group serves as the organic molecule's connection with the water molecule in forming hydrogen bonds. In addition, the aromatic hydrocarbons are generally far less soluble than the chlorinated alkanes and alkenes.

Aromatic & Polycyclic Aromatic Hydrocarbons

Chlorinated Alkanes & Alkenes

The ability of an organic molecule to remain as a non-aqueous liquid or dissolve is reflected in its octanol/water partitioning coefficient (K_ow). The greater the solubility, the less likely it is to remain as an undissolved non-aqueous liquid.

C. Volatilization: Relations Between Liquid and Gaseous Phases, Take II

Non-aqueous liquids and liquids dissolved in water can evaporate. This is called volatilization. The ability of an organic molecule to vaporize is measured by its vapor pressure. The greater the vapor pressure, the greater number of organic molecules that volatilize. Vapor pressure can be thought of as the solubility of a molecule in a gas.

The volatilization of a non-aqueous liquid is measured in terms of Raoult's Law while the volatilization of a dissolved organic molecule is measured in terms of Henry's Law.

Roualt's Law ............Henry's Law

P_org = x_org P⁰_org .......P_org = K_h C_aq

P_org partial pressure of the vapor in the gas phase

x_org mole fraction of the organic solvent

P⁰_org vapor pressure of pure solvent

K_h Henry's Constant

C_aq concentration of the aqueous phase

Vapor Pressure Data for Assorted Organic Contaminants

D. Relations Between Liquid & Solid Phases

The solution, precipitation, complex formation, hydrolysis, sorption activities, reduction-oxidation reactions of solids are highly important in the generation of ground water quality.

1. Solution, Precipitation (& Hydrolysis)

Ground water, especially of a more acidic pH, is capable of dissolving minerals. Congruent solution occurs when a mineral is totally dissolved in the ground water.

Incongruent solution occurs when the mineral and water react but leave a new solid along with parts of the old mineral in solution.

Congruent Solution

Halides, sulfate, carbonate minerals dissolve quite readily, over relatively short periods of time, in acidic ground water. In addition, quartz and gibbsite also dissolved by in much smaller concentrations and over greater spans of time.

\

Incongruent Solutions & Hydrolysis

The feldspars and many of the dark silicate minerals dissolve and undergo hydrolysis, the reaction of a mineral with water. These reactions put Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺ and other cations in solution but also leaves produce a new solid phase, clay minerals. These reactions are incongruent in that there is a new solid phase produced by the reaction.

NaAlSi₃O₈ + H₂O = K⁺ + OH^- + HalSi₃O₈

Albite Feldspar Kaolinite Clay

Hydrolysis reactions also involve organic compounds. In this case an organic molecule reacts with water forming a new organic molecule and several ions. Some organic compounds are susceptible to hydrolysis. These include the alkyl halides, amides, carabates, carboxylic acid esters, epoxides & lactones, phosphoric acid esters, and sulfonic acid esters.

Precipitation

Ions in solution may precipitate due to changes in pressure, pH and/or Eh. These precipitates, such as calcite and siderite deposition may result in massive formations such as stalactites, stalagmites, columns, flowstones, and travertine mounds. Quartz deposition in hot springs may form massive siliceous sinter deposits. Gypsum, and other evaporites, deposition may result in the formation of large crystals in evaporating lakes and seas. Limonite deposition results in massive "bog iron" deposits in wetlands. All of these are types of precipitation.

However, cementation, one of the processes that turns sediments into rocks, is also a precipitate. The most common cements are silica (SiO₂), calcite (CaCO₃) and limonite (Fe₂O₃.xH₂O). These cements coat less soluble grains, gradually expanding to neighboring grains, thereby connecting one grain to its neighbors.

The precipitation of minerals reduces the concentration of those minerals from ground water.

III. Complexation Reaction

A complex is an ion (i.e., cation, anion, charged compound) that forms by combining simpler cations, anions, or molecules. It is composed of a metal ion combined with simple anions (e.g., Cl^-, F^-, SO₄^2-, PO₄^3- and CO₃^2-) or organic molecules (e.g., humic acid, the most abundant fraction of naturally occurring organic matter dissolved in water). The segment connect to by the metal is called a ligand. A simple complex would be a single cation attached to a simple anion.

Mn²⁺ + Cl^- = MnCl^-

Complexes can get more complex when a complex forms a new complex with another ligand.

Cr³⁺ + OH^- = Cr(OH)²⁺

Cr(OH)²⁺ + OH^- = Cr(OH)₂⁺

Metals tend to be mobilized by complexes at low pH. This is effective in mobilizing iron, mercury and radium for example.

IV. Sorption : Reactions of Grain Surfaces

Sorption involves the retention of solute on grain surfaces. It is adsorbed when the solute is attached to the grain surface and is desorbed when is exits from the grain surface. Solutes get partitioned between being in solution and attaching to grain surfaces. It is this phenomenon that is utilized by water filters in screening out metals and organic compounds.

A. The more hydrophobic the solute, the more it will cling to a grain surface than remain in solution. These are mainly non-polar molecules that attach to organic matter. The organic matter may occur as grains, films or stringers. This is assessed by means of a distribution coefficient (K_d).

K_{d =} K_oc f_oc

K_oc partitioning coefficient between organic carbon and water

f_oc weight fraction of organic carbon

B. If the organic molecules in solution carry a charge, they may be attracted to grain surfaces by those surfaces' electrostatic charges.

C. Metal adsorbs to charged mineral surfaces. Clays and oxides provide the charged mineral surface. Cations in solution can be exchanged with those adsorbed to the mineral surface.

Cation Exchange Capacity (CEC) described the quantity of exchangeable cations sorbed to the surface or the negative charge of the mineral particle. There is a general sequence of exchangeable cations going from the least to greatest affinity for an exchange site. The greater the charge, the greater the affinity.

LI⁺ < Na⁺ < H⁺ < K⁺ < NH₄⁺ < Mg²⁺ < Ca²⁺ < Al³⁺

V. Oxidation-Reduction : A Little Help From Our Friends

Oxidation-Reduction reactions (i.e., Redox) are mediated by bacteria. Microorganisms speed up these reaction. Using these reactions for metabolic energy. Even so, concentration of microorganisms is generally low so that these are still slow relative to other chemical processes. Oxidation involves the loss of an electron while reduction involves the gain of an electron. The term pe is used to describe the electron activity while Eh, the redox potential, is a voltage measurement. Eh-pH diagrams relate the type of ion or molecule that exists at a particular level of proton activity (H⁺ ion concentration) and electron activity.

VI. Isotopic Processes

Some elements are isotopically unstable and spontaneously decay to more stable isotopes. This decay is of three types. a -decay causes a decline in atomic mass and number. b -decay causes a decline in atomic number only. g -emission which is emitted when the isotopic drops to a less unstable state. Atomic decay is accessed by half-life (t_1/2).

t_1/2 = 0.693/k, where k is the decay constant

Half-life is significant as a degradation process for radioactive contaminants and is also the means to date ground water. ³H, ¹⁴C, ³²Si, and ³⁶Cl are isotopes commonly used to age date ground water.