Lecture 35

Inorganic Chemicals in Ground Water

Stringer 2450

Water

Humans need approximately 3 L of water/day.

The average household uses 200-300 gal/day

Flagstaff, Arizona

~ 125 gal/day (winter)

~ 170 gal/day (summer)

In the U.S.

~ 40 % ground water

~ 60 % surface water

In Arizona (1985)

~ 48 % ground water

~ 52 % surface water

Used in Arizona (1985)

~ 87 % Agriculture

~ 9 % Domestic/commercial

~ 3 % Industrial/mining

~ 1 % Electric/power

Physical properties of water and interactions of water with the environment are dependent on

1. Atomic structure
2. Molecular structure

Water is an unusual substance.

Abundance of water in all three phases makes Earth unique as a planet in the Solar System.

Molecular structure

2 - Hydrogen atoms - 1 e^- in outer shell

1 - Oxygen atom - 6 e^- in outer shell

Forms a covalent bond ---> Very strong!!

Water has an asymmetric molecular structure.

Water is a polar molecule.

Results in many unusual properties of water.

For instance, the polarity produces the ability for water to form hydrogen bonds between other water molecules.

Also, this allows for surface tension, the ability for water to be attracted to itself ---> capillarity

Chemical constituents can exist in the subsurface as

1. Separate gas or solid phases (CO₂ or minerals)
2. Separate liquid phases (crude oil)
3. Mass dissolved in water (Na⁺ or Cl^-)
   
   Historically, hydrogeologists have studied this occurrence of mass.
   
   
   
   

Inorganic chemicals in Ground Water

Dissolved mass undergoes transport and reactions.

Cations - positively charge species (Ca²⁺, K⁺)

Anions - negatively charged species (HCO₃^-, Cl^-)

(Dissolved organic molecules are usually not electrolytes)

Ground-water naturally dissolves many constituents as it flows through soil and geologic materials.

Units of measure

Weight of solute per volume solvent

milligrams per liter (mg/L)

micrograms per liter (µg/L)

Equivalent weight

formula weight divided by electrical charge

milliequivalent per liter (meq/L)

Mole - formula weight of a substance in grams

a 1-molal solution has 1 mole of solute in 1,000g of water

a 1-molar solution has 1 mole of solute in 1 liter of solvent

Example: A ground-water sample has a SO₄^2- concentration of 85.0 mg/L.

Chemical activities

Molal concentrations can be used to determine equilibrium and solubility of very dilute aqueous solutions.

Chemical activity (a) is equal to the molal concentration (m) times the activity coefficient (l).

a = l m

The behavior of a solute is dependent on the types and amounts of other solutes in solution.

Therefore, need to know the ionic strength of a solution.

where
I = ionic strength of solution
m_i = molality of the ith ion
z_i = charge of the ith ion

For example, a 0.1-molal solution of MgCl₂ is

I = 1/2(m_Mg2+× 2² + m_Cl-× 1²)

I = 1/2[(0.1)(4) + (0.2)(1)] = 0.3

Once you know the ionic strength of a solution, you can then calculate the activity of individual ions.

Debye-Hückel equation

One equation for calculating activity coefficients

Valid for solutions with I < 0.1 (approx. 5,000mg/L)

_i = the activity coefficient for ionic species i,
z_i = the charge on ionic species i,
I = inoic strength of the solution,
A = constant equal to 0.5085 at 25^oC,
B = constant equal to 0.3281 at 25^oC, and
ai = the effective diameter of the ion.

There are other equations for calculating activity coefficients when the ionic strength is > 0.1

One way of classifying water is by the total amount of dissolved solids (TDS).

Fresh water 0 to 1,000 mg/L

Brackish water 1,000 to 10,000 mg/L

Saline water 10,000 to 100,000 mg/L

Brine water > 100,000 mg/L

EPA drinking water standard 500 mg/L

Seawater 35,000 mg/L

Dissolved inorganic constituents in ground water by

abundance (after Davis and DeWiest, 1966)

Law of Mass Action

The rate at which a chemical reaction occurs is proportional to the active masses of substances that participate in the reaction.

Two reactants (A and B) react to form two products (C and D)

Some reactions are at equilibrium or in progress

Equilibrium - no chemical energy available to alter the relative distribution of mass between the reactants an products (time independent)

Kinetic - some reactions are not at equilibrium and are dependent on time.

Kinetic reactions are slow relative the velocity of ground water and transport.

If some mass transfer is slower than the ground-water or transport velocities (not instantaneous), then the system must be described with as a kinetic, not an equilibrium system (for instance, radioactive decay)

Most all reactions that we will consider occur instantaneously.

pH

Water is capable of undergoing a dissociation reaction

The equilibrium constant for this reaction is

pH is a measure of the number of protons in solution

Eh a measure of the number of electrons in solution

- is the oxidation potential of an aqueous solution

Calculated with the Nernst equation

Eh = oxidation potential of the solution (volts)
E^o = standard potential of redox reaction (volts- determined thermodynamically)
R = gas constant, 0.00199 Kcal/(mole·K)
T = temperature (Kelvin)
F = Faraday constant, 23.06 Kcal/V
n = number of electrons in half reaction
[ ] = activity of products and reactants

For example, the oxidation of ferrous iron to ferric iron is

4Fe²⁺ + O₂ + 4H⁺ 2H₂0 + 4Fe³⁺

The reaction is described by two half reactions

4Fe²⁺ 4Fe³⁺ + 4e^- [oxidation]

O₂ + 4H⁺ + 4e^- 2H₂0 [reduction]

Eh-pH diagrams

- Used to define the stability of various forms of an element using thermodynamics

Sequence of reduction reactions in neutral soils

Metal Complexes

Most metallic ions do not exist in solution as an isolated ion, ie. Cu²⁺

Most are bound to water molecules, Cu(H₂0)₆²⁺

Ligands - anions in close association with a metal cation forming a coordination compound

- water is a ligand bound to metal ion
- can be bond covalently or elctrostatically

Complexes form as a result of

- chemical equilibrium reactions
- oxidation-reduction reactions

Chelating agent

- a ligand that has more than one site that can bond

Ligands can be either inorganic or organic

- EDTA or citric acid can increase mobility

Important nonmetallic inorganic contaminants

Fluoride, Chloride, Bromide, Sulfur, Nitrogen, Arsenic, Selenium, Phosphorus

Displaying geochemical data

1. Stiff (1951) diagram
   - shows major cations and anions
   - units are meq/L
2. Piper (1944) trilinear diagram
   - shows major cations and anions and mixing
   - units are %meq/L
   - two triangular fields, one for cations and anions
   - point projected from triangles to diamond field
   - may show pathways of chemical evolution
   - used for classifying different types of water
3. Simple two-component mixing diagrams