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Groundwater Chemistry

1. Inorganics from natural geochemical sources (As, Se, Ba)

2. Radionuclides (Radon)

3. Hardness

4. Sulfate

5. Dissolved Iron and Manganese

6. Taste and Odor

7. Total Dissolved Solids

Natural water quality problems in Groundwater

Groundwater Chemistry

Where do the major ions in Groundwater come from?

Anthropogenic Water Quality Problems

1. Inorganics

2. Volatile Organic Carbons (VOCs)

3. Synthetic Organic Carbons (SOCs)

Mineral SourcesMajor Cations Mineral SourcesMajor Anions

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Groundwater Chemistry – Some Definitions

Hardness =

Alkalinity = Acid neutralizing capacity of a water. The amount of strong acidneeded to bring the pH of a water to near 4.7.

Dissolved Organic Carbon =

Total Inorganic Carbon =

Ionic Strength =

Activity and Ionic Strength

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Groundwater Chemistry is Carbonate Chemistry!

approx. 360 ppm (by volume) of CO2 in clean atmosphere

CO2 (aq) = CO2 (g) KH = 10 1.5 atm / (mol/L)

CO2 (aq) + H2O (aq) = H2CO3 (aq)

H2CO3 = H+ + HCO3-1 Ka1 = 10 -6.3

HCO3- = H+ + CO3

-2 Ka2 = 10 -10.3

CO2 (g) + H2O(aq) = H+ (aq) + HCO3- (aq)

Groundwater Chemistry is Carbonate Chemistry!

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Carbonate Equilibrium in Different Environments

Unsaturated Zone (Roots)

Atmosphere

Groundwater (saturated zone, carbonate dissolution)

pHpCO2

pHpCO2

pHpCO2

Chemistry along Groundwater Flow Path

Ca2+

ALKTDS

ISO4

2-

Cl-

Groundwater Flow Path Distance or Age of Groundwater

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Bar/Pie Diagrams to Represent GW Chemistry

Check water analysis by checking charge balance

Carbonate Equilibrium Calculations

ALK =

TIC =

Combine with Equilibrium Expressions:

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Carbonate Equilibrium Calculations

Is Groundwater Saturated wrt to a Mineral?

Important question because if it is: 1 - undersaturated, then gw is dissolving aquifer material2 – supersaturated, then mineral is precipitating in aquifer

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Mineral Solubility Products

Solubility Example. A groundwater analysis reveals that there is 134 mg/L of calcium (Ca2+) and 429 mg/L of sulfate (SO42-). The ionic strength is 0.025 M. Calculate the molar concentration and activity of each ion to estimate the ion activity product. Is the groundwater is saturated with respect to anhydrite (CaSO4(s))? What is the saturation index?

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Example: Is the GW saturated with respect to Calcite?

GW sample has a pH of 7.5, hardness (due to primarily Ca2+) of 100 mg/L, temperature of 10C, and an alkalinity of 250 mg/L as CaCO3(s).

Groundwater Reactions: Physical, Chemical, and Biological

1- Physical volatilization, phase transfers, sorption, colloid transport

2 - Chemicaldissolution / precipitation, acid/base, complexation, hydrolysis, redox

3 – Biological biotransformations, biologically-mediated redox, phytoremediation

soils, sediments

Solids water

Liquids

Gas

air, void space

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Sorption (Adsorption or Absorption)

Important because:

1 - controls the movement of chemicals in soil and aquifers

Hydrophobic expulsion

Electrostatic attraction

Complexation

Definitions:

Sorbent / Adsorbent: Solid phase (soils, sediments)Sorbate / Adsorbate: Chemical sorbing to the solid phaseSolvent: Liquid phase (water)

Table 3.1 Major adsorption mechanisms and terminology

Mechanism Other Terminology Examples

hydrophobic expulsion partitioning Nonpolar organics

electrostatic attraction outerspherenonspecificphysisorptionphysicalion exchange

Some anions(SeO42- , NO3-)

alkali and alkalineearth metals (Ba2+,Ca2+)

complexation reaction innerspherespecificchemisorptionchemicalligand exchange

Transition metals(e.g. Cu2+, Pb2+,CrO42-)

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How to Measure Sorption Isotherms

0.06

0.05

0.04

0.03

0.02

0.01

0.006543210

Measure and Predict How Much Will Sorb with Sorption Isotherms

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6

4

2

0

Cs (

mg

cont

amin

ant/g

soil)

20151050

Ce (mg contaminant/L)

Freundlich

Linear

Langmuir

Cs = f(Ce)

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Inorganic Compounds: Metals (cations) and Ligands (anions)

Organic Compounds

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Fraction of Organic Carbon, foc

Kd = foc Koc

Koc = f(Kow)

Use Kd to estimate R:

Isotopes in Ground Water

Z, atomic number = # of protons (fixed!)mass number = # of protons + # of neutrons (varies => isotopes)in a neutral atom, # of protons = # of electrons

O168

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Redox Elements in Ground Water

MajorCarbonOxygenHydrogenNitrogenIronSulfurManganese

COHNISM

Redox active PLUS sufficiently high concentrations

MinorArsenicCopperLead ChromiumMercurySelenium, etc.

Redox active, but typically controlled by COHNISM elements

Redox Conventions and Definitions

OX is the oxidized species.

OX + e- = RED

- takes up electrons and makes another compound lose electrons (i.e., it is an electron acceptor).

- gives up electrons and makes another compound gain electrons (i.e., it is an electron donor).

RED is the reduced species.

Oxidation = Electrons are produced Reduction = Electrons are consumed(lose electrons) (gain electrons)

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Overall Redox Reactions = Sum of Two Half-Reactions

3Fe2+ + CrO42- = 3Fe3+ + Cr(OH)3

Light + 6CO2(g) + 6H2O = C6H12O6 + 6O2 (g)

NO3- + HS- + H+ + H2O = NH4

+ + SO42-

Redox Scales

ΔGrxn = -nFE

F = Farayday constant = 96,485 C/molcoulomb = A x s (charge)

Joule = C x V (energy)

pe = -log{e-}

ΔGrxn = -2.3RT log Keq

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