GEOS 4430/5310 Lecture Notes:Groundwater Chemistry

Dr. T. Brikowski

Fall 2012

Vers. 1.33, Printed: December 3, 2012

Introduction

1

What use is Groundwater Geochemistry?

Example applications of low-temperature aqueous

geochemistry:

Simpson-Arbuckle Aquifer a potential “new” source of water

for Oklahoma city. See USGS Factsheet1

• how good is this water for drinking

• how much can be taken out safely (i.e. where does water

come from)

• significance of nearby saline and sulfurous wells? (can this

water be avoided?)

Southern Nevada Region Groundwater System a contaminant1http://www.owrb.ok.gov/studies/groundwater/arbuckle_simpson/pdf/

ArbuckleSimpsonFactSheet2008.pdf2

risk (Nevada Test Site)? A source of water for thirsty Las

Vegas2??

• can shallow contaminated water reach the currently-clean

deep aquifers?

CO2flooding A common secondary recovery method in

petroleum reservoirs. Why CO2, will it accidentally reduce

porosity? Examples include DOE-20093, Shiraki and Dunn

[2000]

2http://www.latimes.com/news/nationworld/nation/la-na-radiation-nevada13-2009nov13,0,3038881.story?track=rss

3http://www.netl.doe.gov/technologies/oil-gas/publications/EP/small_CO2_eor_primer.pdf

3

Basic Principles: Measurement

• concentration generally specified as milligrams mgL , this is

essentially equivalent to ppm (except in saline waters, where

TDS>∼ 7000 mg

L )

• thermodynamic measures [Sec. 9.2, Fetter, 2001]:

– molality: one mole of solute per 1000 gm of solvent is a 1

molal solution

– molarity: one mole of solute per liter of solvent is a 1

molar solution

• comparative chemical behavior: millequivalents:

– concentration of ionic species in mgL , multiplied by ionic

charge, and divided by molecular (formula) weight4

– a quantitative measure for comparing the chemical

behavior of dissolved species

5

Reactions

• Reversible reactions reach equilibrium easily. Typically

equilibrium is assumed in hydrochemistry, therefore

reversibility is implied

• dissociation: most common type, separation of molecules

into individual ions (e.g. NaCl)

• solvent (water) can directly participate in reaction (e.g.

carbonation reactions)

• oxidation-reduction: exchange of electrons between ions, e.g.

electrons appear in the reaction equation, and one or more

cations change atomic charge6

Equilibrium Constant (Law of Mass Action)

• an equilibrium constant can be expressed for an arbitrary

reaction as

cC + dD ⇀↽ xX + yY

K =[X]x[Y]y

[C]c[D]d(1)

where [X] is the molal concentration of X for an ideal

solution, and is the chemical activity a of X for non-ideal

solutions

• a = γm, where m is the molal concentration

• activity coefficient γ can be calculated using the Debye-

Huckel equation [Eqn. 10-17, Fetter, 2001] if the ionic7

strength I of the solution is known [Eqn. 10-16, for TDS<∼

5000 mgL Fetter, 2001]

• K values are tabulated at standard T & P (STP), can be

determined under other conditions using the expression

K = exp(−∆Go

r

RT

)where ∆Go

r is the Gibbs Free-Energy change of the reaction

at STP

• solubility product:

– equivalent to the equilibrium constant for dissolution of

a mildly-soluble salt, e.g. NaCl ⇀↽ Na+ + Cl− (at8

equilibrium):

Keq =[Na][Cl][NaCl]

= [Na][Cl] ≡ Ksp

– because [NaCl]≡ 1, in reality the solubility product is

equivalent to the equilibrium constant at equilibrium

• ion activity product

– Kiap is the numerator of the equilibrium constant (1)

– when Kiap > Keq, the salt is supersaturated and will

precipitate (ignoring kinetic effects)

• common-ion effect9

– is simply that when multiple sources for an ion exist, less

of at least one of those sources will dissolve than if it was

the only source

– for this reason, quantitative calculations involving

groundwater chemistry must consider all dissolved and

solid species

10

pH

• pH is the negative logarithm of [H+] ion concentration (really

H3O)

• a neutral solution has equivalent [H+] and [OH−], i.e. the

concentrations are controlled by water dissociation (Fig. 1)

• since equilibrium constants vary considerably with T, so does

the neutral pH (at 0◦C neutral pH is 7.5, at 25◦C it is 7.0,

Fig. 2)

• should be measured when water sample is collected, since it

can change after a few minutes of atmospheric contact

• acids are proton donors to the aqueous solution. Strong11

acids tend to completely dissociate, weak acids generally

dissociate partially. E.g. dissociation of carbonic acid [eqn.

9.21A-B, Fetter, 2001]

H2CO3 ⇀↽ H+ + HCO−3 , K = 10−6.4

HCO−3 ⇀↽ H+ + CO2−3 , K = 10−10.3

• pH is important because it controls/indicates the distribution

of many species. E.g. the three-component system C-H-O

contains seven species H+, OH−, H2CO3, HCO−3 , CO2−3 ,

CO2, H2O. Given the concentration of the components and

the pH, the concentrations of the species can be determined.

12

Water Autoionization

Figure 1: Autoionization reaction of water.

13

pH vs. T

Figure 2: Variation of pH vs. temperature.

14

pH of Common Solutions

Figure 3: pH of common solutions (after Purves et al., Life:

The Science of Biology, 4th Edition).15

Alkalinity

• ability of solution to neutralize acid (i.e. to react with H+)

• generally reported as meq of bicarbonate (HCO−3 ) +

carbonate (CO2−3 )

• CO2−3 stable only in highly basic water, so alkalinity is

essentially [HCO−3 ] [Table 9.5, Fetter, 2001]

• in common usage also a measure of the tendency to form

carbonate scale

16

Oxidation potential (Eh)

• in effect Eh is the negative log10 of [e−]

• field-measured ORP is essentially (Eh − 200mV ) (e.g. see

YSI manual4)

• oxidation refers to the removal of electrons from an

atom, increasing its oxidation number (the net charge of

a compound), reduction is the addition of an electron

• redox pairs generally control (buffer) conditions, e.g. Fe+2–

Fe+3 in near-surface oxygenated waters (Fig. 5)4http://www.ysi.com/media/pdfs/T608-Measuring-ORP-on-YSI-6-Series-Sondes-Tips-Cautions-and-Limitations.

pdf17

• in nature such redox reactions are invariably mediated by

bacteria, which use the reactions as an energy source

• difficult to obtain accurate value in the field, since any

contact with O2 dramatically changes Eh

• often analyzed using Eh-Ph diagram, for determining stability

of redox pairs (e.g. iron)

O2 + 4Fe+2︸ ︷︷ ︸Reduced

+ 4H+ ⇀↽ 4Fe+3︸ ︷︷ ︸Oxidized

+ 2H2O (2)

O02 + 4FeII + 4H+ ⇀↽ 4FeIII + 2H2O−II

• every redox reaction can be expressed as a pair of half-18

reactions, e.g. (2) can be expressed as:

O2 + 4H+ + 4e−⇀↽ 2H2O

4Fe+2 ⇀↽ 4Fe+3 + 4e−

• important in many remediation schemes (e.g. air sparging

can clog local aquifer with iron oxide precipitates)

• presence of organic compounds leads to reducing conditions

by consumption of oxygen e.g. carbohydrate breakdown

[section 12.5, Domenico and Schwartz, 1998]

O2(g) + CH2O ⇀↽ CO2(g) + H2O (3)

19

Redox in Batteries

Figure 4: Redox in a crude battery. Shown is an early battery called the “Daniell Cell” (for a true battery the two

beakers are connected by a salt bridge, allowing ions to flow between them, and avoiding explosive H2 gas formation). Zinc is

oxidized at the anode, donating an electron e− as it dissolves. Cu2+aq accepts electrons (is reduced), plating onto the cathode.

The total reaction is Zn(s) + Cu2+(aq)

→ Zn2+(aq)

+ Cu(s).

20

Redox Ladder

Figure 5: Redox pairs vs. Eh, most oxidizing environment on

the top, electron donor on the right, acceptor on the left. After

NIEH Meeting Report5.21

Eh-pH Diagram

Figure 6: Eh-pH diagram, showing effect on the solubility of

metal ions (Fe), after [Fig. 9.4, Fetter, 2001].22

A Civil Action: Woburn PCE

• a famous lethal groundwater chemical plume court case,

memorialized in the movie, jury blamed the most innocent

party

• see Woburn Spreadsheet Model Homework6

• see also animation of best model results7

6http://www.utdallas.edu/~brikowi/Teaching/Hydrogeology/Homework/woburn_spreadsheet_model.pdf

7http://serc.carleton.edu/files/woburn/resources/tce_animation.avi23

Dissolved oxygen (DO)

• critical for redox reactions and biological processes

• in last decade became standard to measure DO, wasn’t done

much before that

• easily measured with a specific probe, useful indicator of Eh

• can remain in groundwater far from recharge sites

24

Major Ion Chemistry

25

Water Analyses

• collection methods important, see USGS manual [USGS,

1998]

• generally reported in concentrations of actual ions, some (like

SiO2, nitrate NO3) are lumped together, and/or reported as

oxides

• Analytic methods standardized for EPA and environmental

applications in general [USGS, 1979, WEF, 1998]

• check error in analysis by performing charge balance (sum

of cations and anions expressed as milli-equivalents). This

sometimes fails, e.g. for strongly colored fluids which may

contain organic complexes)26

Graphical Analysis

• Stiff Diagram Fetter [Fig. 9.10, 2001] or Parkhurst et al.

[Fig. 11, 1996], Fig. 7

– plot most common milli-equivalent cations on one side of

a line, most common anions on the other

– intended to give distinctive geometric pattern, allowing

mapping of groundwater bodies/facies

– Typically plot Cations on left: Na+K, Ca, Mg, Fe; right

side anions: Cl, HCO−3 , SO4, CO2−3

– try free software from TWDB8

• Piper diagram8http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/

Open-File/Open-File_01-001.htm27

– plot natural groupings on two trilinear diagrams (one for

cations, one for anions), the combination of these two

plots is made by projecting these onto the quadrilateral

diagram above (Fig. 8)

– classification of the water chemistry is based on the sum

of cation and anion classifications Fetter [Fig. 9.9, 2001]

– evolution of waters along flow path is often revealed by

these diagrams [e.g. Floridan aquifer, p 377-9, Fetter,

2001], or water sources (e.g. Floridan aquifer9)

– try free GW-Chart software10 from USGS

• activity-activity plots

– usually used in modeling studies, or determination of9http://sofia.usgs.gov/publications/wri/02-4050/distsources.html

10http://water.usgs.gov/nrp/gwsoftware/GW_Chart/GW_Chart.html28

mineral reaction effects

– plot chemical activity (often ratios) of two ions in the fluid

and superimpose mineral stability fields [Fig. 2 Parkhurst,

1995]

29

Stiff Diagram Example

Figure 7: Stiff diagram example, after [Fig. 9.10, Fetter, 2001].

30

Piper Diagram Example

Figure 8: Piper diagram example, after [Fig. 9.8, Fetter, 2001].

31

Carbonate Equilibrium

• most common reaction series in groundwater (any water in

contact with atmosphere equilibrates with CO2: rainwater

reaching equilibrium with atmospheric CO2 acquires acidic

pH ∼ 5.7; sufficient to dissolve limestone/dolomite)

• Important reactions

– solution of CO2

H2O + CO2 ⇀↽ H2CO3 (4)

KCO2 =aH2CO3

PCO2

where PCO2 is the partial pressure of CO2 (equivalent32

to its activity, essentially mole fraction times total gas

pressure)

– dissolution (hydrolysis) of calcite. The following reactions

are equivalent, for modeling the first equation would be

used

CaCO3 ⇀↽ Ca2+ + CO2−3

CaCO3 + H2O ⇀↽ Ca2+ + HCO−3 + OH−

CaCO3 + CO2 + H2O ⇀↽ Ca2+(aq) + 2 ·HCO−3 (aq)

– dissociation of bicarbonate

HCO−3 ⇀↽ H+ + CO2−3 (5)

33

– dissociation of carbonic acid

H2CO3 ⇀↽ H+ + HCO−3 (6)

• all of (4)– (6) are strongly influenced by pH (e.g. Fig. 9)

(4), (5), (6) control fluid pH for fluids in contact with the

atmosphere

• since atmospheric reservoir of CO2 is enormous, fluid is

buffered with respect to PCO2

34

Carbonate Species vs. pH

0

0.2

0.4

0.6

0.8

1

2 4 6 8 10 12 14

Fra

ctio

n of

Spe

cies

Pre

sent

pH

Dissolved Inorganic Carbon Species at 20oC

H2CO3 HCO3- CO3

2-

Figure 9: Carbonate species vs pH.

35

Ion exchange

• important process in groundwater chemistry, especially where

clays prominent (e.g. many soils)

• typically Ca exchanges for Na in the clay, naturally

“softening” the groundwater [e.g. Central Oklahoma aquifer,

Parkhurst et al., 1996] Figs. 10–11

• can be a problem in irrigation, eventually defloculating the

clays (destroying soil texture)

36

Cross-Section, Central Oklahoma Aquifer

Figure 10: E-W cross-section through Central Oklahoma

Aquifer, after [Fig. 5, Parkhurst et al., 1996]. Henessy Group

contains abundant clays, providing ion exchange medium.37

Stiff Diagram, Central Oklahoma Aquifer

Figure 11: Stiff diagram, Central Oklahoma Aquifer, after [Fig.

11, Parkhurst et al., 1996]. Significant Ca-Na exchange is

visible along flow path (down vertical axis of diagram).38

Applications of Major IonChemistry

39

Flow Path Determination

• Nevada Test Site/Yucca Mountain, NV [Winograd and

Thordarson, 1975], [Case Study: Great Basin (p. 250-254),

Fetter, 2001]. Also Death Valley Springs Study11

• Central Oklahoma Aquifer [Parkhurst et al., 1996]

– covers central OK, from Oklahoma City eastward about 50

miles [Fig. 2, Parkhurst et al., 1996]

– mostly composed of Permian red-bed aquifers (sandstone),

some Qal & Qt [Fig. 3, Parkhurst et al., 1996]

– unconfined to East, confined to West by Hennesy Group

(clays)11http://hydrodynamics-group.net/yucca.html

40

– groundwater flow from W to E, most streams gain from

aquifer; 100-1000 ft. of freshwater above saline (thinnest

at aquifer edges)

– Ca-Mg-HCO−3 waters in unconfined, Na-HCO−3 in confined

(ion exchange of originally atmospheric-influenced Ca-Mg-

HCO−3 recharge waters)

– high DO, nitrates, 3H, indicating relatively rapid travel

time

– Chemical reactions:

∗ after recharge, soil CO2dissolves, allowing dolomite

dissolution, generating Ca-Mg-HCO−3 water. Usually

dolomite ± calcite saturated after short travel distance

[Parkhurst, 1995]

∗ then Ca+Mg exchange for Na (natural softening) [Table

11, Parkhurst, 1995].41

– flow path calculations based on this conceptual model

indicate 50 mile flow paths requiring 103–105 years

– core analysis supports the model, indicating much of the

soluble carbonate is gone from parts of the aquifer

– mass-balance calculations also clarify the evolution of the

groundwater (e.g. probable early ET⇒ increased N, TDS)

[Fig. 119, Parkhurst et al., 1996], (Fig. 12)

42

Rock Reaction Model, Central Oklahoma Aquifer

Figure 12: Rock reaction model, Central Oklahoma Aquifer, after [Fig. 11, Parkhurst et al., 1996]. As Ca-water is

added, exchange capacity of rock is eventually exceeded at about 100 pore-volumes of water added. Note reduction in mobile

arsenic (As) by surface complexation.43

Remediation: Active Barriers

• One option is to bury a passive, permeable medium that can

remove pollutants

• Example, Elizabeth City, NC chromate plume EPA project

[Puls et al., 1996].

• Remediated by permeable iron wall (grid) reducing the

chromate and precipitating it (Fig. 13)

• Wall is 150’ long, 24 ft high, positioned to intercept the Cr

plume, made up of Fe cylinders (trench filled with iron chips

became impermeable, this design is the second try, Fig. 14.44

• Barrier is effective at removing Cr, and most of the TCE (a

common poly-chlorinated solvent, Figs. 15

45

Elizabeth City Location Map

Figure 13: Location of plume and permeable Fe wall, Elizabeth City, NJ. Hydraulic gradient is directly toward river,

contaminant is Cr-bearing TCE (solvent). After Puls et al. [Fig. 1, 1996].

46

Design of Elizabeth City Fe Wall

Figure 14: Design of permeable Fe wall, Elizabeth City, NJ,

after Puls et al. [Fig. 2, 1996].47

Chemical Changes With Time, Elizabeth City

Figure 15: Chemical changes with time, Elizabeth City, NJ,

after Puls et al. [Fig. 6, 1996]. Note immediate drop

in Cr to near zero, order-of-magnitude drop in TCE, and

seasonally-varying changes in Fe.

48

Bioremediation

• typically need to maintain nutrients (N, P), substrate (the

contaminant) and electron donor (O2)

• provides natural attenuation of many petroleum-based

contaminants [Semprini et al., 1995, Weaver et al., Sept.

11-13, 1996]

• e.g USGS study of degrading TCE plume12

• EPA natural attenuation13 overview12http://pubs.usgs.gov/sir/2006/5030/images/fig03.png13http://www.clu-in.org/techfocus/default.focus/sec/Natural_

Attenuation/cat/Overview/49

• natural attenuation in Macondo Well leak, Gulf of Mexico,

2010 (previously unknown bacteria save the Gulf, Fig. 16).

This interpretation controversial14.

14http://dx.doi.org/10.1126/science.119969750

Natural Attenuation, Macondo Well, 2010

Figure 16: Natural attenuation, Macondo Well blowout, Gulf of Mexico, 2010. FTIR shows development of petroleum

breakdown products (left), these are spatially-correlated with bacteria (right). From Hazen et al. [2010].

51

Isotope Hydrology

52

Introduction to Isotope Hydrology

Very important for determining recharge setting of and

impacts of surficial processes on any water; can be used to

estimate travel time for many waters

53

Light-Stable Isotopes

Typically these are18O16O

, DH

• usually expressed as a normalized ratio (relative to a

standard) in per-mil ( ooo). Increasing values mean enrichment

in the heavy isotope

• lighter isotope partitions into gas phase. Evaporation leads

to increasing δ18O and δD at a slope of about 5:1 on a

δ18O –δD diagram [Fig. 12-12, Domenico and Schwartz,

1998], see Fig. 17

• meteoric water line shows T-dependence of precipitation (low

T/high elev ⇒ lower δ18O –δD )54

• various processes are indicated by scatter/mixing in samples

(Fig. 18)

• local meteoric water lines can develop resulting from local

climate effects (usually rain shadows). These are invariably

more depleted than the world meteoric water line

• examples: Claassen [1985], δ18O & 14C indicate spring

waters recharged during last glacial maximum. See also

Fetter [Fig. 9.7, 2001]

55

Process of Isotopic Fractionation

Figure 17: Isotopic fractionation processes in the water cycle.

56

Isotopic Fractionation Trends

Figure 18: Isotopic fractionation trends, on δ18O -δD diagram,

after [Fig. 12.12, Domenico and Schwartz, 1998].57

Radiogenic isotopes

• 14C

– generated by cosmic radiation in the atmosphere, decays

with approximately 5000 yr half-life

– can date groundwater using14C12C

, use13C12C

ratio to correct

for isotopically “dead” carbon 12C input during subsurface

flow

• 3H (Tritium)

– produced as fallout from atmospheric nuclear testing, 13

year half-life

– 3H in precipitation peaked around 196715, useful as15http://gwadi.org/sites/gwadi.org/files/diagram1.gif

58

“natural” tracer of groundwater [Fig. 14.28, Domenico

and Schwartz, 1998]

– most often used qualitatively to discern water recharged in

the last 40 years, and to make crude velocity estimate

• 36Cl - also from fallout. Useful for waters up to 2 million years

old. Requires particle accelerator for analysis (expensive,

usually done at National Laboratories). Examples: Yucca

Mtn notes16, Scanlon [1991]

16http://www.utdallas.edu/~brikowi/Teaching/Applied_Modeling/GroundWater/LectureNotes/YuccaMtn/yuccaMtn.pdf

59

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60

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