Chapter 9 – The continental environment

Simpler Box Model of Hydrologic Cycle

Weathering: the various physical and chemical processes that lead to the decomposition of minerals and the breakdown of rocks to form soil.

4 Types of Chemical Weathering

Oxidation of minerals containing reduced elements4FeS2(pyrite) + 15O2 + 8H2O 2Fe2O3(hematite) + 8H2SO4

Congruent dissolution by water – ions go directly into solutionNaCl(halite) Na+ + Cl-

Congruent dissolution by acid (written as carbonic)MgSiO4 (olivine) + 4H2CO3aq 2Mg2+ + 4HCO3

- + H4SiO4aq

Incongruent dissolution by acid (breaks down to ion and a different mineral)

2NaAlSi3O8(Na-plagioclase) + 2H2CO3aq + 9H2O Al2Si2O5(OH)4(kaolinite) + 2Na+ + 2HCO3- + 4H4SiO4aq

Primarymineral

secondarymineral

Dissolution of Silica (quartz and amorphous silica)

SiO2 + H2O H4SiO4 aq

Log Ksp = 1.8814 – 2.028 x 10-3T – 1560.46 / T

Log Ksp = 0.338037 – 7.8896 x 10-4T – 840.075 / T……opal

……quartz

SiT = [H4SiO4]Ka1

[H+]+

Ka1

[H+]2

Ka2

1 +

Equ

atio

ns f

or s

ilica

dis

solu

tion

Figure 9-2. function of pH. pH = 9.83 and pH = 13.17 correspond to the first and second dissociation constants, respectively, of silicic acid.

Solubility of silica increases at higher pHOpposite of CaCO3 containing mineralsFossilization = SiO2 replacement of CaCO3

Dissolution of Al and Fe hydroxides

Al(OH)3 Al3+ + 3OH- General eq.

Total solubility of Al is determined by the sum of free Al + all the Al-OH aqueouscomplexes in solution. Complexes: AlOH2+, Al(OH)2

+, Al(OH)3aq, Al(OH)-4

Overall Al solubility eq:

Al = Ksp / K3w ([H+]3 + K1[H+]2 + K2[H+] + K3 + K4/[H+])

Figure 9-3. Total concentration of aluminum in solution, as a function of pH, for a solution in equilibrium with gibbsite.

Al hydroxide solubility depends on the exact mineral but generally has a nearmid pH minima increasing asymmetrically at lower and higher pHs.

Stablility diagramsRepresent equilibria between minerals and aqueous solutionsDerived from thermodynamic data

Figure 9-4b. Mineral stability fields as delineated by equilibrium equations plotted in Figure 9-4a. The labeled curve indicates the changes in chemistry of a solution in equilibrium with albite during weathering in a closed system. See text for discussion.

For the weathering of silicateminerals

X and Y axis describe solutionChemistry

Fields denote the ‘stable mineral’ inEquilibria with particular solution chemistry

ABCetc… represents the path of weathering starting with gibbsiteIn a closed system

Flushing rates in non-closed systemsWill effect solution chemistry during weatheringAnd control final weathering product. HenceDominance of different minerals in different Soil types

Groundwaters

Water chemistry modified bythe vadose zone, and during transport in the saturated zone

In general, the older the waterthe higher the dissolved ionConc. = higher conductivity

Surface and Groundwaters

Graphical representations Stiff diagram

Piper Diagram

Data used to generate the Piper Diagram is Found on Eby p. 324, Table 9-7

Plots each ion as a valuenormalized to 100%

Data on the 2 triangles is Also projected on the quadrilateral

Ca Cl

SO4Mg

Na

HC

O3

Figure 9-7. Hydrochemical facies. After Back (1966).

Piper Diagram

Data used to generate the Piper Diagram is Found on Eby p. 324, Table 9-7

Plots each ion as a valuenormalized to 100%

Data on the 2 triangles is also projected on the quadrilateral

Piper diagrams also provideindications of mixing of water masses

1

2

mix

Straight line= mixture

RiversControls on river chemistryGibbs Approach

Precip – high (Na and Cl), tropical rivers

Weathering – rock dominance, depends on rock type, climate, relief

Evaporation and fractional crystallization

Controls on River chemistry

Rock weathering controls River chemistry

Stallard and Edmond approach

Table 9-9. Sources of major ele ments in river water (% )*

Atmosphere W eathering

Species Cyclic Salt Carbonates Silicates Evaporites Pollution

Ca 2+ 0.1 65 18 8 9

HCO

3 <<1 61 37 0 2

Na+ 8 0 22 42 28

Cl- 13 0 0 57 30

SO24 2 0 0 22 54

Mg2+ 2 36 54 <<1 8

Na+ 1 0 87 5 7

H4SiO4 <<1 0 >99 0 0

*From Berner and Berner (1996)

Ca 2+

HCO

3

Na+

Cl-

SO24

Mg2+

Sources of ions in river water by general rock type

Ocean source

+

-

pH in groundwater and surface water controlled by:

Ion exchangeCarbonic acid systemWater-mineral interactions

Remember the carbonic acidsystem? As pH decreasesbuffering decreases

Water-mineral interaction

H+ + CaCO3 Ca2+ + HCO3- …

replaces bicarbonate ion needed for buffering, increases calcium conc.(and Mg is dolomite is present), raises pH

Calcite

Silicates

Al2SiO5(OH)4kaolinite + 6H+ 2Al3+ + 2H4SiO4aq + H2O

NaAlSi3O8albite + 4 H2O + 4H+ Na+ + Al3+ + 3H4SiO4aq

KMg1.5Fe1.5(AlSi3O10)(OH2)biotite + 10H+ K+ + 1.5Mg2+ + 1.5Fe2+ + Al3+ + 3H4SiO4aq

Water-mineral interaction

H+ + CaCO3 Ca2+ + HCO3- …

replaces bicarbonate ion needed for buffering, increases calcium conc.(and Mg is dolomite is present), raises pH

Calcite

Silicates

Al2SiO5(OH)4kaolinite + 6H+ 2Al3+ + 2H4SiO4aq + H2O

NaAlSi3O8albite + 4 H2O + 4H+ Na+ + Al3+ + 3H4SiO4aq

KMg1.5Fe1.5(AlSi3O10)(OH2)biotite + 10H+ K+ + 1.5Mg2+ + 1.5Fe2+ + Al3+ + 3H4SiO4aq

All the silicate weathering rxns release silicic acid and free aluminium. High free Aluminum conc. in acid lakes. Free Al is toxic to aquatic critters.

Read about the ‘Killer Lakes’ of Cameroon,Eby p. 336, Case Study 9-4

Eutrophication: a process whereby water bodies, such as lakes, estuaries, or slow-moving streams receive excess nutrients that stimulate excessive plant growth (algae, periphyton attached algae, and nuisance plants weeds). This enhanced plant growth, often called an algal bloom, reduces dissolved oxygen in the water when dead plant material decomposes and can cause other organisms to die. Nutrients can come from many sources, such as fertilizers applied to agricultural fields, golf courses, and suburban lawns; deposition of nitrogen from the atmosphere; erosion of soil containing nutrients; and sewage treatment plant discharges. Water with a low concentration of dissolved oxygen is called hypoxic.

Structure and mixing of lake waters

CiFi + ChFh = CoFo + CeFe + RPe

where Ci is the concentration of the substance in the inlet water, Fi is the flux of water into the lake, Ch is the concentration of the substance in the hypolimnion, Fh is the flux of water from the hypolimnion to the epilimnion, Co is the concentration of the substance in the outlet water, Fo is the flux of water out of the lake, Ce is the concentration of the substance in the epilimnion, Fe is the flux of water from the epilimnion to the hypolimnion, and RPe

is the rate of

removal of the substance by particles in the epilimnion.The amount of the substance that is ultimately stored in the sediment is:

Rs = RPe + RPh

– Rd

where Rs is the rate at which the substance is sequestered in the sediment, RPh is

the rate of removal of the substance by particles in the hypolimnion and Rd is the rate of re-solution of the substance.

Soufrière Hills volcano (Montserrat)

Mt Ruapehu lahar event - New Zealand (March 18, 2007)

Total dissolved solids > 80%

Tephra: air-fall material produced by a volcanic eruption regardless of composition or fragment size.

Metals sorption

Figure 9-16. Adsorption of metal cations as a function of pH. From AQUATIC CHEMISTRY, 3rd Edition by W. Stumm and J. J. Morgan. Copyright © 1996. This material is used by permission of John Wiley & Sons, Inc.

Depends on pH and charge to size ratio for individual metal

Generally for the transition metals, the lower the pH the less sorption

Metals mobility depends on:ion exchange / sorption-desorptioncomplex formation and chelation

Ion exchange

aAy + bBX bBz + aAX…..’X’ denotes that A or B are stuck on a solid

The exchange ratio between a and b (Ka/b) is:

Ka/b = [Bz]b [AX]a / [Ay]a [BX]b

Example: Exchange between Na and Ca

2Na+ + Casolid Ca2+ + 2Nasolid

KNa/Ca = [Ca2+]1 [Nasolid]2 / [Na1+]2 [Casolid]1

Metals chelation

Ligands – net negatively charge molecules that associate (attract) metals

M2n+aq + nC2O4

2- M(C2O4)n aq

metal Ligand(oxalate)

Multidentate ligand has more than one binding atomChelation occurs when complexes are formed with a multidentate ligand

Oxalate is a common multidentate ligand produced biologically

Metal-oxalate chelates have low solubility and will precipitate out.

Therefore biological processes assist in metals precipitation through the oxalate chelation process.

Metal Cycles

Geosphere atmosphere viaVolcanoes, dust

Geosphere hydrosphere via ocean vents

Short residence time in atmosphere

Hydrosphere transport controlled by redox, pH, mineralogy

Biosphere important in establishing redox, pH, etc

Heavy metalsAt # > 20

Fate and transport determined strongly by element-type

-Transition metals - Zn, Cd, Pb-As, Se-Hg

Transition metalsUsually occur at divalent, or trivalent cations

With the exception of V, these metals will ppt at high pH (forming oxyhydroxides, or metal carbonates)

Complex with humic material (Mn 2+ <Cd 2+ <Co 2+ <Ni 2+ <Zn 2+ <Pb 2+ <Cu 2+ <VO 2+)

Adsorption decreases with decreasing pH

Arsenic and Selenium

Occur primarily as neutral or negatively charged species(H2AsO4

-, HAsO42-, H3AsO3, H2AsO3

-, SeO4-, HSeO3

-, SeO32-)

Adsorption increases with decreasing pH

When reduced S is present, both As and Se incorporated into S minerals

Changing redox conditions can either immobilize or liberate As or Se dependingon the presence of other elements……difficult to predict mobility

Major anthropogenic source of Se is coal combustion

Mercury

Most redox conditions allow for elemental HgElemental Hg is inert = not immediately hazardous, long half life

High redox = Hg 2+ or Hg(OH)2aq

Low redox + sulfur = HgS (cinnabar)

Global mercury cycling dominated by atm transport and deposition on land

Some anaerobic bacteria methylate HgCH3Hg= methyl mercury, (CH3)2Hg = dimethyl mercury

easy biotic uptakebioconcentrationtoxic

Radioactive materialsMaking fissionable nuclear fuel 235UNatural uranium ore = 99.275% 238U, 0.719 % 235U

Extraction enrichment (1.8 – 3.7% 235U)

Nuclear Fission

235U + 10n fission fragments + 2 or 3 neutrons + energy

neutron

In order to get the reaction to go, the impacting neutron must be slowed down.

Neutrons are slowed either with water or with graphite (Chernobyl) dependingon the type of reactor.

Water-type reactors are self quenching because the ability of water to slow downneutrons decreases as water heats up (negative feedback)

Graphite-type reactors behave the opposite

Radioactive wastes from fissionHigh level = very radioactive and long half-lifeLow level = not so radioactive and short half-life

Table 9-15. Re presentative radioactive isotopes for nuclear wastes

Isotope Half-lifeDecaymode Iso tope Half-life

Decaymode

Fission products Fission products

85Kr 10.8 y β 137Cs 30 y β

89Sr 51 d β 141Ce 33 d β

90Sr 28 y β 147P m 2.6 y β

95Zr 64 d β

95Nb 35 d β Transuranics

99Tc 2.1 x 105 y β 237Np 2.1 x 106 y α

106Ru 1 y β 239Pu 2.4 x 104 y α

131I 8 d β 240Pu 6.6 x 103 y α

133Xe 5.2 d β 241A m 433 y α

Yucca Mountain

Yucca mountain is madeof volcanic tuff

U is more mobile in oxidizingconditions

Mobility of nuclear waste elements

Oklo – the fossil natural nuclear reactor

Uranium ores in Oklo, Gabon contain little 235U because of prior fission rxns

Original U minerals (2 billion years ago) dispersed in a sandstone conglomerate. Relative abundance of 235U vs 238U was much higher then the U minerals were dissolved and redeposited as UO2 at a redox boundary (U concentrated to 50-70%)

Interstitial water moderated the fission like a present day reactor. When the water was steamed away due to the heat release from fission, reaction stopped.

More water infiltrated and reaction fired up again, on off on off etc. for 500,000 yrs

Large quantities of fission products exist today

Site used to study migration and retention of fission wastes

Nonmetals

Carbon – see chapter 5

HalogensNegative charge so sorption is less importantSolubility controls mobility

Fluorine concentrations determined by availbility and solubility of fluorite (CaF2)

Chlorine and bromine used as hydrologic tracer because once dissolved theylargely behave conservatively

-no volatile derivatives-little to no adsorption-little to no biological uptake-care taken to discount anthropogenic inputs

Nitrogen

Oxidized and mobile = NO3-

Reduced and sorptive = NH4+

Phosphorous

Source is weathering of P containing rocks (e.g. apatites)Mobility and abundance controlled by solubility, adsorption, biological uptake, redox

Phosphate (PO43-) strongly adsorbed to ferric (Fe3+) oxides and

oxyhydroxides (Fe(OH)3…..at high redox

Low redox (e.g. anoxia) desorbs phosphate and releases it in the water

Freshwater eutrophication due to phosphateAnoxia associated with eutrophication keeps resupplying phosphate (+ feedback)

Sulfur

HSO4-, SO4

2-, H2S, HS-

Most natural Eh and pH waters sulfate dominates

Low Eh’s sulfide dominates. Which sulfide species at low Eh and high pH?Which sulfide at high Eh and low pH?

Very low Eh (deep sediments) metal sulfide minerals ppt out (e.g. pyrite)

Sulfate mobility controlled by adsorption (divalent anions preferentially adsorbedover monovalent anions like Cl- or NO3

-), and biology.

Similar to phosphate, sulfate is adsorbed by Al and Fe oxides