Unit 11 : Advanced Hydrogeology

Natural Aqueous and Geologic Processes

Mixing

• Mechanical dispersion and molecular diffusion combine to create zones of mixing.

• Mixing is a major agent of chemical change in natural systems.

• Mixing occurs on many scales and as a result of hydraulic, thermal and concentration gradients.

Mixing with Formation Waters• Groundwater systems are recharge by meteoric waters.• These waters commonly displace and mix with original

formation waters.• There are two prime requirements for a meteoric flow

system to develop:– a permeable unit is uplifted so that the outcrops

receives recharge from meteoric water (water can get in)

– the down-dip portions of the unit have outlets through which water can be displaced (water can get out)

Freshwater Noses

• Freshwater “noses” develop around recharge areas.

• Freshwater penetrates down-dip and displaces more saline formation fluids.

inlet

outlet

brine

fresh

mixed

brine

fresh

mixed

Brine Expulsion• Mixing also occurs as a

result of compaction driven flow.

• Brines are expelled where abnormal excess pore pressures are developed.

• Brines can migrate into shallow up-dip aquifers remote from where the pore fluids formed.

Diffusive Mixing• In some areas,

continously varying salinities are consistent with diffusive transport up-dip from salt deposits.

• Modelling suggests that such profiles may require millions of years to develop.

Unsaturated and Saturated Zone Reactions

• In the unsaturated zone it is mainly static mass transfer processes that exert the strongest control over major and minor ion concentrations.– gas dissolution reactions and organic reactions tend to be more

important in the unsaturated zone.

• In the saturated zone, transport processes are more important and the systems are more complex but the chemical reactions are largely identical:– weak acid-strong base reactions– dissolution and precipitation reactions– redox reactions– sorption and cation exchange reactions

Geologic Processes

• The ability of groundwater to dissolve and redistribute large quantities of mass has broad geologic implications:– chemical diagenesis– ore deposition– soil salinity– evaporite deposition– hydrocarbon migration

Chemical Diagenesis

• Chemical diagenesis refers to any chemical change that occurs in a sediment following deposition.

• Newly deposited emerging carbonates are strongly impacted by fresh meteoric groundwater displacing original marine pore water.

• Three distinctive diagenetic settings are found:– shallow freshwater zone

– intermediate mixing zone

– deep marine zone

• Diagenesis advances seawards as the freshwater recharge invades the emerging sediment.

Carbonate Diagenesis

• Modern (newly deposited) carbonates consist primarily of aragonite, high-Mg calcite and a little low-Mg calcite.

• Ancient carbonates are essentially composed of low-Mg calcite.

• The transformation is a multi-stage process but a each stage porosity is lost and the sediment becomes more dense.

Calcium Carbonate Solubility

• If the aragonite-calcite transformation involves dissolution and re-precipitation, it can be treated thermodynamically.

• To carry out quantitative calculations we need to know pH and pCO2.

• Calcium carbonates are more soluble with reducing pH and increasing pCO2.

• Calcite is less soluble (by a factor of 1.38) than aragonite for all pH and all pCO2.

Removal of Soluble Phases• As carbonates dissolve in fresh water, the saturation

of the solution with calcite will always occur before aragonite saturation.

• Aragonite solution will continue as calcite is precipitated.

• Eventually all aragonite must be removed and replaced by calcite.

• High-Mg calcite is more soluble than calcite and less soluble than aragonite.

• In the absence of aragonite, high-Mg calcite is progressively eliminated until the dominant solid phase is the least soluble low-Mg calcite.

Extent of Carbonate Dissolution• Thermodynamic considerations tell us what will happen

when equilibrium is achieved but they don’t predict the extent of the flow system.

• If supersaturation of low-Mg calcite is achieved, the entire carbonate unit will be removed from the geologic record.

• This is not the general case so the supersaturation of invading freshwaters by carbonate must occur in a relatively limited zone relative to the size of the emerging carbonate body.

• How short or how long the zone extends is a question for transport theory.

Emerging Carbonates

• Consider the saturation of CaCO3 in an emerging argonite carbonate body.

• Over some distance, the IAP to precipitate calcite will be reached (Ceq) and calcite will be precipitated while argonite (CA) is still being dissolved.

• Near the recharge source aragonite solution rate will exceed calcite precipitation rate and a zone of calcite supersaturation (Css) will exist.

• Eventually aragonite solution rate balances calcite precipitation and a steady state is obtained.

Distance

CA

Css

Ceq

Co

xs

Co

nce

ntr

atio

n

Calcite Supersaturation

• Up to distance xs, both aragonite and calcite are dissolving.

• After distance xs, aragonite continues to dissolve but calcite is precipitated.

• At greater distances equilibrium is achieved between aragonite solution and calcite precipitation.

• The carbonate is transformed from aragonite to calcite in-situ

Distance

CA

Css

Ceq

Co

xsC

on

cen

tra

tion

Aragonite solution > Calcite precipitation

Aragonite solution = Calcite precipitation

Aragonite and Calcite solution

Saturation Distance• The distance (xs) can be calculated based on the longitudinal

dispersivity (x ) and the advective velocity (vx):

xs= 2xlog[(Css-Co)/(Css-Ceq)]/(-1)

Distance

CA

Css

Ceq

Co

xs

Co

nce

ntr

atio

n

where (1+4kS*x/vx)1/2

S* is the specific surface area of pores per unit volume and k is the reaction rate coefficient (or volume dissolved per unit surface area per unit time).S*kx/vx is the first Damköhler number for the solution reaction and = (1+4DaI)1/2

Controls on Saturation Distance

xs= 2xlog[(Css-Co)/(Css-Ceq)]/(-1)where (1+4DaI)1/2

Distance

CA

Css

Ceq

Co

xs

Co

nce

ntr

atio

n

• xs increases asx increasesDispersion increases the size of the alteration zone.

• xs as DaI 0At low Damkohler numbers advection dominates and there is no time for solution.

• xs 0 as DaI At high Damkohler numbers reaction dominates and a large zone of alteration develops.

Undersaturation Problem

• The meteoric water infiltration problem address how quickly saturation and supersaturation are achieved in carbonates.

• On the other hand, karst systems suggest that water remains undersaturated over large flow distances. How can this occur?

• Experimental work has shown that dissolution rates decrease sharply as calcite saturation is approached.

Calcite Undersaturation• Plummer and Wrigley (1976) found the dissolution rate

to be given by K(Csat-C)n

• At low concentrations (C<<Csat), n=2.• As C Csat n increases to as much as 8.• Trace inhibitors provide a possible reason for this

phenomenon. • Pb2+, Cu2+ and PO4

3- ions in solution have been shown to limit calcite dissolution rates.

• The assymptotic approach to saturation allows slightly undersaturated water to travel for considerable distances.

Dolomitization

• Dolomite CaMg(CO3)2 is a frequent replacement for calcite CaCO3 in rocks.

• This clearly requires Mg2+ to be provided (in a moving fluid).

• Dolomitization is a groundwater reactive transport process.

• The transformation of calcite to dolomite involves a 13% reduction in the solid volume (or a 13% increase in porosity).

Dolomitizing Fluids• Calculations of the number of pore volumes of

fluid required to achieve complete dolomitization range from about 10 to 10,000.

• The controlling factors include initial porosity and the composition of the fluid (both salinity and Ca/Mg ratio).

• The low numbers of pore volumes (10) correspond to hypersaline brines (>100 g/L) with high Mg/Ca ratios.

• The high number of pore volumes (10,000) correspond to diluted seawaters (<4 g/L).

Aqueous Environments

SeawaterRiverLakesAquifersSabkha

Do

lom

ite

Cal

cite

Arag

10-1 1 101

Ca Mg/Ca Mg

Sal

init

y

10-2

10-3

10-1

1

101

Fresh

Saline

• Environments for dolomitization cover a wide range of salinities and Mg/Ca ratios.• In general, the higher the salinity, the higher the Mg/Ca ratio required in the fluid.

Economic Mineralization

• There are four necessary factors (White, 1968) for the formation of an economic mineral deposit by groundwater circulation:– A source area must provide the metals– Mineral dissolution in water must occur– Fluids must transport the metals in solution– Precipitation must occur at the site of

deposition

Metal Concentration

• Groundwater may concentrate metals in several ways:– by selective remobilization to leave zones

enriched in metals– by localized precipitation to create high

metal concentrations (pH or Eh trigger)

• Flow systems may be shallow or deep, local or extensive, geothermal or meteoric.

Groundwater-Related Deposits• Residual laterites/bauxite deposits (Ni,Fe,Al)

– shallow flow systems, weathering, Eh/pH changes at the water table

• Supergene enrichment deposits (Cu)– shallow flow systems, weathering, Eh/pH changes at the water table

• Roll-front deposits (U)– leaching, transport and precipitation in shallow groundwater system

• Unconformity uranium deposits (U)– deep flow systems, precipitation with redox changes at unconformity

• Mississippi valley type deposits (Pb,Zn)– deep compaction flow of brine, precipitation with declining temperature

• Porphyry deposits (Cu,Mo,Au)– convective mixing and cooling of meteoric and magmatic fluids

• Lode gold deposits (Au)– leaching and deep meteoric convection, precipitation on cooling

Roll-Front Uranium• Source leached from surficial volcanic ash.• Uranium and other metals deposited at sulphate-

sulphide redox boundary.• Front continues to advance (rolls) by

remobilization and reprecipitation.• In oxidizing environments, uranyl U(+VI) species

(UO22+,UO2CO3

0,UO2SO40, UO2OH+) species are

mobile.• As Eh falls, U(+IV) containing species coffinite

(USiO4) and uraninite (UO2) are precipitated.

Redox Fronts

Recharge and Leaching

Redox Front

pH

Eh

UO22+

UO

2(C

O3)

22

-

UO

2(C

O3)

34

-

USiO4

UO

2C

O3

00

1.2

14

MVT Deposits

• Carbonate-hosted Pb-Zn deposits are found throughout the world.

• They occur in preferred horizons in carbonate rocks and are approximately conformable.

• Related to regional highs found on flanks of domal structures.

• Hot fluids expelled by compaction from basin to flanks. Gravity driven flow will also work.

• Precipitation occurs as fluids cool in suitable carbonate host rock.

Expulsion of Basin Fluids

Evaporites• Any time that mineralized groundwater gets close

to the surface in an arid climate, there is potential for evaporation and precipitation.

• The quantity of evaporite minerals produced depends on the mass flux and the time available.

• Minerals precipitate in reverse order to their solubility as the solution reaches saturation with respect to the various solid phases.

• Hardie (1991) provides a general overview of evaporite formation.

Soil Salinization• The relative abundances of cations and anions controls

the way in which evaporating discharges evolve.• In east-central Saskatchewan the process leads to

large scale salt efflorescence in soils (salinization) and some economic deposits of mirabilite (Na2SO4.10H2O).

• In west Texas and New Mexico the process leads to gypsum (CaSO4.2H2O) precipitation and eventually magnesite and halite (NaCl).

• In both cases the soils in discharge areas can only support a few salt-tolerant plant species.

Effects of Salinity

• The effect of salinity is to reduce or eliminate plant growth – (1) by reducing or preventing the uptake of water

by plant roots because of increased osmotic tension,

– (2) by direct chemical effects of salt disrupting the nutritional and metabolic process in the plant and/or

– (3) by indirect effect of salt altering the structure, permeability and aeration of the soil.

Soil Salinity Levels

• The effect on plants varies with the salt content of the soil and the plant tolerance itself.

• Generally, at an electrical conductivity (EC) of 200 mS/m only sensitive plants are affected, at an EC of 400 mS/m the growth of many plants are reduced, and only tolerant plants can withstand an EC of 800 mS/m.

Salt Tolerant Plants• Salt tolerant plants such as

Russian thistle, kochia, wild barley (foxtail), goosefoot and salicornia (glasswort) are commonly found in areas of high salt concentration.

Multiphase Flow• When there is more than one fluid phase in a

porous medium, each fluid flows under its own unique driving forces.

• Multiphase flow is complex and can involve both diffuse and segregated flow extremes.

• In diffuse flow, both fluids occupy different paths within the porous medium and on a large scale appear to be mixed.

• In segregated flow, the fluids are separated by a sharp interface and flow in their zones of the medium.

Segregated Flow• We will consider some simple cases of

multiphase segregated flow. • The fluids are separated by a sharp interface

and any mixing zone is assumed to be very narrow.

Diffuse

Segregatedoiloil

waterwater

Two-phase Flow• When two fluids are present, each fluid has its own

driving head

h1 = z1 + P1/g

h2 = z2 + P2/g

• On a common interface both pressure (P) and elevation (z) are the same:

h1 = z + P/g

h2 = z + P/g

h1 = (1/2)h2 – (2 - 1)z / 1

where 2 > 1 so that h1 refers to

the less dense upper fluid (oil).

oil

water

h1

h2

z

Sloping Interfaces• The elevation of the interface (z) is given by:

z = 2 h2 / –1 h1 /

where is the density contrast (2 – 1)• A sloping interface is possible only if one or both fluids are in

motion. The slope is z/s = sin() where s is distance along the interface.sin() = (2 /(h2/s) –(1/(h1/s)

• Applying Darcys Law to write the head gradients as q/K gives:sin() = -(2 /(q2/K2) +(1/(q1/K1)

• Assuming the water is static, q2=0 or q1>>q2: sin() = (1/(q1/K1)

water

oil

Basin Brines

• Static basin brines can be displaced updip in synclinal structures by flowing fresher water.

• When the less dense fluid is in motion the slope of interface is positive in the direction of flow.

• The fresher water passes over the static dense slug at the bottom of the structure.

Hydrocarbon Displacement

• Hydrocarbon can be displaced downdip in anticlinal structures by flowing aquifer water.

• When the denser fluid is in motion the slope of interface is negative in the direction of flow.

• The water passes beneath the static lighter hydrocarbon at the top of the structure.

• Low gradients tend to improves the trapping efficiency of the structure.

• High gradients can lead to flushing of hydrocarbons.

Gas Displacement

• Gas-oil interfaces and gas water interfaces tend to remain close to horizontal because the gas/ term is usually small so sin(is near zero.

• This implies that gas caps are significantly harder to flush than liquid oil traps.

• Separation of a gas cap from it’s associated oil leg can occur as result of aquifer flow.

• High gradients lead to greater separation of gas and oil.