Chapter 1: The Aquifer Geochemical System

 

Geochemical phases

Gas (Hg0)

Solution

suspended particulate (HgS > 0.2 µm)
colloidal (HgS < 0.2 µm)
complexed (HgCl2)
ionic (Hg 2+)

Solid

inroganic (HgS)
organic (CH3Hg)

 

Operational definitions of phases

Total dissolved (< 0.45 µm)
Humic v. fulvic

Steady State v. Equilibrium

"Equilibrium is death" (Picowitz, personal communication)
Steady state:

input = output
dA/dt = dA/dt

Box Model calculations:

residence time = mass/input or output
t = A/(dA/dt)

Concentration units

Mass

mg/L = mg/kg
mg/kg = ppm (parts per million)
dg/kg = pph ? (parts per hundred &endash; I've never seen this used)
= %
g/kg = ppt * (parts per thousand)
salinity (o/oo = psu = unitless)
µg/kg = ppt * (parts per trillion)
ng/kg = ppb (parts per billion)

* always use mg/kg and ng/kg to avoid confusion

Molar

Avogradro's number (6.023 x 1023) = # atoms or molecules in a mole
Atomic or molecular weight = mass of 1 mole
1 mole C = 12 g
1 mole CO2 = 1(12g/M C) + 2(16g/M O)
= 48 g

 

Mass v. Molar

Historic measurements were gravimetric = mass
Geologists think in terms of mass = mass
Chemists think in terms of reactions = molar

Geochemists = confused

Modern = molar

Major v. Minor (trace)

Major (³ g/kg)

Typically includes: Na+, K+, Ca2+ , Mg 2+ (salts or cations) *
Cl - (major salt anion)
HCO3- , CO3 - (bicarcarbonate/carbonate)
SO4- (sulfate)
NO3- (nitrate) **
H4SiO4 (silicate) ***

Minor or Trace (< g/kg)

Typically includes all other elements at some amount
Many are not considered or reported as ND or BDL
ND = Not detected
BDL = below detection limits
e.g., As in drinking water

* error in text Na2+ (Table 1) - please look for and report my errors
** common groundwater contaminant (blue baby syndrome)
*** uncharged dissolved species

Equivalents

 

Groundwater solutions are neutrally charged

 positive charge =  negative charge
 (anions x valence) =  (cations x valence)

ex. NaCl

1 mole Na+ = 1 mole Cl-

ex. CaCl2

1 mole Ca2+ = 2 moles Cl-

equivalents/liter = Conc (mg/L) x Molecular wt (mole/g/mole) x Valence (charge)

ex. 92 mg/L Ca2+ measured in a water sample


equivalents/L = 92 mg x 40.08 g/mole x 2+
= 4.6 x 10-3 equiv./L
= 4.6 millequivalents/L
= 4.6 meq/L *

Cation/anion balance

Must be neutral for groundwater
Sum of positive charge = sum of negative charge
Estimate of accuracy in measuring charge

 

 (cation meq/L) -  (anion meq/L)
_____________________________ x 100%

 (cation meq/L) +  (anion meq/L)

good measurements should have cation/anion charge balance < 5% **

 

what factors can account for a poor cation/anion charge balance ?

page 8 of text

 

* equivalents are commonly reported in meq/L because they typically total 10-3 meq/L
** we and I believe most analytical laboratories are happy with +/- 10% for most measurements

 

Ground water types

Defined by major ion composition

Piper or trilinear diagrams
other methods Hem (1989)

 

pH

why is it a "master variable" for groundwater geochemistry ?

page 11 of text

 

alkalinity (meq/L)

 

total acid-neutralizing capacity of water

 

 concentrations of anions that may titrate H+
= Â anions (charge x moles/L) &endash; H+ (moles/L)

common ground water measurement, reported as mg/L CaCO3
used to calculate speciation of bicarbonate/carbonate

 

acidity (meq/L)

~ hydroxyl neutralizing capacity
not a common ground water measurement

 

Redox potential

Why is it a "master variable for groundwater geochemistry ?

Page 15 of text

Eh/pH plots

Page 16 of text

 

Equilibrium calculations

Equilibrium constants
Activity coefficients
Debye-Huckel equation

 

Chapter 1 and Week 1 in Perspective

Review of Bšhlke and Denver (1995)

How do the authors substantiate their statement of nitrate contamination?

Sampling methods
Analytical accuracy
Analytical precision
Reference materials
Referenced methods
Intercalibrated measurements
Replicate measurements

What does their Piper (trilinear) diagram show and why does that matter?

Explain whether their system in equilibrium or steady state?

How does that state influence nitrate fluxes and residence time calculations?

Explain how speciation is a factor for nitrate contamination within the system?
Inputs and outputs
Residence time
Toxicity

Why are the following parameters measured if the concern is with nitrate contamination ?

Na
K
Ca
Mg
Cl
SO4
SiO2
NH4
 N2
DON
O2
Ar
CO2
CH4 CCl2F2
d 15N
d 13C
tritium (3H)

Bšhlke, J.K. and J.M. Denver. 1995. Combined use of groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds, Atlantic coastal plain, Maryland. Water Resources Research 31: 2319-2339.

 

 

Return to ETOX 144 Home Page

chpt.1.html

Chapter 1: The Aquifer Geochemical System

 

Geochemical phases

Gas (Hg0)

Solution

suspended particulate (HgS > 0.2 µm)
colloidal (HgS < 0.2 µm)
complexed (HgCl2)
ionic (Hg 2+)

Solid

inroganic (HgS)
organic (CH3Hg)

 

Operational definitions of phases

Total dissolved (< 0.45 µm)
Humic v. fulvic

Steady State v. Equilibrium

"Equilibrium is death" (Picowitz, personal communication)
Steady state:

input = output
dA/dt = dA/dt

Box Model calculations:

residence time = mass/input or output
t = A/(dA/dt)

Concentration units

Mass

mg/L = mg/kg
mg/kg = ppm (parts per million)
dg/kg = pph ? (parts per hundred &endash; I've never seen this used)
= %
g/kg = ppt * (parts per thousand)
salinity (o/oo = psu = unitless)
µg/kg = ppt * (parts per trillion)
ng/kg = ppb (parts per billion)

* always use mg/kg and ng/kg to avoid confusion

Molar

Avogradro's number (6.023 x 1023) = # atoms or molecules in a mole
Atomic or molecular weight = mass of 1 mole
1 mole C = 12 g
1 mole CO2 = 1(12g/M C) + 2(16g/M O)
= 48 g

 

Mass v. Molar

Historic measurements were gravimetric = mass
Geologists think in terms of mass = mass
Chemists think in terms of reactions = molar

Geochemists = confused

Modern = molar

Major v. Minor (trace)

Major (³ g/kg)

Typically includes: Na+, K+, Ca2+ , Mg 2+ (salts or cations) *
Cl - (major salt anion)
HCO3- , CO3 - (bicarcarbonate/carbonate)
SO4- (sulfate)
NO3- (nitrate) **
H4SiO4 (silicate) ***

Minor or Trace (< g/kg)

Typically includes all other elements at some amount
Many are not considered or reported as ND or BDL
ND = Not detected
BDL = below detection limits
e.g., As in drinking water

* error in text Na2+ (Table 1) - please look for and report my errors
** common groundwater contaminant (blue baby syndrome)
*** uncharged dissolved species

Equivalents

 

Groundwater solutions are neutrally charged

 positive charge =  negative charge
 (anions x valence) =  (cations x valence)

ex. NaCl

1 mole Na+ = 1 mole Cl-

ex. CaCl2

1 mole Ca2+ = 2 moles Cl-

equivalents/liter = Conc (mg/L) x Molecular wt (mole/g/mole) x Valence (charge)

ex. 92 mg/L Ca2+ measured in a water sample


equivalents/L = 92 mg x 40.08 g/mole x 2+
= 4.6 x 10-3 equiv./L
= 4.6 millequivalents/L
= 4.6 meq/L *

Cation/anion balance

Must be neutral for groundwater
Sum of positive charge = sum of negative charge
Estimate of accuracy in measuring charge

 

 (cation meq/L) -  (anion meq/L)
_____________________________ x 100%

 (cation meq/L) +  (anion meq/L)

good measurements should have cation/anion charge balance < 5% **

 

what factors can account for a poor cation/anion charge balance ?

page 8 of text

 

* equivalents are commonly reported in meq/L because they typically total 10-3 meq/L
** we and I believe most analytical laboratories are happy with +/- 10% for most measurements

 

Ground water types

Defined by major ion composition

Piper or trilinear diagrams
other methods Hem (1989)

 

pH

why is it a "master variable" for groundwater geochemistry ?

page 11 of text

 

alkalinity (meq/L)

 

total acid-neutralizing capacity of water

 

 concentrations of anions that may titrate H+
= Â anions (charge x moles/L) &endash; H+ (moles/L)

common ground water measurement, reported as mg/L CaCO3
used to calculate speciation of bicarbonate/carbonate

 

acidity (meq/L)

~ hydroxyl neutralizing capacity
not a common ground water measurement

 

Redox potential

Why is it a "master variable for groundwater geochemistry ?

Page 15 of text

Eh/pH plots

Page 16 of text

 

Equilibrium calculations

Equilibrium constants
Activity coefficients
Debye-Huckel equation

 

Chapter 1 and Week 1 in Perspective

Review of Bšhlke and Denver (1995)

How do the authors substantiate their statement of nitrate contamination?

Sampling methods
Analytical accuracy
Analytical precision
Reference materials
Referenced methods
Intercalibrated measurements
Replicate measurements

What does their Piper (trilinear) diagram show and why does that matter?

Explain whether their system in equilibrium or steady state?

How does that state influence nitrate fluxes and residence time calculations?

Explain how speciation is a factor for nitrate contamination within the system?
Inputs and outputs
Residence time
Toxicity

Why are the following parameters measured if the concern is with nitrate contamination ?

Na
K
Ca
Mg
Cl
SO4
SiO2
NH4
 N2
DON
O2
Ar
CO2
CH4 CCl2F2
d 15N
d 13C
tritium (3H)

Bšhlke, J.K. and J.M. Denver. 1995. Combined use of groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds, Atlantic coastal plain, Maryland. Water Resources Research 31: 2319-2339.

 

 

Return to ETOX 144 Home Page