Introduction to Groundwater Chemistry
October 04, 2010
Units of Measurements
1. Common units are (1 mg/L = 1 ppm = 1000 ppb).
ppm = 1 part in 1,000,00 (106) parts by mass or volume
2. Molar concentration (molarity) = moles of solute per liter of solution.
molarity = moles of solute/ liter of solution
3. Molality = moles of solute per kilogram of solvent (not solution).
molality (M) = moles solute/kg of solvent
molality = (mg/l) *0.001/formula weight in grams
4. Equivalent weight of a substance is the amount of that substance which supplies or
consumes one mol of reactive species. An element's equivalent weight is its atomic
weight divided by its valence.
meq/l = (mg/l)/valence of ion
Units of Measurements
In order to convert the mass concentration to an equivalent concentration the
following mathematical relationship is used:
(mass concentration) * (ionic charge) / (molecular weight) = (equivalent concentration)
For example, a water with a calcium concentration of 120 mg/L would have the
following calcium equivalent concentration:
(120 mg/L) * (2 meq/mmol) / (40 mg/mmol) = (5 meq/L)
Types of chemical reactions in water
1. Reversible reaction can reach equilibrium with their hydrochemical environment.
The simplest aqueous reaction is the dissociation of an inorganic salt. If the salt
is present in excess, it will tend to form a saturated solution:
NaCl == Na+ + Cl-
If the solution is undersaturated more salt will dissolve.
If it is supersaturated, salt will crystallize.
2. Irreversible reaction: one way reaction, A+B ---- C+D
Law of mass action
Law of mass action: the reaction will strive to reach equilibrium.
cC + dD == xX + yY
Equilibrium constant (K) = [X]x [Y]y / [C]c [D]d
Major ion chemistry
More than 90% of the dissolved solids in groundwater can be attributed to:
Na, Ca, K, Mg, SO4, Cl, HCO3, and CO3
These ions are usually present at concentration greater than 1mg/l.
Silica, SiO2, a nonionic species, is also present at concentrations greater than 1mg/l.
Direct analysis can be done for the first six ions.
Bicarbonate and carbonate concentrations are found by titration with acid to an endpoint
with a pH of about 4.4.
pH, Temperature, and specific electrical conductance are usually made at the time the
sample is collected.
Other naturally occurring ions that may be present in amount of 0.1mg/l to 10 mg/l
include iron, fluoride, strontium, and boron.
Major ion chemistry
Iron and nitrate are typically included in water-chemistry studies, with fluoride,
strontium, and boron being less commonly reported.
Total dissolved solids (TDS) can be determined by evaporating a known volume
of the sample and weighing the residue. TDS can be estimated by summing the
concentrations of the individual ions.
Ion Exchange
Sites Location
selection
Under certain conditions, the ions attracted to a solid surface may be
exchanged for other ions in aqueous solution.
The ion-exchange process can be conceptualized as the preferential
absorption of selective ions with contaminant loss of other ions.
Ion-exchange sites are found primarily on clays and soil organic materials,
although all soils and sediments have some ion-exchange capacity.
A general ordering of cation exchangeability for common ions in
groundwater is: Na > K > Mg > Ca
Ex. Sodium adsorption ratio (SAR):
SAR = Na / [ (Ca+Mg)/2 ]0.5
If SAR between 2 and 10, little danger from sodium
If SAR between 7 and 18, medium hazards
If SAR between 11 and 26, high hazards
Presentation of results of chemical
analysis
A cation-anion balance is usually performed as a check on the chemical
analysis. This is accomplished by converting all the ionic concentrations to
units of equivalents per liter. The anions and cations are summed separately,
and the results are compared.
Charge Balance
For any solution, the total charge of positively charged
ions will equal the total charge of negatively charged
ions. Net charge for any solution must = 0
Charge Balance Error (CBE)
m z m

CBE 
m z  m
c c
a
za
c c
a
za
Tells you how far off the analyses are (greater than 5% is not
good, greater than 10% is terrible…)
Stiff Diagram
Characterizes Water Chemistry
Analyses in meq/l are plotted
on four parallel horizontal lines.
Concentrations of up to four
cations and anions can be
plotted, one each to the left or
right of the center zero axis.
Resulting points are connected
to give an irregular polygon
pattern.
Stiff patterns can be a
relatively distinctive method of
showing water-composition
similarities and differences.
Schoeller Diagram
Line Plot That Characterizes Water
Semi-logarithmic diagram that
represents major ion analyses
in meq/l.
Demonstrates different
hydrogeochemical water types
on the same diagram.
Number of analyses plotted at
one time is limited.
Actual parameter
concentrations are displayed.
Schoeller Diagram
Concentration (meq/l) logarithmic
scale
1000.0000
Groundwater
100.0000
10.0000
Surface Runoff
1.0000
Precipitation
0.1000
0.0100
0.0010
Ca
Mg
K
Si
Na
Cl
F
Major anions and cations
SO4 HCO3
Average concentration of major anions and cations of groundwater, surface-runoff, and rainfall
Piper Diagram
Shows Groupings of Water Types
Major ions are plotted as
cation and anion
percentages in meq/l in
two base triangles.
Total ions are set to
equal 100%.
Data points in the two
triangles are projected to
central diamond.
Allows comparison of a
large number of samples.
Shows clustering of
samples and water type.
ASARCO Data
Station
Name
EP-114
EP-116
EP-119
EP-12
EP-122
EP-13
EP-131
EP-132
EP-133
EP-135
EP-49
EP-51
EP-58
EP-62
EP-68
EP-75
EP-77
EP-81
EP-85
EP-85
SEP-1
SEP-1
SEP-10
SEP-11
SEP-12
SEP-13
SEP-2
SEP-3
SEP-4
SEP-6
SEP-7
Bicarbonate
Alkalinity As Calcium (Ca)
CACO3
TOTAL
400
603
356
284
384
157
876
215
304
182
348
423
520
100
294
221
416
234
500
609
502
556
206
933
860
533
362
165
460
292
464
467
503
65.3
146
89.4
316
121
310
121
260
126
250
125
320
194
340
166
306
143
322
142
338
163
218
105
290
141
222
108
216
106
Carbonate
Alkalinity As
CACO3
UNKNOWN Chloride (Cl) Magnesium (Mg) Potassium (K)
<1
214
222
164
<1
390
55.5
45.5
<1
400
64
52.5
<1
203
67.8
12.1
<1
197
78.9
49.9
<1
946
67.2
87.4
<1
212
36.2
14.2
<1
413
44.6
45.3
<1
324
66
49.4
<1
991
176
16.4
<1
288
130
242
<1
4830
772
55
<1
829
225
226
<1
415
68
59.7
<1
575
119
14.6
<1
299
289
662
<1
120
18.3
16.2
<1
33.8
34.6
10.3
<1
156
52.4
49.7
<1
155
52.3
45.9
<1
378
33.7
14.6
<1
361
33.8
14.6
<1
994
101
20.4
<1
659
72.8
30.6
<1
663
61.7
27.6
<1
651
61.9
24.9
<1
677
71.8
29.2
<1
369
25.5
13
<1
638
61.4
22.6
<1
370
26.3
13.2
<1
362
24.9
13.1
Sodium (Na)
471
766
699
891
560
2560
650
752
1170
1340
1400
1910
1890
829
798
3020
604
105
507
514
468
467
1440
874
880
876
863
432
845
446
435
Sulfate (SO4)
3160
2000
1310
1250
1340
4610
1210
1540
1990
3060
4320
2270
4480
1290
1740
9240
906
382
1000
964
627
568
1770
1360
1280
1360
1490
500
1250
487
504