Page 94 - Hydrogeology Principles and Practice

HYDC03 12/5/05 5:36 PM Page 77

Chemical hydrogeology 77

Table 3.2 Chemical composition of groundwater divided into and can accumulate to undesirable concentrations
major and minor ions, trace constituents and dissolved gases. in unventilated homes, mines and caves. Uranium is
After Freeze and Cherry (1979).
present in crustal rocks (e.g. in the mineral uranite,
UO ), silicates (e.g. in the mineral zircon, ZrSiO )
Groundwater composition 2 4
and phosphates (e.g. in the mineral apatite,
−1
Major ions (>5mgL ) Ca (PO ) (OH, F, Cl)) and is common in granitic
5 4 3
Bicarbonate Sodium rocks but also in other rock types, sediments and soil.
Chloride Calcium Radon and its decay products such as polonium
Sulphate Magnesium 218 216
−1
Minor ions (0.01–10.0 mg L ) ( Po and Po) and ultimately isotopes of lead (Pb)
Nitrate Potassium are harmful when inhaled by humans.
Carbonate Strontium Minor concentrations of the inert gases argon (Ar),
Fluoride Iron helium (He), krypton (Kr), neon (Ne) and xenon (Xe)
Phosphate Boron are found dissolved in groundwater and these can
−1
Trace constituents (<0.1 mg L )
Aluminium Manganese provide useful information on the age and tempera-
Arsenic Nickel ture of groundwater recharge and therefore help in
Barium Phosphate the interpretation of hydrochemical and hydrogeo-
Bromide Radium logical conditions in aquifers (see Section 4.5).
Cadmium Selenium The degree of salinization of groundwater
Caesium Silica
Chromium Silver expressed as the total dissolved solids (TDS) content
Cobalt Thorium is a widely used method for categorizing ground-
Gold Tin waters (Table 3.3). In the absence of any specialist
Iodide Titanium analytical equipment for measuring individual dis-
Lead Uranium solved components, a simple determination of TDS
Lithium Vanadium
Zinc by weighing the solid inorganic and organic residue
−1
Dissolved gases (trace to 10 mg L ) remaining after evaporating a measured volume of
Nitrogen Methane filtered sample to dryness can help determine the
Oxygen Hydrogen sulphide hydrochemical characteristics of a regional aquifer.
Carbon dioxide Nitrous oxide
Equally, a measurement of the electrical conductivity
(EC) of a solution will also give a relative indication
of the amount of dissolved salts, made possible by the
soil water and groundwater to levels in excess of fact that groundwater is an electrolytic solution with
−1
50 mg L (see Box 3.1 for a discussion of the different the dissolved components present in ionic form. For
concentration units used in hydrochemistry). any investigation, it is possible to relate the TDS
Organic compounds are usually present in ground- value to electrical conductivity (usually expressed in
−1
water at very low concentrations of less than 0.1 mg units of micro-siemens centimetre ) as follows:
−1
L as a result of oxidation of organic matter to car-
−1
−1
bon dioxide during infiltration through the soil zone TDS (mg L ) = k · EC (µScm ) eq. 3.3
e
(see Section 3.7). In environments rich in organic
carbon such as river floodplains and wetlands, where the correlation factor, k , is typically between
e
biogeochemical processes can generate anaerobic 0.5 and 0.8 and can be determined for each field inves-
groundwater conditions and the production of dis- tigation. The electrical conductivity for fresh ground-
−1
solved gases such as nitrogen (N ), hydrogen sulphide water is of the order of 100s µScm while rainwater
2
−1
(H S) and methane (CH ). Other dissolved gases is of the order of 10s µScm and brines 100,000s µS
2 4
−1
include oxygen (O ) and carbon dioxide (CO ) mostly cm . Given that ionic activity, and therefore elec-
2 2
of an atmospheric source, and nitrous oxide (N O) trical conductivity, increases with temperature at a
2
from biogeochemical processes in soils and ground- rate of about a 2% per °C, measurements are usually
222
water. Radon ( Rn) gas, a decay product of uranium normalized to a specific temperature of 25°C and
(U) and thorium (Th), is common in groundwater recorded as SEC .
25