2. Hydrogeochemistry
Hydrogeochemistry is a
multidisciplinary field that combines
principles from hydrology and
geochemistry to study the chemical
composition of water, particularly
groundwater.
This branch of science focuses on
understanding the processes that
control the distribution and movement
of chemical elements and compounds
in the Earth's subsurface.
3. Hydrogeochemistry plays a
crucial role in assessing water
quality, understanding water-
rock interactions, and
managing water resources.
4. Groundwater and Its Chemical Constituents
Water
Water is formed by the union of two
hydrogen atoms with one oxygen atom.
The oxygen atom is bonded to the
hydrogen atoms unsymmetrically, with a
bond angle of 105°.
Water in the liquid state, although given
the formula H20 or HOH, is composed of
molecular groups with the HOH
molecules in each group held together
by hydrogen bonding.
5. A classification of the inorganic species that occur in
groundwater is shown in Table 3.3. The concentration
categories are only a general guide.
In some groundwaters, the concentration ranges are
exceeded. The major constituents in Table 3.3 occur mainly
in ionic form and are commonly referred to as the major ions
The total concentration of these six major ions normally
comprises more than 90 % of the total dissolved solids in the
water, regardless of whether the water is dilute or has
salinity greater than seawater.
6.
7. concentrations of the major, minor, and trace inorganic
constituents in groundwater are controlled by the availability
of the elements in the soil and rock through which the water
has passed, by geochemical constraints such as solubility and
adsorption, by the rates (kinetics) of geochemical processes,
and by the sequence in which the water has come into
contact with the various minerals occurring in the geologic
materials along the flow paths.
8. Organic Constituents
Organic compounds are those that have carbon and usually
hydrogen and oxygen as the main elemental components in
their structural framework. By definition, carbon is the key
element. The species , which are
important constituents in all groundwater, however, are not
classified as organic compounds.
Dissolved organic matter is ubiquitous in natural
groundwater, although the concentrations are generally low
compared to the inorganic constituents.
9. Little is known about the chemical nature of organic matter
in groundwater.
Investigations of soil water suggest that most dissolved
organic matter in subsurface flow systems is fulvic and humic
acid.
Concentrations in the range 0.1-10 mg/l are most common,
but in some areas values are as high as several tens of
milligrams per liter.
10.
11. Dissolved Gases
The most abundant dissolved gases in groundwater are N2 ,
02 , C02 , CH4 (methane), H2S, and N20. The first three make
up the earth's atmosphere and it is, therefore, not surprising
that they occur in subsurface water.
CH4 , H2S, and N20 can often exist in groundwater in
significant concentrations because they are the product of
biogeochemical processes that occur in nonaerated
subsurface zones.
12. Water-Rock Interactions
Water-rock interaction is a fundamental process in
hydrogeochemistry that describes the chemical reactions
and exchanges that occur between water and geological
materials, such as rocks and minerals, in the Earth’s
subsurface.
These interactions have a significant impact on the chemical
composition of groundwater and play a crucial role in
shaping the characteristics of natural water systems. Here
are some key aspects of water-rock interaction:
13. Dissolution and Precipitation
Dissolution: Minerals in rocks can dissolve into
groundwater, releasing ions into the water. For example,
carbonate minerals like calcite can dissolve in slightly acidic
water, leading to an increase in calcium and bicarbonate
concentrations.
Precipitation: Conversely, certain ions in the water can
combine to form solid minerals, a process known as
precipitation. This can occur when the concentration of
dissolved minerals exceeds their solubility limits.
14. Ion Exchange
Ion exchange involves the swapping of ions between the
water and minerals in rocks. For instance, cations like
calcium or magnesium in the minerals may be exchanged for
other cations present in the groundwater.
15. Redox Reactions
Redox (reduction-oxidation) reactions involve the transfer of
electrons between different chemical species. In water-rock
interaction, minerals containing elements in different
oxidation states may undergo redox reactions, influencing
the composition of the water.
Example: The reduction of iron (Fe^3+) to ferrous iron
(Fe^2+) in anaerobic groundwater environments.
16. Silicate Weathering
Silicate minerals, such as feldspar and olivine, are common
in many rocks. Weathering of these silicate minerals
contributes to the release of ions like silica, calcium, and
magnesium into the water.
17. Karst Formation
In limestone and other soluble rocks, water-rock interaction
can lead to the formation of karst landscapes. This process
involves the dissolution of carbonate minerals, creating
features like caves, sinkholes, and underground drainage
systems.
18. Ground water quality
Physical Characteristics
Temperature: Groundwater temperature can vary
depending on factors such as depth, season, and proximity
to surface water.
Turbidity: Turbidity measures the cloudiness or haziness of
water, often caused by suspended particles.
19. Chemical Characteristics
pH: pH indicates the acidity or alkalinity of water. It can
influence the solubility and mobility of various chemical
constituents.
Dissolved Oxygen (DO): The amount of oxygen dissolved in
groundwater is crucial for supporting aquatic life and
microbial processes.
Electrical Conductivity (EC): EC is a measure of the water’s
ability to conduct electricity, which correlates with the
dissolved ion concentration.
20. Major Ions
Concentrations of major ions like calcium, magnesium,
sodium, chloride, sulfate, and bicarbonate are crucial
indicators of water quality.
They can influence taste, hardness, and the suitability of
water for various uses.
21. Trace Elements
Elevated levels of trace elements such as iron, manganese,
arsenic, fluoride, and others can have health implications
and affect the aesthetic quality of water.
22.
23. Adsorption and Desorption:
Definition: Adsorption is the attachment of ions to the
surfaces of minerals, while desorption is the release of
previously adsorbed ions.
Example: The adsorption of heavy metals onto clay particles
in soils, affecting their mobility in water.
24. Isotope Hydrology
Isotope hydrology is an integral part of hydrogeochemistry.
Stable and radioactive isotopes of elements like hydrogen,
oxygen, and carbon are used to trace the origin, movement,
and age of groundwater, providing valuable information
about the hydrological cycle.
25.
26. Groundwater Modeling
Mathematical models are often used in hydrogeochemistry
to simulate and predict groundwater flow, solute transport,
and chemical reactions. These models help researchers and
hydrogeologists understand and manage groundwater
systems.
