Hydrogeology
1. Summary
Hydrogeology is the study of groundwater – its origin, occurrence,
movement and quality. Groundwater moves through the sub-surface from areas of
greater hydraulic head to areas of lower hydraulic head. The rate of
groundwater movement depends upon the slope of the hydraulic head (hydraulic
gradient), and intrinsic aquifer and fluid properties. The ground-water system
must be understood in relation to both surface water and moisture in the
atmosphere. Most addition to ground water come from the atmosphere in the form
of precipitation.
2. Aquifer Formations
An aquifer is a geologic unit that can store and transmit water.
Aquifers are generally
categorized into four basic formation types depending on the geologic
environment in which they occur: unconfined, confined, semi-confined, and
perched.
An unconfined aquifer is characterized by a water table which is at or
near atmospheric pressure. A confined aquifer is bounded by its upper and
perhaps lower boundary by a layer of natural material that does not transmit
water readily. A perched aquifer contains unconfined groundwater above a lower
body of groundwater, which is usually a result of clay lenses in the soil
strata.
Figure 1:
Different Aquifer Formations
3.
Main Equations of Groundwater Flow
The basic law of flow is Darcy’s law, which can be written in
differential form:
whereas h is called the hydraulic head [L], and dh/dl is the hydraulic
gradient. The parameter K is known as the hydraulic conductivity, which has the
dimensions of a velocity [L/T].
3.1 Steady State Saturated
Flow
The law of conservation of mass for steady-state flow through a saturated porous medium requires that the rate of fluid mass flow into any elemental control volume be equal to the rate of fluid mass flow out of any elemental control volume. Combining this equation with Darcy’s law the equation for steady-state flow through a homogenous, isotropic medium is:
This equation is also known as Laplace’s equation.
3.2 Transient Saturated Flow
The law of conservation of mass for transient flow in a saturated porous
medium requires that the net rate of fluid mass flow into any elemental control
volume be equal to the time rate of change of fluid mass storage within the
element. If the medium is homegenous and isotropic the equation is:
whereas SS is the specific storage [L-1]
4.
Aqueous Geochemistry
The
major ions in groundwater are Na+, Mg2+, Ca2+,
Cl-, HCO3- and SO42-.
The total concentration of these six major ions normally comprises more than 90
% of the total dissolved solids in the water. Dissolved organic matter is
ubiquitous in natural groundwater, although the concentrations are generally
low compared to the inorganic constituents. The most abundant dissolved gases
in groundwater are N2, O2, CO2, CH4,
H2S and N2O.
From a chemical perspective, groundwater is best thought of as a partial equilibrium system. Some reactions are at equilibrium, however other reactions not at equilibrium are best represented by kinetic concepts. At equilibrium there is no chemical energy available to alter the relative distribution of mass between the reactants and products in a reaction. Away from equilibrium, energy is available to spontaneously drive a system toward equilibrium by allowing the reaction to progress. Transformation reactions involving organic contaminants in ground water depend on the physical and chemical properties of the particular compounds and or the abundance of microbes (for example, biodegradation)
5.
Literature and Guidelines
Calculation and Use of First-Order Rate Constants for Monitored Natural
Attenuation Studies
http://www.epa.gov/ada/download/issue/540S02500.pdf
Suggested Operation Procedure for Aquifer Pumping Tests
http://www.epa.gov/tio/tsp/download/issue15a.pdf
U.S. Army Corps of Engineers, Manual: Groundwater Hydrology
http://www.usace.army.mil/publications/eng-manuals/em1110-2-1421/entire.pdf
Behaviour of hydrocarbons in the subsurface, (including porosity values
of various geologic materials)
http://www.epa.gov/safewater/protect/citguid.pdf
USGS Publication Search
http://usgspubs.georef.org/qbeusgsb.htm
EPA’S Citizens Guide to Ground Water Protection
http://www.epa.gov/safewater/protect/citguid.pdf
Glossary of hydrogeologic
terms: EPA’S Ground Water Primer:
http://www.epa.gov/seahome/groundwater/src/ground.htm
EPA
(2004): EPA'S Risk Screening Environmental Indicators (RSEI), Chronic Human
Health Methodology, RSEI Version 2.1.2, Technical Appendix B.
6. Literature
Domenico, P. A. , Schwartz,
W. (1998): Physical and chemical hydrogeology. Wiley, New York
Freeze, R.A., Cherry, J.A.
(1979): Groundwater. Prentice Hall, Inc.
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Stefan Gödeke