Hydrogeology
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust. The terms groundwater hydrology, geohydrology, and hydrogeology are often used interchangeably, though hydrogeology is the most commonly used.
Hydrogeology is the study of the laws governing the movement of subterranean water, the mechanical, chemical, and thermal interaction of this water with the porous solid, and the transport of energy, chemical constituents, and particulate matter by flow.
Groundwater engineering, another name for hydrogeology, is a branch of engineering which is concerned with groundwater movement and the design of wells, pumps, and drains. The main concerns in groundwater engineering include groundwater contamination, conservation of supplies, and water quality.
Wells are constructed for use in developing nations, as well as for use in developed nations in places which are not connected to a city water system. Wells are designed and maintained to uphold the integrity of the aquifer, and to prevent contaminants from reaching the groundwater. Controversy arises in the use of groundwater when its usage impacts surface water systems, or when human activity threatens the integrity of the local aquifer system.
Introduction
Hydrogeology is an interdisciplinary subject; it can be difficult to account fully for the chemical, physical, biological, and even legal interactions between soil, water, nature, and society. The study of the interaction between groundwater movement and geology can be quite complex. Groundwater does not always follow the surface topography; groundwater follows pressure gradients, often through fractures and conduits in circuitous paths. Taking into account the interplay of the different facets of a multi-component system often requires knowledge in several diverse fields at both the experimental and theoretical levels. The following is a more traditional introduction to the methods and nomenclature of saturated subsurface hydrology.Hydrogeology in relation to other fields
Hydrogeology, as stated above, is a branch of the earth sciences dealing with the flow of water through the subsurface, typically porous or fractured geological material. The very shallow flow of water in the subsurface is pertinent to the fields of soil science, agriculture, and civil engineering, as well as to hydrogeology. The general flow of fluids in deeper formations is also a concern of geologists, geophysicists, and petroleum geologists. Groundwater is generally slow-moving; many of the empirically derived laws of groundwater flow can be alternately derived in fluid mechanics from the special case of Stokes flow.File:EVA- Lanxmeer piézomètre Piezometer 2009.jpg|thumb|293x293px| A piezometer is a device used to measure the hydraulic head of groundwater.
The mathematical relationships used to describe the flow of water through porous media are Darcy's law, the diffusion, and Laplace equations, which have applications in many diverse fields. Steady groundwater flow has been simulated using electrical, elastic, and heat conduction analogies. Transient groundwater flow is analogous to the diffusion of heat in a solid, therefore some solutions to hydrological problems have been adapted from heat transfer literature.
Traditionally, the movement of groundwater has been studied separately from surface water, climatology, and even the chemical and microbiological aspects of hydrogeology. As the field of hydrogeology has matured, the interactions between groundwater, surface water, water chemistry, soil moisture, and even climate have become clearer.
California and Washington both require special certification of hydrogeologists to offer professional services to the public. Twenty-nine states require professional licensing for geologists to offer their services to the public, which often includes work within the domains of developing, managing, and/or remediating groundwater resources.
For example: aquifer drawdown or overdrafting and the pumping of fossil water may be a contributing factor to sea-level rise.
Subjects
One of the main tasks a hydrogeologist typically performs is the prediction of future behavior of an aquifer system, based on analysis of past and present observations. Some hypothetical, but characteristic questions asked would be:- Can the aquifer support another subdivision?
- Will the river dry up if the farmer doubles his irrigation?
- Did the chemicals from the dry cleaning facility travel through the aquifer to my well and make me sick?
- Will the plume of effluent leaving my neighbor's septic system flow to my drinking water well?
In order to further characterize aquifers and aquitards some primary and derived physical properties are introduced below. Aquifers are broadly classified as being either confined or unconfined ; the type of aquifer affects what properties control the flow of water in that medium.
Aquifers
An aquifer A water-bearing layer of rock, or of unconsolidated sediments, that will yield water in a usable quantity to a well or spring. Aquifers can be unconfined, where the top of the aquifer is defined by the water table, or confined, where the aquifer exists underneath a confining bed.There are three aspects that control the nature of aquifers: stratigraphy, lithology, and geological formations and deposits. The stratigraphy relates the age and geometry of the many formations that compose the aquifer. The lithology refers to the physical components of an aquifer, such as the mineral composition and grain size. The structural features are the elements that arise due to deformations after deposition, such as fractures and folds. Understanding these aspects is paramount to understanding of how an aquifer is formed and how professionals can utilize it for groundwater engineering.
Hydraulic head
Differences in hydraulic head cause water to move from one place to another; water flows from locations of high h to locations of low h. Hydraulic head is composed of pressure head and elevation head. The head gradient is the change in hydraulic head per length of flowpath, and appears in Darcy's law as being proportional to the discharge.Hydraulic head is a directly measurable property that can take on any value ; ψ can be measured with a pressure transducer, and z can be measured relative to a surveyed datum. Commonly, in wells tapping unconfined aquifers the water level in a well is used as a proxy for hydraulic head, assuming there is no vertical gradient of pressure. Often only changes in hydraulic head through time are needed, so the constant elevation head term can be left out.
A record of hydraulic head through time at a well is a hydrograph or, the changes in hydraulic head recorded during the pumping of a well in a test are called drawdown.
Porosity
Porosity is a directly measurable aquifer property; it is a fraction between 0 and 1 indicating the amount of pore space between unconsolidated soil particles or within a fractured rock. Typically, the majority of groundwater moves through the porosity available to flow. Permeability is an expression of the connectedness of the pores. For instance, an unfractured rock unit may have a high porosity, but a low permeability. An example of this phenomenon is pumice, which, when in its unfractured state, can make a poor aquifer.Porosity does not directly affect the distribution of hydraulic head in an aquifer, but it has a very strong effect on the migration of dissolved contaminants, since it affects groundwater flow velocities through an inversely proportional relationship.
Darcy's law is commonly applied to study the movement of water, or other fluids through porous media, and constitutes the basis for many hydrogeological analyses.
Water content
Water content is also a directly measurable property; it is the fraction of the total rock which is filled with liquid water. This is also a fraction between 0 and 1, but it must also be less than or equal to the total porosity.The water content is very important in vadose zone hydrology, where the hydraulic conductivity is a strongly nonlinear function of water content; this complicates the solution of the unsaturated groundwater flow equation.
Hydraulic conductivity
Hydraulic conductivity is the ease with which a fluid can move through the pore space, or fracture network. Transmissivity is the product of hydraulic conductivity and the aquifer thickness.Specific storage and specific yield
and its depth-integrated equivalent, storativity, are indirect aquifer properties ; they indicate the amount of groundwater released from storage due to a unit depressurization of a confined aquifer. They are fractions between 0 and 1.Specific yield is also a ratio between 0 and 1 and indicates the amount of water released due to drainage from lowering the water table in an unconfined aquifer. The value for specific yield is less than the value for porosity because some water will remain in the medium even after drainage due to intermolecular forces. Often the porosity or effective porosity is used as an upper bound to the specific yield. Typically Sy is orders of magnitude larger than Ss.