Hydrogeology
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.
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.
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 (the upper 3 m) is pertinent to the fields of soil science, agriculture, and civil engineering, as well as to hydrogeology.
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.
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.
[4] 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.
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.
Porosity (n) 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.
For instance, an unfractured rock unit may have a high porosity (it has many holes between its constituent grains), but a low permeability (none of the pores are connected).
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.
Hydrodynamic dispersivity (αL, αT) is an empirical factor which quantifies how much contaminants stray away from the path of the groundwater which is carrying it.
[11] Diffusion is a fundamental physical phenomenon, which Albert Einstein characterized as Brownian motion, that describes the random thermal movement of molecules and small particles in gases and liquids.
The diffusion coefficient, D[clarification needed], is typically quite small, and its effect can often be neglected (unless groundwater flow velocities are extremely low, as they are in clay aquitards).
Unlike diffusion and dispersion, which simply spread the contaminant, the retardation factor changes its global average velocity, so that it can be much slower than that of water.
These experiments led to the determination of Darcy's law, which describes fluid flow through a medium with high levels of porosity.
The most common means of analytically solving the diffusion equation in the hydrogeology literature are: No matter which method we use to solve the groundwater flow equation, we need both initial conditions (heads at time (t) = 0) and boundary conditions (representing either the physical boundaries of the domain, or an approximation of the domain beyond that point).
Analytic methods typically use the structure of mathematics to arrive at a simple, elegant solution, but the required derivation for all but the simplest domain geometries can be quite complex (involving non-standard coordinates, conformal mapping, etc.).
A quick survey of the main numerical methods used in hydrogeology, and some of the most basic principles are shown below and further discussed in the Groundwater model article.
For example, the first-order time derivative is often approximated using the following forward finite difference, where the subscripts indicate a discrete time location, The forward finite difference approximation is unconditionally stable, but leads to an implicit set of equations (that must be solved using matrix methods, e.g. LU or Cholesky decomposition).
[18][19][full citation needed] Similar to the finite difference method, values are calculated at discrete places on a meshed geometry.
PORFLOW software package is a comprehensive mathematical model for simulation of Ground Water Flow and Nuclear Waste Management developed by Analytic & Computational Research, Inc., ACRi.
This versatile porous flow simulator includes capabilities to model multiphase, thermal, stress, and multicomponent reactive chemistry.
The BEM and AEM exactly solve the groundwater flow equation (perfect mass balance), while approximating the boundary conditions.
As trash is buried, harmful chemicals can migrate from the garbage and into the surrounding groundwater if the protective base layer is cracked or otherwise damaged.
In Owens Valley in central California, groundwater was pumped for use in fish farms, which resulted in the death of local meadows and other ecosystems.
For example, a solar project in San Bernardino County would allegedly threaten the ecosystem of bird and wildlife species because of its use of up to 1.3 million cubic meters of groundwater, which could impact Harper Lake.
50 years ago, the sustainability of these systems on a larger scale began to come into consideration, becoming one of the main focuses of groundwater engineering.
This has made highly complex and individualized water cycle models possible, which has helped to make groundwater sustainability more applicable to specific situations.
As technology continues to progress, the simulations will increase in accuracy and allow for more complex studies and projects in groundwater engineering.
