Week 7
Groundwater
Introduction
Groundwater represents a very small percentage of the total water on the surface of the earth (0.575%) it is critically important to the population of the earth. Groundwater is one of the major sources of freshwater. Although the groundwater system is constantly recharging itself and does display some ability to clean itself with time, it is not a totally replenishable resource, especially in light of the fact that we are constantly threatening it with pollutants.
The Water Table
Groundwater is found both at the surface and below the surface. Typically, as you drill down into the rgound, you will find that the rock or sediment at the top of the drill column is not saturated with water (in its pore space). As you drill further, you will invariably hit rock or sediment that is saturated, meaning its pore space is filled with water. The Water Table is defined as the boundary between the unsaturated zone, called the vadose zone, and the saturated zone. The transition betweem the saturated and unsaturated zones is called the capillary fringe.
The water table and zones
The Water Table
Two of the important parameters controlling the level and nature of the water table, and the movement and distribution of groundwater are the porosity and permeability of the rock, or sediment.
Porosity is defined as the percentage of the volume of a rock or sediment which is void. Porosity has no units, but is defined as a percent of total volume.The Porosity detemines how much water a material can hold. Loose sand sediment may have a porosity as high as 50%, however a lithified sand deposit, in the form of sandstone, may have a porosity of only about 10%. Some rocks, like granite, have porosity values as low as 1-3%.
Permeability is a measure of how easily water is transmitted through a rock. A rock that allows water to pass through it easily is said to be permeable. Permiablity can be defined interms of a velocity (mm/s) at which water flows through a control column of the material, or can be defined in terms poises (dynamic viscosity). Rocks that have low porosity, tend to have low permeability, unless they are fractured. Most rocks with high porosity, tend to be permeable, although some basalts have high porosity, due to gas vesicles, and low permeability, because the vesciles are not connected.
The level of the water table tends to mimic the ground surface and nearly always reaches the surface where there is standing water, like a lake or stream. The water table is recharged when rain or surface water seepes into the ground. Look at the animation in the link below to see this process occurring.
The body of groundwater below the surface is called an aquifer. Look at the link below to experiment with a virtual aquifer. There are two types of aquifers. Confined aquifers are filled completely with water and are under pressure. They are typically separated from the ground surface by an impermeable containing bed. Confined aquifers have no water table. Unconfined aquifers has a defined water table because it is only partially filled with water. Perched water tables are high standing water tables that are trapped above the regional water table by an impermeable rock layer between it and the lower, regional water table.
Click the link below to visit the virtual water table, and learn about recharging, confined aquifers, perched water tables and artesian wells and springs.
