The purpose of this week's lab is to introduce you to the important components of the groundwater system. Water is essential to all life on Earth and particularly important to us as human beings. The typical American uses 50-80 gallons of water everyday. So, one should consider freshwater an important natural resource.
Water can occur on the Earth's surface or under the surface. Water on the surface found in ponds, lakes, rivers or streams is called surface water, whereas water found below the surface is called groundwater. Water can also be stored in solid form as glacial ice.
The hydrologic cycle describes the movement of water from the ocean surface into the atmosphere and ultimately its path back to the oceans. The cycle begins as water is evaporated from the ocean surface (leaving the salt behind). This pure water vapor travels through the atmosphere until conditions are right to form rain droplets (precipitation). The precipitation that falls may be trapped on land surfaces as snow or ice, or drained across the land into ponds, lakes, streams, or rivers forming runoff. Water, which flows over the land surface, is called overland flow.
If water falls onto a porous soil it can infiltrate into the ground via empty spaces called pore spaces. Eventually, the water will reach a depth where all of the pore spaces are filled with water - this is the saturated zone. The top of this zone is the water table. Above the water table is an unsaturated zone. A rock material that contains groundwater is called an aquifer. See this illustration of a generalized aquifer system.
There are several properties of aquifers that determine how much water can be drawn from them. Porosity is the percentage of rock or soil that is void of material. The larger the pore space or the greater their number, the higher the porosity. If pore spaces in sediment or bedrock are interconnected, then water can travel through those easily. This material is said to be permeable. Permeable rock units make good aquifers. This image illustrates some examples of typical porosity and permeabilty for different materials.
An aquifer is a geologic unit that can store and transmit water. Rock units that are known to be good aquifers include unconsolidated sands and gravels, sandstones, limestones and dolomites, basalt flows and fractured plutonic and metamorphic rocks.
Unconfined aquifers are those that are typically close to the surface where groundwater is unhindered by impermeable material. Unconfined aquifer recharge - or replenishment of water - occurs from the downward seepage through the unsaturated zone, through the lateral groundwater flow, or from the upward seepage from underlying bedrock.
A confined or artesian aquifer is one that is bounded above and below by an impermeable layer or confining layer. Clays and silts (mudstone or siltstone) are typically classified as confining layers. Confined aquifer recharge can occur by a very slow downward leakage through a fractured confining layer or in recharge area where the aquifer crops out (meets the ground surface).
A unique situation occurs when ground water flows along sloping units of sediment or rock. This creates a potentiometric surface, which is the surface representative of the level that the water would rise in a water well drilled into that aquifer. If the potentiometric surface of an aquifer is above the land surface, a flowing artesian well may occur. That is, water would flow from the well without a pump. Another unique situation is when a confining unit (such as clay) is found within the unsaturated zone and water is trapped on this forming a saturated zone above the main water table. This is termed a perched aquifer.
