Lecture notes -­‐ Bill Engstrom: Instructor Groundwater GLG 101 – Physical Geology In the hydrologic cycle, what happens to the water that infiltrates into the ground? It becomes groundwater. Fresh water amounts to less than 3% of the water on Earth, and groundwater is only 0.62% of the water on Earth. Although it is a relatively small amount, in the United States, 21% of the fresh water we use comes from groundwater, and in Arizona 50% of our fresh water comes from groundwater. That’s why it is so important, particularly in the dry/arid southwest. What happens to water precipitating on ground surface? •
Over 50% evaporates into atmosphere •
Large amount also transpires from vegetation •
Water infiltrates into the ground from streams (15-­‐20%) and precipitation on Earth’s surface, ending up as groundwater – water flowing underground •
Water that does not infiltrate from streams ends up as runoff to lakes/oceans These are some of the main factors that influence how much water can get into the ground •
Slope – steepness •
Amount of precipitation – rain/snow •
Amount of vegetation •
Surface material – must be porous & permeable Porosity & Permeability are important factors in groundwater movement and storage Porosity = The amount of empty space in a rock measured in percent (%) •
Unconsolidated material= sediment or soil (e.g. gravel, sand and clay) -­‐ porous 25-­‐80% •
Clastic rocks are generally less porous (compaction/cementation). These are some average porosities by rock type. •
conglomerates & sandstones (10 to 30%) •
mudstones (0 to 30%) •
Crystalline rocks (igneous/meta/chem sed) = low porosity-­‐increases with fractures Permeability-­‐ the capacity/ability of a rock to transmit a fluid (allow the flow of water within it). •
Pores or fractures are connected with each other. •
Rock must be permeable to allow the infiltration and flow of water. •
Unconsolidated sands/gravels – These are very permeable •
Clastic sandstones/conglomerates – These are permeable to moderately permeable •
Clay, mudstones, and unfractured crystalline rocks – These are not very permeable. •
Fractured rocks -­‐ These may be permeable if the fracture systems are connected. Important note: A high porosity does not always mean there is a high permeability. For example, clay=low permeability and high porosity, and sand=higher permeability and higher porosity. Aquifer-­‐Aquitard-­‐Aquiclude. We define the rock units by their ability to retain or transmit water. Aquifer -­‐-­‐ permeable rock strata or sediment that transmits groundwater freely (e.g. sands and gravels or fractured limestone). An aquifer is permeable enough to allow for the storage, flow and pumping (extraction) of groundwater. Aquitard -­‐-­‐ an impermeable layer that hinders or prevents water movement (e.g. clay) Aquiclude -­‐-­‐ not porous or permeable and prohibits the passage of groundwater (e.g. unfractured crystalline rocks) Now that we know what the main rock units are called, we can look at the “zones” beneath the surface that form when water infiltrates. Zone of saturation -­‐-­‐ Water reaches a zone where all of the open spaces in sediment and rock are completely filled with water. •
Water within the pores is called groundwater. •
Water table —the upper limit of the zone of saturation Unsaturated zone (Vadose Zone) •
Area above the water table •
Open space is partly occupied by water •
Water cannot be pumped by wells. •
Includes the capillary fringe and the belt of soil moisture Ø Belt of soil moisture — This “belt” is formed by water that is held by molecular attraction on soil particles in the near-­‐surface zone Ø Capillary fringe -­‐ Extends upward from the water table. Groundwater is held by surface tension in tiny passages between grains of soil or sediment. Storage and Movement of Groundwater. These are the terms used to describe the addition or loss of water from the groundwater system, and the flow of water within it. •
Recharge = the addition of water to the ground; where water enters the groundwater •
Discharge = a place where groundwater is lost to the surface •
Groundwater flow = Flows from high pressure to low (usually from recharge to discharge areas) Movement of groundwater •
Movement is exceedingly slow —Typical rate of movement is a few centimeters per day. •
Energy for the movement is provided by the force of gravity. We use Darcy’s law to describe the movement. Darcy was a French engineer who lived in the mid 16th century. Here is a brief description of what his law means and the terms used to describe groundwater movement. Darcy’s law says that if permeability remains uniform, the velocity of groundwater will increase as the slope of the water table increases. In other words, if you think of the water as moving downhill, it moves faster if the hill (water table slope) is steeper. Hydraulic gradient is the water table slope and it is determined by dividing the vertical difference between the recharge and discharge points by the length of flow between these points. This is similar to how you would determine the slope of a hill Hydraulic head —the vertical difference between the recharge and discharge points Darcy also found that flow velocity varied with permeability of sediment/aquifer – faster with higher permeability Darcy put it all together with this formula…. You don’t need to memorize the formula. Discharge (Q) – volume of water flowing through an aquifer KA(h1 − h2 )
Q=
d
Hydraulic conductivity(K) = a factor based on permeability and fluid viscosity h1-­‐ h2/d = hydraulic gradient Cross sectional area of aquifer = A The “bottom line” for all of this…… With increased slope (hydraulic gradient) and permeability….. have increased discharge (volume of water flowing through aquifer) Groundwater Elevations (water table) and Contours. The top of the water table can be contoured, like a topographic contour map of a hill. Groundwater flow is perpendicular to contours. Groundwater Flow Systems -­‐local and regional. Now we can look at how this all works to create flow systems and review some of the features you may already be familiar with, like springs, wells and caves. Springs •
Occur where the water table intersects Earth’s surface •
Natural outflow of groundwater How they form: These may form as an aquitard creates a localized zone of saturation, which is called a perched water table. They can also be related to fractures or cave systems that intersect the surface. There are two primary types of aquifers (unconfined and confined) Aquifer Types -­‐ Unconfined (Open) Aquifer •
Recharge occurs over the entire area-­‐ open to receiving infiltration •
Change of Water Table With Climate (Water table rises & falls seasonally) There are 3 basic types of interactions between the water table and streams 1. Gaining streams — gain water from the inflow of groundwater through the streambed 2. Losing streams — lose water to the groundwater system by outflow through the streambed 3. Combination gaining & losing — Stream gains in some sections and loses in other areas. Important notes: Gaining Streams – common in wet / humid climates Losing Streams – common in arid (dry) climates Aquifer Types -­‐ Confined •
Confined aquifers are bounded above and below by aquicludes or aquitards. •
Water is under pressure from overlying rock and hydraulic pressure from recharge. In confined aquifers we get artesian wells and springs. Artesian well —a situation in which groundwater under pressure rises above the level of the aquifer Types of artesian wells •
Nonflowing — Pressure surface is below ground level. •
Flowing — Pressure surface is above the ground. •
Not all artesian systems are wells; artesian springs also exist. Artificial Artesian Pressure for municipal supplies. When you see a water tower or tank you notice that it is high up in the air or in elevation above the surrounding ground. This creates an artificial artesian situation, where the discharge points in the water supply (aka your faucet) are all below the water level in the tank, creating pressure so the water will flow. Here is some more information about groundwater wells…. To ensure a continuous supply of water, a well must penetrate below the water table. Pumping of wells can cause: •
Drawdown (lowering) of the water table •
Cone of depression in the water table Groundwater Contamination (Pollution). We can’t review groundwater without mentioning groundwater pollution. This is a great concern, particularly in Arizona where we depend so heavily on groundwater for our drinking water supply. Water can infiltrate, and so can contaminants. This can happen from a variety of sources, such as landfills, gas stations, industrial wastewater and domestic wastewater discharges, etc. Contaminants percolate downward and dissolve in groundwater. If enough of a pollutant percolates downward, some of it may not dissolve right away. Instead, the pollutant may “float” (e.g. gasoline) or “sink” (e.g. chemical solvents) depending on whether they are lighter (less dense) or heavier (more dense) than water. Once contaminants dissolve, groundwater and pollutants flow “downgradient”in what is called a “plume”. Pumping can also create a lowering of the water table, which can draw pollutants into a well. The aquifer type also plays a role in the movement of pollutants. For example, there can be rapid movement in fractured limestones, leaving less time for the groundwater to be “purified”. Movement is slower in sandstones, leaving more time for purification. In addition, some pollutants degrade/break down more quickly than others, and many break down naturally. However, it can take decades or much longer for some to degrade. Once contaminants are located, decisions can be made about protecting drinking water supply wells, and whether or not the additional cleanup (remediation) is necessary. If natural processes are not enough, man-­‐made technologies may need to be used to reduce or remove the contaminants. Saltwater contamination is another problem that can be associated with groundwater withdrawl. •
Excessive groundwater withdrawal causes saltwater to be drawn into wells (saltwater intrusion), thus contaminating the freshwater supply. •
Primarily a problem in coastal areas Additional Problems Associated with Groundwater Withdrawal •
Treating groundwater as a nonrenewable resource. We need to understand that groundwater is not in unlimited supply. In many places, the water available to recharge the aquifer falls significantly short of the amount being withdrawn. •
Subsidence -­‐ The ground sinks when water is pumped from wells faster than natural recharge processes can replace it (called “overdraft”). This is a problem in many areas, including Arizona. Earth cracks (fissures) can be formed, particularly in arid climates. Geologic Work of Groundwater Groundwater dissolves rock – REVIEW. Remember that….. •
Groundwater is often mildly acidic. •
Contains weak carbonic acid -­‐ Forms when rainwater dissolves carbon dioxide from the air and from decaying plants •
Carbonic acid reacts with calcite in limestone to form calcium bicarbonate, a soluble material. Caverns •
Most caverns are essentially “holes/empty space” created by acidic groundwater dissolving soluble rock (calcite/limestone) at or just below the surface in the zone of saturation. •
They form in the zone of saturation. Features found within caverns •
Composed of dripstone (travertine) •
Calcite deposited as dripping water evaporates. •
Collectively, they are called speleothems. •
Includes stalactites (hanging from the ceiling) and stalagmites (form on the floor of a cavern) Karst topography -­‐ Landscapes that have been shaped mainly by the dissolving power of groundwater Some common features include: •
Irregular terrain •
Sinkholes or sinks (formed by groundwater slowly dissolving the bedrock often accompanied by collapse). Sinkholes can develop when cave roofs collapse. Sometimes this has occurred in residential areas (e.g. in Florida) •
Striking lack of surface drainage (streams) Hot Springs and Geysers Hot springs •
Water is 6 oC to 9 oC warmer than the mean annual air temperature of the locality. •
The water for most hot springs is heated by the cooling of igneous rock. Geysers •
Intermittent hot springs •
Water erupts with great force. •
Occur where extensive underground chambers exist within hot igneous rock •
Groundwater heats, expands, changes to steam, and erupts. Geothermal energy – Just as geysers are created from volcanism, steam can be used in areas that are naturally hot to drive turbine generators to produce electricity. For example, in Iceland, which is located above the mid-­‐Atlantic ridge, geothermal is a major source of energy. 8/2011