A Hidden Reserve: Groundwater

Transcription

1 A Hidden Reserve: Groundwater Much of the global H 2 O, resides below the surface of the Earth in what geologists refer to as groundwater. Although in the subsurface, it has a large impact on society and the features that we see on the surface of the Earth. In general, surface water gets into the subsurface by infiltration. The Winter Park, Florida sinkhole Central-pivot irrigation utilizing groundwater, Jordan

2 Groundwater: Why do We Care? Groundwater is an important resource. Drinking water for people and livestock. Irrigation. Industry. It has been used for millennia.

3 The Underground Reservoir Some precipitation enters the subsurface via infiltration. Soil properties and vegetation govern infiltration rate. Infiltrated water adds to soil moisture and groundwater. Soil moisture wets the soil. Some is wicked up by roots, some is evaporated.

4 The Underground Reservoir Some infiltrated water percolates to a deeper level. It is added to water that fills subsurface void spaces. This is groundwater.

5 Porosity Only a very small amount of the global groundwater flows in subterranean rivers or lakes Most groundwater resides in pores, small open spaces within rocks Porosity refers to the total volume of a rock that is empty space and is usually quoted as a percentage (e.g. rock A has 25% porosity). Therefore, the porosity of a rock is the percentage of the rock that is empty space Porosity greatly varies between different rock types

6 Primary vs. Secondary Porosity Primary Porosity This is the porosity of the rock after it first lithifies/forms based on the spaces between grains. Fine grained sediment has a lower porosity because the little grains can fill in the spaces. Crystalline rocks have very low primary porosity. Secondary Porosity New pore space created in the rock at some time after the rock formed E.g. Joints, Faults, Dissolution. Because of secondary porosity any rock could potentially have some porosity.

7 Porosity vs. Permeability If solid rock completely surrounds a water-filled pore, then the water cannot flow. For groundwater to flow, pore spaces must be interconnected. The ability of a rock to allow a fluid to flow through an interconnected network of pores is called Permeability. If a rock has a high porosity, it does not necessarily have a high permeability. The pores must have interconnected conduits! E.g. porous cork, is nearly impermeable Permeability depends on: Number of available conduits Size of conduits Straightness of conduits

8 Aquifers and Aquitards Hydrogeologists distinguish between rocks that transmit water easily and rocks that do not easily transmit water. Aquifer A rock that easily transmits water Aquitard A rock that does not transmit water easily (i.e. retards water motion) Aquiclude A rock that does not transmit water at all Unconfined Aquifer An aquifer that has direct access to the surface of the Earth Can be quickly recharged by meteoric water Confined Aquifer An aquifer that is trapped below an aquitard or aquiclude

9 Hydrogeologic Zones Unsaturated Zone / Vadose Zone The portion of the subsurface where some of the pores are filled with only air. Saturated Zone / Phreatic Zone The portion of the subsurface where the pores are completely filled with water. Water Table The boundary between these two zones How deep does the saturated zone go? Hydrogeologists are not sure At some depth (10-20km) water is utilized for metamorphic reactions.

10 Topography of the Water Table The water table is not a flat surface that never changes It may have seasonal oscillations (wet dry seasons) and rise and fall Underneath mountains and hills, the water table follows a similar but subdued shape

11 Perched Water Tables A locally present aquitard may create a Perched Water Table, a localized phreatic zone (saturated) above the regional water table. Can form springs if there is topography.

12 Recharge and Discharge Groundwater flows downward in areas of Recharge, and upward in areas of Discharge. But what causes groundwater to flow?? Hydraulic Head: A measure of the potential energy available to drive the flow of a given volume of groundwater. Groundwater flows from locations of high hydraulic head to low hydraulic head.

13 Hydraulic Head and Hydraulic Gradients In an unconfined aquifer: Hydraulic head = the weight of the water above it. (Similar to air pressure) Water will always flow from regions of high to low head In a confined aquifer: Hydraulic head is measured by drilling holes into the ground and measuring the level to which water fills the hole. Hydraulic Gradient: the change in head from one location to another. Controls groundwater velocity

14 Groundwater Discharge and Darcy s Law Groundwater discharge: Henry Darcy (French Engineer), coined what we now call Darcy s Law. If you know: the hydraulic gradient (Δh/j) the hydraulic conductivity (K) the area through which the water is flowing (A) Discharge = Q = K(Δh/j)A Sometimes we simplify this and say that Discharge = Slope of Water Table * Permeability

15 How Fast Does Groundwater Flow? Water in an ocean current ~ 3 km/hr (1.8 Mph) Water in a river - up to 30 km/hr (18 Mph) Groundwater m/day (~4-500 m/yr) Why so slow? - Conduits are very curved and small, so groudwater must flow in a very crooked path and friction with conduit walls slows it down. Hydrogeologists measure flow in some regions by injecting tracers (a dye, radioactive element, or bacteria) and monitor its movement. Some groundwater may emerge after months or years, but some may not emerge for thousands to tens of thousands of years.

16 Wells How We Get To Groundwater Since water is important to society, access to groundwater is important. We access groundwater through wells and springs (where groundwater percolates out at the surface of the Earth). An ordinary well penetrates to a depth below the water table where an aquifer allows access to flowing water. We then either pump it out (right) or manually pull up the water (below).

17 Well Drawdown & Cones of Depression When a well is drilled to access an aquifer, drillers like to make the well as shallow as possible to save $. If a well pumps out water faster than it is replaced by normal groundwater flow, it draws down the water table in what is called a Cone of Depression. Cones of depression can make nearby wells temporarily dry. So when drilling a well, drillers must consider both the flow rate in the aquifer and the pumping rates of nearby wells.

18 Artesian Wells In some places, groundwater does not need to be pumped out of a well; if water freely flows out of the well it is called a flowing artesian well. To see why this happens, lets look at a city water supply and water towers. Cities first pump water from the local aquifer/source into a high reservoir tank. This high tank is connected to the houses in town by a network of underground pipes. The pressure in the elevated tank provides the push to make water rise out of the pipes of town. The level to which the water will rise is called the Potentiometric Surface. Therefore, the water company doesn t have to pump water to your house, just to the raised tower. So if you are on city water, you are likely getting it because you have an artesian connection to the city water.

19 Artesian Wells in Nature An artesian well can occur in nature when a well penetrates a confined aquifer that is under great pressure. If the potentiometric surface is above ground, the well will be a flowing artesian well. If the potentiometric surface is above the water table, but below the ground, it will be a non-flowing artesian well.

20 Springs What Conditions Cause Them to Form? Spring A location where groundwater is discharged from the ground The springs can form in various hydrologic scenarios Impermeable layer

21 Springs What Conditions Cause Them to Form? The various conditions shown here cause a spring to form. Note that an Artesian Spring is a natural feature while and artesian well is drilled by man.

22 Oasis...Mirage or Geology? Folded aquifers and faults can cause an Oasis to form. These are important stops for people traveling across the Sahara. Faults can bring deep water up to the surface of the Earth forming a hot spring.

23 Karst Landscapes and Groundwater Groundwater can dissolve calcite bearing rocks such as limestone. When CO 2 mixes with water is makes a weak acid called carbonic acid that speeds this process. Over time, changes in the water table may form complex networks of caves. If a large cave becomes near to the surface of the Earth (usually by erosion), it can collapse forming a sinkhole. Terrain dominated by sinkholes is called Karst Landscape or Karst Topography.

24 How Does Groundwater Flow Through Limestone? Although limestone is nearly impermeable, it is commonly jointed. The joints provide a secondary porosity and allow groundwater to flow through.

25 Karst Landscapes Arecibo Observatory, Puerto Rico

26 Groundwater Infinite or Finite? Although on scales of tens of thousands of years, groundwater is renewable, if usage is high, it can be a big problem on scales of years to hundreds of years. An Industrial well, lowers the water table and dries up a river

27 Large wells can change the direction of groundwater flow moving contaminants into unsafe places. Groundwater Problems

28 Groundwater Problems Saltwater is more dense than freshwater so it stays below the fresh water table. Pumping and drawdown can cause saltwater influx into what would naturally be freshwater aquifers.

29 Groundwater Problems Saltwater influx is a huge problem in Florida.

30 Groundwater Problems Groundwater pressure holds grains of rock apart. When water is removed, the once wet layer may become compacted, causing subsidence above the aquifer.