G102 Groundwater and Structural Geology Module PDF

Groundwater and Structural Geology Module Videos Porosity and Permeability https://www.youtube.com/watch?v=8mfBomrw0rs Transcript: (00:01) Groundwater represents about a quarter of the US freshwater supply. Most of which is used for domestic purposes or for agriculture. And there are two essential...

Groundwater and Structural Geology Module Videos Porosity and Permeability https://www.youtube.com/watch?v=8mfBomrw0rs Transcript: (00:01) Groundwater represents about a quarter of the US freshwater supply. Most of which is used for domestic purposes or for agriculture. And there are two essential properties of groundwater that we really need to investigate to understand how it works and those are porosity and permeability. Today we are going to describe what we mean when we use the term groundwater And we are also going to examine those two properties, porosity and permeability, and how they effect how much groundwater is there and how easy it is to get it out of the ground. (00:27) All the water on Earth is linked together by the hydrologic cycle. In brief, this cycle begins when water evaporates from the oceans. Water vapor rises into the atmosphere and condenses to form clouds. Clouds lose their moisture through precipitation. Rain falling on land can run off into streams and lakes or may infiltrate through the soil and into the rocks or sediment below. (00:49) This is groundwater. We want to examine the properties of earth materials that allow this water to be present underground. When people think of groundwater they often imagine water flowing through a cave system or maybe an underground lake While groundwater does exist in these forms, it is not that common. (01:04) Most usable groundwater is actually stored in the tiny spaces between grains of sand and gravel. Porosity and permeability control the distribution of this water. We will consider each of these separately, starting with porosity. We have filled this beaker with about 300 milliliters of relatively unsorted gravel. (01:23) Notice that there are grains of different sizes that loosely fill the container, leaving several visible open spaces. These spaces represent the porosity of the sediment. Porosity is the proportion of the volume of an earth material that is composed of void spaces. We can do a brief experiment to determine the proportion of space in the gravel occupied by porosity. (01:45) We have 200 milliliters of water in this smaller beaker. We dyed the water blue with food coloring to make it easier to see. When we pour the water into the beaker, it fills up the empty pore spaces from below and the water eventually rises to the top of the gravel. Now, let’s look to see how much water we used. (02:05) Remember that we started with 200 milliliters and now we have about 80 ml left. So we added 120 milliliters of water to a beaker containing what appeared to be approximately 300 ml of gravel. That tells us that about 40% of the gravel mixture was composed of air spaces that we subsequently filled with water. (02:23) We can try the same experiment with smaller, better sorted sand grains. In this case, it is more difficult to make out spaces between the smaller individual sand particles. What proportion of the sand do you think is composed of empty spaces? This time the sand mixture accommodates 100 ml of water, indicating that the estimated porosity of this sand is about 33%, a little less than that of the gravel. In both cases, the water lies in the spaces between the grains. (02:54) There are a few big, visible spaces in the gravels, and lots of small, less visible spaces in the sand. Depending how well these materials are sorted, they can have similar porosities. These porosity values are not unreasonable for unconsolidated gravels and sands near Earth’s surface. About 80% of shallow groundwater systems in the US are composed of these materials, sand and gravel. (03:19) In most cases we can extract this groundwater using wells, in much the same way that we could extract the water from the gravel mixture using a straw. However, before we make this seem too simple, we have to consider the role that permeability, plays in controlling how groundwater moves through rocks and sediment. (03:38) Permeability represents the capacity of water to flow through earth materials. It is not sufficient that groundwater is present; it must also be able to flow into our well so that we can extract it. Many rocks have pretty good porosity but their permeability values will differ. For example, some igneous rocks contain preserved gas bubbles that are not connected. (03:57) These rocks would have good porosity but low permeability. We have designed a little experiment to demonstrate how permeability varies among gravel, sand and clay, common sediments at or near Earth’s surface We have taken a funnel and filled it with each type of sediment. We added a tiny piece of filter paper to prevent the sediment flowing through the funnel. (04:16) Then we poured a constant amount of water into each set up and watched to see how long it took to collect in the beaker below. The faster the flow of water, the higher the permeability. Let’s see what happened. Let's see what happened. As you can see, water quickly passes through the gravel and almost all of the original water collects in the beaker below. (04:49) Water pools on top of the clay, and is unable to flow downward between the tiny clay particles, making it essentially impermeable at the scale of this experiment. Finally, water passes through the sand more slowly than the gravel and only about 75% of the original water makes it to the beaker during the time of the demonstration. (05:16) The permeability of these three materials decreases as we move from the gravel on the left to the clay on the right. Sand and gravel make for excellent groundwater sources because of their combination of good porosity and permeability. Other materials, such as sandstone, some limestones or fractured igneous rocks may also have high porosity and permeability values and serve as good groundwater reservoirs under specific circumstances. (05:44) Material like clay or fine grained sedimentary rocks like shale, or unfractured metamorphic or igneous rocks such as granite have such low permeability values that they often act as barriers to groundwater flow. We had three learning objectives for today’s lesson, how confident are you that you could complete these tasks? Where is the Water Table? https://www.youtube.com/watch?v=UfgyJkmZgK8 Transcript: (00:01) In this lesson we want to take a close look at the water table, a key feature that gets discussed a lot when we talk about the availability of groundwater. Our learning objective is to use some simple models to define what the water table represents and take you on a short field trip to explain how the position of the water table varies relative to the land surface. (00:21) Water on the land surface may infiltrate through the soil and into the sediment and rock below. This is groundwater. This water is moving slowly downslope, eventually returning to the ocean to complete the hydrologic cycle. For more about the hydrologic cycle, see our video on Porosity and Permeabilty. In order to quantify how much groundwater is available, we need to figure out the position of a feature known as the water table. (00:45) As water infiltrates into the ground it fills up connected spaces in sediment or rock formed by fractures or small gaps between grains known as pores. We are going to add colored water to this gravel-filled beaker to illustrate how the position of the water table is dependent upon which pore spaces are filled with water. (01:08) Where the pore spaces may are full of water is known as the saturated zone. Closer to the surface, the spaces are empty or are only partially filled. These earth materials are in the unsaturated zone. The top of the saturated zone is known as the water table. OK, let’s go and try to find the elevation of the water table in some nearby wells. (01:28) This hillside slopes down to a small pond that you can see through the trees. There are two wells here, one is visible; the other is hidden in the shadows further up the slope. The relative positions of the wells are illustrated on this cross section. If we look around we can find outcrops of bedrock not far from the wells. (01:49) The bedrock here is a type of metamorphic rock. Normally we wouldn’t think of metamorphic rock as an ideal groundwater source as these types of rocks lack interconnected pore spaces. However, as you can see from this outcrop, these rocks contain numerous fractures providing lots of pathways for water to enter the groundwater system. (02:20) We are going to demonstrate how to measure the water level in the wells starting with Well #1. Shining a flashlight down the well shows the reflection off the top of the water. The water level meter is essentially a measuring tape attached to a probe that will make a beeping sound when it touches the water. (02:55) The probe is carefully lowered down the well until.. Now we measure the depth by reading the tape. We will do the same again for Well #2. This time the water level was 3.39 meters below the edge of the casing. So let’s look to see what all this means. The two wells are about 50 meters apart with well #2 about 2 meters higher in elevation. (03:41) From our measurements, we know the depths to the water table in each well. Based on our interpretation we can see that the water table is sloping gently downhill, roughly parallel to the slope of the ground surface. Groundwater is flowing downslope toward the small creek below Well #1. The water table is the minimum depth that we would need to drill to ensure a consistent supply of groundwater. (04:06) The greater the water supply, the higher the elevation of the water table. We expect the water table to show some seasonal fluctuations and to rise during wet periods and fall during dryer months. For example, this graph illustrates how water levels changed in a well in eastern North Carolina. Note that in 2015 the water table was typically within a few feet of the surface but it dropped to nearly 8 feet deep during October before bouncing back due to winter precipitation. (04:36) However, we can see a steady decline in the depth of the water table if groundwater is consumed more rapidly than it is replenished. This graph illustrates ground water levels over the course of more than 40 years for a well in western Kansas. This well is located above the High Plains aquifer which experiences relatively low precipitation and heavy agricultural groundwater consumption. (04:58) The water table was 40 feet below the surface in the 1970s but is around 90 feet deep today. You can find out information about groundwater levels in wells in your state by visiting the US Geological Survey’s Groundwater Watch site. We had two learning objectives for this lesson. How confident are you that you could complete these tasks. What is an Aquifer? https://www.youtube.com/watch?v=g7R0yLX0V9E Transcript: (00:02) In this lesson we want to take a closer look at aquifers. The underground features that supply drinking water for millions of people every day. After the lesson you'll be able to describe two major types of aquifers and identify some of the common materials that make up aquifer systems in the U.S. Let’s start with a definition: An aquifer is an underground body of rock or sediment that serves as a storage reservoir for groundwater. (00:26) Aquifers are made up of materials that contain interconnected spaces. These spaces are essential as they can both store water and let it easily flow in and out of the aquifer. As we will see, depending on where you are located, your local aquifer may be composed of different types of rock or sediment. The opposite of an aquifer is a confining unit. Materials such as clay are have poor porosity and/or permeability and will restrict or prevent the flow of groundwater. (00:55) Layers of clay, shale or other low permeability materials act as a barrier for groundwater flow and may separate aquifer systems. Water may still pass through these layers but much more slowly than through the aquifer. Most aquifers fall into one of two types. Unconfined or open aquifers are directly connected to Earth’s surface. (01:16) In contrast, confined, or closed aquifers, are separated from surrounding rock layers by confining units above and below the aquifer. Let’s take a closer look at each type. Open or unconfined aquifers are supplied by water that filters down from the land surface. It is much like pouring water into a beaker of gravel. The water easily flows between the grains and the level of the water in the beaker is dependent upon the water supply. (01:41) The water table represents the upper surface of an unconfined aquifer. The greater the water supply, the higher the elevation of the water table. We expect the water table to show some short-term fluctuations related to storms and longer-term seasonal variations. There is also the potential for a decline in the depth of the water table if groundwater is consumed more rapidly than it is replenished. (02:08) Water doesn’t enter confined aquifers as easily as it does an unconfined aquifer. The overlying confining layer prevents water from flowing directly into the confined aquifer from above. Instead these aquifers are supplied by streamflow or precipitation in places where the aquifer materials crop out at the surface. (02:31) Groundwater in the confined aquifer is under pressure from water upslope in the same layer. This can produce what are known as artesian wells that shoot the pressured water upward, no pumping needed. This colorful map shows the most significant aquifer systems in the U.S. The type of aquifer present under different locations is largely a consequence of the local geology. (02:54) Lets take a look at some of the most common types of U.S. aquifers. The most common unconfined aquifers are piles of sand and gravel typically found within 100 feet or less of the surface. These shallow aquifers can provide an abundant supply of groundwater but may also be susceptible to contamination. (03:13) Many of these deposits were left behind by glaciers that covered northern states during the last ice age. In comparison to loose sand, sandstone has been compacted and its grains have been cemented together. This reduces the porosity of the original sand deposit but it still represents a pretty good choice for an aquifer in many locations. (03:35) Sandstone aquifers are common in western states, parts of the Midwest and throughout the Appalachians. The line on the map represents the southern edge of glacial deposits. North of the line sand and gravel are more likely to serve as the primary groundwater source than sandstone. Limestone can be dissolved away leaving large spaces to fill with groundwater. (03:57) Aquifers composed of limestone are not as common as some other but can be found in states like Missouri, Ohio and Florida. Groundwater may enter the carbonate aquifer system in Florida in its unconfined state where it is buried at shallow depths and may then flow downslope through connected fractures and cave systems to where it forms a confined aquifer further south. (04:24) Finally, igneous and metamorphic rocks should not be good aquifers as they contain no natural porosity or permeability. However, these rocks are often fractured during their formation or during episodes of tectonism. Fractured lava flows in Oregon, Washington and surrounding states and fractured crystalline igneous and metamorphic rocks in the Appalachians are two examples of these rock types that can result in productive aquifer systems. (04:52) So water flows through the majority of U.S. aquifer systems by way of natural porosity dissolved spaces or sets of fractures sand and gravel may make up simple unconfined aquifers but confined aquifers can contribute to groundwater supply in many locations. These are our learning objectives for today How confident are you that you can respond to these statements? Science of Folding and Faulting https://www.youtube.com/watch?v=F2-fHccQUb0 Transcript: (00:01) hello everyone this is our science teacher Tim Martin and in this video I want to introduce you to the geologic concepts of folding and faulting when we look for faults and folds in the earth what we're really doing is looking for past evidence of plate tectonic motion let's take a minute to review the basic motions of plate tectonics the first of the three motions is divergent motion this occurs when magma comes up from deep within the earth this typically happens in the middle of the ocean it generates earthquakes as the platelets (00:33) move apart from each other this may also happen on continental plates as well the next type of motion convergent motion happens when it plates collide here we can see an oceanic plate subducting beneath a continental plate has the plates abducts it cracks and breaks generating earthquakes finally we have transform motion this occurs where tooth plates slide or grind past each other again this generates earthquakes now let's take a look at the structures identified with each of these three types of motion to start off with let's (01:08) talk about rifts or cracks when the earth is pulling apart or forces are acting on it in opposite directions the crust will thin stretch and possibly crack these cracks are often known as rifts this is caused by tension or divergent motion a normal fault also happens with tension or divergent motion if the rock is more rigid and less likely to stretch it may simply crack and when it does one block may drop lower than the other this is what's called a normal fault opposite the normal home we have a thrust or reverse fault this occurs from (01:47) compression or convergent motion when two plates are pushing against each other one plate may go down or another one you may go up this creates a thrust or reverse fault finally a transform fault where a strike-slip fault occurs with lateral stress when two parts of the Earth's surface are moving in opposite direction with respect to each other one may slide causing a transform or a strike-slip offset while sometimes app rocks break with faults sometimes they're soft enough to fold a fold occurs with compression or convergent (02:26) motion sometimes rock layers may fold with an upward Bend other times they may form with a downward Bend both types of folds occur with compression or convergent motion and up fold is referred to as an anticline the downward or u-shaped fold is referred to as a sim climb let's take a look at what this looks like with real geology can you identify the motion from these real images let's take a look at these five different features Fingal your national park in iceland is one of the classic locations where the earth is splitting (03:02) apart on the right hand side of the picture we see the North American plate on the left side the European plate as the plates are splitting apart this rift or gap is opening up this occurred through divergent motion this picture of the Hayward Fault was taken in Fremont California in Central Park you can see when the road planners built this road they didn't intentionally offset the sidewalk in curving what happened here is part of the road was built on the North American plate and the other part of the road was built on the Pacific (03:34) plate here's the fault line and we can see the further section is moving northward where the side that the photographer is standing on is moving towards the south my own fault is a dramatic fault that was revealed in a road cut just outside of Arches National Park in Utah here we can see several very large faults as we identify the previously aligned layers we can see that this brown stripe used to all be the same layer but now has been offset with several normal faults this represents divergent motion or tension (04:07) Browns Canyon of the arkansas river goes through Salida Colorado here we can see rock layers that had been folded this must have occurred through compression or convergent motion it's impossible to make a fold by pulling going from a large scale to a rather small scale in mosaic canyon in Death Valley National Park California we can see this small layer was offset The Fault indicated in red and these yellow layers are offset with a little bit of overlap this indicates that this was a reverse fault caused from compression or (04:42) convergent motion so let's take a little quiz can you identify the motion in these next five images feel free to pause the video between each image number one is kotoba knob located in the Cedar Mountain recreation area in the San rafeal swell in Utah number two is Rock structure that I found at noosa heads in Queensland Australia number three a road cut near order Ville Utah number four rock layers west of Wild Rose campground on Wild Rose Road in Death Valley National Park number five is specifically called (05:25) earthquake fault located just outside Mammoth Mountain ski area in Mammoth Lakes California so could you identify all five of these images what sort of plate motion caused each are they normal faults reverse faults folds or rifts let's take a look at each number one kotoba knob in the San rafeal swell in Utah first we identify the fault plane located here then we identified the layers that used to line up here and here so we can see because of the overlap this must have formed from compression or convergent motion (06:01) number two noosa heads in Australia again the first thing to it do is identify the fault plane located here then the layers that you as they used to match up here and here because of the lateral offset we understand this must have been transformed or a strike-slip fault number three again this is a road cut in order Ville Utah let's identify the fault plane leaning from left to right across the image see the double brown lines towards the bottom of the image these used to match up with each other thus this is a normal fault formed (06:37) through tension or divergent motion number four along wildrose road in Death Valley National Park we can obviously see folds these folds must have formed from compression or convergent motion finally number five the earthquake fault near Mammoth Lakes California again this is a great example of a rift where the earth was experiencing tension and cracked open causing this split in the rocks would you like a little more practice identifying folds and faults you can download a slideshow of images that I've taken from around the world of (07:14) places where we can see convergent-divergent and transform plate motion thanks for watching and I hope to see you again on another team art science video Lastly… Webpage I want the students to check out to get an idea of how complicated plate tectonics can be. https://serc.carleton.edu/NAGTWorkshops/geophysics/visualizations/PTMovements.html