Geology 101, Spring 2008 Name(s): Lab 10: Groundwater and resource extraction Groundwater As many parts of the world are becoming increasingly aware, groundwater is an economic resource, as strategic and necessary as petroleum and hard-rock mineral deposits. In the Middle East, for example, the Israeli-occupied West Bank of the Jordan River is disputed territory in part because of the groundwater reserves there. Closer to home, the fossil Snoqualmie aquifer is being tapped heavily by the expansion of the towns of North Bend and Snoqualmie; as a fossil aquifer, it has no chance of being recharged and problems will ensue when that resource runs out. In almost all cases, groundwater does not flow as “underground rivers”. Most commonly, it flows within the pore space between sediment grains. Groundwater can flow more rapidly through fractured rock or through cave systems in carbonate (mostly limestone) rocks. The amount of pore space in a rock is called its porosity; how well these pore spaces are connected to each other is defined as permeability. 1. Compare sediment samples W8 and W9 in the drawer. a. Describe the grain size of each. W8 W9 b. Which sample is more porous? 2. Compare sediment samples W10 and W11 in the drawer. a. Describe the sorting (well-sorted to poorly-sorted) of each. W10 W11 b. Which sample is more porous? c. Which sample is more permeable? 3. Based on your observations on the questions above, circle the characteristics of a good sediment for an aquifer: poorly-sorted or well-sorted coarse-grained or fine-grained poorly-cemented or well-cementedSurface water seeps into a layer of sediment or rock below the ground surface (such as the glacial gravels in many North Seattle backyards) and becomes groundwater. The water continues to seep deeper into the ground until it reaches the saturated part of the sediment (remember the B soil horizon?) or rock unit. The top of the zone of saturated sediment or rock is called the water table. The water table often, but not always reflects the ground surface topography, lying at higher elevations under hills than under valleys (see the figure below). Figure 10-1. Diagram showing the relationship of the water table (shaded) with the ground surface (upper line). Barely visible curved arrows show direction of water flow. Groundwater flow Groundwater, like surface water, flows downhill, which in the case of groundwater is defined as the direction of the lower water table (recall that the water table is not flat, but is higher under hills than valleys). Unlike surface water, groundwater encounters far more friction and resistance to flow, and thus moves much more slowly than surface water, which is why the water table does not “level out”. In the figure above, the groundwater will flow from under the hill toward the stream valley; some of the water will flow out of the ground and into the stream. The surface of the water table can be mapped using water wells — the height to which water rises in a well is the height of the water table there (note that you will need a pump to bring the water the rest of the way to the surface). Surface water features such as streams and lakes also show where the water table is — the shore of a stream or lake is coincidentally the water table elevation in that area. A spring or seep is an area where the water table is at ground level and thus water comes literally out of the ground. The level of the water table depends on the amount of water soaking (percolating) into the ground versus the amount of water being removed via streams, springs and wells. If more water is removed than is recharged, the water table will drop. For example, irrigation projects have so depleted the Central Valley of California’s aquifer that the water table has dropped 100 feet in some places. A body of rock or sediment that yields useful amounts of water is called an aquifer. Typically, aquifers are made of gravels, sands or sandstones. A body of rock or sediment through which water cannot flow is called an aquiclude. Typical aquiclude materials are clays, shales, igneous and metamorphic rocks. An aquifer that is at the surface is called an unconfined aquifer. An aquifer that lies below an aquiclude is called a confined aquifer. Note that a confined aquifer is not necessarily closed off from recharge by rain or snow; “confined” simply means that at some points, the aquifer is under an aquiclude, though the recharge area of the aquifermay be open to the air (see the figure on the next page). A fossil aquifer is a confined aquifer with no recharge area. Figure 10-2. The mechanics of a confined aquifer reveal how it can be confined and recharged at the same time, and where an artesian well might be drilled. Because the recharge area of a confined aquifer may be many hundreds of feet in elevation above other parts of the aquifer, and because even groundwater tries to run “downhill”, the lower parts of a confined aquifer can build up a significant amount of water pressure. If a well were drilled through the aquiclude into the confined aquifer, it is quite possible that the pressure (called the hydraulic head) might be sufficient to bring water to the surface without a pump; this is called an artesian well. An artesian spring is a naturally-occurring artesian well. The pressure of the confined aquifer can be measured by drilling a special type of well called a piezometer into the aquifer. By measuring the pressure in the well (really the level to which the water will rise in the well), a potentiometric surface or a hypothetical water table can be mapped, similar to a map of the unconfined water table. Figure 10-3. Four wells drilled near Palm Springs, California.4. On figure 10-3, water levels are shown in black for each of four wells in the Palm Springs, California, area. Assume the shale and granite are impermeable. a. Label the confined aquifer, the unconfined aquifer, and the aquicludes. b. Draw a solid line representing the water table of the unconfined aquifer. c. Draw a dotted line representing the "hypothetical water table" (sometimes called the "potentiometric surface") of the confined aquifer. d. In which direction is the water in the unconfined aquifer flowing? e. In which direction is the water in the confined aquifer flowing? f. In the summer, well A goes dry if it is pumped for more than a few hours. The owner of well A sues