Groundwater flow can be driven by thermal gradients, particularly in areas of high heat flow, such as tectonically active regions (Ingebritsen et al., 2006). This is known as a flowing artesian well. What makes water flow from the recharge areas to the discharge areas? As is the case with surface water, or a ball rolling down a hill, the water flows in the direction of the steepest gradient, meaning that it flows perpendicular to equipotentials. That is the energy that was lost to friction as the groundwater flowed from the top of the hill to the stream. Potentiometric surface map of the Spring Creek Groundwater Basin ca. This is known as the unsaturated zone. The level of water in the hole represents the water table, which is the surface of the saturated zone. The potentiometric map also provides clues about the rate of groundwater flow. Hydraulic Head and the Direction of Groundwater Flow, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Recall that water is flowing in pores where there is friction, which means it takes work to move the water. If you think back to Darcy material and our in-class activity from last week, you will recall that groundwater flow rate depends on the head gradient (i.e. Water movement patterns relative to a contoured water table can be shown by drawing short … In situations where there is an aquitard of limited extent, it is possible for a perched aquifer to exist as shown in Figure 14.7. It is forced upward by the pressure differences, for example, the difference between the 112 and 110 equipotential lines. That is equivalent to 0.000048 m/min, 0.0029 m/hour or 0.069 m/day. Here water behaves in a similar way to any other groundwater, and it flows according to the hydraulic gradient and Darcy’s law. Groundwater flow problems may be solved quickly and easily (with practice) by sketching flow nets. The dashed orange lines are equipotential, meaning lines of equal pressure. equipotential lines) and flow direction (i.e. A mass balance must be performed, and used along with Darcy's law, to arrive at the transient groundwater flow equation.This balance is analogous to the energy balance used in heat transfer to arrive at the heat equation.It is simply a statement of accounting, that for a given control volume, aside from sources or sinks, mass cannot be created or destroyed. Please send comments or suggestions on accessibility to the site editor. Groundwater is below our feet flowing in the sediments or rocks. That means it would take 1,450 days (nearly four years) for water to travel the 100 m from the vicinity of the well to the stream. In fact it would likely take longer than that, because it doesn’t travel in a straight line. Like surface water, groundwater flow moves downward in whichever the direction the water table slopes. Learning Designer: April Millet, The Pennsylvania State University. Diagram courtesy of the Idaho Division of Environmental Quality In a simple one-dimensional Darcy tube experiment, the head gradient is just the difference (h1-h2)/L. In areas with topographic relief, the water table generally follows the land surface, but tends to come closer to surface in valleys, and intersects the surface where there are streams or lakes. It’s critical to understand that groundwater does not flow in underground streams, nor does it form underground lakes. River water generally flows thousands of times faster. Creative Commons Attribution 4.0 International License. The site editor may also be contacted with questions or comments about this Open Educational Resource. River water generally flows thousands of times faster. The red dashed line in Figure 14.6 is the potentiometric surface for the confined aquifer, and it describes the total energy that water is under within the confined aquifer. All flowing fluids are always losing energy to friction with their surroundings. This is illustrated in Figure 14.8. Groundwater moves slowly, and that is a reasonable amount of time for water to move that distance. Caves above the water table are air-filled conduits, and the water that flows within these conduits is not under pressure; it responds only to gravity. It flows from areas of higher hydraulic head to areas of lower hydraulic head, and this means that it can flow “uphill” in many situations. If we assume that the permeability is 0.00001 m/s we get: V = 0.00001 * 0.08 = 0.0000008 m/s. The water loses this gravitational energy as it flows from the recharge area to the discharge area. To first approximation, groundwater flows down-gradient (from high to low hydraulic head). Groundwater flow The direction of groundwater flow follows a curved path through an aquifer from areas of high water levels to areas where water levels are low; that is from below high ground, which are recharge areas, to groundwater discharge points in valleys or the sea. 1 , :: ” 2.6 Recommendations for Remedial Measures > 3. Unit 1: Fresh Water: Scarcity or Surfeit? Groundwater can take over 10 years to move one mile, so depletion and pollution problems are not quickly remedied. Using V = K * i, estimate the velocity of groundwater flow from Joe’s to the stream, and determine how long it might take for contaminated groundwater to flow the 80 m to the stream. The graphical representation of the head (i.e. flowlines) is called a flow net. Source: USGS Scientific Investigations Report 2005-5091. The dashed lines red lines are no-flow boundaries, meaning that water cannot flow across these lines. Each contour, or equipotential, represents a line of equal hydraulic head. Groundwater flows at right angles to the equipotential lines in the same way that water flowing down a slope would flow at right angles to the contour lines. Mass balance. The water in a well drilled into the confined aquifer in this situation would rise above ground level, and flow out, if it’s not capped (well C in Figure 14.6). That’s not because there is something there to stop it, but because there’s no pressure gradient that will cause water to flow in that direction. As shown in Figure 14.6, there is always a water table, and that applies even if the geological materials at the surface have very low permeability. The area at the stream or lake to which the groundwater is flowing is a discharge area. A cross-section of a typical hillside with an unconfined aquifer is illustrated in Figure 14.5. We can apply this equation to the scenario in Figure 14.5. Groundwater tends to flow under the influence of gravity, that is, "downhill". equipotential lines) and flow direction (i.e. 7. She calls in a hydrogeologist to find out how long it might take for the fuel contamination to reach the nearest stream. Darcy’s equation, which has been used widely by hydrogeologists ever since, looks like this: (where V is the velocity of the groundwater flow, K is the hydraulic conductivity, and i is the hydraulic gradient).
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