Groundwater Assignment updated

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University of Arkansas *

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Geography

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Feb 20, 2024

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docx

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LAB 6: GROUNDWATER 100 points Due at the beginning of lab next week in physical or digital forms. LEARNING OUTCOMES At the end of this lab, you will be able to 1. Identify what causes groundwater to flow in the subsurface. 2. Describe how water tables and groundwater are affected by pumping. 3. Explain what makes a good, productive aquifer. 4. Describe how contamination moves in an aquifer. INTRODUCTION Groundwater is water that infiltrates through soil and is stored in or moves through openings in rocks and unconsolidated material (e.g., sediment). An aquifer is a permeable body of rock or unconsolidated material that stores and transmits enough water to supply wells or springs (Figure 1). A unit of rock that will store water but not transmit it is called an aquitard or aquiclude. If there is an upper layer that creates a barrier to flow along the top of an aquifer, a confining layer, it is called a confined aquifer. If there is no confining layer present, then it is called an unconfined aquifer. In a confined aquifer the groundwater can build up pressure beyond the weight of water above it (i.e., greater than hydrostatic pressure), forming artesian wells if tapped. The potentiometric surface is a pressure level in a confined aquifer to which water will rise in wells, this is also known as the piezometric level. The potentiometric surface is the combination of the groundwater elevation in a well caused by any built up pressure. Groundwater flows in the direction of decreasing pressure or, if converted to units of length using density and gravity, it is called hydraulic head. Groundwater flows from high hydraulic head to low hydraulic head. The most important parameter for setting how groundwater flows through the subsurface is the hydraulic conductivity (K). Hydraulic conductivity describes the permeability of geologic materials and includes information about the fluid, as well. Geologic materials that allow a lot of water to flow through them are termed permeable, with a relatively higher value of hydraulic conductivity. Examples of permeable materials would be gravels, sandstones, and overall rocks with a lot of connected open spaces (i.e., pores) or high porosity rocks. Lower permeability or impermeable materials are those that do not allow as much water to flow through them (e.g., shales, clays, unfractured granites/basalts). Groundwater is almost always flowing, but it frequently flows very slowly. The average rate of groundwater flow is about the same as how fast your fingernails grow. The best way to think of groundwater flow is how water would move through a column of sand – water moving in between and around grains of sand. A special type of groundwater flow happens in caves, where there are large openings with groundwater flowing more like water in a pipe. That type of flow is not as common, 1
except in karst terrains (i.e., limestone and dolomite rock types that have weathered to form caves and conduits). The normal type of groundwater flow (e.g., through sands) can be described with Darcy’s Law, developed in the mid 1800’s in France by an engineer named Henry Darcy: Darcy’s Law: Q = K*A*Δh/L Q = Discharge or groundwater flow in volume per time (e.g., ml/min) K = Hydraulic conductivity, the ease at which water can move through the ground, measured in length per time (e.g., liters/day) A = Cross-sectional area, the area of the aquifer that is perpendicular to the flow (e.g., inches 2 ) Δh = Hydraulic head change, the vertical difference between two water table measurements (h 2 -h 1 ), units of length (e.g., inches) L = Length, the horizontal distance between the two head or water table measurements (e.g., inches) Figure 1. Diagram and components of the groundwater sand-tank model. 2
LAB 6: GROUNDWATER FLOW MODEL Name: _____________________________ Section: _____________________________ Using your lecture textbook and in-text figures, answer the following questions [100 pts]. Groundwater Flow Model Perform the experiments and answer the questions. 1. Examine the different layers in the groundwater model. a. Describe the geologic material for each layer (gravel, clay, sand, coarse grained or fine grained?). (5 points) b. Rank the materials by hydraulic conductivity (i.e., permeability) from most permeable to least (5 points). c. Is the confining layer permeable? (5 points). 2. Starting with the model drained, the “water table” of the model should be flat and located at the same height as the lowest available outlet of the model (e.g., the lake or river, Outlet 1 in Figure 1). Take one of your water bottles and carefully flip it into the open column on the lefthand side of the model (using the orientation shown in Figure 1). Watch how the water table across the model and in the wells changes. Make sure that you have a bucket receiving the outflow from your lowest model outlet. a. In which well does the water level rise the fastest? And the slowest? (5 points) b. Why do some wells appear to fill faster than others? (5 points) 3
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