Study Guide for the Groundwater Final Exam NOTE: Read all questions, apparently identical ones from the mid-term have may have been changed. Also, the final will be graded strictly so prepare your complete answers in advance. Final will have 35 of these questions so be prepared to answer efficiently.

Hydrological Cycle

(6pts) As discussed in class and summarized in the book, the hydrologic cycle describes the storage and transport of water on the earth. Our class examined the cycle in terms of the role of groundwater. One way to think of groundwater within the hydrological cycle is the conservation of mass: Input – Output = Change in Storage. Describe the fluxes to and from groundwater that control the change in water storage (groundwater) in the context of the hydrological cycle. Do not worry about seasonal changes or spatial variations; just discuss the “average” case.

(5pts) Groundwater storage is an important factor in stream flow and the storage is often dependent on the geologic conditions. For two different geologic environments but subject to the same precipitation event, briefly describe the differences in streamflow response caused by different groundwater characteristics resulting from the different geologic settings

(3pts) The hydrologic cycle for Mono Lake was altered due to diversions of stream flow away from the lake to the Los Angeles Aqueduct. What happened to the lake and why?

(5pts) Evaporation and evapotranspiration represent losses from groundwater storage. Define these two processes. Are they the same, different? Also, do they affect all basic types of groundwater aquifers the same, or is one more affected than another? Explain.

(2pts) Explain evapotranspiration and why it varies diurnally and seasonally.

(3pts) How are evaporation, evapotranspiration, and precipitation measured?

(5pts) Discuss infiltration capacity and its effects relative to rainfall intensity. Give two different examples.

(6pts) In the beginning of the quarter we discussed groundwater in terms of infiltration, interflow, and base flow. By the end of the quarter we were talking in terms of unsaturated flow, unconfined aquifers, and confined aquifers.

(5pts) What is the importance of groundwater study? Please include in your description the various sources of water for human consumption, effects of population growth, and climate change.

Surface-water Groundwater Interactions

(6pts) Please draw a stream hydrograph that has responded to a precipitation event. Separate the hydrograph (on the same plot as the hydrograph) into its 3 component parts to illustrate your knowledge of surface-groundwater interactions. Label the axes and each curve. Take some care to insure that the sum of all components equals the total stream hydrograph.

(3pts) Redraw the total stream hydrograph in the previous question for the same precipitation event but for the case of a disconnected stream. Briefly explain why it is different.

(3pts) Redraw the total stream hydrograph if impervious bedrock were exposed at the surface. Explain why it is different from the hydrograph in the previous question.

(3pts) Draw and briefly describe the hyporheic zone and its importance to stream chemistry.

(6pts) Draw and briefly explain the spatial variations in pressure variations at the stream channel boundaries that cause flow in the hyporheic zone.

(6pts) Explain gaining, losing, connected, and disconnected streams. Provide a diagram of each case to help illustrate your answer.

(5pts) Explain the overland flow, interflow, and baseflow and how do they interact to produce stream flow. Also describe where these three flows occur given your understanding of vadose and saturated zones,

(3pts) What are recession curves and how are they useful?

Aquifer Properties

(2pts) Define porosity, be precise:

(4pts) Define secondary porosity and give an example of how it can increase and decrease.

(2pts) Define hydraulic conductivity in words:

(2pts) Give a geologic example of a rock formation with high porosity and low hydraulic conductivity and explain.

(2pts) Give a geologic example of a rock formation with low porosity and high hydraulic conductivity and explain.

(2pts) Generally, porosity is related to permeability, but not always. Why?

(2pts) Define intrinsic permeability:

(3pts) Depict an anisotropic, inhomogeneous aquifer using permeability vectors. Give one geologic example

(3pts) Depict a homogeneous anisotropic aquifer using permeability vectors. Give one geologic example.

(3pts) Define transmissvity in mathematical terms. Identify all terms and give units for each.

(6pts) Define Darcy’s Law in mathematical terms. Identify all terms and give units for each. Be sure that all the units work out. Then describe in words what it means. Also, under what flow conditions does it apply?

(3pts) What is the equation for average pore flow velocity? Identify all terms and give units for each.

(2pts) The hydraulic conductivity for an aquifer consisting of horizontal layers of different conductivity is larger in the horizontal than the vertical. Why?

(5pts) Write out Bernoulli’s equation (equation 4.8 in the book) and explain the physical meaning of each group of terms (there are 3) in it and why their sum can be considered constant.

(3pts) Modify Bernoulli’s equation (equation 4.8 in the book) for the ground water situation, explaining which terms drop out and why. Then explain the physical relevance of the final equation.

(5pts) In groundwater we deal with fluid potential, known as total head, rather than pressure. The reason is that a sloping confined aquifer may have the same pressure at two different locations but the total head is different. Please diagram this situation showing the confining units and the aquifer. Draw two wells and identify total head (h) and the two component parts that make up the head. Remember that we derived this from Bernoulli’s equation for flowing water.

(5pts) Show, using the mathematical expression for fluid potential in groundwater, that water doesn’t flow the box shown below. Do not attempt to draw the potential. The box is for your reference and provides a distance scale. Instead, calculate the fluid potential at various points in the box. Show where the points are. Define where your datum (z=0) is located. The distance interval, defined by the dashed lines is 100m, both in z and in x. The vertical is z and horizontal is x

Arrows point to the water surface. The remaining 3 sides are
impermeable...A vertical baffle separates the near surface.

(5pts) When water is pumped from a confined aquifer, the aquifer is still saturated with water. We can’t get something from nothing, so what is happening? In other words, how does the aquifer medium mechanically respond such that it remains saturated? What is the term used to account for this process?

(4pts) Explain storativity. Give the equation for a confined aquifer, define all the terms and make sure all the units work out.

(3pts) Specific yield is a characteristic of unconfined aquifers. Briefly describe what it is and the physical process that it accounts for. Also, how does it differ from storativity?

(5pts) What is the main difference between the boundary conditions of unconfined and confined aquifers? Explain its relevance. That is, why do we care? Further, under what circumstances might you use water from an unconfined aquifer versus a confined one (and vice versa) given the potentially large difference in cost in drilling one versus the other?

(4pts) Looking at the big picture, is the volume of water from a confined aquifer limitless? What are the practical limitations to obtaining water from the aquifer over the long term? How is it recharged? (Input-Output = storage change).

Flow Equations

(6pts) What are the time-dependent and steady-state (Laplace) forms of the groundwater flow equation for confined aquifers in 3-dimensions? Define all terms.

(10pts) Using the groundwater flow equation in one dimension, evaluate the following two situations. Which is in steady-state and which is not? Explain your answer for both in mathematical and in words.

h
x x

(5pts) Explain the following equation in words:

What does the left hand side of the equation mean, physically, and what does the right hand side mean physically? Give one graphical example of how it applies and explain.

(5pts) Give the numerical form of the steady-state Laplace’s Equation.

(3pts) The flow equations differs between confined and unconfined aquifers. Why?

Solutions to the Groundwater Flow Equations

(5pts) One solution to the groundwater flow equations is graphical, using streamlines and equipotential lines. Describe the concept of each line and explain why this solution only works for steady flow and not for time-dependent flow. What is the geometric relationship between the streamlines and equipotential lines for an isotropic aquifer and an anisotropic aquifer?

(3pts) When water flows from one stratum to another with different hydraulic conductivities, why do the flow lines refract?

(5pts) We also solved the Laplace equation using the finite difference form. Please take one of the derivative terms in Laplace’s equation and express it in the finite difference form. In so doing, show how the finite difference form results from a difference in gradients (slopes). It will probably help to draw and label 3 nodes in a finite difference grid.

Pump Tests and Modeling

(4 pts). Pumping water from wells is great. If you do it right, not only do you get the water but you can also use the well to infer some hydraulic properties of the aquifer. Two basic interpretations of pump tests are the Theis and Jacob methods. Both methods apply to non-equilibrium conditions and require the use of the well function {W(u)}. But each method is different in their sensitivity to the aquifer. How so? Although both methods typically give similar results, what characteristic of an aquifer would cause the results to differ?

(6 pts) Briefly describe the fundamentals of the slug method to test the hydraulic properties of an aquifer. Also describe one advantage and one limitation to the method.

(6 pts) Explain the general idea behind the use of image wells to calculate well draw down near a hydrologic boundary. Also, draw an example of how you use an image well to estimate draw down near a no-flow (impermeable) boundary. Please draw the figure carefully and label the important components. Be sure to show the resulting curve of the final estimated draw down.

Unsaturated Zone

(10 pts) For a homogeneous and isotropic medium compare the hydraulic characteristics between unsaturated and saturated zones. For the first 4 characteristics identify whether it is constant (C) or variable (V) in each zone. For the last characteristic (pressure head) define the controlling factor. In other words, the pressure head is a function of what?

Hydraulic Characteristic Unsaturated Saturated

porosity

water content

hydraulic conductivity

pressure head (y)

(6 pts). One of the difficulties in the unsaturated zone is the hysteresis between pressure head and water content. Please draw a graph of this relation, labeling the axis and important parts of the curve. Also, explain its importance to water flow and modeling water flow in the unsaturated zone.

(3 pts). What is the fundamental physical process that makes the flow processes in the unsaturated zone so different from the saturated zone? Explain.

Hydrogeology

(6 pts). This requires 3 answers to illustrate your knowledge of storativity. Which aquifer would tend to have a greater storativity, one composed of silt/sand or one of competent rock? Why? How would this relate to the effects of pumping in a flood plain compared to an adjacent area of bedrock?

(6 pts). The hydraulic conductivity typically varies over several orders of magnitude within the same aquifer. What natural geologic conditions during the formation of the aquifer cause the hydraulic conductivity to be anisotropic and non-homogeneous? Draw a geologic cross-section that causes anisotropic and non-homogeneous groundwater conditions. Be sure to label the cross-section and include the conductivity vectors. Also describe in words the processes that formed the cross-section you drew.

(4 pts) Compare the important hydrogeologic differences between an unconsolidated aquifer composed of valley alluvium and a lithified aquifer of carbonate rock that includes karst.

(4 pts) Water flow in the Columbia River Basalt (CRB) formations of Washington and Oregon is complex. What are the main causes of this complexity? That is, what are the geologic structures in the CRB’s that cause complex water flow paths?

(4 pts) How does the groundwater geology of the Upper Deschutes Basin influence the flow of Deschutes River? Include in your answer a short description of the recharge zone and the role of the irrigation canals.

(3 pts) What is permafrost and how does it affect groundwater?

(4 pts) Tell me about the Ogallala Aquifer.