Introduction
All too frequently today, we hear about trucks or railroad tank cars that accidentally overturn, spilling their contents onto the ground. If the material spilled is liquid, it can easily soak into the ground or flow into a nearby body of water. The environmental concern here is that the liquid could reach into water supplies that people, livestock, or wildlife use and contaminate it with potentially toxic or harmful compounds. If such a spill does occur, people need to react quickly to clean it up and to prevent movement of the liquid from the spill site to the surrounding water supply. As more and more chemical substances are manufactured and transported around the world, spills of this kind will likely increase. Dealing with these serious environmental problems requires a variety of skills, among them the ability to assess possible subsurface water contamination. To do this, environmental geologists rely on their knowledge of groundwater dynamics and movement, properties of soils and earth materials, and general topography and subsurface structures.

Understanding Groundwater
The earth’s surface is covered by tremendous quantities of water, including those waters
exposed in rivers, lakes, and oceans, and even frozen in glaciers and polar ice caps. But deep beneath the earth’s surface lies another important storehouse of this precious resource that often goes unrecognized. While some precipitation falls to earth and runs off into streams and rivers, another portion seeps slowly through the soil into the upper layers of the earth’s crust, filling the air-filled voids, or pores, between rock and sediment particles. This underground water is called groundwater.

To better understand how groundwater is stored and transported, consider a jar full of sand (see Figure 1). The spaces between the sand grains are the pores. As water is poured into the sand, it seeps down to the bottom of the jar and eventually starts to fill up all the pore spaces in the sand from the bottom up. The pore spaces in the sand in Zone A become completely filled with water up to Point Y. The term used to describe the region where all the pore spaces are filled with water is called the saturated zone. Zone B is called the unsaturated zone or zone of aeration. Here, some of the spaces are filled with water and the rest are filled with air. The surface between the saturated and unsaturated zones is called the water table.

Groundwater is stored underground in the pore spaces of saturated soil and rock materials. An underground unit of soil or rock through which large volumes of water can flow and be stored is called an aquifer. Groundwater flows through interconnected pore spaces in aquifers. Using the example from Figure 1, visualize what happens when the jar is tilted and the water runs out of the zone of saturation (see Figure 2). In this case, the water is flowing out of the jar, demonstrating that water can move through interconnected pores in an aquifer. Eventually, the saturated zone in the jar will be drained dry.

Groundwater may flow at different rates in different types of aquifers. Aquifers are not always uniform, either horizontally or vertically, because of differences in composition and other properties. Aquifers may also be separated by layers of rock or soil that do not transmit much water. These layers are called confining layers or aquitards. If a confining layer exists above an aquifer that is fully saturated, this aquifer is called a confined or artesian aquifer. Aquifers without a confining layer above them are called unconfined aquifers.

Groundwater Movement
When studying aquifers and aquitards, geologists need to know something about the
subsurface geology. The term “profile” is often used to describe the shape or slope of the land, but it is also used to describe the shape of a water table surface. Remember the term “water table” describes the top of the zone of saturation. Generally, the water table profile is a replica of the surface topography, being deeper below the surface under hills and closer to the surface in areas where water flows out to form springs or accumulate in depressions to form ponds or supply water to rivers. Notice also that the water table surface slopes (see Figure 3).

Hydraulic gradient is the term used to express the slope of the water table, and is equal to the vertical distance between two points on the water table divided by the horizontal distance between these two points. The vertical measurement is called the hydraulic head.

Since surface water moves downslope from higher elevations to lower, it stands to reason that groundwater can be expected to move through an aquifer in a similar manner. Water flows through the zone of saturation; it is not stagnant in the ground. This is why water flows from a spring. In fact, to see how fast water can seep or flow through an aquifer, geologists can inject a harmless dye into a well (e.g. Point X on Figure 3) and measure the time it takes for the dye to appear in the water coming out at the spring near the pond. This downslope movement of groundwater is an important principle to keep in mind when determining the direction of flow for polluted water in the groundwater system.

The type of aquifer shown in Figure 3 is called an unconfined aquifer. Notice in this case the porous soil or rock extends right up to the earth’s surface. Surface water seeps slowly through interconnected pores or fractures in soil and rock until it reaches a zone where it saturates the soil or rock to create an aquifer. This seeping process is called recharge. The area on the surface where the water begins its journey into the aquifer is called the recharge area. Figure 4 depicts another type of aquifer known as a confined aquifer. Here the layer saturated with water is sandwiched between two aquitards. Notice the difference between these two types of aquifers. In the unconfined aquifer, there is no overlying aquitard and there is a defined water table at the upper surface of the saturated zone. On the other hand, the entire layer of rock or soil in a confined aquifer is saturated with water—there is no water table as such. This layer is like a large underground sponge saturated with water.

Figure 5 shows the recharge area in a typical unconfined aquifer. Any place that rain falls on the surface in this diagram is a place of recharge. The arrows indicate the path of water
downward toward the zone of saturation. Since the profile of the water table slopes both toward the left and the right, any liquid pollutant introduced into the soil at Zone A will tend to move toward the spring at Point C. Figure 6 shows the recharge area in a typical confined aquifer in which the rock layers are dipping toward the right. Notice the difference in this scenario. Rain falling to the left of the recharge area, indicated by the arrows, could flow on the surface toward Point X and then into the aquifer. Rain falling immediately over the recharge area would seep directly into the confined sandstone aquifer and move downslope. Water falling onto the surface near Point Z would flow downslope along the surface and would not seep into the ground to reach the aquifer. Why? Shale is an aquitard composed principally of clay particles that act as a barrier to water flow through it. If a polluted liquid entered the recharge area at Point X, would Well #1 become polluted? How about Well #2? These are the kinds of questions that environmental geologists and hydrologists need to answer to help solve groundwater pollution problems. In this instance, Well #1 would be polluted before Well #2; however, if the polluted water was removed from the aquifer before the flow reached Well #2, then Well #2 would remain clean. Stopping this flow of polluted water is part of solving the environmental problem.

Subsurface Geology and Groundwater Principles
We’ve already examined some of the basic ways water enters and moves about in the
groundwater system. However, to more fully understand this process, we need to explore other geological factors, including rock and soil compositions, textures, and the important properties of porosity and permeability. How water travels through an aquifer is determined by a number of factors. We’ve stated that open spaces or pores exist between subsurface rock or soil particles, and that water may fill these pores and totally saturate the soil and rock. The percentage of open spaces or pores in a given volume of rock or sediment is called porosity. Porosity determines the total amount of water a rock or soil will hold, and varies from one material to another. The greater the number of pore spaces a rock or sediment material contains, the higher its porosity, and the more water it can hold. Porosity is largely influenced by factors of particle size, shape, assortment, and packing.

The ability of a rock or soil to allow water to flow through it freely is called permeability. The rate at which a material transmits water depends not only on its total porosity, but also on the size of the passageways between its openings. To be considered permeable, the open spaces in a rock must be connected. Water will only flow through the aquifer if the pores are connected. In Figure 7, notice that the flow of water droplets through the aquifer is possible because most of the pores are interconnected. There is nothing to clog the pores and prevent flowage. The rock along paths A and B is said to be permeable and allows flow. In contrast, the movement of water along path C is abruptly stopped and redirected when it encounters a zone where the soil or rock particles are tightly packed. This impermeable zone prevents flowage because the pore spaces here are not
interconnected, thus preventing the water from getting past this barrier. This diagram clearly illustrates the difference between an aquifer and aquitard in situations where rocks (and in some cases soils) are not fractured. However, fractures may cut through both permeable and impermeable layers and provide pathways for water to flow, regardless of the original permeability of the rock.

When surveying the general geology of an area to solve a groundwater problem, geologists
need to consider a variety of factors. Are the rocks igneous, metamorphic, or sedimentary?
What is the specific rock type (e.g. limestone, sandstone, etc.)? What is the soil composition? Is it gravel, sand, or clay? Is the rock fractured? Is the rock porous or permeable? What is the direction of slope of the land and the subsurface layers? Determining the answer to this last question is particularly important because it indicates the direction toward which the polluted water will flow. If, for example, the subsurface layers are horizontal (see Figure 8), the polluted water would move away from the point source of the pollution equally in all directions. If the subsurface layers are tilted or dipping (see Figure 9), the water would naturally flow down dip. Notice the example in Figure 9 shows a confined aquifer situation. In both examples, the polluted water would seep through the aquifer. The rate of seepage would depend upon both the porosity and permeability of the aquifer.

It is quite possible that in the early stages of a spill, the polluted water zone, or plume, may only move as far as Point A in Figures 8 & 9. However, without immediate attention at its source to stop the pollutant from leaking into the ground, the plume may migrate to Point B or farther, to Point C. The plume of polluted water mixes with pure water in the aquifer as it travels. To better predict plume migration and direction, geologists need to obtain information about the slope of the land and subsurface layers. To do this, they will construct a topographic profile of the land surface and a geological cross-section depicting the rock or soil layers in the subsurface.

Constructing a Topographic Profile
Geologists start by obtaining a standard topographic or contour map of the site area.
Elevations at the earth’s surface are depicted on topographic maps by means of lines of equal elevations called contours. The vertical distance between adjacent lines is called the contour interval. Looking at the simplified topographic map in Diagram 1, let’s assume you want to construct a topographic profile from point X to point Y to determine the slope of the land. First, lay a strip of paper along a line from point X to point Y on the map as shown in Diagram 2. The distance from point X to Y is the horizontal distance, that is, the distance between these two points on the ground. In this example, the distance represents 6 miles. Now, wherever the top of the strip of paper intersects a contour line, or point between contour lines, draw a mark or arrow on the strip of paper and label the elevation at that point (see Diagram 2). The final result should look like Diagram 3.

You now know the elevations of all the key points along line X-Y. You also know their horizontal positions relative to each other on the topographic map. You can now construct a 2-dimensional representation that is a profile of what the land slope looks like in the field. Set up a simple graph as seen in Diagram 4, and establish a vertical scale to represent the land elevations. Any vertical scale will do, but usually this scale is somewhat exaggerated and doesn’t equal the horizontal scale. Place the strip of paper with your marked elevations along the horizontal scale at the bottom of the graph (see Diagram 5). Now draw a line up from each elevation mark on the strip of paper until it intersects a line on the vertical scale that has the same elevation. Draw a dot at this point. Now connect all the dots with a smooth curve, and you’ve completed your topographic profile (see Diagram 6). In this example, notice that the area slopes from higher ground at Point X to lower ground at Point Y.

Constructing a Geological Cross-Section
Whereas topographic profiles provide geologists with the slope and general view of the land surface as seen from a particular line on the ground, a geological cross-section provides a more detailed picture of the regional geology and the specific rock types and structures present both at and below the surface. Let’s assume that you stop along the road to view a rock outcrop that you find particularly interesting. You want to describe the outcrop to a friend back home, so you sketch a drawing of it and label the rock types on your sketch (see Diagram A). While at the outcrop, you get a good idea of the rocks and soil in the region. The rock is solid granite, and the soil on top of it is sandy. You decide to plot the location of this particular outcrop on a road map so that you and your friend may visit it again (see Diagram B).

During your drive in the area, you notice along Routes 5 & 10 (west of your outcrop location) that granite outcrops at the surface, and there is no overlying sandy soil—just bare rock. However, to the east of your outcrop location on Route 10 and along Routes 20 & 30, the land is quite flat and you see only sandy soil and no granite at the surface. You mark these observations on your road map (see Diagram B). But where is the granite on the eastern portion of the map? You suspect it is below the surface someplace, but how far? One way geologists obtain subsurface data is by drilling. Oil and water well drillers monitor how deeply they drill and record the soil and rock types and thicknesses they find at various depths. Suppose you find out that a driller had to drill through 40 feet of sandy soil before hitting granite when putting in a well at Point Z (see Diagram B). You now know that 40 feet of sandy soil is overlying the granite at this location. From your observations at the original outcrop (see Diagram A), you know the thickness and trend of the sandy soil in the outcrop at Points A and B. Therefore, you determine as you travel east from the outcrop on Route 10 along the flat sandy soil, that the granite surface is sloping below you to the east and is 40 feet below the surface at Point Z.

If you plot the surface distribution of granite (X) and sandy soil (Y) on the map of the area, you will construct a simple geologic map (see Diagram C). This map only tells you the type and position of rock and soil at the surface. To find out what’s taking place below the earth’s surface at any given point, we will need more data to construct a true geologic cross-section. Fortunately, several other wells have been drilled in the area at locations #1 through #4 (see Diagram C). From the well data, we know the depth below the surface to the granite body at each location. For the sake of simplicity, we will also assume that the surface elevation is the same in all places (1000 feet). At Well #1, the granite lies 10 feet below the surface; at Well #2 it is 20 feet; at Well #3 it is 30 feet; and at Well #4, the granite is 40 feet below the surface. From this information, you can definitely see that the granite surface slopes to the east, and from the elevation measurements, you can also tell the thickness of the sandy soil at each well site.

In Diagram A, you made a sketch of a simple geological profile of the original outcrop. It was not drawn to scale like your topographic profile, but it gave you a rough idea of the geology of the area. When solving a groundwater problem, you will want to draw a more accurate crosssection. To do this, you will follow the same basic principles used in constructing a topographic profile. Diagrams D and E outline the steps in preparing a geological cross-section for the area used in our example.

To begin, lay a strip of paper on the geologic map. Mark a few key land elevations on the map—for example, the granite elevations at the surface and the elevation of the sandy soil at the surface. Use an X to plot the elevation of the granite at the surface (see GRANITE AREA) and at the subsurface in each well. For example, the granite at Well #1 is 10 feet below the surface at an elevation of 990 feet.

Proceed as before in constructing a topographical profile, only this time, plot both the surface elevations and the depth elevations to granite on the same graph as shown. Connect the plotted points to draw a smooth line and complete your geological cross-section showing both surface and subsurface profiles.

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