GROUND-WATER CONTAMINATION

With permission adapted from PL Garvin Cornell College for AGI by NP Flynn Ph.D.

OBJECTIVES

• Understand groundwater flow
• To determine, by preparing and analyzing contour maps, which way a plume of contaminated         ground water will move, which drinking water wells will be affected.
• Determine how long it will take the contaminants to reach a well.
• Determine the influence of a cone of depression on surrounding wells.

MATERIALS

• Calculator
• Pencil
• 2-3 color pencils.

SKILLS

• Understand how to determine the depth of the water table from well data.  
• Distinguish the zone of aeration from the zone of saturation.  
• Understand porosity and permeability and how they relate to the transportation of ground         water.  
• Recognize the connection between the water table, ground water and surface water.
• Draw equipotential contour lines from well data.  
• Using the contour lines determine and draw flow direction lines.  
• Determine well contamination potential.  
• Calculate Darcy’s velocity when given hydraulic conductivity.  
• Modify the Darcy velocity when conductivity and slope vary.  

BACKGROUND

The hydrologic cycle represents the movement of the water on Earth.  This water is in constant motion, moving from one portion of the cycle to the next. Solar energy from the sun evaporates water mainly from the oceans (as they cover two-thirds of the earth’s surface), lakes, streams, soil and plants (transpiration).  This moisture condenses to form clouds in the atmosphere, which eventually precipitates as snow, ice and rain returning the water to the oceans, lakes, streams, soil and plants. Some of the water on the ground runs off as overland flow such as streams and rivers, another portion infiltrates the soil and becomes groundwater (figure 1).  All the water on the land, including groundwater is moving down elevation (due to gravity) toward the oceans which are the largest reservoir of water.  Most of Earth’s water is salt water and is found in the oceans – nearly 97%.  Almost 2% is frozen freshwater stored as glaciers. Groundwater represents less than 1% of the water on Earth (figure 2). Groundwater is a small but vital portion of the hydrologic cycle, because it is the main source of public drinking and agricultural water and a portion of industrial water use.   All of the portions of the hydrologic cycle are interconnected.  Water is constantly moving between them. 

Figure 1. The hydrologic cycle showing the interactions between the many locations of water. Image: Howard Perlman and John Evans USGS Water Science School Public Domain. https://www.usgs.gov/media/images/water-cycle-natural-water-cycle

TABLE 1. The volume and percentage distribution of the Hydrologic Cycle.

Figure 2. A representation of the Earth’s water.  The 1-liter jug is filled with salty (3.5%) water representing the oceans which are 97% of the earth’s water store. The ice cube (2%) represents fresh glacial ice. The 2 teaspoons (less than 1%) represent the groundwater portion of the hydrologic cycle and the 3 drops from the dropper combine to represent all the fresh water in lakes, streams, wetlands, and in the atmosphere. Image: S Earle https://opentextbc.ca/geology/

Groundwater is a valuable resource and differs from the more familiar surface water (lakes and streams) as it is stored in the open spaces within the soil and rocks of the ground and is also called the saturated zone.  The open spaces may be called: voids, interstices, pores or pore spaces. It can be difficult to visualize water underground as it very rarely flows in underground streams.  Groundwater is the fully saturated pores in soil and substrate – rocks that make up the land. It moves down elevation due to gravity and interacts with surface streams as it moves toward the oceans. Groundwater is replenished by precipitation of snow/rainwater, as well as groundwater from higher elevations. The shallow depths contain water but are considered unsaturated zones if all the pore space is not filled with water, but also contain air. If you have ever dug a hole in the ground, you may have noticed that the soil was dry to moist.  This upper layer represents the unsaturated zone.  If you dig deep enough, perhaps you were at the beach, water may begin to fill the hole.  This may be water from the saturated zone filling in the open space. The boundary between the top unsaturated zone and the lower saturated zone is known as the Water Table (figure 3).  

Figure 3. The water table represents the boundary between the shallow unsaturated zone where both water and air spaces filled the pores in rocks and the saturated zone where all the open pore spaces are filled with water. Image: Public Domain USGS.gov https://www.usgs.gov/media/images/groundwater-saturated-zone-soilrock-below-land-surface

https://prd-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/s3fs-public/thumbnails/image/wss-gw-how-groundwater-occurs-diagram.png

How Does Groundwater Move? - Porosity and Permeability

The percentage of open spaces to be filled with water represent the porosity of the rock substrate.  This relates to the amount of water that can be stored by the rock material. Think about how much water a sponge or a piece of paper towel can hold.  For the saturated zone this amount varies greatly from 1-2% for unfractured igneous rocks such as granite to 10-30% for some sedimentary sandstone to up to 50% for some loose sand materials and silt (figure 4).  The percentage of water that a rocky material can hold is different from the amount that can be extracted.  Think again about that sponge or paper towel.  If you attempt to squeeze it you will not get all of the original water back out. This represents the yield of the rocky material and is related to the permeability. Permeability describes how the pores and open storage spaces are shaped and interconnected to one another allowing the water to be transmitted or stored. This ease of movement is highly variable (figure 5).  

Figure 4. Variations in porosity of materials.  Blue lines represent solid rock materials, and the red lines represent loose or unconsolidated materials. Image Steve Earle  https://opentextbc.ca/geology/wp-content/uploads/sites/110/2015/08/Variations-in-porosity.png

Figure 5. Rock material can be permeable if the open spaces are well connected or impermeable if the spaces are not connected. Image:  USGS.gov Public Domain US Dept. of the Interior. https://pubs.usgs.gov/of/1993/ofr93-643/

Groundwater is always moving down elevation.  This is not as simple as it sounds because groundwater is just the portion of water found between the spaces in rocks, so they may not be connected, as in fractured rocks or they may have to move around the loose gravel material to continue moving downward.  This characteristic of permeability is commonly called hydraulic conductivity (K) and can have a variety of units involving distance over time such as feet per day or meters per second (figure 6). It is important to note that some materials may be very porous or have equal levels of porosity but have very different permeabilities or conductivities. Sediments such as clay and silt hold tightly to the water as their silicate minerals hold tightly to the water molecules, while quartz-rich sandstone minerals do not.

Figure 6. Hydraulic conductivity levels of both solid rocks (blue) and unconsolidated materials (red). Image: Steve Earle https://opentextbc.ca/geology/

Determining Groundwater movement

Ultimately glacial, surface waters and groundwater are moving toward the oceans. Like surface streams and rivers, groundwater moves from higher elevation to lower elevation though the path may be highly variable.  Groundwater and surface water share a strong connection (figure 7).  Have you ever noticed that a stream continues to have water and flow even when it has not rained for a while?  Most streams are surface representations of the water table.  Streams are either providing water to the ground or the ground is providing water to the stream (gaining stream).  As a result, the elevation of most streams is generally the elevation of the water table.

In order to determine the velocity and direction of groundwater movement, data from wells is necessary.  Groundwater elevations can be determined by subtracting the elevation of the surface at the well from the depth of the saturated zone or water table.  For example, if the well is located 100 feet above sea level and the depth down the well to reach the water table is 20 feet below, the elevation of the water table at the individual well is 80 feet (100 ft -20 ft = 80 feet).

Figure 7. A cross-section of the surface and water table showing the link between the water table and streams. Also shown is the determination of the elevation of the water table and the direction of groundwater flow. Image: Steve Earle https://opentextbc.ca/geology/.

Darcy’s Law and groundwater movement rates

In 1856 French Engineer Henri P.D. Darcy (1803-1858) carried out some experiments to estimate the rate of groundwater flow in porous material. Supplying adequate fresh water from wells, especially in urbanized areas had been an issue in the 1800’s.  His experiments involved water moving through sand filled tubes to simulate a measure of resistance to free-flowing water. This type of movement within an aquifer became known as Darcy’s Law.

        Darcy’s law calculates the discharge rate over an area Q=KA(h1-h2)/L:  

        Q represents discharge,

        K = the hydraulic conductivity

        A = area of interest

        (h1-h2)/L is the slope of the area.  In essence, how steep is the water table dipping.  This is a function of change in elevation over distance (you may know this as rise over run).

His law is frequently simplified to V = Ki.  Where V = the velocity of the groundwater, K = the hydraulic conductivity of the material and is a function of the porosity and permeability of the material, I represents the hydraulic gradient, which is basically slope (elevation change divided by the distance).  

PROCEDURE

1. Underlying a military base in northeastern Michigan is a shallow sand gravel aquifer, a subsurface layer that is permeable enough to conduct ground water and to yield water readily to wells and springs.  The water table lies between 10 and 25 feet below the ground surface.  A leak in a buried storage drum has allowed a toxic organic liquid to enter the aquifer.  This contamination is a potential threat to drinking water supplies on the base. Using the data contained in Table 1, determine the elevation of the water table for the remaining wells.  Most have been completed.

Table 1. Lists the ground-surface elevation and depth to the water table for 55 wells on the military base.  English units are used rather than metric units, because ground elevation data are given in feet.  Conversion to metric units gives fractional elevation data that are more difficult to use. The wells are drilled for various purposes, and their locations are shown on the map of the base (Worksheet 2). Note: There are different types of wells. This lab focuses on production and monitoring wells.

PROCEDURE

        2. On the map or on a tracing overlay.  Carefully plot, in pencil, the elevation of the water table at each well. Wells Y5, and R84 have been recorded for an example.  Write small and clear and as close to the dot (represents the actual well).

        3. Lets practice drawing contour lines on the graph below.  Each of the dots         represents the elevation of the water table as determined from well data.  Draw in contour lines for elevations: 515, 510, 505, 500, 495, 490.  Recall that contour lines cannot cross each other.  Each line represents the same elevation and therefore they may curve around and between the numbers. Elevation 490 is not continuous on the map and 515 is pre-drawn.

        4. Using a pencil, contour the water table elevation on the map or overlay.  Using         a contour interval of one foot each.  The contour lines you have drawn are called         equipotential lines and show the general location of the water table.  Equipotential lines must increase/decrease in order and can never cross each other.  Once you have checked for errors, color over the lines with a colored pencil. Contour elevations 604, 603, 602, and 588 have been drawn as an example.

        5. The direction of ground water flow is generally perpendicular to the         equipotential lines, moving from higher to lower elevations.  They are not necessarily straight lines, as water will flow in a curved path as long as it is moving generally down elevation.  Using a second contrasting color pencil, draw arrows (flow lines) at several places on the map to show the directions of the ground         water movement.  Figure 8 is an example of how curved groundwater flow lines         can look.

Figure 8. Equipotential lines and ground water flow lines perpendicular. Image: Steve Earlehttps://opentextbc.ca/geology/wp-content/uploads/sites/110/2015/08/unconfined-aquifer.png

        6. Let's practice determining the gradient between two points. On figure 9,                 below Joe’s building is 37 meters above sea level and the stream is 21 meters above sea level. If the distance between the two is 80 meters what is the gradient?

Figure 9. A diagram showing the distance between two points and the elevation differences. Image: Steve Earle opentextbc.ca/geology

QUESTIONS

1. Based on the direction of ground water movement, if the storage drum marked with a DOT is leaking, which of the drinking wells is likely to be contaminated by the plume, or outflow, from the leaking storage drum?  

The leaking storage drum will quickly spread to a width of 500 feet in the vicinity of the well.  Using the distance scale to estimate width at the bottom of the map, shade in the area of the pollution plume. Assume no interaction (a noted oversimplification).

2. The velocity of ground water movement can be determined from the simplified version of Darcy’s Law Darcy’s Law: V= Ki.  

This equation shows that the velocity (V) of ground water is a product of the horizontal hydraulic conductivity (K) of the underlying rock material (look up several factors) and represents the ease with which the water can flow between the spaces, K takes into account the porosity and permeability of the substrate, thus it is a slowing factor and is frequently represented as a negative number, horizontal hydraulic gradient (i) this is effectively slope, change in elevation over distance.  

Velocity (V) is a function of distance/time. Such as feet/day or cm/sec. and represents the rate at which groundwater can move through the aquifer.  

Hydraulic conductivity (K) is a measure of soil permeability.

Horizontal hydraulic gradient (i) is slope.

For this lab activity the hydraulic conductivity (-K) is 100 feet/day. 

3. Determine the horizontal hydraulic gradient between the storage drum and the threatened well (in feet/mile).  Note: 1 mile = 5,280 feet.

4. Calculate the velocity of ground water flow from the storage drum to the well (feet/day).

5. Using the formula time= distance/velocity, determine how long it will take the contaminants to reach the well, assume no sorption or dispersion of contaminants.

6. Now let’s vary the factors involved in the velocity of the GW movement.  Let us reduce (-K) to 10 feet/day.  How does this change the velocity? And let us increase the slope by 2 times.  How does this influence the velocity?

HOMEWORK: Due before the next class.

Using the second worksheet:  A well is scheduled to be drilled at location P.  Because of the high pumping requirement, the water table at the well is expected to be lowered by 15 feet.  Table 2 shows water table lowering within the cone of depression, the area around the well where the water table will be affected.  The cone of depression will extend to a radius of 1,000 feet around the well. Lightly measure and mark 250, 500, 750 and 1,000 feet distances from P, and lower the known water table levels by the appropriate amount.  For example, well G17 was 596 feet and is within 250 feet of P it is lowered by 10 feet and its new elevation is 586.

1. On the second worksheet #2, use a third color pencil to re-contour that portion of the water table surface affected by the new well. Use a two feet contour interval.

2. Show with arrows the new direction of ground water flow.

3. What effect will the new well at P have on the direction of movement of contaminants from the storage drum?  What are the consequences of the change?