Groundwater Lecture Notes 6y10

Groundwater Hydrologic Cycle Rainfall can:  flow into rivers and streams,  return to the atmosphere by evaporation or transpiration  seep into the ground to become part of the subsurface water. 

Aquifers As water percolates down through cracks and pores of soil and rocks, it es through a region called “unsaturated zone”. Unsaturated zone consists of both air and water in the spaces between soil particles. Water in the unsaturated zone is called vadose water and this water is unavailable for human use since it cannot be pumped. Plants can however use this water. Below is the saturated zone, where all spaces between soil particles are filled with water. Water in the saturated zone is called groundwater and the upper boundary of the saturated zone is called water table. There is a transition region between these two zones called the capillary fringe. Here water rises into small cracks due to attraction between water and rock surface.

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AQUIFER: it is a saturated layer that is permeable enough to allow water to flow easily through it. An aquifer is located above a relatively impermeable layer that greatly restricts the movement of ground water. The illustration shows an unconfined aquifer. A well drilled into the saturated zone of unconfined aquifer will have water at atmospheric pressure at the level of water table. Groundwater also occurs in confined aquifers where it is underlain and overlain by impermeable layers. Water in such aquifer can be under pressure and when a well is drilled to such an aquifer, the water in the well will rise above the level of the aquifer and water will gush out of this well to a certain height. Such well is called artesian well. A line drawn at the level to which water would rise in an artesian well is known as the peizometric surface or potentiometric surface.

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If there is a local impermeable layer in the middle of an unsaturated zone above the main body of groundwater. Downward percolating groundwater is trapped above this layer, creating a perched water table. The amount of water that can be stored in a saturated aquifer depends on the porosity of the rock that makes up the aquifer.

Porosity, η =

𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒗𝒐𝒊𝒅 𝒕𝒐𝒕𝒂𝒍 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒊𝒅𝒔 𝒂𝒏𝒅 𝒗𝒐𝒊𝒅

Although porosity controls the water bearing capacity of a geologic formation, it is not a good indicator of how much water can be removed from the formation. Some water will always be retained as a film on the rock surfaces and in very small cracks and openings. The volume of water that can be drained from an unconfined aquifer per unit area and per unit decline in the water table is called the specific yield or the effective porosity. Representative values of porosity and specific yield for various formations are given below. Material / Formation Clay Sand Gravel Gravel and sand Sandstone Limestone, shale Quartzite, granite

Porosity (%) 45 34 25 20 15 5 1

Specific yield (%) 3 25 22 16 8 2 0.5

The amount of water that can be stored in a saturated aquifer depends on the porosity of the rock that makes up the aquifer. Although porosity controls the water bearing capacity of a geologic formation, it is not a good indicator of how much water can be removed from the formation. Some water will always be retained as a film on the rock surfaces and in very small cracks and openings. The volume of water that can be drained from an unconfined aquifer per unit area and per unit decline in the water table is called the specific yield or effective porosity. Example: (pp. 233, Masters and Ela) For an aquifer of sand, what volume of water would be stored in a saturated column with a cross sectional area equal to 1.0 m2 and a depth of 2.0 m? How much water could be extracted from that volume?

Hydraulic Gradient In an unconfined aquifer, the slope of the water table, measured in the direction of the steepest rate of change is called the hydraulic gradient.

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The groundwater flow is in the direction of the hydraulic gradient and at a rate proportional to the hydraulic gradient. Hydraulic head is the vertical distance from some reference datum (usually seal level) to the water table. If we imagine two wells, the hydraulic gradient is simply the difference in head divided by the horizontal distance between them. Hydraulic gradient = change in head/ horizontal distance = (h2-h1)/L = dh/L

L

h2 h1

Determination of hydraulic gradient Heath (1983) has suggested a graphical method for finding the hydraulic gradient between three closely spaced wells. 1. Draw a line between the two wells with the highest and lowest head and divide it into equal intervals. Find the place on the line between these two wells where the head is equal to the head of the third well. 2. Draw a line from the third well to the point on the line between the first two wells where the head is same as the third well. This line is called an equipotential line. Ground water will flow in a direction perpendicular to the equipotential line. 3. Draw a line perpendicular to the equipotential line through the wells with the lowest or highest heads. The ground water will flow in a direction parallel to this line. This is called flow line. 4. Calculate the gradient as the difference in head between the equipotential line and the lowest/highest head well divided by the distance from the equipotential line to the corresponding well. Example: (Davis, pp. 5.38; example 5-10.) For the wells shown in the plan view below, determine the direction of flow and the hydraulic gradient. The totals heads of the three wells A,B, and C are as follows: Well A= 10.4 m, Well B = 9.9m, Well C = 10.0 m

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Darcy’s law for flow through porous media Established by French hydraulic engineer, Henri Darcy. Flow rate, Q is proportional to the cross sectional area A and the hydraulic gradient. Q = KA (dh/dL) Q = flow rate (m3/d), K = hydraulic conductivity, or coefficient of permeability (m/day) A = cross sectional area (m2) Ks or standard coefficient of permeability is defined as the flow of water at 60 0F (15.5 0C) in gallons per day through a medium having cross-sectional area of 1 ft2 under a hydraulic gradient of 1 ft/ft.

Hydraulic conductivity: Measure of the ability of an aquifer to transmit water under a sloping peizomtric surafce is called hydraulic conductivity. Discharge that occurs through a unit cross section of auifer (1 m2) under a hydraulic gradient of 1.00. Permeability of a porous medium refers to the ease with which a fluid will through it. Coefficient of permeability depends both on the medium and also the fluid. Darcy’s law is valid only for laminar flow. When the flow is turbulent, it breaks down. For example, turbulence occurs in the vicinity of a pumped well. It is also invalid for flow through extremely fine grained materials such as colloidal clays. It is valid only when the medium is fully saturated with water. Aquifers having same hydraulic conductivities throughout are homogeneous and those where hydraulic conductivity differs from place to place are heterogeneous. Aquifers having hydraulic conductivity same in all the directions are called isotropic and different in different directions are known as anisotropic. Hydraulic conductivities of different types of rock medium are given below. Rock type Clay Sand Gravel Gravel and sand Sandstone Limestone, sahle Quartzite, granite

Hydraulic conductivity (m/d) 0.0004 41 4100 410 4.1 0.041 0.0004

Example 5.12 (Masters, pg. no. 238) A confined aquifer 20 m thick has two monitoring wells spaced 500 m apart along the direction of groundwater flow. The difference in water level in the wells is 2.0m. The hydraulic conductivity is 50m/day. Estimate the rate of flow per meter of distance perpendicular to the flow.

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Flow velocity By considering the relationship between flow rate, velocity, and cross-sectional area, v = Q/A Using Darcy’s law v = K dh/dL The velocity as determined using the above relationship is called Darcy velocity. This is not a real velocity since it assumes that the full cross sectional area is available for water to flow through. Since much of the cross sectional area is made up of solids, the actual area through which all of the flow takes place is much smaller and as a result, the real groundwater velocity is faster than the Darcy velocity. Q = Av = A’v’ Solving for v’ and introducing an arbitrary length of aquifer L: v’ = Av/A’ = ALv/A’L (multiply with L in both numerator and denominator) A’ is the area representing void (containing water, allowing flow of water). v’ = ALv/A’L = Total volume x v/ Void volume v’ = v/Porosity = v/ η v’ , average linear velocity, v’ = K/ η (dh/dL)

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Fig.

Example (5.13, Masters and Ela; pp. 239) The aquifer in the previous example has become contaminated upgradient of the two wells. Consider the upgradient well to be a monitoring well whose purpose is to provide the early detection of the approaching plume to help protect the second, drinking water well. How long after the monitoring well is contaminated would you expect the drinking water well to be contaminated? The aquifer has a porosity of 35%. Make the following assumptions: 1. Ignore the dispersion and diffusioin of the plume (that it does not spread out) 2. Assume the plume moves at the same speed as the ground water.

Transmissivity The coefficient of transmissivity (T) is a measure of the rate at which water will flow through a unit width of vertical strip of aquifer extending through its full saturated thickness under a unique hydraulic gradient. Its unit is m2/s. Value of T varies between 1.0×10-4 to 1.5×10-1 m2/s. Groundwater movement Darcy’s law: Q = K A dh/dr The equation has been solved for steady state and nonsteady or transient flow. Steady state is a condition under which no changes occur with time. It seldom occurs in practice. May be approached after very long periods of pumping. Assumptions: 1. well is pumped at a constant rate. 2. flow towards the well is radial. 3. initially the peizomtric surface is horizontal (water table) 4.the well fully penetrates the aquifer is open for the entire height of the aquifer. 5. aquifer is homogeneous in all directions and extends for infinite horizontal extent.

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6. water is released from the aquifer immediately with drop in peizomteric surface.

Fig. Drawdown for a unconfined aquifer Cone of depression

Fig. Cone of depression When a well is pumped, the level of the peizometric surface will be lowered in the vicinity of the well. This lowering of the water table (also known as drawdown) causes the peizomteric surafce to take the shape of an inverted cone which is known as cone of depression. The drawdown is largest near the well and decreases as the distnce from the well increases untill the slope of the cone merges with the water table. The distance of the well to the point at which there is no change in the water table is called as radius of influence.

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Fig. Effect of overlapping cone of depression

Steady Flow For unconfined water equation for steady flow is given as: 𝑄=

𝜋𝐾(ℎ22 − ℎ22 ) 𝑟 ln( 2⁄𝑟1 )

Where Q is well pumping flow rate, m3/s h = height of the water table from the bottom of the aquifer (m) K = hydraulic conductivity (m/s) r = radius of pumping well (m) r1 and r2 = distance from pumping well Assumptions: 1. pumping has been steady for a long enough time that the shape of the cone is no longer changing; it is under equilibrium conditions. 2. Draw down is small with respect to the depth of the aquifer. Under these conditions, for an unconfined aquifer, for the flow through an imaginary cylinder of radius r and depth hc, and hydraulic gardient (dh/dr) the Darcy’s law can be applied. Q = KA dh/dr = K 2𝜋rh dh/dr

(1)

Where Q is well pumping flow rate, m3/s h = height of the water table from the bottom of the aquifer (m) K = hydraulic conductivity (m/s) r = radius of pumping well (m) We can rearrange equation (1) and get Q dr/r = 2K𝜋 hdh 𝑟2

ℎ2

Integrating ∫𝑟1 Q dr/r = ∫ℎ1 2K𝜋 hdh

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(2)

The radial term is integrated between two arbitrary values r1 and r2 corresponding to heads h1 and where r1 and r2 are distance of two monitoring well from a pumping well. Integrating we get: Q ln (r2/r1) = 𝐊𝝅 (𝒉𝟐𝟐 -𝒉𝟐𝟏 ) Re-arranging, 𝑄 =

𝜋𝐾(ℎ22 −ℎ12 ) 𝑟 ln( 2⁄𝑟1 )

(3)

Thus, we can calculate the value of K if we know the pumping rate Q of a pumping well and the hydraulic head of two monitoring wells at two known distances from the pumping well.

Example: A 0.50 m diameter well fully penetrates an unconfinedn aquifer which is 30 m thick. The drawdown at the pumped well is 10.0 m and the hydraulicm conductivity of the gravel aquifer is 6.4×10-3 m/s. if the flow is steady and the discharge is 0.014 m3/s , determine the drawdown at a site 100 m from the well.

Example: A well of 0.30 m diameter is pumped at a rate of 6000 m3/day for a long enough time that steady state conditions apply. An observation well located 30 m from the pumped well has been drawn down by 0.10m. Another well at 100 m is drawn by 0.50 m. The well extends completely through an unconfined aquifer 30.0 m thick. Determine: a. the hydraulic conductivity K, and b. the drawdown at the well.

Contaminants in the ground water Some of the most toxic contaminants are inorganic: lead, chromium, zinc, arsenic, cium, manganese, copper, barium, and Nickel. Many of the organic chemicals causing contamination in water do not dissolve well in the water. They are called non-aqueous phase liquids (NAPL). The presence of NAPL makes the task of restoring an aquifer very difficult or even impossible. The NAPL can be divided into two categories: Dense NAPL (DNAPL) those denser than water. Examples of DNAPL include chlorinated solvents such as trichloroethylene, tetrachloroethylene, polychlorinated biphenyl (PCBs), pesticides, polycyclic aromatic hydrocarbons (PAHs). Light NAPL (LNAPL) are those with density less than water. They include hydrocarbon, crude oil, gasoline, and benzene.

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a. LNAPL

b. DNAPL Fig. Non-aqueous phase liquids in aquifers

a. LNAPLS float on top of aquifer; b. DNAPLS form pools in the bottom of the aquifers As NAPLs enter the unsaturated zone, some may dissolve into water in the pores, some may volatalise and become mixed with air in other pore spaces, and some may adsorb onto soil particles. When the LNAPLs reach the water table, they do not dissolve well and instead spread out to form a layer of contaminant floating on the top of the saturated zone (fig. a.)

DNAPLs also sink, dissolve, adsorb and volatilize in the unsaturated zone, but when they reach the water table, they keep on sinking until they reach a layer of relatively impermeable material. They form pools and can overflow and sink to the next impermeable layer (fig. b). Portions of DNAPLs tend to form small globules that become trapped in pore spaces and making them virtually impossible to remove by pumping. As they accumulate in pools on the bottom of the aquifers, they slowly dissolve into ing ground water. Their solubility is so low that, removal by dissolution can take centuries. Attempts to remove DNAPLS by extracting groundwater can be nearly impossible.

Table. Specific gravity and aquesous solubility of some NAPLs. Pollutants (NAPLs)

Specific gravity

Aquesous solubility (mg/L)

DNAPLs Carbon tetrachloride

1.58

7.57x102

Trichloroethylene

1.47

1.10x103

Tetrachloroethylene

1.63

1.50x102

Phenol

1.24

9.3x104

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LNAPLs Benzene

0.873

1.75x103

Toluene

0.862

5.35x102

p-Xylene

0.861

1.98x102

Example: Estimated time to remove a NAPL from aquifer Suppose 1 m3 of aquifer is contaminated with 30 L trichloroethylene (TCE). The aquifer has porosity of 0.3, groundwater moves through it with an actual speed of 0.03 m/day, and the TCE has a dissolved concentration equal to 10 % of its aqueous solubility. a. Find the mass of dissolved TCE and the mass of undissolved DNAPL. b. Estimate the time for TCE to be removed.

Contaminant transport Dispersion and diffusion When there is difference in concentration of a solute in groundwater, molecular diffusion will cause movement from regions of high concentration to regions of lower concentration. That is even in the absence of groundwater movement, a blob of contaminant will tend to diffuse in all directions. A second process that causes contaminant plume to spread out is dispersion. The contaminant plume follows a irregular pathways as it moves, some find large pore spaces in which it can move quickly, while other portions of the plume have to force their way through more confining voids, there will be a difference in speed of an advancing plume that tends to cause the plume to spread out. The diffusion and dispersion both tend to smear the edges of the plume. Both the process together are referred to as “hydrodynamic dispersion”.

Retardation As the contaminant move through an aquifer, some are absorbed by solids along the way, and some are adsorbed. Both the processes are referred to sorption.

Ground water Remediation technology There are two remediation alternatives for ground water. i.

Treatment of the water after it has been extracted (ex situ treatment)

ii.

Treatment of water in place in the aquifer (in situ treatment)

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Pump and treat system This is an ex situ approach where the water is pumped from the aquifer and then treated on the surface.

In situ treatment Soil vapor extraction (SVE) This system is implemented to remove organic vapors from the unsaturated zone. The schematic of a SVE system is shown in the figure below.

Fig. schematic of soil vapor extraction system.

SVE system consists of an injection well, an extraction well, a blower/ vacuum pump, and vapor treatment unit. The extraction well consist of slotted, plastic pipe set into a permeable packing material. They may be placed vertically or horizontally depending on circumstances. Horizontal systems are often used when the contaminant is near the surface. The air injection system pushes the vapor towards the collection wells. The air injection system can be connected to the vapor treatment unit and thus forming a close loop system.

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SVE systems are effective in removing volatile organic compounds that have leaked from underground storage tanks. They may also help remove some NAPL in the unsaturated zone. A combination of SVE and pump and treat system can help remove LNAPLs that sit on the surface of the saturated zone. When pumping system lowers the water table, some of the LNAPLs are left behind. These LNAPLs get sorbed (adsorbed + absorbed) onto the particles of the porous aquifer medium which are now in the unsaturated zone. So these can be removed by the SVE system.

The performance of the SVE system is dependent on the characteristics of the subsurface and the contaminant. Low permeability zones in the subsurface are not easily flushed. It is usually difficult to remove the contaminant from such low permeable zone. Contaminants that are strongly sorbed to the subsurface particles will resist removal. Highly volatile contaminants located in permeable soils are easily removed by the SVE system.

SVE systems can be augmented with air sparging systems for removing the contaminant from the saturated zone. (figure below). An air sparging system consists of a compressor that drives air into the saturated zone through an injection well. The injected air rises through the contaminant plume and captures volatile compounds as it moves. Adsorbed contaminants can desorb directly into the rising air stream. Dissolved volatile compounds can move from liquid phase to into the vapor phase and then vacuumed out by the SVE system. Air sparging systems have been shown to be very effective removing substantial quantities of volatile organic compound such as benzene, and gasoline.

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Fig. Schematic of air sparging with SVE

In situ Bioremediation In situ systems are designed to degrade subsurface pollution in place without the need to capture and deliver contaminants to an above ground treatment system. By treating the contaminants in situ, the need for excavation, above ground treatment, and transportation to a disposal site is eliminated and the risk of human exposure to hazardous chemicals is greatly reduced. Other advantages: 

Potential to degrade compounds sorbed to the sub-surface materials.

This technique is based on stimulating the growth of microorganisms (primarily bacteria), that are indigeneous to the sub-surface and that can bio-degrade the contaminants. Provided with oxygen and nutrients, they can degrade a number of common soil and water contaminants, especially petroleum based hydrocarbons, volatile compounds.

For shallow contamination, the system can be as simple as an infiltration gallery that allows nutrient- and oxygen- rich water to percolate down through the contamination zone. For deeper contamination, an injection system as shown in Figure can deliver the nutrient- and oxygenrich water to the desired depth.

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The oxygen needed for aerobic decomposition can be supplied by bubbling air or oxygen into the injected water or by adding hydrogen peroxide. Nitrogen and phosphorous nutrients are needed for hydrocarbon contaminants. Extraction well may be added to the system for better control of flow of amendment water. In-situ bioremediation has also been applied for chlorinated solvents, and metals. Bio-degradation of chlorinated compounds can be done either aerobically or anaerobically. The aerobic pathways involve methanotrophic bacteria, which derive their energy from methane. To stimulate their growth methane gas is injected into the contaminated aquifer. In the process of consuming methane, the methanotrophs produce an enzyme, which has the ability to transform the contaminant to an harmless compound.

Results of previous field studies : TCE concentration was reduced by 20-30% in a few days. Vinyl chloride was reduced by 90-95% in days

Fig. In-situ bio-remediation

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