Lin S.D. Water and Wastewater Calculations Manual

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USTs contain petroleum products (gasoline and other petroleum prod-

ucts). When the UST leaks and is left unattended, and subsequently con-

taminates subsurface soils, surface and groundwater monitoring and

corrective actions are required.

The migration or transport pathways of contaminants from the UST

depend on the quantity released, the physical properties of the con-

taminant, and characteristics of the soil particles.

When a liquid contaminant is leaked from a UST below the ground

surface, it percolates downward to the unconfined groundwater sur-

face. If the soil characteristics and contaminant properties are known,

it can be estimated whether the contaminant will reach the groundwater.

For hydrocarbons in the unsaturated (vadose) zone, it can be estimated

by the equation (US EPA 1987):

(3.54)

where D ⫽ maximum depth of penetration, m

⫽ a coefficient of retention capacity of the soil and the

viscosity of the product (contaminant)

V ⫽ volume of infiltrating hydrocarbon, m

A ⫽ are of spell, m

The typical values for R

are as follows (US EPA, 1987):

Retention (attenuative) capacities for hydrocarbons vary approximately

from 5 L/m

in coarse gravel to more than 40 L/m

in silts. Leaked gaso-

line can travel 25 ft (7.6 m) through unsaturated, permeable, alluvial,

or glacial sediments in a few hours, or at most a few days, which is

extremely site specific.

The major movement of nonaqueous-phase liquids that are less dense

than water (such as gasoline and other petroleum products) in the cap-

illary zone (between unsaturated and saturated zones) is lateral. The

plume will increase thickness (vertical plane) and width depending on

leakage rates and the site’s physical conditions. The characteristic shape

of the flow is the so-called “oil package.” Subsequently, it will plug the

pores of the soil or be diluted and may be washed out into the water table.

Soil Gasoline Kerosene Light fuel oil

Coarse gravel 400 200 100

Gravel to coarse sand 250 125 62

Coarse to medium sand 130 66 33

Medium to fine sand 80 40 20

Fine sand to silt 50 25 12

D 5

Groundwater 215

The transport of miscible or dissolved substances in saturated zones

follows the general direction of groundwater flow. The transport path-

ways can be applied to the models (laws) of advection and dispersion.

The dispersion may include molecular diffusion, microscopic dispersion,

and macroscopic dispersion.

Immiscible substances with a specific gravity of less than 1.0 (lighter

than water) are usually found only in the shallow part of the saturated

zone. The transport rate depends on the groundwater gradient and the

viscosity of the substance.

Immiscible substances with a specific gravity of more than 1.0 (denser

than water) move downward through the saturated zone. A dense

immiscible substance poses a greater danger in terms of migration

potential than less dense substances due to its deeper penetration into

the saturated zone. When the quantity of released contaminant exceeds

the retention capacity of the unsaturated and saturated zones, the

denser nonaqueous-phase liquid continues its downward migration

until it reaches an impermeable boundary. A liquid substance leaking

from a UST enters the vapor phase in the unsaturated zone according

to its specific vapor pressure. The higher the vapor pressure of the sub-

stance, the more it evaporates. The contaminant in the vapor phase

moves by advection and by diffusion. Vapor moves primarily in a hor-

izontal direction depending on the slope of the water table and the

location of the impermeable bedrock. If the vapor, less dense than air,

migrates in a vertical direction, it may accumulate in sewer lines, base-

ments, and such areas.

6.4 Groundwater treatment

Once a leaking UST is observed, corrective action should be carried out,

such as tank removal, abandonment, rehabilitation, removal/excavation

of soil and sediment, on-site and/or off-site treatment and disposal of

contaminants, product and groundwater recovery, groundwater

treatment, etc.

Selection of groundwater treatment depends on the contaminants to

be removed. Gasoline and volatile organic compounds can be removed

by air stripping and stream stripping processes. Activated carbon

adsorption, biological treatment, and granular media filtration can be

used for removal of gasoline and other organics. Nonvolatile organics are

removable by oxidation/reduction processes. Inorganic chemicals can

be treated by coagulation/sedimentation, neutralization, dissolved air

flotation, granular media filtration, ion exchange, resin adsorption, and

reverse osmosis.

These treatment processes are discussed in detail in Chapter 5, Public

Water Supply, and in Chapter 6, Wastewater.

216 Chapter 3

7 Setback Zones

Section 1428 of the Safe Drinking Water Act (SDWA) requires each state

to submit a wellhead protection program to the US Environmental

Protection Agency (EPA). For example, Illinois EPA promulgated the

Illinois Groundwater Protection Act (IGPA) in 1991. The Act assigned

the responsibilities of existing state agencies and established ground-

water classifications. On the basis of classification, different water qual-

ity standards, monitoring, and remedial requirements can be applied.

Classifications are based on the PCB levels which may be associated

with hazardous wastes.

Groundwater used as a public water supply source is called potable

groundwater. This requires the highest degree of protection with the

most stringent standards. The groundwater quality standards for

potable groundwater are generally equal to the US EPA’s maximum

contamination levels applicable at the tap pursuant to the SDWA. The

rationale is that potable groundwater should be safe for drinking water

supply without treatment.

The state primacy agency establishes a comprehensive program for

the protection of groundwater. Through interagency cooperation, local

groundwater protection programs can help to prevent unexpected and

costly water supply systems. An Illinois community experienced a leak-

ing gasoline underground tank, operated by the city-owned garage. It

contaminated one well and threatened to contaminate the entire well

field. The city has spent more than $300,000 in an attempt to replace

the water supply.

Some parts of the groundwater protection programs, such as mini-

mum and maximum setback zones for wellhead protection, are used to

protect public and private drinking water supplies from potential sources

of groundwater contamination. Each community well must have a set-

back zone to restrict land use near the well. The setback zone provides

a buffer between the well and potential contamination sources and

routes. It will give time for cleanup efforts of contaminated groundwa-

ter or to obtain an alternative water supply source before the existing

groundwater source becomes unfit for use.

A minimum setback zone is mandatory for each public well. Siting for

new potential primary or secondary pollution sources or potential routes

is prohibited within the setback zone. In Illinois, the minimum setback

zone is a 200-ft (61-m) radius area around the wellhead for every water

supply well. For some vulnerable aquifers, the zone may be 400 ft in

radius.

The maximum setback zone is a second level of protection from pol-

lution. It prohibits the siting of new potential primacy pollution sources

within the area outside the minimum setback zone up to 1000 ft (305 m)

Groundwater 217

from a wellhead in Illinois (Fig. 3.11). Maximum setback zones allow the

well owners, county or municipal government, and state to regulate

land use beyond the minimum setback zone.

The establishment of a maximum setback zone is voluntary. A request

to determine the technical adequacy of a maximum setback zone deter-

mination must first be submitted to the state by a municipality or

county. Counties and municipalities served by community water supply

wells are empowered to enact maximum setback zone ordinances. If

the community water supply wells are investor or privately owned, a

county or municipality served by that well can submit an application on

behalf of the owner.

7.1 Lateral area of inﬂuence

As described in the previous section, the lateral radius of influence

(LRI) is the horizontal distance from the center of the well to the outer

limit of the cone of depression (Figs. 3.1 and 3.12). It is the distance from

the well to where there is no draw of groundwater (no reduction in

water level). The lateral area of influence (LAI) outlines the extent of

the cone of depression on the land surface as shown in the hatched area

in Fig. 3.12.

218 Chapter 3

Figure 3.11 Maximum and minimum setback zones.

The LAI in a confined aquifer is generally 4000 times greater than the

LAI in an unconfined aquifer (Illinois EPA, 1990). If a pollutant is intro-

duced within the LAI, it will reach the well faster than other water

replenishing the well. The slope of the water table steepens toward the

well within the LAI. Therefore, it is extremely important to identify

and protect the LAI. The LAI is used to establish a maximum setback

zone.

7.2 Determination of lateral radius

of inﬂuence

There are three types of determination methods for the LRI. They are

the direct measurement method, the use of the Theis equation or volu-

metric flow equation, and the use of a curve-matching technique with

the Theis equation. The third method involves the interpretation of

pump test data from observation wells within the minimum setback zone

using a curve-matching technique and the Theis equation to determine

the transmissivity and storativity of the aquifer. Once these aquifer

constants have been obtained, the Theis equation is then used to com-

pute the LRI of the well. It should be noted that when the observation

Groundwater 219

Figure 3.12 Lateral area of influence and the cone of depression under normal operation

conditions (Illinois EPA, 1990).

well or piezometer measurement was made outside of the minimum

setback zone, the determination could have been conducted by the direct

measurement method.

Volumetric flow equations adopted by the Illinois EPA (1990) are as

follows. For unconfined unconsolidated or unconfined nonfractured

bedrock aquifers, the LRI can be calculated by

(3.55)

where r ⫽ lateral radius of influence, ft

Q ⫽ daily well flow under normal operational

conditions, cfs

t ⫽ time that the well is pumped under normal operational

conditions, min

H ⫽ open interval or length of well screen, ft

n ⫽ aquifer porosity, use following data unless more

information is available:

If pump test data is available for an unconfined/confined unconsoli-

dated, or nonfractured bedrock aquifer, the LRI can be calculated by

(3.56)

where r ⫽ lateral radius of influence, ft

T ⫽ aquifer transmissivity, gpd/ft

t ⫽ time that well is pumped under normal operational

conditions, min

S ⫽ aquifer storativity or specific yield, dimensionless

u ⫽ a dimensionless parameter related to the well function W(u)

and

(3.57)Wsud 5

Tsh

2 hd

114.6Q

r 5

u T t

2693S

Sand 0.21

Gravel 0.19

Sand and gravel 0.15

Sandstone 0.06

Limestone primary dolomites 0.18

Secondary dolomites 0.18

r 5

4524nH

220 Chapter 3

where W(u) ⫽ well function

– h ⫽ drawdown in the piezometer or observation well, ft

Q ⫽ production well pumping rate under normal

operational conditions, gpm

Direct measurement method. The direct measurement method involves

direct measurement of drawdown in an observation well piezometer or

in an another production well. Both the observation well and another

production well should be located beyond the minimum setback zone of

the production well and also be within the LAI.

Example: A steel tape is used to measure the water level of the observation

well located 700 ft from a drinking water well. The pump test results show

that under the normal operational conditions, 0.58 ft of drawdown compared

to the original nonpumping water level is recorded. Does this make the well

eligible for applying maximum setback zone?

Answer: Yes. A municipality or county may qualify to apply for a maximum

setback zone protection when the water level measurements show that the

drawdown is greater than 0.01 ft and the observation well is located beyond

(700 ft) the minimum setback zone (200 to 400 ft). It is also not necessary to

estimate the LRI of the drinking water well.

Theis equation method.

The Theis equation (Eqs. (3.56) and (3.57))

can be used to estimate the LRI of a well if the aquifer parameters,

such as transmissivity, storativity, and hydraulic conductivity, are

available or can be determined. The Theis equation is more useful if

the observation well is located within the minimum setback zone when

the direct measurement method cannot be applied. Equations (3.52) and

(3.53) can be used if pump test data are available for an unconfined/

confined, unconsolidated or nonfractured bedrock aquifer (Illinois

EPA,1990).

Example: An aquifer test was conducted in a central Illinois county by the

Illinois State Water Survey (Walton 1992). The drawdown data from one of

the observations located 22 ft from the production well was analyzed to deter-

mine the average aquifer constants as follows:

Transmissivity: T ⫽ 340,000 gpd/ft of drawdown

Storativity: S ⫽ 0.09

The pumping rate under normal operational conditions was 1100 gpm. The

volume of the daily pumpage was 1,265,000 gal. Is this aquifer eligible for a

maximum setback zone protection?

Groundwater 221

solution:

Step 1. Determine the time of pumping, t

Step 2. The criteria used to represent a minimum level of drawdown (h

– h)

is assumed to be

– h ⫽ 0.01 ft

Step 3. Calculate W(u) using Eq. (3.57)

Step 4. Find u associated with a W(u) ⫽ 0.027 in Appendix B

u ⫽ 2.43

Step 5. Compute the lateral radius of influence r using Eq. (3.56)

Answer: Yes. The level of drawdown was estimated to be 0.01 ft at a distance

of 1980 ft from the drinking water wellhead. Since r is greater than the min-

imum setback distance of 400 ft from the wellhead, the well is eligible for a

maximum setback zone.

7.3 The use of volumetric ﬂow equation

The volumetric flow equation is a cost-effective estimation for the radius

of influence and is used for wells that pump continuously. It is an alter-

native procedure where aquifer constants are not available and where

the Theis equation cannot be used. The volumetric flow equation

5 1980 ft

2.43 3 340,000 gpd/ft 3 1150 min

2693 3 0.09

r 5

u T t

2693S

5 0.027

340,000 gpm/ft 3 0.01 ft

114.6 3 1100 gpm

Wsud 5

Tsh

2 hd

114.6Q

5 1150 min

t 5

1,265,000 gal

1100 gal/min

222 Chapter 3

(Eq. (3.55)) may be utilized for wells in unconfined unconsolidated or

unconfined nonfractured bedrock aquifers.

Example: A well in Peoria, Illinois (Illinois EPA, 1995), was constructed in

a sand and gravel deposit which extends from 40 to 130 ft below the land sur-

face. The static water level in the aquifer is 65 ft below the land surface.

Because the static water level is below the top of the aquifer, this would be

considered an unconfined aquifer. The length of the well screen is 40 ft. Well

operation records indicate that the pump is operated continuously for 30 days

every other month. The normal discharge rate is 1600 gpm (0.101 m

/s). Is

the well eligible for a maximum setback zone protection?

solution

Step 1. Convert discharge rate from gpm to ft

Q ⫽ 1600 gpm

⫽ 308,021 ft

⫽ 8,723 m

Step 2. Calculate t in min

t ⫽ 30 days

⫽ 30 days ⫻ 1440 min/d

⫽ 43,200 min

Step 3. Find the aquifer porosity n

n ⫽ 0.15 for sand and gravel

Step 4. Calculate r using Eq. (3.55)

Yes, the well is eligible for maximum setback zone.

Note: The volumetric flow equation is very simple and economical.

5 213.4 m

5 700 ft

308,021 ft

/d 3 43,200 min

4524 3 0.15 3 40 ft

r 5

4524n H

H 5 40 ft

5 1600

gal

min

1 ft

7.48 gal

1440 min

1 day

Groundwater 223

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224 Chapter 3