GROUNDWATER


(Potable Water Supply)

Precipitation is the ultimate source of all fresh water on the Eastern Shore of Virginia, whether the source is groundwater obtained from Aquifers which have been recharged by precipitation or surface water obtained from ponds or streams which capture and carry precipitation runoff. The water that the Town of Parksley, Accomack and Northampton Counties residents rely on for their everyday needs comes from groundwater.

Potable Water (Miriam-Webster®)

Potable:

"A liquid that is suitable for drinking water.The liquid that descends from the clouds as rain, forms streams, lakes, and seas, and is a major constituent of all living matter and that is an odorless, tasteless, very slightly compressible liquid oxide of Hydrogen H2O which appears bluish in thick layers, freezes at 0° C and boils 100° C; has a maximum density at 4° C and high specific heat; is feebly ionized to Hydrogen and Hydroxyl ions and is a poor conductor of electricity and is a good solvent."

Groundwater:

"Is a commodity that is not completely understood and is taken for granted. It is always there when the faucet is turned on. However, it is often said that the water table is depleting. The probable cause is over-pumping and misuse. The groundwater beneath the Town of Parksley, Accomack and Northampton Counties is discussed below.

To understand the groundwater, we must ask the following questions:

1.Where does the water come from that Parksley uses?

2.How does the water get to the Town of Parksley?

The Town of Parksley relies on for their water needs comes mainly from an unconfined Aquifer deeper than 90 feet and shallower than 300 ft deep, the Upper Yorktown - Eastover Aquifer. This Aquifer contains ample amounts of groundwater for the Town’s needs during normal conditions but during high pumping and/or during drought conditions, the Town of Parksley water requirements may also have to rely on the Columbia Aquifer and/or the Lower Yorktown - Eastover Aquifer along with the Middle Yorktown - Eastover Aquifer for their water requirements. These Aquifers consist of sand, gravel, and shell material of sufficient saturated thickness to yield significant quantities of water. The residents and industries located in Northampton and Accomack Counties also rely on their water needs from these Aquifers.

Aquifer Definitions

Aquifer:

A permeable geologic unit that is capable of providing an economic water supply Confined Unit A less permeable geologic unit that is incapable of providing an economic water supply but is significant on a regional basis due to leakage Intervening Confining Unit Allowing water to recharge lower Aquifers Unconfined Aquifer Allowing water to enter from other sources Aquifers below the Town of Parksley

-

Columbia Aquifer:

The Columbia Aquifer was deposited during the Pleistocene age some 10,000 to 15,000 years ago and generally supplies sufficient quantities of groundwater for domestic purposes and irrigation ponds. The Columbia Aquifer provides much of the water needed for agricultural purposes. In upland areas, the quality of water in this Aquifer is generally within drinking-water standards if wells are not located down gradient of potential sources of contamination. In low-lying and poorly drained areas, the water quality is worse than in upland areas, reflecting the nearness of saltwater bodies and contamination from land uses.

Upper Yorktown-Eastover Aquifer and Confining Aquifers:

The Upper Yorktown-Eastover Aquifer was deposited during the Miocene era between 5,000,000 and 23,000,000 years ago. The confining unit consists of gray, greenish-gray, or brownish clayey silt or silty clay. The Upper Yorktown-Eastover Confining Unit ranges in thickness from 26 feet to 109 feet.

The Upper Yorktown-Eastover Aquifer consist of varying mixtures of fine-grained to very coarse-grained, white to greenish-gray, shelly, glauconitic, and pebbly quartz sand. With the range of fine-grained to very coarse-grained sediments in this Aquifer and the variable thickness, this aquifer possesses moderate permeability with transmissivities ranging from less than 1,000 GPD/ Ft2 to 40,000 GPD/ Ft2. This Aquifer ranges from 15 feet to 110 feet thick.

-

Middle Yorktown-Eastover Aquifer and Confining Units:

The Middle and Upper Yorktown-Eastover Aquifer sediments are similar therefore the hydraulic properties are similar. The Middle Yorktown-Eastover Aquifer ranges in thickness from 12 feet to 124 feet. The confining unit consists of gray, greenish-gray, or brownish-gray clayey silt or silty clay and ranges in thickness from 8 feet to 76 feet.

Lower Yorktown-Eastover Aquifer and Confining Units:

This primarily consists of sediments from the Miocene Eastover Formation and are chiefly fine-grained to very fine-grained, greenish-gray, clayey, silty, and shelly quartz sands. The Eastover Formation typically contains finer-grained sediments than the Yorktown Formation; therefore, the lower Yorktown-Eastover Aquifer generally is less transmissive than the Upper and Middle Yorktown-Eastover Aquifers. The Aquifer thickness ranges from 22 feet to 40 feet. The lithology of the confining unit is similar to the middle and upper confining units and consists of gray, greenish-gray or brownish-gray clayey silt or silty clay. The thickness ranges from 10 feet to 74 feet

St. Mary’s Confining Unit:

The sediments are middle to late Miocene in age and consist of interbedded silty and sandy clay with varying amounts of shells and are typically bluish-gray to gray in color. This confining unit thickness ranges from 50 feet to 350 feet. This massive clay unit is effectively a lower bond for the fresh groundwater-flow system on the Eastern Shore.

Hydrogeologic Sections for the Eastern Shore

Sea level
Aquifer
-50 feet
Upper Yorktown - Eastover Confining Unit
-140 feet
Upper Yorktown - Eastover Aquifer
-180 feet
Middle Yorktown - Eastover Confining Unit
-200 feet
Middle Yorktown - Eastover Aquifer
-250 feet
Lower Yorktown - Eastover Confining Unit
-300 feet
Lower Yorktown - Eastover Aquifer
-350 feet
St. Mary's Confining Unit

Linear Velocity is the rate at which water travels through Aquifers either vertically and/or horizontally.

Terms
Definitions

Transmissivity

The horizontal rate of water flow through the width of an Aquifer with a hydraulic gradient of 1.0. This is the product of Aquifer thickness and hydraulic conductivity. Reported in feet square per day (ft2/d), a mathematical reduction of the unit cubic feet per day per square foot times feet of Aquifer thickness [(ft3/d/ft2 )*ft].

Hydraulic Conductivity

The rate of water flow (typically horizontal) through a unit under a hydraulic gradient of 1.0. Hydraulic Conductivity is reported in feet per day (ft/d), a mathematical reduction of the units cubic foot per day per square foot [(ft3/d)/ft2].

Porosity

The ratio, usually expressed as a percentage, of the volume of a material’s pores, as in rock, to its total volume.


The natural flow of the groundwater can be calculated. Without the impact of heavy pumping, based on an observed natural gradient of approximately 0.003, estimated hydraulic conductivity of 8.6 ft/day (Lower Yorktown-Eastover Aquifer) and estimated porosity of 0.4, the average linear velocity of groundwater at this state is:

Kin = Kinematic Viscosity, how thick or sticky a liquid is.
V = Kin
V = (0.0038 x 8.6 x 0.4)
V = 0.01032

The distance across the widest part of the Town of Parksley is approximately 2,640 feet. The estimated time for a drop of water to cross the Town Horzionaly without the aid of pumping would be:

(2640 feet / 0.01032 feet) / 365 = 701 years

The U. S. Geological survey conducted in 1991 indicated the horizontal hydraulic conductivities (transmissivity of water in the Aquifer) were as follows:

Horizontal Hydraulic Conductivities

Upper Yorktown-Eastover
Middle Yorktown-Eastover
Lower Yorktown-Eastover
51.8 ft/d
43.2 ft/d
8.6 ft/d


Permeability for Different Classes of Soils

Clean Gravel
Clean Sand; Mixtures of Clean Sand and Gravel
Very Fine Sand; Silt; Mixture of Sand, Silt, and Clay; Glacial Till; Stratified Clays; etc.
Un-weathered Clays
Nature of Soils
Good Aquifers
Good Aquifers
Poor Aquifers
Impervious
Flow Characteristics
Good Draining
Good Draining
Poor Draining
Non-Draining
Retention Characteristics


Leakage depends on the conductivity and thickness of a confining unit .The illustration below shows two confining units. The illustration shows the confining unit on the left is thinner than the unit on the right, thus demonstrating that it will take longer for water to pass through the confining unit on the right thus decreasing the amount of water available to the lower Aquifer.

Leakage between Aquifers

Wells Aquifer Confining Unit Aquifer Leakage

Groundwater Flow

Accomack and Northampton Counties, the groundwater is recharged with freshwater primarily through precipitation. The precipitation infiltrates into the sediments because there are no major surface-water bodies in Accomack and Northampton Counties. It is estimated that the annual rainfall in Accomack and Northampton Counties is 43 inches. Of the 43 inches of rainfall, it is estimated that 8˝ to 15 inches is recharge for the Columbia Aquifer. The remaining 28 to 34.5 inches of rainfall is either surface runoff or evaporation. It is estimated that the average recharge rate is 12 inches/year for the Virginia part of the Eastern Shore. The estimated natural recharge to the unconfined Columbia Aquifer is 257 million gallons per day. The water in the unconfined Columbia Aquifer flows vertically into the lower parts of the Columbia Aquifer and laterally through the unconfined Aquifer toward discharge sites such as springs, streams, marshes, estuaries, the Chesapeake Bay and Atlantic Ocean. Eventually, water that is moving vertically encounters the upper Yorktown-Eastover Confining Unit and much of the flow is forced to move laterally through the unconfined Aquifer. Under natural (before pumping of groundwater) conditions, a comparatively small amount of water is able to flow (leakage) through the less permeable confining unit into the Confined-Aquifer system. It is difficult to get an accurate volume of rainfall in a particular location without placing accurate rain gauges in all locations.

Below is the average rainfall per month for Wallops Island for the past 17 years. Every inch of rain that falls on the Town of Parksley 100 acre site equates to approximately 2,715,425 gallons. Since Wallops Island is 15.5 miles from the Town of Parksley. This data is readily available and it was used to determine the rain fall for the Town of Parksley.

Accomack County Average Rainfall

Wallops Island, Virginia

Monthly Rainfall in Inches
Month
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
Average
January
5.51
2.19
8.07
4.06
3.96
6.54
2.45
2.79
2.11
3.95
2.73
2.19
1.70
1.89
2.92
3.01
2.12

3.42

February
2.98
4.86
7.01
2.74
1.92
3.07
0.93
6.28
1.95
2.26
0.75
2.60
3.34
0.45
3.33
1.64
2.12

2.98

March
3.01
4.31
4.93
6.89
4.78
5.48
5.96
6.20
2.12
3.16
0.28
1.73
2.04
4.14
7.12
2.74
2.10

3.94

April
4.59
3.42
2.13
3.35
5.01
2.43
2.75
5.35
5.06
2.94
2.35
3.69
3.63
4.22
0.89
1.60
3.78

3.36

May
3.69
1.81
6.77
1.13
4.06
3.51
1.60
6.47
1.47
2.79
1.94
1.35
6.36
2.98
0.04
1.31
3.69

3.00

June
4.05
1.86
4.26
3.48
3.04
4.33
3.82
2.27
2.16
5.66
10.06
6.15
4.28
2.82
1.63
4.41
2.72

3.94

July
8.20
4.88
3.34
2.98
8.79
8.31
3.64
6.97
11.91
2.63
5.09
2.62
2.90
8.82
1.90
2.93
3.46

5.25

August
6.27
1.01
1.43
2.85
5.25
2.06
3.97
6.72
11.19
4.47
1.36
2.98
1.63
7.14
7.78
8.38
5.97

4.73

September
5.29
0.83
5.86
9.95
3.02
2.14
1.78
7.89
2.79
0.83
8.92
1.50
5.81
7.87
3.66
3.27
5.76

4.54

October
6.09
4.04
1.20
5.97
0.24
1.03
6.88
3.10
2.04
4.89
5.96
3.33
0.92
7.36
4.71
3.59
10.4

4.22

November
3.82
7.84
1.52
1.36
1.58
0.07
4.71
1.82
4.52
2.51
4.78
0.78
5.36
5.42
2.71
1.98
0.56

3.02

December
10.68
3.85
5.12
2.86
2.82
2.19
4.19
7.17
3.31
3.58
2.96
4.94
5.88
8.55
1.84
2.12
3.85

4.47



17 Years of Rain Fall

(1996 through 2012)

Total Rain (17 years)
Yearly Average Rain (Inches)
Monthly Average Rain (17 years)
796.94
46.88
3.91


The predicted rainfall for Accomack County is 42.96 inches per year. Calculating the differences between the averages predicted rainfall and the actual rainfall from 1996 through 2012 demonstrates the yearly differential.

Accomack County Average Rainfall

Wallops Island, Virginia

Yearly Rainfall in Inches
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012

Average

64.18
40.9
51.64
47.62
44.47
41.16
42.68
60.03
50.63
39.67
47.18
33.86
43.85
61.66
38.88
36.88
48.95

46.88

Groundwater Movement toward Wells

The movement of groundwater towards a well can be formulated in accordance with the principles of Dupuit and Forchheimer. This principle states when the well penetrates to the sole of the Aquifer, the flow is steady and the water table or the piezometric (pressure of material subjected to hydrostatic pressure) surface, as the case may be, is horizontal. Actually the water table or the piezometric surface is rarely horizontal and flow is seldom steady. Changes in pumping and recharge rates and in amounts of water stored in the Aquifer interfere. However, the usefulness of Dupuit’s formulation can be expressed by introducing potential flow theory to cover confined Aquifers in which the piezometric surface is inclined. The theory of steady flow has also been extended to leaky confining units and other situations.

The illustration above demonstrates the movement of water through the Aquifer to a well. There is a downward and upward conning effect at the well inlet. This is caused by the removal of more water from the well field than can be replaced. The causes of the severity of the conning area are directly related to the soil type and condition. In areas with highly permeable soil and high production wells, the cone could reach for miles and be very shallow. Low permeability soil can cause more problems because more water is drawn from a smaller area which causes the up-conning to be more severe. This can increase the possibility of saltwater infiltration.

Over-pumping of an Aquifer is common when the well is withdrawing more water than the Aquifer can supply. This is commonly referred to as “the well is dry” or “the well is out of water.” Over-pumping occurs more frequently in low permeable soil. This happens when the Aquifer cannot supply the pump with the amount of water that is needed to be withdrawn and the Aquifer does not have sufficient time to recharge. When this happens, it may be necessary to turn off the pump for several days to allow replenishing of the water supply.



In 1980, the State Water Control Board (Department of Environmental Quality) drilled observations wells in 11 locations in Accomack County on Perdue's Farms property. These wells are located 3 miles from the Town of Parksley's well field. These observation wells have been monitoring the effects of groundwater withdrawal on the water table and the possibility of saltwater infiltration into the fresh water Aquifers in Accomack.

The wells were drilled at various levels; 40 feet, 60 feet, 135 feet, 210 feet and 313 feet below the surface. The state has collected groundwater level readings on three of the wells on a continuous basis since 1980. The 135 foot, 210 foot and 313 foot wells are equipped with chart recorders. The State has stopped collecting data from the 40 foot and 60 foot wells 10 years ago.

Below is a graph of State Water Control Board Observation Well 112-B. This well is located in the Middle Yorktown-Eastover Aquifer which is the same Aquifer from which Broadwater Academy’s water is drawn. Each point on the graph is the static water level on January 1 of that year.



The graph below indicates the differences between sea level and actual water level of Observation Well 112-B. Sea level is 30 feet below the ground level at the Observation Well 112-B location. The level indicating a minus equals feet below sea level. Each point on the graph is calculated from the water level on January 1 of that year.

Wells

Well or water well is defined as any excavation constructed by any method for the purpose of extracting water from or injecting water into the underground. the Town of Parksley has three well. One of the well is out of service (well #1) and two wells are used for the Town's potable water needs.

(1996 through 2012)

Well
Drilled (feet)
Screen Location (Inches)
Pumping Capacity (gpm) center>
Pump
4
218
198 - 218
160
Submersible
5
218
198 - 218
160
Submersible


Wells are typically constructed in similar manner. Below is a schematic of a typical well construction and a submersible turbine well pump

Typical Well Construction



Submersible Turbine Well Pump

Town of Parksley Groundwater Withdrawel

Monthly Totals

Monthly Groundwater Withdrawel in Gallons (1994 - 2012)

,

Month
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2006
2007
2008
2009
2010
2011
2012
2013
Average
January
2,488,000
1,701,000
2,163,000
1,903,000
1,754,000
2,333,000
2,725,000
2,470,000
2,175,000
3,318,000
2,447,600
1,309,300
2,575,000
2,360,900
2,361,700
1,770,100
1,962,200
2,428,000

2,291,000

February
1,635,000
1,586,000
2,442,000
1,739,000
1,575,000
2,113,000
2,336,000
2,026,000
2,006,000
2,538,000
2,573,500
1,155,300
2,333,900
2,102,400
2,159,700
3,809,000
1,484,000
1,673,900
1,830,900

2,059,000

March
1,840,000
2,190,00
1,948,000
1,883,000
2,443,000
2,386,000
2,248,000
2,221,000
2,288,000
2,520,700
2,248,400
2,083,800
2,353,500
2,436,100
3,195,300
1,777,100
1,727,200
1,892,200

2,205,000

April
1,745,000
2,264,000
1,919,000
2,263,000
1,964,000
2,356,000
2,302,000
2,223,000
2,238,000
2,314,800
2,037,600
2,000,100
1,986,900
1,900,700
1,827,300
1,804,400
2,015,300
2,122,200

2,071,000

May
3,002,000
1,992,000
2,133,000
2,002,000
2,083,000
2,931,000
2,629,000
2,461,000
2,513,000
2,764,100
2,556,600
2,247,000
1,963,000
1,907,100
1,893,300
2,227,700
2,158,600
2,236,300

2,317,000

June
6,018,000
2,156,000
1,940,000
2,293,000
3,345,000
2,835,000
2,611,000
2,414,000
2,806,000
2,450,900
2,520,500
2,260,700
2,427,300
2,367,700
2,127,400
2,593,100
1,122,300
2,172,000
2,062,000

2,501,000

July
5,490,000
2,731,000
1,502,000
2,475,000
2,497,000
3,148,000
2,554,000
2,762,000
3,033,000
2,625,700
2,552,300
2,722,700
2,277,300
2,333,000
2,287,200
2,409,300
2,173,500
2,460,900

2,669,000

August
5,963,000
2,485,000
1,985,000
2,277,000
2,533,000
2,688,000
2,463,000
2/461,000
3,725,000
2,272,700
2,438,200
2,640,700
2,394,500
1,861,900
2,024,200
2,027,100
2,423,200
2,125,500

2,599,000

September
2,408,000
2,051,000
1,861,000
2,009,000
2,305,000
2,417,000
2,297,000
2,295,000
2,761,000
2,196,400
2,351,700
2,136,700
2,263,900
1,969,600
1,764,200
1,797,200
1,774,600

2,156,000

October
1,929,000
1,923,000
1,039,000
2,017,000
2,275,000
2,568,000
2,357,000
2,211,000
2,441,000
1,992,800
2,473,100
2,262,900
1,848,900
1,755,500
1,604,900
1,973,000
2,061,900

2,043,000

November
1,704,000
1,793,000
1,800,000
1,796,000
2,130,000
2,594,000
2,990,000
2,205,000
2,334,000
2,234,800
2,320,400
1,956,500
1,592,400
1,735,700
1,918,900
1,830,000
1,954,500

2,052,000

December
1,798,000
1,967,000
1,852,000
1,716,000
2,000,000
2,081,000
2,540,000
2,216,000
2,360,000
2,274,800
2,073,400
2,057,800
2,198,500
1,872,400
2,106,300
1,815,000
1,735,700

2,039,000

Average
3,108,000
2,041,000
1,902,000
2,037,000
2,112,000
2,542,000
2,516,000
2,333,000
2,551,000
2,582,000
2,403,000
2,179,000
2,229,000
2,103,000
2,045,000
2,369,000
1,850,000
1,985,000
2,095,000

2,254,000





DISINFECTION



Disinfection is the process of killing microorganisms in potable water that might cause disease. Disinfection is usually synonymous with chlorination because Chlorine addition is by far the most common form of disinfection used today.

Types of Disinfections



Disinfection can be accomplished by a variety of methods. Some are economical, convenient or easier to apply than others while some are extremely hazardous. All methods fall into one of the following types:

1. Heat treatment
2. Radiation treatment
3. Chemical treatment

The term Chlorine is common terminology used throughout the water treatment field that refers to either the gas, liquid and/or solid form of Chlorine. The Town of Parksley uses a liquid form of Chlorine (Sodium Hypochlorite) and throughout this paper Sodium Hypochlorite and Chlorine will be used interchangeably.