Underground Water

 

I. Introduction

A. Look again at the world's water reservoirs. As you remember, the ocean is by far the largest reservoir, with subsurface water coming in next. Although these volumes are relatively constant, significant amounts of water move continuously in and out of these reservoirs. 

B. This circulation of water from one reservoir to another is called the hydrologic cycle.

1. Water moves out of the ocean via evaporation and is carried by atmospheric currents of air over the land masses where precipitation occurs in the form of rain and snow. 

2. Water is carried by run-off along the surface of the ground and eventually runs into rivers or seeps down into the ground to become part of the subsurface water reservoir.

3. Water flows in rivers and in underground conduits from higher to lower elevations, towards the sea.

4. Water also evaporates directly from the land surface or is returned to the atmosphere by plants in a process collectively called evapotranspiration.

C. Look at the cycle quantitatively for a moment for the USA.

1. There is an average of 76 cm (30 in.) of precipitation each year. 

2. Of that amount approximately 53 cm (21 in.), 70%, returns directly to the atmosphere by evapotranspiration

3. Only 23 cm (9 in.)=30%, runs off in streams directly to the sea. Of the total run-off nearly 40% escapes by the Mississippi River -- an impressive fraction of the continental supply.

4. Where does groundwater come from, then if the budget balances as closely as these figures indicate???

a. The answer is that although the amount of water entering the ground by infiltration (seepage) is slight, perhaps as little as .25mm (.01in)/year, with the passage of many millennia, great quantities of water slowly collect in the ground.

5. It is that vast reservoir built up gradually over thousands of years that we draw on today—unfortunately in some areas more rapidly than it is replenished.

a. E.G. Tucson-Phoenix, Arizona or California's Great Valley: Groundwater == nonrenewable resource that will be depleted some time in the future.

D. Half of the USA gets its drinking water from groundwater

E. Having briefly considered the hydrologic cycle and the movement of water in the oceans we come next to the subsurface waters.

 

II. Historical views of groundwater

 

In Xanadu did Kubla Khan

 

A stately pleasure-dome decree:

 

Where Alph, the sacred river, ran

 

Through caverns measureless to man

 

Down to a sunless sea.

 

         Samuel Taylor Coleridge (1772-1834)

 

A. This reflects the remarkable image that many people have of water within the Earth. They often think of underground rivers flowing for miles beneath the parched surface of deserts and to some of us springs are nearly as mysterious as they were to people long ago.

B. There were two leading schools of thought on the origin of springs. One held that springs drew their water from the sea. The other belief was that springs and streams had their own origin within subterranean caverns, large enough perhaps to have atmospheres of their own from which water condensed as a sort of rain to feed them. Aristotle was one who supported this idea of the origin of springs and groundwater.

C. We now know that the primary source of groundwater is rainfall and that groundwater feeds springs, but it was not until the 1600's that the connection was established.

 

III. Actual distribution of groundwater

A. When rain sinks into the ground from the surface it passes through an upper zone called the ZONE OF AERATION in which the pore spaces between the soil, sediment or rock particles are only partially filled with water. The water moves out of this zone due to three major processes:

1. Used by plants

2. Evaporated back up into the atmosphere

3. Passes on down into the zone below

B. Beneath the zone of aeration lies the ZONE OF SATURATION in which openings are filled with water.

C. The surface that separates the two zones is the WATER TABLE. The water below this surface is what we will be most concerned about in this section. This is also the surface at which water stands in wells drilled down into the zone of saturation.

D. The thickness of the zone of aeration varies from place to place and therefore the elevation of the water table changes from place to place. In humid regions where rainfall is abundant, the water table is most likely to be a blurred replica of the ground surface, rising under hills and sinking under valleys. It intersects the surface at lakes, streams and springs.

E. The zone of high water content under the water table does not continue indefinitely downward. In other words drilling a well to greater depths will not necessarily increase the flow of water. With increasing depth, the pore spaces in the rocks close up and their water-bearing capacity diminishes until the water is in low supply and virtually immobile.

1. For example, the upper levels of deep mines may require constant pumping to avoid flooding, whereas the lower levels may be so dry that water has to be piped down for use in drilling.

 

IV. Movement of groundwater

A. The flow of water in streams on the surface can be measured in terms of 100's of centimeters/second. BUT FOR GROUNDWATER THE FLOW RATES ARE MUCH, MUCH, LESS IN CM/DAY, OR IN SOME AREAS EVEN CM OR M/YEAR.

B. The reason for this is that the water must flow through small cavities in the rock or soil, if it can move at all.      

C.Porosity = the porosity of a rock is measured by the percentage of its total volume that is occupied by voids or interstices. If one half of a material is taken up by pores it has 50% porosity.  The more porous a rock is the more open space it contains.

1. Many factors determine the porosity of a rock. In clastic sedimentary rocks:

a. Packing arrangement

b. Degree of sorting

c. Amount of cementing material filling the pores

d. Grain size does not influence porosity of these rocks. BB's or basketballs if packed in the same manner, would have identical porosities. In fact, relatively fine-grained material such as silt, may have higher porosities than seemingly open material such as gravel.

e. A sand deposit composed of rounded quartz grains with fairly uniform size has a high porosity.  But if mineral matter enters the deposit and cements the grains into sandstone, the porosity is reduced by an amount equal to the volume of the cement.

f. A deposit of sand poorly sorted with finer particles of silt and clay mixed in has low porosity because the smaller particles fill up much of the space between the larger particles.

2. The porosity of igneous and metamorphic rocks is largely determined by the joint frequency because the rock itself is so dense (1% or less porosity).

3. The porosity of earth materials varies widely.

              Recently deposited muds    =   90%

              Solid igneous rocks        =    1%

D. Permeability

1. Porosity is not the only property of the subsurface material that determines its ability to transmit water. If the pore space in the rock is completely isolated, that is if the pores are not connected then water can not move from space to space and therefore the material can not transmit water. The ability of a material to transmit water is termed its PERMEABILITY.

2. In this respect, the size of the openings in the rock is much more important than the percentage of open space

a. For example, a silt or clay may have a higher porosity than a gravel, but since the void spaces are so small the permeability is less. Water passes more readily through larger passages because the molecular attraction on the water is much stronger in the tiny openings of the clay.

3. A permeable material that actually carries groundwater and readily yields that water to a well is an AQUIFER.

a. Most effective aquifers:

1) Unconsolidated sand and gravel

2) Sandstone

3) Some limestones-the permeability of LS is usually due to dissolution that has enlarged the fractures and bedding planes.

 

                CaCO3 + 2H+ = Ca2+ + 2HCO3-

 

b. "Sometime" aquifers:

1) Fractured igneous rocks

 

c. "Never" aquifers:

1) Clay

2) Shale

3) Most metamorphic/crystalline igneous rocks when unfractured

 

d. Two common kinds of aquifers -

1) Unconfined aquifer-water-bearing surficial layer of permeable material (i.e. sand & gravel)

2) Confined or pressurized aquifer-a permeable layer such as sandstone, between  layers of impermeable material such as clay or shale.

 

E. Driving force

1. GRAVITY drives downslope mass movement, currents in the ocean, downhill movement of rivers, and also drives the flow of groundwater.

2. Just as streams need a slope or elevation gradient to flow, so does groundwater.  Gradient = the inclination of a slope, or the "dip" of the slope.  For groundwater this is the slope of the water table and is called the HYDRAULIC GRADIENT.  It is measured in terms of the difference in elevation of the water table between two points divided by the distance between the two points.  The elevation of the water table at a given point is called the HEAD.

 

                       Higher head - lower head      h2 - h1

  Hydraulic gradient =-------------------------- or ---------

                             length of flow             l

 

where h2 is the greater of the two elevations, h1 is the lesser,  and "l" is the distance  between  the  two points.

3.  The equation used to express the velocity of water through rock is DARCY'S LAW, which is expressed as:

 

(greater head - lesser head)       K(h2 - h1)

Velocity  = K ------------------------------- or ----------

length of flow                   l

 

where "K" is a coefficient that depends on the permeability of the material, the acceleration of gravity, and  the viscosity  of the water. (viscosity is "resistance to flow")

4. You can see that [Velocity = K(hydraulic gradient)] so in material of constant permeability, the velocity will increase as the hydraulic gradient increases, just like water or any material will flow downstream faster if the slope is steeper.

a. Rates

Average flow rates:  1.5 m/day - 1.5 m/year

Extreme flow rates:  A few cm/year - 120 m/day.

5. As we saw with mass movement, although the force of gravity is directly downward towards the center of the Earth, material can not always move vertically into the Earth, in which case it moves down the slope taking the easiest and quickest route it can find. In homogeneous material the groundwater follows a path of broadly looping curves, as suggested in this slide.

F. Surface manifestations of groundwater flow

1. Spring-location where the water table crops out at the surface and water flows out more or less continuously. The following two figures are examples of geological situations that give rise to springs.

a. A thermal spring consists of warm or hot water issuing from the ground.

b. Warm spring-deep circulation of groundwater in areas devoid of geologically young volcanism.

c. Hot spring-groundwater heated by young, still hot volcanic rock.

d. Geyser-liquid water plus steam, heated to the boiling point for a given pressure, erupts from a complex vertical tube or fissure system.

2. Well-an artificial opening cut down from the surface into the zone of saturation. Only successful if drilled into permeable material and penetrates below water table

a. Some wells that pump confined aquifers flow at the surface of the ground.  They are called "artesian wells".  In them the water is under pressure because a confined aquifer has been penetrated.  The aquifer is also inclined so that one end is exposed to receive water at the surface and there is enough head to force the water above the aquifer wherever it is tapped.  When a well is driven into such an aquifer, the water that is tapped is under the pressure exerted by all the water piled up between the well and the source of the water at higher elevation.

1) The Castle Hayne limestone under eastern NC is a pressurized or artesian aquifer.

 

V. Recharge of groundwater

A. The ultimate source of groundwater is rainfall that infiltrates into the ground. In unconfined surface aquifers the water table rises or falls in response to discharge at the downstream end or recharge at the upstream end. So the water table in unconfined aquifers rises and falls in response to the amount of precipitation.

B. A number of factors decrease the amount of rainfall that penetrates into the ground to recharge the aquifers.

1. Time of year during which the rain falls.

a. During times of high evaporation (high temperature) more evaporates and less penetrates.

2. During rapid, heavy rainfalls more water runs off into the streams instead of soaking into the ground.

3. High slopes encourage run-off

4. Lack of vegetation promotes run-off

5. Impermeable surfaces (parking lots) near the surface promote run-off.

 

VI. Problems associated with groundwater withdrawal

A. If a well is pumped heavily and water is taken out of the ground faster than it can be replenished then the water table is pulled down in the form of an inverted cone centering on the well.  This is known as a CONE OF DEPRESSION. Wells located within a cone of depression have their water levels lowered, which requires lowering the pumps.

B. Heavy pumping can also result in LAND SUBSIDENCE, which is due to sediment compaction. The highly productive, but very dry areas of California are examples of regions that have suffered substantial land subsidence due to groundwater withdrawal.

C. Pollution from numerous sources including leaking underground fuel tanks, improperly installed and maintained septic tanks, chemical spills at ground surface, improperly maintained or installed wells which provide a conduit for surface pollutants down to the aquifer, landfills or other waste disposal with ineffective liners, etc.

D. In coastal areas overpumping (removing the water faster than it is replaced by natural groundwater flow) lowers the freshwater level in wells and allows seawater to migrate inland and pollute the well water. Called saltwater intrusion

E. Prolonged excessive overpumping of an aquifer throughout a region can cause dewatering. When the water level in an aquifer drops below the top of the aquifer, the aquifer material compacts thereby leaving less space for water to be stored. A dewatered aquifer can not be rehabilitated and has diminished storage capacity.

 

VI. Groundwater in areas of limestone bedrock

A. Calcium carbonate will slowly dissolve as groundwater flows through fractures and openings in the limestone. CAVES are the most impressive examples of dissolution of LS by groundwater.

1. Dripstones, stalactites (grow downward), stalagmites (grow upward), columns.

B.  Another feature developed in such regions that often cause considerable problems for local people are SINKHOLES.

1. These form when the cave roof collapses and the land          surface subsides.  They can also form when the carbonate

immediately below the soil surface dissolves.

C. An area with numerous sinkholes exhibits KARST TOPOGRAPHY.

D. Regions of karst topography also show DISAPPEARING STREAMS that lose their water down into a sinkhole that develops along the course of the stream. Sometimes the stream flow reappears at the surface farther downhill.

 

VII. Groundwater Resources of North Carolina

A. Groundwater supplies more than 3 million people or about half the state's population.

B. Besides withdrawals for public supply, the largest groundwater withdrawals are for mining and quarrying operations and process water for a number of textile and chemical industries.

C. General setting

1. N.C. is divided into 3 physiographic provinces-the Coastal Plain, Piedmont, and Blue Ridge.

2. The Coastal Plain (CP) aquifers are generally unconsolidated sand and gravel, or limestone layers, which are separated by clay.

a. These strata dip and thicken southeastward and comprise a wedge overlying crystalline bedrock. Under Cape Hatteras this wedge of sediments and sedimentary is at least 10000 feet thick. Under Greenville it is about 800 feet thick.

3. The Piedmont and Blue Ridge provinces are mostly underlain by massive crystalline and metamorphic rocks and derive their groundwater from them. Unlike the situation in the CP, groundwater suppliers in these provinces rely on finding interconnecting fractures to yield water.

4. Recharge is derived from precipitation (44-54in/yr-C.P. and Piedmont, 40-80in/yr-Blue Ridge)

a. In the C.P. less than 1 in of this precipitation recharges the deeper aquifers every year.

D. Principal aquifers of the Coastal Plain

1. The principal aquifers in the North Carolina CP are the Surficial, Yorktown, Castle Hayne, and Cretaceous.

a. Except for the surficial aquifers of sand, silt, clay, and gravel these aquifers are generally confined

2. The Surficial Aquifer is immediately below the ground surface and is, therefore, easily contaminated, however, it is a major source or water for rural homeowners who install individual wells.

a. Historically, surficial aquifer sands were the only source of freshwater for most of the barrier islands. However, heavy coastal development in recent years has required the exploitation of deeper, somewhat saltier water from the Yorktown and Castle Hayne. The salt is being removed from this groundwater at reverse osmosis plants.

3. The Yorktown Aquifer provides adequate supplies for single families on individual wells. In some places in northeastern NC well fields containing numerous wells are also supplying small municipalities.

4. The Castle Hayne is the most productive aquifer in N.C. Wells yielding more than 1000 gallons/minute can be readily developed and yields may exceed 2000gpm. It sometimes contains enough dissolved solids to require treatment before use.

5. The major withdrawals from the Castle Hayne Aquifer are to dewater the PCS Phosphate mine down the Pamlico (south bank) below Washington, NC. About half of the 75% of the water withdrawn from the Castle Hayne for industrial uses is accounted for by PCS.

a. The sudden drop in water levels in wells installed in the Castle Hayne in 1965 corresponded to the Aurora operation coming on line. About 65 million gal/day were withdrawn from this confined aquifer to reduce the artesian pressure, thereby facilitating dewatering of the overlying phosphate ore beds. This permitted dry pit mining. Water levels in the Castle Hayne have declined by at least 5 feet and by more than 150 feet over an area of 1300 square miles in response to this pumping. A small percentage of the water is recirculated in the mine and used for processing the phosphate ore, but most is dumped directly into the Pamlico River.

1) Various plans have been proposed to use this water as a public supply but the expense of piping it from Aurora to the major potential users to the west has so far prevented their implementation.

b. Changes in water level since the late 1960's are the result of fluctuating pumping rates and movement of the center of pumping as different areas of the property have been mined. Since about 1966 the cone of depression has not expanded indicating that the Castle Hayne can support this extremely high pumping rate without becoming dewatered.

5. The Cretaceous Aquifer is the principal aquifer in much of the central and southern Coastal Plain. Water is drawn from the Peedee, Black Creek, and Cape Fear Aquifers - all of Cretaceous age. This water is extremely pure - frequently requiring only chlorination before delivery to consumers. It is, therefore, a very desirable source. Because it is cheap to treat and deliver it has been extensively pumped, and recently, severely overpumped, so that drops in water levels have exceeded a dozen feet per year. Unlike the Castle Hayne, which has an extremely high hydraulic conductivity, water flow in the Cretaceous Aquifers is much slower, so it cannot replace groundwater as quickly as it has been withdrawn. In the Central Coastal Plain continued withdrawals will soon begin to dewater the aquifer. As a result the State has delineated the "Central Coastal Plain Capacity Use Area" (CCPCUA). Within the CCPCUA a progressive decrease in withdrawals from the Cretaceous Aquifers has been mandated culminating in a 75% decrease in about 15 years.