1. Groundwater
Water in the saturated region of the subsurface is groundwater
Groundwater is a major source of usable water in many parts of the world. In some cases it is the only source of freshwater. Even in areas surface water is available, groundwater may be the preferable source because of water temperature, quality, or accessibility.
Forest have appreciable effect on groundwater, particularly when the water table is near the surface
Streamflow can be maintained in perennial streams between precipitation events through baseflow
Water moving down the unsaturated zone will percolate and feed groundwater

2. Nomenclature
Aquifer: Geologic stratum that can hold or transmit water.
If the aquifer is bounded above by water table it is an unconfined aquifer. The water level in a well penetrating the unconfined aquifer reveals the water table surface.
The slope of the water table determines the direction of flow, which could change during the year with recharge and discharge rates, particularly in flat terrain (coastal plains, swamps, etc.)

3. Nomenclature
A pumped well in an unconfined aquifer creates a cone of depression.
Drawdawn of the water table creates a hydraulic gradient in the direction of the well.
Permeability determines the rate at which water will move to the pumped well and thus the yield of the well.
Wells in Southeastern Piedmont yield only 20 to 40 Liters per minute, while wells in limestone aquifers may yield 8 to 11 m3/min

4. Nomenclature
Confined aquifer (artesian aquifer): Water is confined below a relatively impermeable geologic formation.
Water in a confined aquifer is under pressure and as a result water will rise into wells penetrating the confined formation.
An imaginary line projected across the water levels in a series of artesian wells is the piezometric surface.

5. Nomenclature
A flowing well results when the piezometric surface is above the ground surface.
Recharge to a confining aquifer occurs generally where the confining formation emerges at the surface, often in the higher ground many kilometers away. Many confined aquifers are huge and supply all the water used by man in some regions. National Atlas
Forests have little influence on confined aquifers except in the recharge zone

6. Nomenclature
The water table generally emerges at a stream channel or springhead.
A stream fed by groundwater is called an effluent stream.
A stream losing water to groundwater is called influent stream.
Perched aquifer is a special case of the unconfined aquifer. It occurs when a relatively impermeable zone such as a clay lens traps water as it percolates toward the main groundwater body.
They are frequently the source of springs. Perched water table fluctuates during wet periods and may disappear completely during dry periods.

7. Nomenclature
Aquiclude: A hydrogeologic unit which, although porous and capable of storing water, does not transmit it at rates sufficient to furnish an appreciable supply for a well or spring
Aquifuge: A hydrogeologic unit which has no interconnected openings and, hence cannot store or transmit water
Aquitard: A confining bed that retards but does not prevent the flow of water to or from an adjacent aquifer; a leaky confining bed. It does not readily yield water to wells or springs, but may serve as a storage unit for ground water

8. Forests and Groundwater
Because forest soils maintain high infiltration, forest land favors high quality groundwater.
At the same time, forests prevent some recharge and take drafts from shallow aquifers by high ET rates.
In coastal plains, forests lower water tables by ET and maintain a relatively swamp-free condition during most of the year.
It is common in these regions for water tables to rise following forest cutting, due to reduced ET
High water tables usually have adverse effect on tree growth, and growth usually improves when the water table is lowered by a network of drainage ditches
Drainage also permits improved vehicular access during wet seasons, and economic advantage during timber harvesting

9. Example
Determine the rate of seepage if hydraulic conductivity of the pervious medium is 0.25 ft/hr.
Solution:

10. Example
Determine (a) the rate of flow through the aquifer and (b) the travel time from the head of the aquifer to a point 4 km downstream Assume average width = 5 km
K = 50 m/day
f = 0.2
Solution:

11. Steady Flow in unconfined aquifers
Let discharge per unit width be qx = qh, where
q: Darcian flux
h: depth of flow
Integrating it from x = 0 where h = h1 to x = L where h = h2
If there is a uniform recharge with a rate Re then qx = Re x. Then

12. Steady Flow to a Well
Initial piezometric level is horizontal, so no initial groundwater motion
As water is being pumped water flows toward the well lowering the piezometric surface and creating drawdown
For constant Q steady state is eventually achieved
, integrating from r1 to r2 with T = bK (transmissivity)
It is applicable in the range rw ≤ r ≤ R, where R is radius of influence. Drawdown z(r) is given by

13. Steady Flow to a Well
In the case of unconfined aquifer, the saturated depth b is not constant and decreases toward the well
Integrating similarly from r1 to r2
Note that using T = K(h1+ h2)/2 in the equation developed for confined aquifer results in same equation
For small drawdowns, i.e. z << H, confined and unconfined aquifers have same equations.

14. Flow Nets Head decreases linearly with distance from left boundary
h = hL – qx/K
On a plan view of the channel-aquifer system, the contour lines of the potentiometric surface are parallel to the channels
Spacing of the contours indicate slope of this surface
Once these lines of equal hydraulic head or equipotentials have been established flow direction can be found by drawing perpendiculars called streamlines
Together equipotentials and streamlines constitute a flow net

15. Flow Nets
Consider an idealized valley in which groundwater flow from the hillslopes is perpendicular to the axis of the valley
Flow net from a cross-section shows U-shaped paths from the ridge to valley bottom
Note that water flow near the stream is not horizontal, but has a significant upward component
Construction of accurate flow nets require considerable practice, usually done using numerical solutions to GW equations
They provide not only visualization of groundwater flow paths, but also information on the rate of groundwater flow in a particular region

16. Quantifying GW flow using flow nets
Lets’ return to the horizontal flow through an aquifer bounded by two channels separated by distance L
We find that
Discharge per unit length = qb where b is aquifer thickness
The area between two streamlines is called streamtube
Total discharge through a streamtube
Because dm = ds , Qs = Kbdh
If we know K (or T=Kb), Qs is simply contour interval (dh) multiplied with transmissivity. Total Q, is Qs multiplied with number of streamtube.

17. Example Hydraulic gradient is created beneath the dam.
Low-permeability layer exists at depth, which represents bottom of the aquifer beneath the dam
Assume 100 m wide dam, and K =10-10 m/s.
hup = 26 m, hdown = 10 m, and dh = 2 m
Qs = Kbdh = (10-10)(100)(2) = 2·10-8 m3/s
Total number of streamtubes = 3
Total flow = 6·10-8 m3/s  1.9 m3/yr
Fairly small, but that’s what we would hope for

18. Recharge and Discharge Areas
A recharge area occurs where water is crossing the water table downward, hence recharging the groundwater system
A discharge area occurs where groundwater is moving upward across the water table, thereby discharging into the unsaturated zone above or to the land surface or a surface water body
Equipotentials and streamlines may both intersect a non- horizontal water table
And the orientation of these lines provides a means of delineating recharge and discharge areas

19. Land Subsidence
A common misconception concerning confined aquifers has been that confining layers are perfectly impervious
No geologic formations are truly impervious and confining units such as aquitards (e.g. clay) may contain large amounts of water that can move given enough time
When overlying units are clay pumping the confined aquifer de-waters clay units
Clay shrinks when it is de-watered (consider the cracks that appear on he bottoms of mud puddles when dried up)
In Mexico City lowering of the potentiometric surface resulted in removal of water from overlying clays
Land surface subsided by some 7.5 m in central part of Mexico City

20. Other issues
Salt Water intrusion: The recharge zone of an aquifer near the seacoast is likely to be inland, often at considerable distance. In these coastal areas, a lowered water table may induce sea water to reverse the flow toward the sea. Sea water moving inland is called a saltwater intrusion
Mining: Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries. When the usage of water is greater than the recharge, it is referred to as mining water (the water is often called fossil water because of its geologic age). Under those circumstances it is not a renewable resource.
Pollution: Not all groundwater problems are caused by over-extraction. Pollutants released to the ground can work their way down into groundwater. Movement of water and dispersion within the aquifer spreads the pollutant over a wider area, which can then intersect with groundwater wells or find their way back into surface water, making the water supplies unsafe.
e.g: Arsenic problem in Ganges Plain of northern India and Bangladesh