PART 7 and aquitards

Definitions

Aquifer - geological formation which contains and yields water.

- saturated, permeable geologic unit which can transmit significant quantities of water.

Aquitard - saturated, permeable geologic unit which cannot transmit significant quantities of water (but can transmit small quantities). Also called a semi-pervious formation or leaky formation.

Aquiclude- geologic formation which may contain water, but is incapable of transmitting water.

Aquifuge - geologic formation which neither contains nor transmits any water.

In reality, there are no aquicludes and aquifuges and these terms are no longer used.

Types of aquifers

(1) Unconfined is one whose upper boundary is the water table, i.e., where pressure is zero (p=0).

Look ate the total head h: h = z + p/γ

γ At the top of the aquifer, htop = ztop + ptop/ but because ptop = 0, we have:

htop = ztop which means that if the head (h) increases, groundwater table rises.

Hydrogeology, 431/531 - University of Arizona - Fall 2002 Dr. Marek Zreda Aquifers and aquitards 46

There are four types of unconfined aquifers:

(a) aquifer

VALLEY AQUIFER

water table

vadose zone

phreatic aquifer

bedrock divide

yyyyyy Sources of water: infiltration of rain water; surface water bodies; lateral influx.

Sink of water: rivers.

Water table reflects topography.

Saturated zone - below water table.

Unsaturated (vadose) zone - above water table.

Examples: High Plains aquifer (Ogallala Formation), Coastal Plains aquifer (Atlantic, Gulf).

Hydrogeology, 431/531 - University of Arizona - Fall 2002 Dr. Marek Zreda Aquifers and aquitards 47

(b) valley aquifer in arid zones Surface recharge is negligible because of high evapotranspiration rates. Only in valleys, rivers may carry water from mountains and recharge the aquifer.

VALLEY AQUIFER (IN ARID ZONES)

flooded wadi

yy

yy

water table

divide

Here, contrary to valley aquifers (in humid, temperate climatic zones), water table is highest beneath rivers.

Examples: North African aquifers (Nubian Aquifer), aquifers in the American Southwest (Ari- zona, New Mexico).

Hydrogeology, 431/531 - University of Arizona - Fall 2002 Dr. Marek Zreda Aquifers and aquitards 48

(c) alluvial aquifer River Along streams. Usually in equilibrium with the stream, i.e., alternately drains and recharges streams along their length and at different times.

yyy Example: Rhine River

Stream may be either gaining water from the aquifer or losing water to the aquifer.

yyy

Stream

losing part gaining part (upstream) (downstream)

red = equipotentials green = flow directions

(d) perched aquifer Located on impermeable lenses or discontinuous layers.

yyyy

yyy perched water table

yyyclay lens sand

yyyy water table

yyyy

Hydrogeology, 431/531 - University of Arizona - Fall 2002 Dr. Marek Zreda Aquifers and aquitards 49

(2) Confined aquifer is one in which the top of the saturated zone is confined (bounded) by an ≠ aquitard, i.e., at the top of the aquifer, pressure is not zero (ptop 0).

γ htop = ztop + ptop/ thus, ≠ htop ztop which means that if the head (h) increases, the pressure (p) also increases.

In a confined aquifer, the piezometric head (or water level in an observation well, or a piezometer) is higher than the upper boundary of the aquifer.

If the head is higher than the surface elevation, the aquifer is artesian.

Piezometric surface - a conceptual, imaginary (!) surface joining the water levels in all piezome- ters in the aquifer. In a phreatic aquifer, it was the water table and it had a physical meaning.

recharge area piezometric surface

well

yyyyyy

discharge area

yyyyyy artesian well

Examples of artesian aquifers: the in , Milk River aquifer in

Alberta (Canada).yyyyy

Hydrogeology, 431/531 - University of Arizona - Fall 2002 Dr. Marek Zreda Aquifers and aquitards 50

Water seeks the most efficient way to lose potential (or energy). Therefore, in isotropic and homo- geneous media, water flows in the direction perpendicular (normal) to the equipotential lines, i.e., normal to the piezometric (potentiometric) contours.

Isotropic Anisotropic aquifer aquifer

h1 h2 h1 h3 h2 no preferred h3 direction _ q preferred direction

_ q

Flow line (along the direction of specific discharge q) is a compromise between the direction of applied gradient J and the preferred direction (or the direction of least resistance to flow).

Hydrogeology, 431/531 - University of Arizona - Fall 2002 Dr. Marek Zreda