7.1 Groundwater occurrence, porosity/permeability & aquifers

The subsurface water profile

Below the land surface, water is distributed between two zones separated by the water table. The unsaturated (vadose) zone extends from the surface down to the water table; here pores hold both air and water, and the water is held under tension (pressure less than atmospheric). The saturated (phreatic) zone lies below the water table, where all interconnected voids are completely filled with water at pressures greater than atmospheric. The water table is the surface at which pore-water pressure equals atmospheric pressure — the level to which water rises in a shallow well screened in an unconfined aquifer.

Just above the water table, capillary forces draw water upward into the capillary fringe, a nearly saturated band whose thickness increases as grain size decreases — a few centimeters in gravel but up to a meter or more in silt. Although effectively saturated, this fringe is still under tension and is counted as part of the vadose zone. Recognizing it matters when interpreting shallow soil moisture and the true depth to usable water.

Porosity versus permeability

Porosity (n) is the ratio of void volume to total volume, expressed as a fraction or percent:

n = V_v / V_total

Porosity measures how much water a rock or sediment can store. Well-sorted sands have porosities of 25–40%; clays can exceed 50%; unfractured crystalline rock may be under 1%.

Permeability measures how easily fluid moves through the material — it depends on the size, shape, and interconnection of pores, not merely their volume. This distinction is central: clay has very high porosity but very low permeability because its pores are microscopic and poorly connected, so water clings to grain surfaces and barely moves. Gravel has lower porosity than clay yet enormous permeability. Intrinsic permeability (k, in m² or darcys) is a property of the medium alone, whereas hydraulic conductivity (K) also incorporates fluid properties (see 7.2).

Worked example — porosity

A 1,000 cm³ sample of dry sand is fully saturated and takes 320 cm³ of water to fill its pores. Porosity n = 320 / 1,000 = 0.32, or 32%.

Specific yield versus specific retention

Not all stored water drains by gravity. Specific yield (S_y) is the volume of water released per unit bulk volume of aquifer under gravity drainage. Specific retention (S_r) is the water held back against gravity by molecular attraction and surface tension. Their sum equals porosity:

n = S_y + S_r

Coarse gravel drains almost completely (S_y near 0.25, small S_r). Clay, despite n of about 0.50, has S_y near 0.02–0.05 because nearly all its water is retained on grain surfaces — most of its porosity is unavailable to wells. Specific yield, not total porosity, controls how much water an unconfined aquifer can actually deliver.

Worked example — specific yield

A silty sand has porosity 0.35 and specific retention 0.12. Then S_y = n − S_r = 0.35 − 0.12 = 0.23. A 2-m-thick, 1-m² column would yield 0.23 × 2 × 1 = 0.46 m³ of drainable water.

Aquifers and confining units

An aquifer is a saturated unit permeable enough to yield usable quantities of water to wells and springs. It is defined relative to less-transmissive units:

• Aquitard — low-permeability unit that transmits water slowly but can leak substantial volumes across large areas (silt, sandy clay).
• Aquiclude — effectively impermeable; transmits negligible water (idealized clay or shale).
• Aquifuge — neither stores nor transmits water (unfractured, unweathered crystalline rock).

Unconfined (water-table) aquifer: its upper boundary is the water table, open to the atmosphere through the vadose zone, so its level rises and falls directly with recharge. A perched aquifer is a saturated lens held above a discontinuous aquitard, separated from the main water table.

Confined (artesian) aquifer: bounded above and below by confining layers, it holds water under pressure greater than atmospheric. Water in a well tapping it rises above the top of the aquifer to the potentiometric (piezometric) surface — the imaginary surface defined by hydraulic head. If that surface lies above the ground, water flows freely: a flowing artesian well. The potentiometric surface is the confined-aquifer analog of the water table; the two must not be confused.

Recharge and discharge

Recharge adds water to the saturated zone — from precipitation infiltrating through the vadose zone, from losing streams, or from artificial injection. Recharge areas for confined aquifers are commonly distant outcrops where the permeable unit reaches the surface. Discharge removes water — via springs, gaining streams, wetlands, evapotranspiration where the water table is shallow, and pumping wells. Over the long term natural recharge balances natural discharge; pumping that exceeds recharge lowers the water table and produces aquifer depletion.

Representative porosity and hydraulic conductivity

The table captures the porosity–permeability paradox: clay tops the porosity column yet bottoms the conductivity column, while gravel does the reverse. For the ASBOG exam, be ready to rank common materials by both properties and to explain why a productive aquifer requires adequate storage and adequate transmission.