Hydrogeology: Porosity, Permeability, Water Tables & Aquifers

Porosity

Porosity is the percentage of void space (pores) in a material compared to its total volume.

Primary Porosity

Original pore spaces formed during rock/sediment formation.

Secondary Porosity

Pore spaces created after formation (fractures, dissolution).

High Porosity Materials

Well-sorted sediments (uniform grain size), unconsolidated materials (sand, gravel), fractured rocks. Examples: Sandstone, unconsolidated sand (20-40% porosity).

Low Porosity Materials

Poorly sorted sediments (mixed grain sizes), highly cemented rocks, crystalline rocks. Examples: Granite, shale (1-10% porosity).

Permeability

Permeability is the ability of a material to transmit fluids through its pore spaces.

Factors Affecting Permeability

Pore size Larger pores Sorting Fracturing Fractures can greatly increase permeability.

Key Point on Permeability

High porosity doesn't always mean high permeability (e.g., clay has high porosity but low permeability due to tiny pore sizes).

Capillary Water

Water held in soil pores by surface tension and capillary forces, above the water table.

Relationship with Soil Texture

Fine-textured soils (clay): High capillary water retention due to small pore spaces and strong surface tension; Medium-textured soils (loam): Moderate capillary water retention; Coarse-textured soils (sand): Low capillary water retention due to large pore spaces and weak surface tension.

Rule of Capillary Water

Smaller particles = more surface area = stronger capillary forces = more capillary water.

Soil Texture

The relative proportions of sand, silt, and clay particles in soil.

Particle Size Classification

Clay: < 0.002 mm; Silt: 0.002 - 0.05 mm; Sand: 0.05 - 2.0 mm.

Water Table

The upper boundary of the zone of saturation where groundwater pressure equals atmospheric pressure.

Characteristics of Water Table

Fluctuates with seasons and precipitation; Generally follows surface topography; Depth varies by location and geology.

Subsurface Water Zones

Zone of Aeration (Vadose Zone): Above the water table, pores contain both air and water, includes soil water and capillary fringe; Zone of Saturation (Phreatic Zone): Below the water table, all pore spaces completely filled with water, where groundwater exists.

Unconfined Aquifer

Water table is the upper boundary, direct connection to surface, water level in wells equals water table, recharge occurs directly from surface.

Confined Aquifer

Bounded above and below by impermeable layers, water under pressure, water level in wells rises above aquifer top, recharge occurs at distant locations where aquifer outcrops.

Aquitard

A low-permeability layer that restricts groundwater flow but doesn't completely stop it. Examples include clay layers, shale, and silt layers.

Artesian Wells

Formation requires a confined aquifer, recharge area at higher elevation, hydraulic pressure from recharge area, and well penetration into confined aquifer.

Flowing artesian well

Water flows to surface naturally.

Non-flowing artesian well

Water rises but doesn't reach surface.

Good Recharge Areas

Characteristics include high permeability, gentle slopes, adequate precipitation, minimal development, permeable surface materials, and deep water table.

Water Budget Calculation

Basic equation: Inflow - Outflow = Change in Storage.

Inflows

Include precipitation (P), surface water inflow, and groundwater inflow.

Outflows

Include evapotranspiration (ET), surface water outflow, and groundwater outflow.

Water Contour Maps

Purpose is to show the elevation of the water table or potentiometric surface using contour lines.

Contour lines

Connect points of equal water table elevation.

Flow direction

Perpendicular to contour lines, from high to low elevation.

Hydraulic gradient

Change in elevation divided by horizontal distance.

Closely spaced lines

Indicate a steep hydraulic gradient.

Widely spaced lines

Indicate a gentle hydraulic gradient.

Key Applications of Water Contour Maps

Include determining groundwater flow direction, locating potential contamination paths, designing well placement, and calculating flow rates.

Hydraulic head

Water flows from high to low hydraulic head.