EOS305_2024_2025_Lecture_Slides_008

Groundwater Hydrogeology

  • Groundwater Hydrogeology is defined as the study of the laws governing the movement of subterranean water. It includes the mechanical, chemical, and thermal interactions of this water with porous solids, and the transport of energy, chemical constituents, and particulate matter by the flow.

Volume of Water on Earth

  • The total volume of water present in, on, or above the Earth’s crust is approximately 1,388,003,940 km³ or 1.388 x 10²¹ liters, which represents about 0.1% of the Earth’s total volume.

Water Cycle and Reservoirs

  • Water is in a constant flux between various reservoirs:

    • Seawater: 97.5%

    • Freshwater: 2.5%

      • Ice caps and glaciers: 68.7%

      • Groundwater: 30.1%

      • Remaining: 1.2% found in rivers, lakes, the atmosphere, living organisms, soil, and permafrost.

Subsurface Water Zones

  • There are two main zones when discussing groundwater:

    • Unsaturated Zone: This is the zone above the water table (WT) where water pressure is less than atmospheric pressure. The moisture content is below field capacity, meaning the water fills only part of the void spaces in the soil or rock.

    • Saturated Zone: This is located below the water table where water pressure is greater than atmospheric pressure and porosity equals effective porosity.

Porosity

  • Definition: Porosity (n) is the percentage of void space in a rock or sediment, calculated as:

    • n = Volume of Void Spaces / Total Volume

    • Effective Porosity (ne): This refers to the interconnected pore spaces available for fluid flow, where effective porosity is always greater than or equal to total porosity (n ≥ ne).

  • Types of Porosity:

    1. Primary Porosity: The void spaces that formed simultaneously with the rock or sediment (e.g., sand dunes).

    2. Secondary Porosity: The void spaces that develop after the formation of the rock or sediment (e.g., faults, karst features).

    • Examples: Beach sand and lava tubes exhibit primary porosity, while faulting and jointing represent secondary porosity.

Controls on Porosity

  • Porosity is influenced by sorting and shape of particles, rather than the size of the pores or the material itself.

  • Water Table: The water table is defined as the underground surface at which the water pressure is equal to atmospheric pressure. Mapping the water table facilitates understanding groundwater flow dynamics.

  • Specific Yield (Sy): Refers to the volume of water that drains from a saturated medium due to gravity.

  • Specific Retention (Sr): The portion of water that remains in the pore spaces against the force of gravity.

Investigating Porosity Through Experimentation

  • An example experiment could involve filling a container with fine sand, adding water, draining it, and recording the quantities of water relative to the porosity.

    • If 1 liter of fine sand retains 100 ml of water, this translates to:

      • Porosity of: 30%

      • Yield of: 200 ml (20%)

      • Retained: 100 ml (10%)

    • The equation n = Sy + Sr examines the relationship between porosity and the ability to retain water.

Measuring Water Levels in Wells

  • Depth to Water (DTW): When observing water levels in wells, DTW refers to measuring how deeply the water is positioned below the surface. Understanding these water levels allows for establishing hydraulic head across different reference points, which helps in predicting groundwater flow and saturation levels.

  • Hydraulic Head (h): Total head (h) combines pressure head (ψ) and elevation head (z), calculated as h = ψ + z. A barometric measure of 1 bar is approximately equal to 10 meters of water.

Flow Direction and Hydraulic Gradient

  • Water level data from multiple wells can indicate local flow conditions and general flow direction. The hydraulic gradient can be determined by the formula dh/dL, indicating the change in hydraulic head with distance.

    • The basic flow equation is given by Darcy’s Law, Q = -KA(dh/dl), where:

      • Q = discharge (volume of water per time)

      • A = cross-sectional area of flow

      • K = hydraulic conductivity

      • dh/dl = hydraulic gradient.

Hydraulic Conductivity (K)

  • K is the measure of the ease with which fluid moves through a medium, influenced by both the properties of the fluid (density and viscosity) and the medium's characteristics. K values change based on the medium type:

    • Gravel: 26m/d – 260m/d

    • Igneous & Metamorphic Rock: 0.0009mm/yr – 0.6mm/yr

    • Clay: 0.3mm/yr – 0.15m/yr

Aquifer Types and Properties

  • An aquifer is defined as a geologic unit that can transmit significant quantities of water under ordinary hydraulic gradients. There are two main types of aquifers:

    1. Unconfined Aquifer: Directly open to the surface, with water levels fluctuating with the pressure.

      • The water table can rise and fall due to changing external conditions.

    2. Confined Aquifer: Trapped between impermeable layers, with water held under pressure. The potentiometric surface represents the potential water level in such aquifers.

Additional Concepts in Aquifer Dynamics

  • Storativity (S): Represents the volume of water an aquifer releases from storage per unit area per unit change in hydraulic head. Confined and unconfined aquifers display different storativity behaviors due to their geological constraints.

  • Transmissivity (T): Refers to the ability of an aquifer to transmit water through its saturated thickness, computed as T = Kb, measured in metrics per time (e.g., m²/day).