Hydrogeology

1. Groundwater and Aquifers

• Groundwater: Water located beneath the Earth's surface in soil pore spaces and fractures of rock formations.
• Aquifer: A geological formation that can store and transmit groundwater.
  • Unconfined aquifer: Water table is open to the atmosphere through permeable material.
  • Confined aquifer: Bounded above and below by impermeable layers (aquitards or aquicludes).
• Water table: The upper surface of the saturated zone in an unconfined aquifer.

2. Porosity and Permeability

• Porosity ($n$): Percentage of void space in a rock or sediment that can hold water.
• Permeability ($k$): Ability of a material to transmit fluids; depends on pore connectivity.
• Effective porosity: Portion of total porosity that contributes to fluid flow.

3. Darcy’s Law

Describes the flow of groundwater through porous media:

$$Q = -K A \frac{dh}{dl}$$

• $Q$: discharge (volume/time)
• $K$: hydraulic conductivity (length/time)
• $A$: cross-sectional area
• $\frac{dh}{dl}$: hydraulic gradient (change in hydraulic head per unit length)

1. Hydraulic Head

• Sum of pressure head and elevation head.
• Determines direction of groundwater flow: from high to low hydraulic head.

2. Flow Regimes

• Laminar flow: Smooth, orderly flow in porous media (valid for most groundwater).
• Turbulent flow: Rare in groundwater, occurs in large conduits or fractures.

3. Flow Nets

• Graphical representation of flow lines and equipotential lines.
• Used to analyze two-dimensional steady-state groundwater flow.

1. Recharge

• Process by which water infiltrates from surface to replenish aquifers.
• Sources: precipitation, rivers, lakes, irrigation return flow.
• Influenced by soil type, vegetation, land use, and climate.

2. Discharge

• Groundwater exits aquifer to surface or subsurface.
• Common discharge points: springs, rivers, wetlands, wells.

1. Natural Factors Affecting Quality

• Mineral dissolution (e.g., calcium, magnesium)
• Redox conditions (aerobic vs anaerobic)
• Temperature and pH

2. Anthropogenic Contaminants

• Nitrates from fertilizers
• Industrial chemicals (solvents, heavy metals)
• Pathogens from sewage
• Hydrocarbon spills

3. Contaminant Transport Processes

• Advection: transport with groundwater flow.
• Dispersion: spreading due to velocity variations.
• Diffusion: molecular movement from high to low concentration.
• Sorption: adsorption/desorption on soil particles.
• Degradation: chemical or biological breakdown.

1. Well Drilling and Pumping Tests

• Wells provide access to groundwater for sampling and extraction.
• Pumping tests measure aquifer properties (transmissivity, storativity).

2. Geophysical Methods

• Electrical resistivity, seismic refraction, ground-penetrating radar to map subsurface features.

3. Monitoring Techniques

• Observation wells for water level measurement.
• Sampling for chemical and isotopic analysis.

1. Overexploitation Risks

• Lowering of water table (drawdown)
• Land subsidence
• Reduced surface water flow
• Degradation of water quality

2. Artificial Recharge Techniques

• Infiltration basins
• Recharge wells
• Managed aquifer recharge (MAR)

3. Legal and Institutional Frameworks

• Water rights and permits
• Protection zones around wells
• Integrated water resources management (IWRM)

Key point: Groundwater is a critical natural resource whose sustainable management requires understanding its geology, flow dynamics, quality, and human impacts.