HYDROGEOLOGY NOTES
INTRODUCTION
a) Hydrogeology
Hydrogeology is a branch of geology that focuses on the study of the distribution,
movement, and properties of groundwater in the subsurface. It involves understanding the
geological, hydrological, and chemical aspects of groundwater systems.
Key Concepts: Hydrogeologists investigate aquifer properties, groundwater flow,
recharge and discharge areas, and the impact of human activities on groundwater
quality and availability.
b) The hydrological cycle
The hydrogeological cycle, also known as the water cycle or hydrological cycle, is a
continuous process that describes the movement and distribution of water on Earth. It
involves various stages, including evaporation, condensation, precipitation, infiltration,
runoff, and groundwater flow. Here's an explanation of the hydrogeological cycle:
1. Evaporation: The cycle begins with the process of evaporation, where the sun's heat
causes water from oceans, lakes, rivers, and other surface water bodies to turn into
water vapor. This water vapor rises into the atmosphere.
2. Transpiration: Transpiration is the release of water vapor from plants into the
atmosphere through small openings in their leaves called stomata. This is another way
water enters the atmosphere.
3. Condensation: As water vapor rises into the cooler upper atmosphere, it undergoes
condensation to form tiny water droplets. These droplets combine to form clouds.
4. Cloud Formation: Clouds are formed by the aggregation of water droplets or ice
crystals in the atmosphere. Clouds play a crucial role in the water cycle as they
eventually lead to precipitation.
5. Precipitation: Precipitation occurs when condensed water droplets in clouds combine
and become heavy enough to fall to the Earth's surface. This can take the form of rain,
snow, sleet, or hail.
6. Infiltration: Once precipitation reaches the Earth's surface, it may either infiltrate into
the soil or flow over the land as runoff. Infiltration is the process by which water
penetrates the ground and becomes part of the soil moisture or percolates deeper to
recharge groundwater.
7. Groundwater Flow: Water that infiltrates the ground may continue to move downward
through soil and rock layers until it reaches the water table, forming groundwater.
Groundwater flows laterally through permeable rock layers, creating aquifers.
8. Runoff: Runoff occurs when precipitation exceeds the infiltration capacity of the soil,
leading to the flow of water over the land surface. Runoff contributes to rivers, streams,
lakes, and other surface water bodies.
9. Surface Water Bodies: Water in rivers, lakes, and other surface water bodies may
evaporate directly into the atmosphere, completing the cycle, or it may continue to flow
through the landscape.
, 10. Return to the Atmosphere: Water returns to the atmosphere through evaporation from
surface water bodies and transpiration from plants, continuing the cycle.
The hydrogeological cycle is a dynamic and interconnected system that redistributes water
across the Earth's surface and subsurface. It plays a crucial role in maintaining the planet's
water balance, sustaining ecosystems, and providing freshwater resources for human
activities. The cycle is driven by solar energy, and its various components are influenced
by factors such as climate, topography, geology, and human activities.
Types of water
1. Meteoric - found in circulatory system of hydrologic cycle
2. Connate - fossil interstitial water out of contact with the atmosphere for appreciable
length of time.
3. Juvenile/magmatic - originates from volcanic emanations
4. Metamorphic - associated with heat, pressure and re-crystallization that created
metamorphic rocks.
c) Basic geochemistry and geophysics principles
a) Porosity: Porosity refers to the amount of open space within a rock or sediment
There are two types of porosity;
Primary Porosity: Type of porosity that develops in rocks during its formation.
This includes intergranular pores between mineral grains in sedimentary rocks.
Secondary Porosity: Porosity that develops after the rock has formed due to
geological processes such as fracturing, faulting, or dissolution. Examples include
fractures and solution cavities in carbonate rocks.
, b) Permeability: Permeability is the ability of a material to transmit fluids. It quantifies the ability
of a material to transmit fluids and is a crucial factor in understanding groundwater
c) Darcy's Law: Darcy's law is a fundamental principle in fluid mechanics and hydrogeology
that describes the flow of fluids (usually water) through porous media.
It states that the rate of groundwater flow is directly proportional to the hydraulic
gradient (the slope of the water table) and inversely proportional to the material's
hydraulic conductivity (a measure of its permeability).The law is particularly
applicable to situations where water moves through soils, rocks, or other permeable
materials. Here are the key concepts of Darcy's law:
1. Permeability (K): Permeability is a measure of how easily water can flow through a
porous medium. It is a property of the material and depends on factors such as pore
size, connectivity, and the nature of the fluid. Permeability is a crucial parameter in
Darcy's law and is denoted by the symbol K.
2. Hydraulic Gradient (dh/dl): The hydraulic gradient is the slope of the water table or
the piezometric surface in the direction of flow. It represents the change in hydraulic
head per unit distance. In Darcy's law, the hydraulic gradient is denoted by dh/dl,
where h is the hydraulic head and l is the distance along the flow path.
3. Flow Velocity/discharge (q): The flow velocity, denoted by the symbol q, represents
the volume of water passing through a unit cross-sectional area per unit of time. It is
proportional to the product of permeability (K), the cross-sectional area (A) through
which the water is flowing, and the hydraulic gradient (dh/dl). Mathematically, it is
expressed as q = -KA(dh/dl), where the negative sign indicates that flow occurs in the
direction of decreasing hydraulic head. Darcy's Law Equation: q=−KA(dl/dh) where:
q is the flow velocity,
K is the permeability,
A is the cross-sectional area,
dh/dl is the hydraulic gradient.
Sample question 1: an aquifer possesses hydraulic conductivity K of 0.02m/s, cross-sectional area
of 4m2, and a discharge Q of 0.1m3/s determine the hydraulic gradient.
The formula for Darcy's law is:
Darcy's Law is given by:
Q=−kA(Δh/l)
We can rearrange the equation to solve for the hydraulic gradient (Δℎ/L):
Δh/l=−Q/kA
