Introduction to Groundwater Flashcards
Hydrogeology
The sub-discipline of geology dealing with the distribution and movement of water in the Earth’s crust
Common Applications of Hydrogeology
- Extraction of water: drinking, irrigation, manufacturing
- Dewatering: construction, quarrying, flooding
- Contamination: industrial discharge, spills, leaks from buried tanks, infiltration of agricultural runoff
- Assessment of extraction effects
- Quantification and management of groundwater resources
What are aquifers
- Saturated rock/soil layer/s forming a reservoir for groundwater
- contains pores or open spaces to hold water
- pores or spaces generally interconnected
Forms of aquifers
- Unconfined
- Confined
- Semi-Confined (Leaky)
- Perched
Unconfined Aquifer
the water table forms the upper boundary and is exposed so is free to rise and fall
Confined Aquifer
groundwater is isolated from the atmosphere by much less permeable layers (aquitards and/or aquicludes) and is subject to higher pressures than atmospheric pressure
Aquitard
layer «_space;permeable than aquifer, not impermeable
Aquiclude
an impermeable layer (aquifuge if water-free)
Semi-Confined Aquifer
intermediate between unconfined and confined
Perched
discontinuous (in space and time) saturated conditions overlie unsaturated conditions
2 Categories of aquifer properties
Storage and Flow properties
What are the storage properties and what do they dictate
- dictate how aquifers store and release water
- Porosity, specific storage, specific yield and storativity
What are the flow properties and what do they dictate
- dictate how water moves through aquifers
- hydraulic conductivity, intrinsic permeability, transmissivity
Porosity
- % of aquifer consisting of pore spaces (voids)
- Larger pore space (more voids) = greater porosity therefore aquifer has greater water bearing capacity
- provides indication of volume of water in aquifer
Permeability
capacity of aquifer to transmit water through pore space and is related to connectivity of pores
What are the 2 forms of porosity and what type of rock do they dominate
- Primary porosity: pore space created during the formation of aquifer and they generally dominate in clastic sedimentary rocks
- Secondary porosity: pore space formed through fracturing and/or weathering of aquifer after formation and generally dominate in non-clastic sedimentary and non-sedimentary rocks
What is primary porosity controlled by
- grain size (larger = higher)
- grain shape (rounded = higher)
- sorting (well sorted = higher)
- cementation (less cement = higher)
What is secondary porosity controlled by
- degree of weathering (more = higher)
- persistence of fracture (higher = higher)
- spacing of fractures (closer = higher)
- openness of fracture (greater = higher)
- filling of fractures (less = higher)
- connectedness of fractures (more = higher)
Hydraulic head
- Height above a datum, e.g. mean sea level, of a column of water that can be supported by the hydraulic pressure at a given point in an aquifer; units L
- elevation to which water will rise in a borehole connected to a point in an aquifer under pressure
- provides indication of the ground water flow direction
Hydraulic gradient
- Rate of change in hydraulic head (h) per unit of distance of flow in a given direction
- Equates to slope of water table or piezometric surface
- Provides indication of groundwater flow direction
Groundwater flow
- equipotential lines are contours of h and flow lines indicate flow direction in water table maps
- Flow lines always move down i, i.e. from high h to low h
- Steep to vertical i in recharge and discharge areas
- Gentler sloping to horizontal i in between
- Downward flow in recharge area
- shallower groundwater has higher h so a downward i forces water down into the aquifer
- Upward flow in discharge areas
- deeper groundwater has higher h so an upward i forces water up out of the aquifer
Flownet
set of intersecting equipotential lines and flow lines representing two-dimensional steady flow through porous media
Darcy’s Law
Flow (Q) = - hydraulic conductivity (K) x are of pipe (A) x hydraulic gradient (dh/dl)
Hydraulic Conductivity
- The rate of flow through a unit cross-sectional area of a media under a unit hydraulic gradient; volume/area/time – units L/t
- calculates rate of groundwater movement
- not a measure of permeability
Iso/Anisotropy
- If K is independent of the direction of measurement at a point in a geologic formation the formation is isotropic
- If K varies with the direction of measurement at a point in a geologic formation the formation is anisotropic
Homo/Heterogeneity
- If K is independent of the position within a geologic formation the formation is homogeneous
- If K is dependent on position within a geologic formation the formation is heterogeneous
Causes of anisotropy and heterogeneity
- sedimentation: grain size and distribution; sphericity; nature of packing
- stratification: bedding; lenses
- structure: faults
Groundwater Discharge calculation
calculated using Darcy’s law
Darcy’s Law at a macroscopic level
−Qis defined as flow through 1m2 of aquifer, i.e. a unit cross-sectional area of saturated porous material−This area comprises voids and matrix
−vD(specific discharge or darcy velocity)not an accurate portrayal of true microscopic velocity of water along winding flow paths through voids
−Average linear velocity (v) greater; gained from knowing ne (effective porosity) of the aquifer material
−Accounts for flow through voids only
