Lecture 3: subsurface water Flashcards
Describe the difference between the unsaturated and saturated zone.
Unsaturated - above water table (water in vadose zone) Saturated - below water table (aka groundwater/ phreatic zone - water not available for evaporation, variable and slow moving and has long residency time).
What is the water table?
Where porewater pressure is equal to atmospheric pressure.
How does movement of water contrast in the vadose and phreatic zones.
Vertical in vadose, lateral in phreatic.
Describe the importance of groundwater.
- Earth’s largest accessible store of freshwater (94%) with over half of this within 800m of the surface. - Sustains river flow during periods of dry weather and is a major water source for many.
What constitutes a groundwater aquifer?
- Large deposits of till and unconsolidated material that is sufficiently saturated to hold vast quantities of water. - Consolidated rock such as sandstone, limestone, granite or lava in which water flows through pores, cracks and openings in the rock.
Describe an unconfined aquifer.
Upper boundary is defined by the water table, which is free to rise and fall depending on volume of water in the aquifer. Water in a well would sit at the height of the water table. (vulnerable to contamination from activities on land).
Describe a confined aquifer.
Has an upper and lower boundary that constricts water flow and puts water under pressure. Water in a well would rise above the constricted upper boundary.
What is the confining layer typically made of?
Clay (as opposed to porous sandstone)
What underlies the main water table and what might it be made of?
The aquiclude (shale)
What might cause the movement of groundwater?
- Chemical, electrical, or most commonly a hydraulic gradient.
Darcy’s law describes the direction and rate of movement of water in the saturated zone for which two given values?
- The hydraulic gradient - The hydraulic conductivity of the saturated medium (k)
What relationship did darcy discover between total discharge (Q), hydraulic head difference, length and Area.
Q (m^3/s) varies in direct proportion to Area and hydraulic head difference, and inversely to L: Q = KA ((Ha-Hb)/L)
What is saturated hydraulic conductivity?
The ability of a porous media to transmit water, it varies spatially at all scales and can be difficult to measure.
What affects K in groundwater?
Pore size and interconnectivity, geometry of rock particles, geological processes (folding/faulting), fluid density and viscosity(influenced by temp and salinity change).
What does a negative K show?
Flow is in direction of decreasing hydraulic head.
What factors affect hydraulic head?
The porewater pressure and height above sea level.
What are the limitations of darcys law?
-Only applicable to homogeneous saturated porous media - Cross section needs to be greater than dimensions of the microstructure.
If inflow is equal to outflow, what does the slope of the hydraulic gradient reflect?
The hydraulic conductivity
What is the total potential energy of a fluid equal to?
The pressure head + gravitational head
What is porosity?
The percentage volume of the rock that is represented by voids or interstices.
What are original interstices compared to secondary ones?
Original - intergranular spaces in sedimentary rocks Secondary - Faults / joins
How is porosity related to aquifer retention?
Determines the max amount of water an aquifer can hold if fully saturated.
Define specific yield.
The amount of water that is available to freely drain from saturated rock/soil under gravity in order to sustain aquifer. (as % of total aquifer vol.)
What is the specific retention?
The volume of water that is retained (i.e. by surface tension forces as films around grains, or in capillary openings)
Draw a graph to show how the void ratio of specific yield, specific retention and porosity vary with grain size,
Describe recharge and discharge of aquifers.
Recharge - replenishment from surface Disharge - groundwater outflow or pumping
What is total head equal to in unconfined aquifers?
The elevation head (as upper bound is the water table, so pressure head is negligible)
In unsaturated flows, what is total potential a function of?
Osmosis, gravitational and pressure potentials
What governs water flow in the unsaturated zone, where water flows from high to low total potential areas?
-Soil-water interactions -Forces other than gravity -K (varies with the water content) -Rention forces (depend on moisture content and and soil texture and cause attraction between soil and water)
What are the three water retention forces?
- Adsorption (electrostatic forces cause the polar water molecules to be adsorbed on the charged faces of the solid particles) - Capillarity (surface tension at the interface between soil air and soil water) - Osmotic pressure due to the solute in soil water (important when there is a difference in solute concentrations across a permeable membrane, e.g. plant root surface)
