Groundwater Flashcards
Groundwater recharge
precipitation (comes from high inland reaches)
Losing stream
Sits above the water table. May exit into groundwater system
Gaining stream
Sits below water table, receives groundwater
Groundwater flow
- downhill through aquifers (regions of high porosity)
Aquifers
Regions of high porosity
Aquitards
Inhibit the flow of water
Confined aquifer
aquitard above it, can cause water to flow uphill
Unconfined aquifer
connected to the water table, no aquitard above it
Ocean discharge
can discharge into ocean or a seawater-freshwater interface can form
Perched aquifer
water gets stranded and cant flow out
Conceptual models
- topography
- aquifers
- aquitards
- water table
- flow direction
- piezometric surfaces
- position of wells
Aquifer material
gravels, limestone
Aquitard material
clay, silt, bedrock, glacial till, shale
Seepage face
When water in an aquifer breaks the surface
Protecting a spring for water supply
Horizontal, vertical
- slanted covers, rock entrance, overflow, outflow
Artificial springs (groundwater well)
- recharge
- discharge
- ground surface
- piezometric surface
- piezometers
How to get ground water
- drill a hole
- keep bad water out (casing, grout)
- let good water in (screen)
- get good water out (punmp)
- improve (sanitary seal, gravel pack, sand trap)
Groundwater depletion
pump lots of water out of wells, piezometric surface depresses, springs dry up, well levels and yields affected
Groundwater head
elevation that water will rise to or settle at in a semi-infinite straw
Contours
fitted through measurements of equal head
water flows perpendicualr to contours
K (Darcys law)
conductivity [m/s]
dh/dx
hydraulic gradient
A (Darcys law)
cross-sectional area
Water table in an unconfined aquifer
piezometric surface
Leakage
underlying aquifer is imperfectly confined. Direction depends on difference in head between layers
Storage
The amount of water in a groundwater system that we can extract
ΔV (storativity)
volume of water removed
Δh (storativity)
drawdown (m)
A (storativity)
area over which drawdown occurs
Extracting water from unconfined aquifer
easy (drinking while letting water in)
Extracting water from confined aquifer
hard, need large suction forces
Storativity
the volume of water that is extracted for each unit decline, for each unit area that decline occurs over
Storativity in an unconfined aquifer
= porosity = specific yield
Storativity in a confined aquifer
= thickness x specific storage or elastic yield
Magnitude of storativity
Confined aquifer Ss small, need large Δh to produce small q
Unconfined Sy large, need small Δh to produce large q
Water balance (without pumping)
Recharge
- precipitation
- upgradient groundwater
Discharge
- spring/stream
- downgradient groundwater
Water balance changes with pumping
- steeper hydraulic gradient = larger upgradient recharge
- shallower hydraulic gradient = smaller downgradient recharge
- reversal to spring (discharge to recharge)
Why use welltests
- can’t look at groundwater directly
- need to estimate aquifer properties
- understand aquifer geometry
Transmissivity eqn
T=Kb
Pump test
- temporary drawdown is intentionally induced to measure aquifer properties
- drawdown caused by pumping at fixed rate or series of fixed steps
- analyse using theis solution
Pump test solution
- plot drawdown on semi-log graph paper
- draw line through linear portion (start is full theis soln, end suggests flow boundary)
- use gradient and y-intercept (=log(1))
When is Theis solution not appropriate
- confined aquifer with leakage
method: Hantush-Jacob solution
parameters: c = hydraulic resistance of aquitard - Unconfined aquifer
method: Neuman-Moench semi analytical soln
parameter: Sy= specific yield
Physical meaning of super position
drawdown evolution around a well is linear so it can be added for wells at different locations- including imaginary wells
Flow barriers and recharge
lateral constraints on drawdown- solve by superposition of image wells
Well performance
- Sustainable yield
- well efficiency
Rorabaugh method
y = mx + c
- choose value of β that gives linear relationship between x and y
- use β to find T and Sy by guessing S and checking T (using t[test])
- Then estimate Qsus that limits drawdown to the maximum value (using t[pump])
Well components
- sanitary seal
- borehole
- grout
- pump
- pump intake
- screen
- casing
- sandtrap
- gravel pack
Drilling
old: percussion driven steel casing into ground
new: rotary cutting drill driven by mud circulation. Mud returns cuttings to surface for analysis
Screen
Open section of well, prevents particle entry, minimise entrance velocity
Screen slot size
Small enough to exclude particle sizes without causing entrance velocity to exceed requirements
Gravel pack
- removes fines around the well
Screen length
as long as possible BUT need to avoid drawdown, deep enough to avoid surface contamination and below top of gravel pack
Pump selection
Need adequate flow rate and head
Other well improvements
grout
- cement between casing and borehole (prevents contamination and anchors)
sandtrap
- empty section below screen for sand and silt to settle
sanitary seal
- prevent contaminants entering the well, provides access to well
Wellfield design issue
Well interface (overlapping cones of depression)
Wellfield design goal
Use as few pumps as possible with minimum drawdown to meet total pumping requirements
Source protection
prevent contaminated water reaching well by measuring time of travel
Source protection zones
Immediate zone (well head), middle zone (microbial viral attenuation), outer zone (persistent pollutants)
Time of travel contour lines
asymmetric (more uphill than downhill)
Adquate time of travel
50-100 days
c meaning
celerity- translation of wave while holding shape constant
D1 meaning
diffusion constant- rate at which wave spreads out
