Ground Water Flashcards
Hydraulic Head
Measure of the mechanical energy of water at a location
- equals pressure head + elevation head
Elevation Head
Equal the elevation above some chosen reference level
- Base reference is sea level datum elevation
Pressure Head
Equals the length of the column of water above the screen
- Common practice defines the water pressure at the free interface between air and water as zero
- Function of density and gravity
Gaining Stream
Receives water from local, intermediate, or regional ground water flow.
- Seepage face at water table height
Losing Stream
Connected to the water table but loses water to the ground water
Perched Losing Stream
Loses water to ground water but is perched above it
Flow Through Stream
Connected to the water table where it gains water from one side of cross-section and loses it on the other side
Darcy’s discharge/flow equation
Discharge = area cross-section of flow path (length^2) x Hydraulic conductivity (1/T) x Hydraulic Gradient (Change in Hydraulic Head/distance between the 2 points)
Why is hydraulic conductivity relevant for calculating discharge?
Relevant for pore (matrix) flow in a saturated, porous medium
- lower in clays than gravels
Hydraulic gradient
- Change in head (elevation) between 2 points at the top of the groundwater table
- i = change in h/L (distance between 2 points)
- Function of Hydraulic head
- Gradient slope of the topic of the groundwater table (indicates direction of movement)
Darcy’s Flux Equation
- Volume of fluid passing through a unit cross-sectional area of A during a unit length of time of L
- q = Darcy’s flow (Q)/Area = L^3T^-1/L^2 = L/T
Three steps for finding how water moves through the subsurface
Step 1: Find discharge (Q)
Step 2: Find Flux, q = (Q/A (length))
Step 3: Find Effective (seepage) velocity (q/effective porosity n)
Step 4: Divide length by effective velocity
5 General rules for groundwater flow
- Water table reflects topography
- Equipotential contour lines perpendicular to divides, parallel to boundaries
- Streamlines perpendicular to equipotential contour lines
- Both horizontal and vertical flow components (down in uplands, up in lowlands)
- Flow nets complicated by changes in geology and topography
GRACE
Satellite that can measure groundwater (gravity as a proxy for GW)
- Gravity on Earth’s surface varies depending on material at the subsurface
- Can’t measure soil water b/c soil water is very small value
How does ground water flow vertically?
- Along potentiometric gradient from high to low
- i = change in hydraulic head/ change in elevation head between wells or piezometers
How does ground water flow horizontally?
horizontal i = change in h/ change in x
What are equipotential contours?
Lines of the same flow energy
- Lines of equal hydraulic head
What defines a flow tube?
Flow streamlines that are perpendicular to equipotential contour lines
What are some issues with ground water flow nets?
- Geological environment can affect flow greatly
- Doesn’t work well in fractured rock because water will preferentially flow along fractures and not so much equipotential lines
What are the 2 main methods that ground water is recharged naturally?
Infiltration and percolation
What are the 3 ways that recharge inputs are transferred?
- Flow to adjacent areas via through flow
- Re-surface as return flow (springs) or base flow (rivers)
- long term storage in deep aquifers
What is an important application of GRACE?
It can give a draught indicator from the wetness value of earth’s subsurface over time
How is GW in BC derived?
Infiltration and snowmelt
What are some potential challenges with GW in BC
- shallow and confined aquifers are vulnerable
- flow routes, rates, and residence times are poorly understood and unmodelled
- land use impacts changing recharge, storage, discharge
What are some impacts on GW in BC relating to quality and quantity?
- Quality: Contamination
- Quantity: Drainage diversion, artificial recharge, and withdraw
Point Source
- Easier to identify, has a single known location
- Landfills, gas stations, mines, disposal sites etc.
Non-Point Source
- Difficult to identify, protect against, fix, and legislate
- May not have boundaries, exact source unknown
- Agriculture, runoff of manure/fertilizer, herbicides, pesticites, urban runoff containing oil/soaps
California Aquaduct
- Artificial water feature to mitigate subsidence in california
- Gravity driven flow from Sierra Nevada Mnts. and power to pump water to some locations
- Power usage creates problems but getting the water is still more important
What are the two principal sources of toxic organic chemicals in water?
- Improper disposal of industrial and household wastes
- Pesticide runoff from farm fields, forests, roads, golf courses, lawns, etc.
Walkerton Ontario Incident
- E. coli from cow manure spread into cracked water well due to prolonged heavy rains
- Shared responsibility of Government and farms/company (privatization, incompetence, unconfined manure, public deception, etc.)
~1500 illnesses and fatalities
What conditions are needed for effective recharge?
- Water needs to move through the vadose zone
- Water needs to move in the aquifer, away from infiltration site so as to raise water build up of groundwater mound or ridge
What characteristics are desirable for infiltration recharge systems?
- Permeable soil
- Permeable vadose zone free of clay
- Confined aquifer that is permeable and thick to avoid groundwater mounds
- GW table must be deep (at least 10m below surface)
What are suitable sites for infiltration recharge?
- Flood plains of rivers, sand dunes, alluvial fans, permeable vadose zones, glacial outwash plains
- higher concentration of larger soil particles (faster infiltration)
What is the solution to building an infiltration basin where there is a clayey surface?
- Surface clay can be removed, infiltration basin can then be built in underlying permeable deposits
What is artificial recharge?
- Artificially increasing the amount of water that enters a groundwater reservoir
What is artificial recharge used for?
- Used for waste disposal, secondary oil recovery, land subsidence problems, and water resource management
How is artificial recharge engineered?
- Injection wells (like a reverse well)
- Infiltration ponds
- Drainage ditches
Infiltration Basin
- Collects water and gives it opportunity to infiltrate GW in optimal places (above unconfined aquifer)
- Goal is to build up mound of GW to create pressure difference and stimulate flow to a recovery area
What happens if a groundwater mound in an infiltration basin gets too big?
- The water won’t infiltrate further
- Water flow can reverse direction and won’t feed the recovery area
What are some purposes of artificial recharge?
- Conserve and dispose of runoff-flood water
- Supplement natural recharge
- Reduce or balance salt water intrusion
- Suppress (but can’t reverse) subsidence
- Store water in off-seasons to use during growing season
- Geothermal applications
- Remove suspended solids by filtration through ground
Saltwater Intrusion
- Marine sediments have saltwater in their pores and high hydraulic head that can push saltwater inland
- Salt water is more dense and can intrude further under fresh GW
- Pumping will exacerbate this and can draw saltwater in
How can saltwater intrusion be combated?
- Artificial recharge of fresh water
- Take uncontaminated fresh water from wells further away inland and channel to contaminated we and recharge to hold off salt intrusion
Iraq and Persian Gulf salt water intrusion problem
- Wetlands reducing due to climate change, drought, land use changes (damming, agriculture)
- Led to salt water intrusion from gulf
Ground subsidence
- land sinking as GW is drawn out faster than it can recharge
- Venice due to pumping out of GW but is exacerbated by climate change and sea level rise
