5 Equipotential maps and protection zones Flashcards
Definition of Equipotential map
2-dimensional map: lines of equal ground water levels (potential), extrapolated from point values (wells and observation wells)
Informations obtainable from equipotential map
- gradients
- direction of flow (perpendicular to equipotential)
- cones of depression (wells)
- interaction with surface water bodies
Definition of equipotential line or surface
line or surface of equal potential, e.g. hydraulic head (third dimension!)
Esential preconditions for flow nets (and groundwater equipotentials):
- flow paths and equipotentials at right angles (90°)
- equipotentials at right angle to impermeable boundaries
- equipotentials always parallel to infiltrating boundaries (constant head)
- flow paths cannot cross
Flow nets are a solution of the 2D-Laplace-equation. Assumptions:
- homogeneous, isotropic aquifer
- fully saturated aquifer
- potential field constant over time (stationary)
- aquifer and water incompressible
- laminary flow (DARCY)
- known boundary conditions
How to make an equipotential map?
1. Measure distance to groundwater from surface. #if possible all wells on same day #Usually given in “metres below surface“ or “metres below well cap“
2. Correction for ground level elevation. #Elevation of ground level or well cap –distance to groundwater level = elevation of groundwater #usually given in “metres above se level“ [masl]
3. Connecting the equipotential dots. # Taking into account: •elevation of ground surface •water levels in surface waters •pumping wells •geology (formations of differing conductivity, faults …)
Ground und surface water relationship
1. Gaining stream # Groundwater level is higher than water level of river. Groundwater feeds the river. Typical base flow example.
2. Losing stream (saturated zone) # River fully saturated between river bed and aquifer. Water level on the river is higher than the the groundwater water level
3. Losing stream (non saturated flow) # Water only flows through certain of pores
4. No contact # No interaction at all between river and groundwater. Might be because of clogging, plugging or there's clay layer on river bed surface.
Note: The condition can change over space and time
Ground and surface water Image example
slide 12
Left image: # upstream: river to groundwater # downstream: groundwater to river
Right image: # upstream: groundwater to river # downstream: river to groundwater
Measurement and quantification of exchange
See image slide 13
- Ushape tube
(a) Water level in river is higher than groundwater, so water leaves the river
(b) Water level in groundwater is higher than river, so water flows to river - Drum cut in a half, make a hole and put plastic bag with certain amount of water. Plastic bag gains or loses water as water seeps out of or into the streambed.
Ideal to use in fine sandy or silty river bed surface.
How not to make an equipotential map
Do not forget to consider:
•infiltrating or exfiltrating surface water bodies
- overlapping cones of depressions of wells
- different aquifers (vertical)
- perched groundwaters
- impermeable boundaries, fault zones
- measurements taken on different days
- etc.
Delineation of groundwater catchments
Possibilities:
- directly “from equipotential map“*
- using analytical or numerical models
- area = pumped volume/ groundwater recharge
Subterranean and superficial catchments
Superficial (EZGo) and subterranean (EZGu) catchments need not be identical but are often similar
Groundwater protection zones
Protection zone 1: 10 m around well, fenced off
Protection zone 2: 50 days underground passage, not always applicable (e.g. deep aquifer), strict rules of land use
Protection zone 3: complete catchment, including all properties touched
