Regional groundwater flow
-Read Fetter Chapter 7
- Simple flow nets provide basis for understanding regional groundwater flow
- flow patterns are dictated by the variation in the shape of the basin and water
table, and spatial patterns of hydraulic conductivity
Recharge area Discharge area - water is crossing the water table upwards into unsaturated
zone, land surface, or base of some surface water body
Natural basin yield - average rate of discharge from a hill slope or basin in the
absence of anthropogenic disturbances
- water table is usually at or near the surface at the discharge area and are
usually the sites of lakes, marshes, etc.
- the line separating recharge from discharge areas is called the hinge line of
(or midline)
Effect of Basin aspect Ratio
- basin shape exerts influence over the groundwater flow pattern
- the basin aspect ratio (length to depth) is one measure of basin shape
-two basins of identical length but different depths will have different
natural basin yields
- the deeper basin will have a higher yield because there is more crosssectional area to pass the water through
- the depth of the basin will also affect the pattern of flow
- in deep basins vertical hydraulic gradient’s exist over a large portion of the
basin
- and shallower basins flow is essentially horizontal over most of a basin
Effect of topography
-Complex land-surface topography should produce complex water-table
topography
- similarly local flow systems are superimposed on top of regional flow
systems
-independently these flow systems appear similar but on different scales
-local vs. regional is subjective, but regional is usually considered to recharge
at a basin divide and discharge at a valley bottom
- local flow system water has less of a residence time underground and
therefore less interaction with geology
- local flow system water temperatures close to mean annual surface, more
active in hydrologic cycle
- spring flows from local systems is more variable than regional
- intermediate flow systems have at least one local flow system between their
recharge and discharge areas
-for shallow basins the regional system may exist but may be attenuation to
as a result of the dominant influence of local flow systems
Stagnation points
Regional Groundwater Flow continued...
Flowing Artesian Wells
-Artesian Wells are usually associated with confined aquifer’s
-not necessary to have confinement
-unconfined aquifer’s can also provide flowing Wells if the well opening is at a
potential that is higher than the ground surface
Flow system mapping (where are the zones of recharge/discharge)
-distribution of discharge and recharge areas can be mapped on basis of
field observation
-five basic types of indicators:
1.
2. Piezometric patterns
-ideally would like to have piezometers everywhere, but not economical or
practical
-open Wells are not good piezometers if they are open to flow throughout
their depth
-however plots of well depth vs. water level can be useful given many wells
in a single topographic region
3
- salinity generally increases along the flow path
-water from recharge areas is generally fresh, water from discharge areas
is relatively saline
4. Environmental isotopes
-most commonly used 2H, 3H, 18O, 14C
i. 18O and 2H (sometimes just D)
Natural water stable isotope composition
1H
= 99.985%
2H = 0.015%
16O
= 99.76%
17O = 0.04%
18O = 0.20%
isotopic ratios between heavy and light isotopes is usually expressed as
δ is usually expressed in ‰ (per mil)
R is the ratio in the standard and samples
For both 18O and 2H, standard mean ocean water (SMOW) was the standard,
now VSMOW (Vienna SMOW) is the standard
the various istoptic forms have slightly different freezing temperatures and
vapor pressures and therefore “fractionation” occurs during phase changes. R
changes during evaporation, condensation, etc.
Harmon Craig introduced SMOW but it never existed
Hypothetical water calibrated to NBS-1 from Potomac River
SLAP used for very light samples
- during evaporation, both 18O and 2H become “heavier”
- therefore, can measure degree of evaporation of water
- useful for delineating recharge from discharge areas in arid regions but
not more humid ones (humid region gw isotopic signals are similar to mean
annual precipitation)
- arid regions have heavier signals as flow proceeds
ii. Tritium
-tritium (3H, or sometimes just T) is used to look for water that has entered the
system since 1953 (initiation of weapons testing)
-distribution of tritium maps distribution of post 1953 water
- tritium (3H) concentration in water not affected by chemical reactions
- natural precip has range of 5 to 20 TU
1 tritium unit (TU) = 1 3H in 1018 H atoms
- has in the last 30 years been up to 2 to 3 orders of magnitude higher
- water with less than 10 TU is prebomb (pre-1953)
-can also be used to map average linear velocities
-concentrations can sometimes be directly related to the atmospheric trend in
tritium
-
iii. Radiocarbon
Two stable isotopes (12C, 13C) and one useful radioactive isotope (14C)
- 14C produced in the upper atmosphere by comic ray bombardment (nitrogen
transmutation) 14N7 + n ==> 14C6 + p
- previous to bomb testing this was only source
- 14C very rare, ~0.000000001% of carbon in atmosphere
- half-life of 5730 years
- amount of carbon in a sample is often referred to as percent modern carbon
(PMC), 100% being from a standard wood sample from before bomb testing
-abundance since bomb testing had almost doubled, now dropping back
(presently ~120 percent modern carbon)
- besides H2O, water has other constituents, e.g. dissolved inorganic carbon
(DIC)
-
- some transformation of the DIC can occur as water moves through the root
zone and interacts with carbonaceous minerals (problem)
- “hardwater effect” needs to be assessed before information from 14C can be
used. Often the δ13C can be useful in determining transformations
-14C can be used to distinguish zones of old vs. new water
- radio carbon dating of the dissolved inorganic carbon of groundwater can
extend to several thousand years
- radiocarbon dating developed by Libby in 1950 at U of Chicago
5. Soil and land surface features
- direct observation of Springs, seeps, and other discharge phenomena
- sometimes referred to as “ groundwater outcrops”
-in semi-arid region’s, outcrops may be saline soils, playas, etc.
-vegetation can also be a clue
- in discharge areas salt-tolerant phreatophytes prevail
-
Phreatophytes are plants that have adapted to arid environments by growing
extremely long roots, allowing them to acquire moisture at or near the water table.
The mesquite's roots are considered the longest of any desert plant and have
been recorded as long as 80 feet
Plants that have adapted by altering their
physical structure are called xerophytes.
Xerophytes, such as cacti, usually have
special means of storing and conserving
water. They often have few or no leaves,
which reduces transpiration.
Joshua Tree
Some are a combination of phreatophyte and xerophyte
e.g. the Creosote Bush It has tiny leaves that close their stomata (pores)
during the day to avoid water loss and open them at night to absorb moisture.
Creosote has an extensive double root system -- both radial and deep -- to
accumulate water from both surface and ground water.
Other strategies:
1. P
2. Annuals bloom during the wet season, release their seeds and die
Quantitative Interpretation of Regional Flow Systems (how much
recharge/discharge)
- using flow net construction we can calculate rates are recharge and discharge
throughout the basin profile
Recharge-discharge profiles
- can be created above flow nets
- also a good way of checking flow that construction since in steady state recharge
must equal discharge
- 3-D maps possible as well
Rates of recharge from infiltration:
Recharge from infiltration more likely to occur when:
1. soils have high conductivity and/or moisture levels over a large range of
tension head
2. Water table is shallow
3. Soil is relatively wet
4
Ground water balance
- goal to measure RI from increases in water-table elevations (Δh+) in one or
more wells over Δt
Sy is the specific yield of the aquifer (often assumed value for entire aquifer)
ratio of water that drains under gravity to volume of sample
Δh+ not always associated with recharge. Can also occur by:
1. fluctuations in atmospheric pressure expanding and contracting trapped gas
2. Thermal effects, including freeze thaw
3. Pressurization of capillary fringe can occur where water table near surface.
Quick rise in water table without change in storage
4. Evaporation/phreatophytic consumption
5. External loads
G terms difficult to obtain
- one method to find G terms uses well observations at boundaries and
applying a modification of Darcy’s law:
G is appropriate flow rate at Gin or Gout boundary
T is the transmissivity of the aquifer (T=bK)
L is the width of the boundary
A is the area of the region
i1 and i2 are the hydraulic gradients at the boundary at the beginning and end
of the observation period
-other G term method in Dingman Chapter 8
Other Ri methods involve using geochemical tracers (CFC’s, stable isotopes,
etc)
Fluctuations in Groundwater Levels not related to Recharge
- many causes of fluctuations
- focus on four:
1. Evapotranspiration and Phreatophytic Consumption
- may be possible to measure ET in discharge area just from diurnal
fluctuations of groundwater
- daily draw-downs by phreatophytes
r is the hourly rate of groundwater inflow (e.g. mm/h)
s is the net rise or fall of the table during 24-hrs
Sy in this case should be the “readily available specific yield” which is 0.5
time the true Sy. Best to base Sy on lab experiments and take value for first
24 hours.
2. Air entrapment during groundwater recharge
- causes anomalously large rise in water levels during heavy rain storms in
shallow unconfined aquifer
- not related to recharge, but because it is associated with heavy rain it is often
associated with it
- water level rises to rainfall depths of 20:1 have been observed
-
3. Atmospheric pressure effects
- mostly in confined aquifers
- increased pressure causes decreases in observed water levels
recall that total stress is counteracted by effective stress and water pressure
before we considered the atmospheric pressure to be essentially constant
if we add the weight of the atmosphere:
 T  pa   e  pw
since σT will remain constant in this problem:
so if we add weight to the overburden (dpA) effective stress and water
pressure have to increase to counteract it
note that dpA must be > dpw
In a well the over burden at a point level with the base of the confining
layer is (point Y)
A
w
where γ is the weight density (ρg)
p    p
so the water pressure at that point is related to the pressure from the
overlying mass of water and atmosphere
after the pressure increase this becomes:
combing these two equations:
dp A  dpw   (   ' )
since the left side must be > 0, then the right side inside the parentheses
must be > 0
- therefore an increase in atmospheric pressure will lead to a decease in
pressure head in a confined aquifer
- in unconfined aquifers small decreases in water level can be caused by
increases in air pressure “squeezing” entrapped gas
4. External loads
- passing of trains, construction blasting, earthquakes cause oscillations in
water levels
- similar effect to atmospheric pressure on confined aquifers
-
Groundwater-Lake/Wetland interactions
-hydrologic regime of a Lake is strongly influenced by the regional
groundwater flow system in which it sets
- large permanent lakes almost always discharge areas for regional
groundwater systems
- small permanent lakes in upland portions of watersheds usually discharge
areas for local or intermediate flow systems
-where water table elevations are higher than Lake levels on all sides,
recharge-lake only possible if high permeability at depth
Ways to observe ground-water interactions in lakes:
1. Piezometers
- use several shallow piezometers all around lake to define local water table
elevations
- find height of mounds
- to find if stagnation point exists, put piezometers to different depths in the
downslope end of the lake
-
Drive-head piezometer
no
outflow
inflow
flow
2. Seepage meters
- rates of seepage into a lake
can be made with seepage
meters
- inverted pipes/drums stuck in
the bottom of the lake, with an
expandable balloon-like
collection devices
3.
Groundwater that is “noncyclical”
- some groundwater by and large does not take part in flow systems:
connate water: Water which was deposited, by geological means,
simultaneously with the surrounding rock formations and held without flow.
- This water usually occurs deep in the earth, and is high in mineral content due
to long contact with rock.
connate water and fossil water often get interchanged
- some definitions have connate water as any water that is out of contact with the
surface for a long time
magmatic water: may contain water that was subducted previously and juvenile
water
springs
-constant or variable
discharge
-ephemeral or perennial
-variable temperatures
-high or low flows
Character depends on the
local geology, whether the
discharge originates from
the local or regional flow
systems