Quantitative Interpretation of Regional Flow Systems how much rechargedischarge

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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
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Water balance relations the long-term average
water balance for a ground-water in a basin
is RI is recharge from infiltration RSW is
Recharge from surface water bodies Gin is
groundwater flow inti a basin (recall, usually
negligable) CR is movement of groundwater into
capillary fringe (capillary rise) QGW is the
groundwater contribution to stream flow Gout
water leaving basin as ground water Therefore,
net recharge is
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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. 4. Water input rate is
low and duration long
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Soil-Water Balance Can attempt to arrive at RI
through detailed soil water balance over some
?t QSW is overland flow ?S is the change in
soil water over time -as with all other balance
approaches, accuracy depends on ability to
measure all parameters in the equation
accurately - should be applied to small plots
over short ?t - for basic applications P and
QSW measured directly ET calculated RSW and CR
assumed minimal ?S measured using e.g.
tensiometers at several depths
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Ground water balance - goal to measure RI from
increases in water-table elevations (?h) in one
or more wells over ?t ?sy is the specific
yield of the aquifer (often assumed value for
entire aquifer) ?h not always associated with
recharge. Can also occur by 1. fluctuations in
atmospheric pressure expanding and contracting
trapped gas 2. 3. Pressurization of capillary
fringe can occur where water table near surface.
Quick rise in water table without change in
storage G terms difficult to obtain
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- one method to find G terms uses well
observations at boundaries and applying a
modification of Darcys law G is
appropriate flow rate at Gin or Gout boundary T
is the transmissivity of the aquifer (TbK) 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
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Geochemical tracers - natural chemical tracers
occasionally exist that must be a. Measurable
in precipitation, or if solid deposition must be
soluble b. Conservative in the vadose zone for
a column of vadose zone water in which there is
no hroizontal flow of water, the balance for a
natural chemical tracer would be where RI
infiltration recharge rate W is water input Cw
is the concentration of the tracer as it first
enters the vadose zone Cgw is the concentration
at the water table -
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CFCs - specific chemical tracer -
Chlorofluorocarbons (CFCs) are an anthropogenic
constituent in the atmosphere produced only since
the 1930's - good tracers of the movement of
water masses since 1. 2. Exhibit only
limited degredation or sorption
(conservative) 3. an easily detected
analytically - 3 most abundant are CFC-11
(CCl3F), CFC-12 (CCl2F2), and CFC-113 (C2Cl3F3) -
CFC-113 hard to measure so CFC-11 and CFC-12 most
commonly used - can use CFC concentration of
water to accurately date when water was last in
contact with atmosphere
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• Stable Isotopes
• - 18O or 2H
• - precipitation may have different signal than
  groundwater and therefore balance of precip
  entering ground water can be calculated
• - need to make measurements over long periods
  (expensive)
• Radioactive isotopes
• - samples should be taken near groundwater divide
• velocity of infiltrating water is
• e.g. for Tritium
• Where ZB is the maximum depth where post-bomb
  was found
• tB is

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Recharge rate is then where n is
porosity other methods see chapter 8 in
Dingman on reserve
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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
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• -if water table mound exists between two lakes,
  likely no movement between lakes
• -recharging lake can leak through part or all of
  bed
• -where width of lake greater then hi-K deposits
  on which it sits, groundwater seepage (in or out)
  is concentrated nearshore
• - in many cases groundwater-Lake interaction is
  not steady state
• - e.g. hummocky glaciated terrain, temporary
  ponds during spring snow melt

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- some lakes are site of recharge and
discharge, almost exclusively water flows in at
shallow depths and out of the bottom Flow-through
Lakes - recharge at one end and discharge at
other - exist where lake is outcrop of a
continuous sloping water table wetlands are
usually around edges of lakes or were lakes
previously - hydrologically similar to
lakes Bogs are typically groundwater recharge
areas
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Lake A is a ground-water dominated
flow-through (seepage) lake Lake B also
ground-water dominated but drained by
streamflow Lake C fed by streamflow and
groundwater, with inflow low enough that it can
be drained by groundwater alone Lake D is
surface water dominant with minimal groundwater
inflow and outflow Lake E is an artificial
reservoir where ponded water has risen well above
natural water table. - a lot of seepage
loss Lake F is terminal lake with water loss only
by evaporation
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Numerical modeling studies by the USGS has shown
that stagnation zones are likely beneath lakes
where the water table is higher on all sides -
under homogeneous conditions Add heteorgeneity
at depth ( a higher K) layer and the same
situation will be a laking losing water through
its bed, even with water table mound
downslope Multiple lake systems even with high K
layer can have some lakes draining at base and
some not -
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Establishment of the stagnation zone depends
on 1. Height of adjacent water table mounds
relative to lake level - water table
configuration around the lake most important
control - bigger the down-gradient water table
mound is, bigger the possibility of stagnation
zone - however, bigger the slope between the
downslope water mound and regional discharge
area, bigger chance of leakage 2. Position and
value of k in high conductivity zones - presence
increases chance of outseepage 3. isotropic
state - if KxKz is less than 100, stagnation
point likely 4. regional ground water slope -
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5. Depth of the lake - deeper more likely to
have outseepage - if outseepage is occurring, it
should be in a radial pattern at the downslope
end of the lake, with inseepage occurring
elsewhere
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Great Lakes
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Near Green bay
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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 - if everywhere, head is above the lake
surface, stagnation
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Drive-head piezometer
inflow
outflow
no flow
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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. Direct observation of underwater seeps
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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 juvenile
water
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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
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Depression Springs -topographic lows -where
water table meets the surface -instantaneously
remove Lake water for most lakes and youll find
depression Springs Contact spring -Springs
formed at contact between high and low K units -
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Fault spring -faulting may cause impermeable
rock to interfere with flow and force flow to the
surface Sink hole spring - -caverns of
interconnecting weathered caverns can be under
artesian pressure and drive flow to the surface
through sink holes Joint or fracture springs
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Fluctuations in Groundwater Levels - 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.
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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 - dissipates within hours or
days owing to lateral escape of trapped air
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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
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since sT 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 gt dpw In a well the over burden
at a point level with the base of the confining
layer is (point Y) where ? is the weight density
(?g)
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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 since the
left side must be gt 0, then the right side inside
the parentheses must be gt 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
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4. External loads - passing of trains,
construction blasting, earthquakes cause
oscillations in water levels - similar effect to
atmospheric pressure on confined aquifers -
transient changes in total stress cause transient
changes in water pressure