groundwatersfrc.ufl.edu/ecohydrology/fwr/lecture6_2018.pdfgroundwater = k a Δh/l darcy’s law...
TRANSCRIPT
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Groundwater
P = Q + ET + G + ∆S
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What is an Aquifer?• Rocks and sediments have pores (spaces)
– Rock (limestone, granite, sandstone etc.)– Sand and gravel
• When pores are full of water that media is saturated• Contiguous areas of saturated media form an aquifer• Aquifers can be layered
– Confined vs. Unconfined
• Aquifers have the capacity to transmit water through interconnected pores
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Floridan AquiferExtent
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High Plains Aquifer
(Ogalalla)
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Confinement
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Cross-Section– Floridan Aquifer
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Transmissivity
• How much water can be moved horizontally– Function of thickness
and Ksat
– Good measure of well productivity
– Floridan is the most transmissive aquifer in the world
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Loss of Potential
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Artesian Springs• Where a confining layer exists, there may be a
pressure potential in the aquifer HIGHER than the gravity potential of the surface.
• When tapped, water flows upwards
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Potentiometric Surface
• Elevation of “free water surface”
• Where this surface and the ground intersect (and there’s no confinement) water seeps
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Relevant Questions
• Where is the water going?– Potentiometric (piezometric) surface
• How much water is moving? How quickly?– Potentiometric surface and Darcys Law
• What level of natural assimilation is occuring?– Water quality modeling
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Gainesville’s Well Field
Also Lake City
Jasper
Geology
Land use
Conservation Easement
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Murphree Wellfield Cone of Depression
1988 (Observed) 2010 (Predicted)
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How to make a sinkhole
Pray for lots of Rain
Suck a lot of water
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Freeze Protection• To protect strawberry yield during a January
freeze in 2010, ~ 2 billion gallons per day of water pumped over a 5 day period. Voila.
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Invasion
Fire
Subsidence
Degraded wildlife habitat
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Soil water movement across a watershed boundary.
P=Q+ET+G+ΔS
Qgroundwater= K A ΔH/L
Darcy’s Law
Darcy’s law can calculate vertical leakage through a clay layer AND lateral flow through a seepage face.
Q is water crossing the defined area of the boundary in m3/day
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K is hydraulic conductivity, or the capacity of the media to transmit water
Most meaningful as saturated hydraulic conductivity Ksat (m/day). Why?
Q= K A ΔH/L
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Measuring Ksat
• Constant Head Method– Constant depth of water
(~50mm) on top of a saturated soil column with known dimensions (diameter, length)
– Outflow volume collected over a period of time (e.g., 5 hrs)
– Darcy’s Law to solve for Ksat
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Measuring Ksat• Hvorslev Method
– Field measurement in screened wells
K only determined
2 4 6 8 10minutes
.1
1
.2
.3
.4
.5
.6
.7
.8
H/H o
t37
Log scale
Linear scale
H/Ho=.37
casin
g
Gravel pack
ScreenLe
Le/R must be >8
Rhigh K material
r
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Approximate Ksat and Uses
Ksat (cm/h) Comments
>50 Beach sand/Golf Course Greens
5 Very sandy soils, cannot filter pollutants
0.5 Suitable for most agricultural, recreational, and urban uses
0.05 Clayey, Too slow for most uses
<0.005 Extremely slow; good if compacted material is needed
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ΔH is the difference in H between two points
Water flows from high pressure to low pressure (could be “up”)
H = level of water in an open well above some datum
HA = ? HB = ? ΔH = ?
Flow Direction?!?
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H @ D?
H@F?
ΔH?
Direction?1098
7
654
32
1
0
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ΔH AD? Dir?
ΔH BF? Dir?
ΔH AF? Dir?
ΔH/L BF?
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Q=K A ΔH/LK? A? ΔH? L?
Vertical leakage problem
Ksat=0.001m/d
Watershed=100 ha
0
1
2
3
4
5
6
Datum
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Q = K * A * ΔH/L
K = 0.001 m/d
A = 100ha = 1,000,000 m2
ΔH = 5-3 = 2m
L = 2m
Q = 0.001m/d x 1,000,000m2 x 2m / 2m
Q = 1,000 m3/day or 365,000 m3/year
Q surface depth = 365,000 m3 / 1,000,000 m2= 0.365m
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Q=K A ΔH/L
K=0.1m/d
A=100m x 50m=5,000m2
ΔH=108m-105m=3m
L=1,000m
Qm3/d= 0.1m/d * 5,000m2 *0.003 = 1.5m3/d
Lateral leakage problem
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Groundwater Flowpaths at Streams
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Groundwater Discharges
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Groundwater Flowpaths
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Next Time…
• Soil Water Storage