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Unit 01 : Advanced Hydrogeology
Review of Groundwater Flow
Malcolm ReevesCivil and Geological Engineering
What is hydrogeology?
• Hydrogeology is the study of the laws governing the movement of subsurface water,
• the mechanical, chemical and thermal interaction of this water with the porous solid,
• and the transport of energy, chemical constituents and particulates by the flow.
Domenico and Schwartz, 1997
Laws governing movementDarcy’s Law• Q = -A.K dh/dx
Q is the flow, K is the hydraulic conductivity, dh/dx is the head gradient and A is the cross sectional area normal to x.
• Q/A = q = -K dh/dxq is called the specific discharge
• v = q/n = -(K/n) dh/dxv is the advective (flow) velocityn is the porosity
Mechanical, Thermal and Chemical Interactions
• Mixing caused by– Hydraulic (mechanical) gradients (dh/dx)– Thermal gradients (dT/dx)– Chemical gradients (dC/dx)
• Interactions of fluids with the porous medium
Groundwater Transport
• Groundwater transports fluids. The process is called advection.
• Advection of fluid also carries:– Solutes (metals, organics, nutrient, etc)– Particulates (colloids, bacteria, etc)– Energy (mainly heat)
Elements of the Hydrologic Cycle• Condensation• Precipitation• Evaporation• Transpiration• Interception (Interception refers to
precipitation that does not reach the soil, but is instead intercepted by the leaves and branches of plants and the forest floor.
• Infiltration• Percolation• Runoff
Water Profile
Bound Water in Minerals
Capillary Water
Intermediate Vadose Water
Water in Unconnected Pores
Groundwater
Soil Water
Inte
rsti
tial
Zo
ne
Sa
tura
ted
Un
sat
ura
ted
Subsurface Flow
• Infiltration• flow entering at the ground surface
• Percolation• vertical downward unsaturated flow
• Interflow• sub-horizontal unsaturated and perched saturated flow
• Groundwater flow• sub-horizontal saturated flow
Soil Moisture
J F M A M J J A S O N D
Mil
lim
etre
s o
f W
ater
Spring Recharge Fall
Recharge
Soil MoistureDepletion
PotentialEvaporation
AdjustedPrecipitation
Infiltration Capacity
• Water supplied to the soil surface at an increasing rate will eventually runoff.
• Water supplied to the soil surface at a constant rate infiltrates at a rate that decreases with time to a limiting rate.
• This limiting rate (when the soil is saturated) is called the infiltration capacity of the soil surface.
Field Capacity
• Flow of water in an unsaturated soil cannot take place until a limiting moisture content is reached.
• This limiting moisture content is called the field capacity of the soil in soil science and the residual water saturation in hydrogeology.
InfiltrationIn
filt
rati
on
Rat
e
0% Moisture Content 100%
Infiltration Capacity
Fie
ld
Ca
pac
ity
Infiltration Capacity [ LT-1 ]Field Capacity [ % ]
Hydrograph Components
• Stream flow hydrographs can be broken down into three components:– Runoff (overland flow)– Interflow (unsaturated sub-horizontal flow)– Baseflow (groundwater flow)
• Each component has a characteristic recession (decay) rate.
Baseflow
• The decline of the flow in a stream in the absence of input is called recession
• Empirically, recession curves are exponential decay functions Q = Qoe-kt.
• After long periods without precipitation, the recession rate is called baseflow and is characteristic of the groundwater system feeding the stream.
• The groundwater recession constant is given by the equation k = ln(Qo/Q)/t
Hydrograph Analysis
• Point A is minimum Q gradient is determined from recession rate
• Point B is maximum Q• Point C is Q at time T*
after the peak:T* = An
where A is the drainage area and n
is an empirical power.If A is in km2, n = 0.14
If A is in mile2, n = 0.20Time
Dis
char
ge
AB
C
T*
Global Hydrological Equation
Input – Output = Change in Storage
P – E –T – Ro = DS
P precipitation
E evaporation
T transpiration
Ro runoff
DS change in groundwater storage
P = I (DS) + R (Ro) + E (T+E)
Elements of the Basin Cycle
Surface
Soil
Aquifer
F
P ETs
Rs Ta
Str
eam
Ch
ann
els
Ro
Qo
Qi
Qs
Qa
For the groundwater sub-systemRs + Qi – Ta – Qa = DS
Aquifer Types
• Unconfined - storage LARGE depends on specific yield• Confined - storage SMALL depends on compressibilities
=κρεμάμενος υδροφόρος
Porosity
Material Porosity(%)
well-sorted sand or gravel 25-50sand and gravel, mixed 20-35glacial till 10-20silt 35-50clay 33-60
Specifics of Aquifer Storage
Unconfined
Sy = n - Sr
n porosity
Sy specific yield (gravity drainage)
Sr specific retention (like field capacity)
Confined
S = b.Ss
b thickness
Ss specific storage
Ss = g.(a + n. b )g specific weight
a matrix compressibility
b water compressibility
Specific YieldMaterial Specific Yield
(%)Max Min Mean
coarse gravel 25 12 22medium gravel 26 13 23fine gravel 30 21 25gravelly sand 35 20 25coarse sand 35 20 27medium sand 32 15 26fine sand 28 10 21silt 19 4 18sandy clay 12 3 7clay 5 0 2
Hydraulic Conductivity
MaterialHydraulic
Conductivity(m/s)
well-sorted gravel 10-4 to 10-2
well-sorted sand 10-5 to 10-3
silty sands, fine sands 10-7 to 10-5
silt, clayey sand, till 10-8 to 10-6
clay 10-11 to 10-8
Steady-State Flow• q = -K dh/dx
K is hydraulic conductivity [ LT-1 ]h is hydraulic head [ L ]
• q = -(kg/m) dh/dxk is intrinsic permeability [ L2 ]m is absolute viscosity [ FL-2T ]
g is specific weight [ FL-3 ]
For horizontal flow gdh/dx = dp/dx• q = -(k/m) dp/dx
p is fluid pressure [ FL-2 ]
Vertical Flow
• For vertical flow
q = -K dh/dz
h = p/g + z
dh/dz = (1/g) dp/dz
q = -(kg/m) dh/dz
q = -(k/m)(dp/dz + 1)
Density-Dependent Flow
• For density-dependent flow
q = K dh/dz
h = p/g + z
dh/dz = (1/g)(dp/dz – (p/g)dg/dz + 1)
q = -(kg/m) dh/dz
q = -(k/m)(dp/dz + 1 – (p/g)dg/dz)
Unsaturated Flow
For unsaturated flowq = -K(y) dh/dz h = y + z y is the pressure head
z is the elevation headh is the total hydraulic head
The pressure head, , y depends on saturation. At full saturation, y increases with depth. In the unsaturated zone, y is negative and is called suction pressure.
Soil Water Characteristic Curve
- Pressure +
Dep
th WaterTable
0% Saturation 100%
+
P
ress
ure
-
WaterTable
Hyd
raulic C
on
du
ctivity