4932 AOM - Subsurface Water Subsurface water - Water occurring below land surface. Groundwater - Water under positive pressure in the saturated zone of earth materials. - constitutes approximately 30% earth's fresh water approximately 99% liquid fresh water Overall residence time for global groundwater reservoir is centuries to millenia because rate of groundwater movement is generally slow ( < 1 m/day). This is in contrast to 9 day residence time for atmospheric water completely different time scales. Classification of soil-rock profile:
soil water UNSAT'D ZONE intermediate vadose water capillary water p < atm n
p < atm = n water table p = atm p > atm
SAT'D ZONE
groundwater
=n
volume voids volume voids porosity (n) n) = porosity ( = total volume total volume volume water water content (soil moisture content) ( ) = volume water total volume water content (soil moisture content) ( ) = total volume soil water - Ground surface to bottom of root zone depth depends on soil type and vegetation. May become saturated during periods of rainfall otherwise unsaturated (soil pores partially filled with air). Plants extract water from this zone. Evaporation occurs from this zone.
intermediate vadose zone - Between soil water zone and capillary fringe. Unsaturated except during extreme precipitation events. Depth of zone may range from centimeters to 100s of meters. capillary zone - Above saturated zone. Water rises into this zone as a result of capillary force. Depth of this zone is a function of the soil type. Fractions of a meter for sands (mm) to meters for fine clays. All pores filled with H2O, p < 0. Effect seen if place bottom of dry porous media (soil or sponge) into water. Water will be drawn up into media to a height above water where soil suction and gravity forces are equal. saturated zone - All pores filled with water, p > 0. Formations in this zone with ability to transmit water are called aquifers. Unsaturated Zone Water can exist in all its phases in the unsaturated zone. Liquid water occurs as: hygroscopic water - adsorbed from air by molecular interaction (H-bonds) capillary water - held by surface tension due to of liquid gravitational water
viscosity
Hygroscopic and capillary waters are held by molecular electrostatic forces (between polar bonds and particles -- surface tension) in thin films around soil particles drier soil, smaller pores hygroscopic and capillary forces. 1. Hygroscopic water unavailable - held at -31 to -10,000 bars. 2. Capillary water - Held at -0.33 to -31 bars. More water filling pores but discontinuous except in capillary fringe. This water can be used by plants. permanent wilting point - Tension (suction, negative pressure) above which (or level of dryness above which) plant root system cannot extract water. Depends on type of vegetation typically -15 bars. field capacity - Maximum amount of water soil can hold against gravity. Tension above which (or level of dryness above which) water cannot be drained by gravity (due to hygroscopic and capillary forces). Depends on soil type somewhere between 0 and -0.33 bars. 3. Gravitational water - Water in unsaturated zone in excess of field capacity which percolates downward due to gravity ultimately reaching saturated zone as recharge. (Note 1 bar = 105 Pa = approximately 1020cm water= approximately 1 atmosphere)
Typical Moisture Profiles a) rain after a long dry period
moisture content root zone direction of moisture movement
depth wilting point hygroscopic field capacity
saturation
1 - 4 typical wetting path b) drying process
evapotranspiration moisture
depth
saturation 1 - 5 typical drying pattern 2 - Drying in upper layers by ET. 3 - Bottom part of wetting front continues . Upper part continues to dry. 4 - At some point and movement results in no moisture gradient 5 - Dry front established. Lower zones are being depleted to satisfy PET at surface.
field capacity
Drying continues until capillary forces are unable to move water to surface. Flow in unsaturated porous media governed by a modified Darcy's law called DarcyBuckingham law : h 1-D flow in z-direction q z = - K ( ) z But now total head
h=+z
P/g gravitational head
- suction head (capillary or head) negative pressure head. Energy possessed by the fluid due to soil suction forces (still P/g but negative). Suction head varies with moisture content, , 0, 0 K() - hydraulic conductivity is a function of water content , K() because more continuously connected pores, more space available for water to travel through, until = n, K(n) = Ksat
-107
Ksat
Soil Suction ()
K() or K()
-10-1
Soil Suction () head measured with tensiometers. Airtight ceramic cup and tube containing water. Soil tension measured as vacuum in tubes created when water drawn out of tube into soil. Comes to equilibrium at soil tension value.
h 1 = z 1 + 1
h1 = 100 cm - 65 cm = 35 cm h2 = 50 cm - 50 cm = 0 cm
1 (negative) (-65 cm) z1 = 100 cm z=0 z2 = 50 cm h2 = z2+ 2 2 (negative) (-50 cm)
Estimate Darcy flux in vertical direction from tensiometer measurements: 35cm - 0cm h - h2 h = -K qz = -K ( ) -K 1 z -z 100cm - 50cm z 1 2
( )
( )
Look up K() for soil type at the moisture content corresponding to the average tension. Flow is in negative direction (i.e. down). Look at components of gradient: h -h ( + z ) - ( 2 + z 2 ) - 2 = -K ( ) 1 q z = -K ( ) 1 2 = -K ( ) 1 1 z -z z - z - K ( ) z1 - z 2 2 2 1 1 - 65 - (-50) = -K ( ) 100 - 50 - K ( ) - 15 = -K ( ) 50 - K ( )
Capillary gradient cause upward flow, gravitational gradient causes downward flow. Net flux is down. What would it take to get net flux upward?
Saturated Zone
recharge area for confined aquifer
potentiometric surface recharge (infiltration)
unconfined aquifer aquitard - confining layer confined aquifer confining layer - aquiclude
aquifer - saturated soil or geologic unit with ability to transmit water aquiclude - saturated soil unable to transmit water aquitard - saturated soil transmits water very slowly aquifuge - has no water therefore does not transmit, impermeable confined aquifer - Saturated zone between two impermeable layers. Fed by exposed recharge area [recharge areas can be remote confined aquifer then contains "fossil water" deposited in past geologic times. No free water surface except in recharge area] , and in some cases recharge through leaky upper confining layer. Well in confining layer water will rise to piezometric level above upper confining layer, sometimes above ground surface (equal to elevation above datum and pressure in aquifer). No free water surface except in recharge area. unconfined aquifer - Also called phreatic or water table aquifer. Water in well in unconfined aquifer will rise to water table level which defines piezometric surface or head of system. Unconfined aquifers have free water surface. Water comes from direct rainfall over the aquifers, connections with surface waters or other aquifers. Whether rock or soil formation is an aquifer, aquifuge or aquiclude depends on its geologic origins and history. - gravel, coarse sand aquifer - silt, clay aquiclude or aquitard consolidated sediments - sandstone, limestones, dolomites aquifer - shale, siltstone, basalt aquifuge Flow in saturated porous media is governed by Darcy's law: dh qx = - K dx unconsolidated sediments
1-D flow in x-direction
Darcy flux or specific discharge volume rate of flow per unit area (L/T)
piezometric head - measured as height above arbitrary datum to which water rises in a tube connecting the point being measured to the atmosphere h = p/g + z (L)
K - saturated hydraulic conductivity (L/T); function of fluid and porous media K = 10-6 cm/sec clay; 10-3 cm/sec sand; 10 cm/sec gravel
h1 p1 = 0 z1 = 100 h = 100
h2
p=0 z2 = 90 h2 = 90
qx z = 0 x1
K= 10 -3 cm/sec
1000m p1 = g(z1) z=0 h = z1 =100 x2
h -h ^ qx = -K 2 1 x -x 2 1
p = g(z2) z=0 h2 = z2 =90
h -h cm 100m - 90m - 5 cm q = - K 2 1 = 10 - 3 = 3.1m / year = 10 x -x sec 1000m sec 2 1 Water table in surficial aquifer typically reflection of topography. Flow from topographic highs to lows. Groundwater often can be assumed hydrostatic but not always (i.e., not in recharge and discharge areas) 1 2 3 4 5 6
h1 > h2 > h3 downward flow recharge
h4 > h5 > h6 upward flow discharge
h1, h2, h3 > h4, h5, h6 flow from recharge to discharge