wastewater and soil due today
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soiltgw_1.pptxSoil Water and Groundwater
Soil Water and Groundwater
Soil Water
(Heath, 1998)
Soil water- water in the unsaturated zone, held to soil particles by surface tension and adhesion
Capillary fringe- zone just above water table, where soil is almost completely saturated (99%), but is held in soil matrix and will not drain under gravity
Water Table- zone where soil is completely saturated
Adhesion- attraction of water and soil
Cohesion- attraction of water to water
Positive side of water molecule attracted to
Negative surface charge of soil particles
Negative side of water molecule attracted to
Positive side of another (different) water molecule
Soil Water
(Brady, 2004)
Saturated- all pores filled with water
Field capacity- water from macropores has drained due to gravity
Wilting coefficient- water held too tightly by soil for plants to extract and plants die
Moist Conditions
Soil and Site. Lindbo et al. DRAFT
3/28/16
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Under moist (aerobic) conditions the bacteria is treated and removed better.
This is because of more liquid/solid contact increasing removal by filtration and adsorption.
Wet Conditions
Soil and Site. Lindbo et al. DRAFT
3/28/16
8
If a saturated soil, all the pores are filled with water. Bacteria (and other pathogens) move easily through the big pores and are not removed in the soil. They may proceed directly to ground water.
Matric force- attraction of water to soil solids, responsible for adsorption and capillarity
Osmotic force- attraction of water to ions and other solutes (like salts)
Gravity- Pulls water down
Soil Water Potential – difference in energy levels of water between two sites
Water moves in direction of decreasing potential
Soil Water Potential Measurements
Tensiometer- water filled tube with one end sealed & the other capped with a porous ceramic tip. Inserted into soil, water flows out of tip until it reaches equilibrium with surrounding soil. As water is drawn out of tube, there is a vacuum that is read with a gauge.
Soil Water Potential
Resistance blocks- porous blocks with a pair of electrodes embedded in porous material and placed in soil- equilibrate with matric suction (tension) of soil water
When soil wets, electrical resistance between electrodes decreases and can be measured with meter
Soil Water Samplers
Lysimeters- tubes with ceramic, porous cups at the end. A vacuum is created in the tube and water is pulled from surrounding soil into the cup.
Water Content Measurements
Volumetric water content – volume of water associated with given volume of dry soil m3/m3
Mass water content - mass of water associated with a mass of dry soil kg/kg
Gravimetric method- most straightforward, often used to calibrate other methods
Soil sample from field (with water) weighed, dried and weighed again
Weight of water/weight of dry soil = mass water content
Neutron attenuation and time domain reflectometry- allows for repeated measurements at a given soil depth, measure changes in moisture over time, changes in moisture with depth
Water Content Measurements
Volume soil water content
m3 H20/m3 soil
TDR, Neutron Attenuation
Matric potential is the amount of suction the soil has on water (expressed in Bars or kilopascals (kPa).
(Brady, 2004)
Tensiometer, resistance blocks
Sandy Loam Clay Loam
Irrigation Water
Distance from center (cm)
45 30 15 0 15 30 45 75 60 45 30 0 30 45 60 75
(Brady, 2004)
Wetting fronts for soils with relatively more micropores (small pores) will be more diffuse than soils with more macropores (larger pores). Smaler Pores “pull” water in first, once the smaller pores are full, medium and then larger pores are filled in response to the pressure gradient.
Groundwater Monitoring/Sampling Devices
Well
Modified
Piezometers
Automated Wells
Ground
Water Table
Screened interval
Groundwater Movement
GW moves in direction of decreasing hydraulic head, along a slope or gradient
Hydraulic head (Hh) is the sum of gravitational head (Hg) and pressure head (Hp)
Datum
Reference point
Hg
Hp
Hh
Hh
Groundwater Movement
Hh (C)
D
C
Datum
Hh (D)
A
B
Downward
Vertical Flow
Upward
Vertical Flow
Hh (B)
Hh (A)
Methods for estimating hydraulic conductivity
* Lab permeameter (small sample)
* Field permeameters
* Hazen method (grain size estimation)
* Slug tests (displacing a known volume
of water (slug) from a well and recording the time
required to return to the original level), the quicker
the return to original level, the higher the Ks
* Pumping tests- pump water from a well and
observe the response of nearby wells
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Slug Tests for Ks Determination
slug
Slug test: remove or add slug,
relatively inexpensive, quick, but
need many to characterize an area.
ECU
Pumping Tests
Pumping test :(monitor drawdown nearby)
Larger sample, more expensive, but better overall estimate of hydraulic conductivity ; need to know if confined or unconfined aquifer aquifer, and if the wells are partial or fully penetrating aquifer.
Ground Water Velocity Determination Darcy’s Law Calculation
Darcy’s Law q = -K(dh/dl)
q = velocity (length/time)
K = Hydraulic conductivity (length/time)
dh/dl = Hydraulic gradient (length/length)
Pore water velocity = q / ne
ne = effective porosity
Porosity – the % of total volume of rock or sediment that consists of pore spaces (including pores and fractures)
Effective porosity- the % of total volume of rock or sediment that consists of pore spaces that are connected
Hydraulic conductivity – a measure of a material’s ability to transmit water
Aquitard – largely impermeable layers that hinder water’s movement (ex. clay), confining layer
Aquifer – permeable rock or sediments that transmit water easily (ex. sand or gravel)
Confined (artesian) Aquifer –pressurized due to confining beds
Groundwater Flow - Terminology
25
Estimating Groundwater Discharge
Groundwater flow (Q) = KA dh/dl
Where K = hydraulic conductivity, A = cross-sectional area of flow
dh = difference in head between upgradient and downgradient
dl = length between upstream and downstream
Use Darcy’s Law.
(Heath, 1998)
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Equation: EA - EC/ac = EA - EB/x
10.1-9.7/100 = 10.1-9.8/x
0.004 = 0.3/x
0.004x = 0.3
x = 75
A (10.1)
B (9.8)
C (9.7)
ac =100
ab = 85
bc = 90
x
Groundwater Flow Direction
GW
Site 1 House
Hoods
Creek
N
