Soil and Groundwater

Soil

A dynamic natural body composed of mineral and organic solids, gases, liquids and living organisms which can serve as a medium for plant growth. 

Dirt

Soil that is out of place

All soil is a combination of ______.

clay, sand, and silt

Soil texture

• Sand - gritty, largest particles  (0.05 – 2.0 mm)

• Silt - smooth, velvety, Intermediate (0.002 – 0.05 mm)

• Clay - slick, sticky, Smallest (< 0.002 mm)

You can determine soil texture by:

texture by feel method, hydrometer or pipette

Soil texture influences the ______.

soil water and air movement

Texture by feel method

USDA Textural pyramid

• shows 12 classes

• Textural groups dependent upon the relative percentage of sand, silt and clay

• all %’s add to 100

Soil horizons

layers of soil with distinct characteristics

• a horizon, e horizon, and b horizon

A-horizon

surface layer, typically

darkened with organic matter from roots,

litter layer, etc.

E-horizon

zone of eluviation (leached through and left horizon causing different properties), where iron, aluminum, organic matter, and/or

clay was translocated to a deeper layer

• not in all soils

B-horizon

layer of accumulation of clay, Iron, aluminum, organic matter

• orange/reddish-brown color from iron

Adhesion

attraction of water and soil

Cohesion

attraction of water to water

Polarity of 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

water in the unsaturated zone, held to soil particles by surface tension and adhesion → some water & air in pores

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

• narrower the opening/pores the more suction → higher it can pull up above the water table in some cases

Water table

zone where soil is completely saturated → all pores are filled with water

Groundwater

water in the saturated zone

Soil Moisture Status

saturated, field capacity, and wilting coefficient

Saturated

all pores are filled with water

Field capacity

water from macropores has drained due to gravity → bigger pores have emptied

Wilting coefficient

water held too tightly by soil for plants to extract and plants die → plants can’t create a vacuum to get water out of the soil

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)

submergence

pressure potential due to weight of water above a point

• below water table → dealing with groundwater

Soil water potential

difference in energy levels  of water between two sites

Gravity _______.

Pulls water down

Water moves in direction of ______.

decreasing potential (from high to low potential)

Dominant forces in determining where water goes

matric and osmotic force

Soil moisture content

high porosity = most open spaces to store water

• on this graph, the soil that has high porosity is clay → why? think a sponge vs a straw →clay and loam retain water more quickly → sand drains so quickly (has large pores that are connected so water can transmit through it easily → effective porosity

Matric potential

the amount of suction the soil has on water (expressed in 

Bars or kilopascals (kPa).

volumetric water content

volume of water associated with given volume of dry soil m3/m3

• a measurement of water content

Mass water content

mass of water associated with a mass of dry soil kg/kg

• a measurement of water content

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

• a measurement of water content

Neutron scattering

• widely used field method for repetitive measurement of volumetric soil water content →a water content measurement instrument

• based on the propensity of water molecules to slow down (thermalize) high energy fast neutrons emitted from a radio active source

• the thermalized neutrons are counted by a detector present in the access tube (along with the source)

Time Domain Reflectometry (TDR)

• a water content measurement instrument

• sends a pulse of electromagnetic energy down the three parallel metal rod

• makes precise picosecond measurements of the speed at which the pulse travels down the rods, a speed influenced by the nature of the surrounding soil

• microprocessors analyze the wave patterns generated and calculate the dielectric constant of the soil (influenced by water content)

• converts the dielectric constant into the volumetric water content of the soil

Tensiometer

• water filled tube with one end sealed and 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 the tube, there is a vacuum that is read with a gauge. 

• if the soil is already saturated → no vacuum

• used a lot in golf courses

• a Soil Water Potential Measurements

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 →aka as soil gets wet, water dissolves the salts inside it, which increases the electrical conductivity (EC) of the soil

• soil water potential measurement

Wetting fronts for soils with relatively more micropores (small pores) will be more _______.

diffuse than soils with more macropores (larger pores). Smaller pores “pull” water in first, once the smaller pores are full, medium and then larger pores are filled in response to the pressure gradient. 

• ex: clay loam has smaller pores than sand loam

Groundwater

Subsurface water in the zone of saturation. 

The capillary fringe is considered _______.

a part of soil water

Groundwater sampling / monitoring devices

• wells = measure hydraulic head → groundwater sampling

• modified Piezometers = increase pressure of the water → measure pressure head

• automated wells

GW moves in direction of ______.

decreasing hydraulic head, along a slope or gradient

Hydraulic head (Hh)

the sum of gravitational head  (Hg) and pressure head (Hp)

Vertical groundwater movement

• downward flow = discharge

• upward flow = recharge

hydraulic conductivity

how water moves in soil → a measure of a material’s ability to transmit water

Slug tests

• measure hydraulic conductivity

• in = 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

• slug out = Measure initial depth to water, remove water from the well, and determine the time To return to initial depth ( in various intervals, using a stop watch and depth meter)

Pumping tests

• measure hydraulic conductivity

• pump water from a well and observe the response of nearby wells pump water from a well and observe the response of nearby wells

• monitor drawdown nearby) Larger sample, more expensive, but better overall estimate of hydraulic conductivity ; need to know if confined or unconfined aquifer, and if the wells are partial or fully penetrating aquifer. 

Darcy’s law

K(dh/dl) (m/day)

• q = velocity (length/time)

• K = Hydraulic conductivity (length/time)

• dh/dl = Hydraulic gradient (length/length)

Pore water velocity

q / n_e

• n_e = effective porosity → must have info on what type of media it is

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

storage

the volume of water an aquifer releases from or takes into storage per unit surface area per unit change in head

aquitard

largely impermeable layers that hinder water’s movement (ex. clay)

aquifer

permeable rock or sediments that transmit water easily (ex. sand or gravel)

Confined (artesian) Aquifer

pressurized due to confining beds

Groundwater flow direction

Equation:  E_H - E_L/HL = E_H - E_I/x

• EH is elevation of highest water level; EL is elevation of lowest; EI is elevation of intermediate.    

• HL is distance from wells with highest and lowest water elevations.

• X is calculated distance from well with highest water elevation towards well with the lowest water elevation, where water elevation is the same as in Intermediate well (water level contour line). Flow direction is perpendicular to the contour line. 

Groundwater Velocity

how quickly groundwater moves