Watershed Hydrology Midterm 2

Soil Water

water moving through the “unsaturated” or “vadose” zone

Absorption

process where a liquid or other substance is assimilated within the bulk of a solid or other liquid

adsorption

Process where soil water accumulates on soil
particle surfaces, forming thin molecular film of
water

Porosity

proportion of material that consists of empty space → measures capacity to hold fluid

pore volume/total volume

small grain size tends to have higher porosity

total porosity

how much water a material can hold in space

• higher = holds more water

effective porosity

pore space in which water can actually move (space - water film - saturated areas)

• very low when pore sizes are very small = clay

permeability

how fast water can flow through a material (sand more than clay) → ability to pass water through

• water is held very tightly in a thin but non-zero thickness layer to all the material’s surfaces

• more surface area = more water films

  • high suction force, low flow

clay: porosity vs permeability

materials with very small pore space have a low permeability despite a high porosity

• low effective porosity due to films created

• clay holds a lot of water, but it is hard for water to flow through as it “sticks”

volumetric water content

measures how much water the material is currently holding

volume of water/bulk volume

saturation

all pores filled with water

field capacity

water in large pores has emptied (still has water film)

permanent wilting point

ground so dry that the water films are held on too tight and roots cannot uptake water

infiltration rate

measure of how quickly water will infiltrate from surface into material

• typically length/time

• faster when there are open spaces (dry → actively pulled in → open gaps)

length/time

unsaturated/vadose zone

zone where void spaces are filled with a mixture of air and water (held unter tension)

saturated zone

zones where voids are all water → water will flow freely

water table definition #1

boundary between saturated and unsaturated zones

groundwater

water in the saturated zone under the water table (unsaturated = soil water)

• ¼ water used in US is groundwater

aquifer

geologic unit that contains a significant amount of water and can conduct water at a reasonable rate

aquiclude

geologic unit incapable of transmitting water (confining layer)

unconfined aquifer

upper boundary of this is the saturated/unsaturated zone

• responds quickly to rainfall

• fast rate of recharge

• relatively short RT

• well water will rise to aquifer level

confined aquifer

upper boundart is an aquiclude

• recharge area small and distant

• long RT

• water in well will usually rise above aquifer level

multiple aquifer system

shallow aquifers usually unconfined → confined usually occur at greater depths

unconsolidated material/sediment

loose sediment, unlithified

• deposited by wind, rivers, etc

• grain size and permeability vary

  • clay → aquiclude

  • sand, gravel, silt → aquifer

igneous rocks

forms from the crystallization of molten rock (ex basalt, granite)

• does not typically make good aquifer unless highly fractured (exception basalt)

metamorphic rocks

forms from the recrystallization of existing rocks under heat or pressure (ex marble, slate)

• does not typically make good aquifer unless highly fractured (exception basalt)

sedimentary rocks

form from lithification of mineral and rock sediments

• often form layers on surface and underground

• if rock is porous and permeable enough, it is a good aquifer

• if not permeable = confining layer

• permeability depends on grain size, compression/cementation/lithification

Primary permeability

pore space between grains due to the original rock forming and depositing

Secondary Permeability

fractures and caverns created after rock formation or sediment deposition

Aquifer Properties: (n) porosity

Vvoid/Vtotal

• water held in molecular films via surface tension, in small pores via capillary action/suction force

Aquifer Properties: specific yield

Sy= volume of water drained under gravity/Vtotal

• water remaining is specific retention

Aquifer Properties: Specific retention

volume of water left after gravity draining (Sr) = feld capacity

Sr = volumetric moisture content after draining/Vtotal

Aquifer Properties: total porosity (n)

Sy + Sr

Effective Porosity (n_e)

volume from which water can drain via gravity

• only large pores are available for transmitting water

• generally, n_e = S_y

groundwater movement

groundwater can flow up against flow of gravity (ex artesian well)

hydraulic head

a measure of total mechanical energy →controls where water goes

• based on bernoulli’s law

moving water particle has three types of energy

• kinetic (negligable in slow flow)

• potential (elevation above datum)

• pressure (function of height and weight)

  → sum of these is constant along streamline or path of flow for incompressible fluid in a frictionless system (constant = hydraulic head)

hydraulic head equation

h = z + p

if water has the same density, elevation head (z) + height of overlaying water (p)

water table definition 2

in an unconfined aquifier where overlaying water p=0; so h=z

potentiometric surface definition

confined aquifer with no sat/unsat zone interference

• p=0

• h can rise above sat zone and even ground surface

peizometer

tube only open at the bottom → measure hydraulic head at specific location

well

intentionally get water our with screen → water rises to average hydraulic head in open area

water flow

not friction-free situation, energy lost to heat

hydraulic head decreases along steamline

• water flows from high to low hydraulic head

hydrostatic

h is the same everywhere, water table or potentiometric surface creates horizontal flow = NO FLOW

hydraulic gradient

slope of water table/potentiometric surface (rise over run)

i = h₁-h₂ / l₁-l₂

(difference in elevation / length)

vertical gradients

calculate gradients by having adjacent piezometers with bottom openings at different depths (piezometer nest)

can also compare watr levels in a peizometer adjacent to a well with an open screen

• if water levels differ, there is a vertical gradient

vertical groundwater flow

“rise” is still change in h, but “run” is now vertical distance between two well openings

• steeper hydraulic gradient, the faster water will flow

change in H/change in Z = h₁-h₂/z₁-z₂

hydraulic conductivity (K)

refers to quantitative measure of the material’s ability

• “permeability” is subjective, qualitative term

high K = “fast” flow, low K = “slow” flow

• can range many orders of magnitude

Darcy’s Law

states that discharge through a porous medium will be proportional to the hydraulic gradient, conductivity, and discharge

Q = KiA

• K gives the Hydraulic Gradient of a volume of water flowing through a 1ft by 1ft cross-sectional area

anistropy and heterogeneity

K is not the same in all directions and rarely the same in space

• material variable

• pores might be better connected one way than another

we are focusing on homogeneous materials

groundwater velocity

how fast are molecules flowing (how fast different materials can transmit water)

• derive from darcys law

water table/potentiometric surface contours

define a surface where p=0; n=z, all mechanical energy is potential energy (at saturation rate)

• lines of constant hydraulic head on that surface

• a type of equipotential (p=0)

• water flow always from high to low and perpendicular to flow lines

can tell you:

• groundwater flow direction

• horizontal hydraulic gradient

2 well situation

• Know h in two locations and distance between the two

• Calculate hydraulic gradient and determine flow direction

  • can use linear interpolation to find water table elevations in between

linear interpolation

elevation₂- elevation₁ / distance = elevation₂-contour/x

3 point situation

1. which well has intermediate hydraulic head

2. where will intermediate contour cross the line connecting the other two wells (linear interpolation)

3. connect linear interpolation point with intermediate well = water table contour

4. flow direction is perpendicular and downslope

5. use drop and distance from contour to either well to get gradient

many well situation

cna create contour mapwith linear interpolation to capture hydraulic gradient and flow direction

constant head boundaries

• equipotential (contour, line of constant h)

• lake or ocean (water table intersects land surface)

• groundwater will flow perpendicular to these boundaries

water table boundaries

p=0 but h wariable

• water table surface

• river

• water will usually flow at an oblique (not right) angle at these boundaries

flow nets

map showing both water table/potentiometric surface contours and flow lines

• flow is perpendicular to contours (constant head boundaries)

• flow is typically oblique to water table boundaries

show horizontal components of flow

potentiometric profile

vertical component of flow maps

water flows in three dimentions, water table contours specifically only characterize surface p=0 in map view

equipotentials

lines of equal hydraulic head where p is equal or greater than 0

• vertical gradients

• open portion of peizometer has same hydraulic head as the rise of the water and the water table

  • perpendicular, downhill to contours

  • in unsaturated zone, water goes directly down

constant head boundaries

water table contour

lake, ocean

PERPENDICULAR

in confined aquifer, equipotentials create perpendicular even lines in confined bed

no flow boundaries

along a streamline

impermeable layer

groundwater divide

PARALLEL to confining unit

flow direction and equipotentials

flow perpendicular to constant head boundaries

flow parellel to non-flow

flow usually oblique to water table

flow moving from high to low hydraulic head levels

groundwater divides

separate areas of an aquifer draining to different points

• important for understanding regional flow

• no-flow boundary; parallel

water table “toppography”

water table generally has the same shape as surface topography, though usually muted

• groundwater discharge zones typically in topographical low spots

• generally, groundwater flow high → low

flow tubes

made from flow nets

• parallel or semiparallel flow between two flow lines → flow doesnt cross flow lines

• discharge is constant down a flow tube (conservation of mass → water cant escape)

• crosssectional areas smaller as we approach the stream → hydraulic gradient increases steeper as area decreases

  • equipotentials closer together

• water tube drops when well is pumping (cone of depression)

recharge and discharge areas

is water flowing up or down relative to water table - angle between equipotential and water table

• acute = recharge = water down

• obtuse = discharge = water up

groundwater divide: peak in water table

discharge zone: dip in water table

regional vs local flow

can be over a vast area, not just water table

water can eventually percolate in deep aquifers (long RT + flow paths)

• through confined aquifer

water supply wells

casing → keeps hole open

seal → prevents surface water from entering

screen → allows water into wells

pump → get water out

typical well diameters

piezometers → 0.5-1 inch

monitor wells → 2 inch

domestic wells → 4-6 inches

“production wells” → 6-20 inches

• large diameter only increases yeild a little bit

• matters more to determine type of pump needed

cable tool drilling

repeatedly lift and drop a heavy drilling tool in the borehole

pros: good for remote settings, low fuel consumption, inexpensive, single operator

cons: slow

auger drilling

helical, sometimes hollow drill stems spins and moves material out; widely used for site investigations, site sampling, construction

pros: inexpensive, fast, easy sediment sample

cons: only works in sediments, hole doesnt stay open

rotary drilling

uses sharp spinning drill bit and downward pressure through sediment or rock - mud or pressurized air brings cuttings to the surface

pros: leaves materials near hole undisturbed, most economical for large wells, dont need casings

cons: need large water supply and mud pit, several operators, labor expensive, large rigs

well pumping

water level in and next to well drops

hydraulic gradient gets steeper

radial flow into well; as cone of depression grows, the radius of influence expands

eventually, gradient is steep enough and roi large enough that flow into well = rate of pumping (equilibrium)

• wont expand or drop under continued, steady pumping

Cone of Depression

draw down and radius of influence

• rate of pumping (Q₀)

• aquifer properties

  • hydraulic conductivity

  • how much water can be released

  • shape/size of aquifer

transmissivity

the volume of water flowing through a crosssectional area of an aquifer that is 1 ft x the aquifer thickness under a hydraulic gradient 1 ft x 1 ft in a given amount of time

hydraulic conductivity multiplied by the thickness of the aquifer

• units of L²/T

• determines if aquifer is good to put wells in

storativity (storage coefficient)

measure of aquifers ability to release water to a well - water table may lower in elevation but the pores in an aquifer won’t fill with air

S = volume of water released from an aquifer per 1 ft surface area per 1ft change in head

• ranges from 0 to specific yield

equations

equilibrium conditions (water levels stabilized)

non-equilibrium (drawdown)

thiem equations

helps determine transmissivity based on equilibrium conditions

• confined vs unconfined

nonequilibrium conditions

“pump tests” - pump from a well at a known rate and see how h responds

• water levels dropping during test

• Knowns: Q, distance from well, drawdown

• solve for T and S

darcy’s law and wells

Hydraulic gradient = change in head w/distance from well (dh/dr)
• A = circumference of circular section, 2(pi)r, times aquifer thickness b

Methods

often use theis equations

graphical = cooer-jacobs method

water level at a location will drop at rate dependent on pumping rate (Q), S and T, distance from well (r )

CJSL Time drawdown method

one pumping well and a single monitor well

• Q in ft³/day

• need r= ft

• measure drawdown in well over certain length of time → graph

• plot drawdown vs time on graph → use graph to determine change in h over one log cycle in time → find intercept where drawdown = 0 → convert to days

CJSL Distance drawdown method

one pumping well and at least 3 monitor wells

• measure drawdown at a single point in time

• meed water levels to drop over one log cycle of distance

• need pumping rate

• need time of measurement sincenstart of pumping t (in days)

• intercept r₀ in ft

Slug Tests

doesnt require pumping

can get properties by dropping slug into well, displace water, measure water seeping into ground aquifer

falling head = drop in

rising head = pull out

polluted/contaminated

if human activity alters water so it is not fit for its original use

hydrocarbons

typically less dense than water, vary in solubility

• petroleum products from gas stations, heating tanks, roads (oil, coal tar, gasoline, creosote)

• solvents (organic compounds)

• pesticides

organic compounds

synthetic substances used industrially - long-lived, traveled slowly, can be toxic

• Volatile Organic Compounds (VOCs), Semivolatile (SVOCs), chlorinated (CVOCs)

Volatile Organic Compounds (VOCs)

high vapor pressure and low water solubility

Chlorinated Volatile Organic Compounds (CVOCs)

compounds that contain chlorine, often used as solvents and degreasers, and for drycleaning and personal hygiene products

ex: Trichloroethylene, Tetrachloroethylene (PCE or “perc”)

SemiVolatile Organic Compounds (SVOCs)

higher molecular weights and boiling points than VOCs, in pesticides, solvents, cleaning agents, etc

ex: polychlorinated biphenyls (PCBs)

DNAPLs

dense non-aqueous phase liquid (sinks to bottom of aquifer)

• CVOC, PCBs, Coal tar

• difficult to remove

LNAPLs

light non-aqueous phase liquid (floats)

• gasoline, diesel fuel, fuel oil

• cause vapor problems

inorganic compounds

• lead (neurotoxin)

• copper (liver and kidney damage)

• mercury (neurotoxin)

• chromium (carcinogen)

• arsenic (neurotoxin + carcinogen)\

Nonmetals (agricultural waste; yard fertilizer, septic, manure)

• nitrates

• phosphates

pathogens

bacteria → e.coli, cholera, giardia

viruses → enteric viruses (polio, hepatitis, meningitis)

many originate from faulty septic and sewer systems and manure lagoons

“natural” contaminents

Radon

• comes from naturally occurring uranium in granitic rocks and granite-sourced sediments

Arsenic

• comes from minerals dissolving from weathered rocks and soils

emerging contaminents

prescription drugs, 1 and 4 dioxane, perchlorate, microplastics, PFAS → forever chemicals

• technology for detecting new contaminents is emerging

• can measure very low concentrations of some (ppt or ppm)

regulations

“MCLs” maximum contmainent levels

• set for drinking water, surface water, groundwater

• determined by EPA and individual states

• goal in remediation is to meet MCLs as cost-efficiently as possible

contaminent sources

point vs nonpoint

point source

enters environment through a discrete conveyence

• well, ditch, pipe

nonpoint source

enters environment through diffuse source

• agricultural, roads, mining, military properties