GEOLOGYTWO

Groundwater

JD Price

Water in the earth

In general, the planet has

an excess of water. It is

likely that free (molecular)

H2O is found throughout

the crust and into the

upper mantle. We’ll focus

on that close to the

surface of the planet, but

realize that there are

deeper reservoirs

(although much less large).

Just below the surface, we

can find significant

quantities of water within

fractures and intergranular

pore spaces.

The near-surface is water-rich, but

most contains too many dissolved

ions to be consumed by many

continental-dwelling organisms,

humans included. To put this more

simply, if the world’s water supply

were only 100 liters, our useable

supply of fresh water would be only

about 0.003 liters or half a teaspoon.

Much of this is not in the lakes and

streams we often consider when we

mention water resources.

“There’s water below us”

Is water an undervalued commodity?

The distribution of surface water

Spring

R.O. Purified

What is water worth?

Water is the resource of life. As such, water is probably undervalued. An

exception is the bottled water craze - but it’s surprising that natural springs

and purified tap water command roughly the same price (perhaps the money

is in the packaging).

The craze is somewhat puzzling - yes

it’s convenient, but the US boasts one

of world’s best standards for municipal

water service.

2-458

The hydrologic cycle

Water runs downhill - it

does so as well

underground. Flow is

not simply

topographical, as

subterranean strata that

transmits water may

have a different

orientation from that of

the land surface.

Rocks like these are responsible for storage and

transports of large reservoirs of potable water (and

other liquids - brines, natural gas, and oil.)

Sandstones, white area are grains of quartz (SiO2),

blue area is a dye to show pore space in rock. The

grains permit large amounts of fluid to be hosted

within the rock - and its interconnected, so it can

move.

Porous Rocks

Porosity: volume of void space

Permeability: connectivity of

void space

It’s not just about pore spaces

- fluids must be able to move

to be a useful reservoir or

aquifer.

Void space: the part of a

material filled with an easily

displaced substance (fluid)

Porous / permeable

Well-sorted rounded grains have excellent

permeability for a given pore volume. Even

when the materials are highly compressed (as

they are in the Earth), they still retain pore

space (perfect spheres would give you 29.7%)

with an interconnected geometry.

Poorly-sorted rounded grains have reduced

permeability for a given pore volume. Porosity

is reduced because smaller grain sizes fill some

of the void space.

Flat grains tend to stack, which can greatly

reduce permeability for a given pore volume.

Porosity may be huge, but there’s no way for

fluids to move easily through the material.

Grain size, sorting, and structure

Fractures

Intergranular pores are not the only porosity -fracture systems tend to

have overall lower porosity, but can by highly interconnected. If the

material can be dissolved into the moving fluid, then porosity may be

enlarged with time.

Groundwater movement

Water is denser than air and will flow down provided there is permeable pore

space. This “fills” permeable strata to a level known as the water table or the

piezometric surface. The region above the surface is the dry or vadose zone.

The surface is not static, and will rise and fall with input and outflow

variations.

Question: What is an

artesian aquifer?

Artesian well

Natural Resources Canada

Example of fracture and intergranular aquifers

Rainwater is weakly acidic, due to CO2 in the

atmosphere, forming H2CO3. Carbonates are

rocks largely composed of calcium carbonate,

and are easily dissolved by acidic fluids.

Although carbonates can be low porosity (and

permeability), fractures can transmit water.

These may enlarge withy times.

Karst

Karst example

Caves

(c) 2005 Kim Knox Beckius

(c) 2005 Kim Knox Beckius

Fracture porosity can see increased porosity

and permeability over time resulting in

karsting and cavernous porosity. Caves (like

nearby Howe Caverns) form from water

movement, often near the water table.

Speleothems

Water moving through

carbonate rock dissolves

CaCO3 - and will precipitate

out if conditions change.

Caverns above water table

have localized deposits of

CaCO3

The type of speleothem depends on cavern

geometry, rock permeability, and fracture and

geometry - New Mexico Bureau of Mines & Mineral

Resources Bulletin 117

Aquifers in the US

Ogallala Aquifer

The Ogallala or High Plains aquifer

provides water for communities,

agriculture,and ranging in this dry high

flatland region.

The aquifer is hosted in river gravels laid

down from

geologically recent

outwash from from

the Rocky

Mountains.

However, the

system is not

recharging - it is a

fossil aquifer.

Withdrawals are

from water left

after the last ice

age.

Use

The primary use of

the Ogallala is

irrigation. This

NASA image of

Kansas shows the

dominance of

central-pivot

irrigation farming in

this part of the mid-

continent.

Central pivot

systems have a single

well and permit

more efficient water

distribution.

This state relies largely on its abundant

surface water. Glaciation has created

numerous lakes. The topography also

lends itself to engineered

impoundments. There are some small

aquifers in use.

Surface

Aquifer

Wells

Water in NY

Saratoga Springs

Nearby Saratoga Springs State

Park hosts several extraordinary

examples of artesian springs.

The source of the water varies

from spring to spring - some are

sweet, most taste of high sulfur

contents. Water is thought to be

sourced as deep transport from

the Adirondacks along a fault

system in the area.

The spring above right has a

piezometric surface that

averages 1.5m above the

surface, although it fluctuates

over the course of a few

minutes. Both pictures show

flow-stone deposits.

Water in the big city…

NY DEC

Long Island is made of loosely consolidated glacial material - porous,

permeable, extensive. This caps two tilted sandy units. It has an

impressive aquifer system. But not without complications.

By the sea

Freshwater is less dense than saltwater - it floats, meaning that the

rainwater from L.I. sits on top of seawater. This can be easily drawn

off with too much pumping - Kings County, 1930’s.

Interactions with the sea

Parts of this state has relatively little natural year-round surface water.

Citizens rely largely on its abundant groundwater and artificial reservoirs.

We will briefly consider how the actions of one water well impacted an

entire city.

Water in Texas

The City of San Antonio vs. The Pucek Catfish farm.

As a city at the western edge of the Gulf Plain, San Antonio

relies solely on groundwater from the Edwards Aquifer.

Water levels within city wells are carefully monitored.

Well level drops to a daily minimum around 7:00 AM when the

majority of people get up and shower. Drops again in the evening

(after work use), and recharges overnight.

Resource controversy:

Edwards Aquifer

The aquifer takes its name from the Cretaceous limestone unit that hosts it.

The limestone layer is locally offset by faulting, which provides an area of

recharge. Below the fault, the limestone unit is bounded by clay-rich rocks.

Edwards is artesian

Recall that artesian aquifers confine the water below a low-permeability

unit, so that pressure builds as the water top of the aquifer is below the

water table in the area open to water input.

Recharge

Water enters the aquifer through sinkholes (above) and fractures at the

surface in the fault zone. The Seco Creek sinkhole above, was not

currently active, but the creek has been diverted to increase recharge into

the aquifer system.

Of course, this use evicted several thousand bats…

Permeability is through

fracture networks. Fractures

enlarge into caves over time

(karst topography).

Cavernous porosity systems

are rare aquifer forms - unique

geology forms a think aquifer

unit bounded by relatively

impermeable clay units.

The Edwards Aquifer is hosted in

within an initially low-porosity

limestone. But limestone does react

with the weak acidity of rainwater.

Over the last 10 ka to 1 Ma years,

water has worked its way into cracks.

Fault in Wonder Cave

Caves

In the region above the

recharge zone, the

water table fluctuates -

much of the porosity can

be above the

piezometric surface.

Caves that have active

speleothems are in the

vadose zone - flowstone

and dripstone features

are the result of

percolating waters from

above encountering the

cavern atmosphere.

Active caves

Pit Cave - Gregg Eckhardt

Water levels

Data from the Bexar monitoring well, City of San Antonio. Levels

are highly affected by year-to-year precipitation in the recharge

area, rainfall events (summer storms), and withdrawals. Daily levels

fluctuate as well, with heavy morning usage in the city.

Comal Springs - USGS

Outflow

Fractures can bring the water

table to the surface (along an

escarpment) - an artesian spring.

A series of springs and pools are

found at this interface.

Water flows from Comal Springs

at 250-280 cfs. This feeds

downstream water usage.

Wells within the artesian zone exploit the fact that

although the Edwards formation is 120 m below,

the water is within 30 m of the surface. The J-17

well (left) is one of the municipal wells for San

Antonio - water is stored in the adjacent water

tower to provide pressure.

Cleansing effect

Despite the large pores (and

the considerable biota hosted

therein), the water coming out

of the spring is of high purity -

save the large concentration of

dissolved carbonate ions.

Certainly suspended

sediments and organic matter

is filtered out - compare the

recharge water (left) with that

from an artesian spring (right).

Use of Discharge

1989, Ron Pucek decided to exploit the Edwards Aquifer southeast of

San Antonio in order to irrigate catfish tanks.

Tapping into the Edwards

This was the largest water well in the

world.! There is an incredible amount

artesian pressure here. When this

well came in, it blew out rocks the

size of basketballs 20 feet into the air.

Water flowed out at 25000 gallons

each minute (55 cfs).

Not potable, but it would be enough

water to support about 250,000

people

The “right of capture,” - beneficial use

- as long as the farm proves that

water is being put to use, then they

have the right to the water under

Texas law.

Under pressure

The air above contains a number of sulfur compounds and smells awful.

Water is allowed to degas,is fed though the catfish tanks, and is released to

the river system.

Living Waters Catfish Farm, 1991

Aquiculture

So what’s the problem? It’s bad water.

The problem is that the Pucek well was draining the aquifer at a much

more rapid rate than water could be replaced. If allowed to persist, the

well would eventually tap the sweet water and lower San Antonio’s

supplies beyond use.

Something for nothing

In Texas Law, (at least at the time) property rights include water and

minerals on or beneath property. Taken literally, this includes the

entire earth to the core. Right of capture for beneficial use - as long

as the water is being used for a purpose, then it is permissible.

This doesn’t allow for dynamics on or beneath the property.

In this case, because scientists were able to quantify the determent

the well placed on the water supply, the city of San Antonio finally

convinced the Pucek farm to close the well in the early 1990’s.

Science vs. the law