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Geology/Aquifers

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Question
Does using water from one aquifer affect the water resources on aquifers beneath it?
So if I use the salt water 10 meters below ground at a rate of 30 liters per second will it have an effect on water resources let's say 60 meters below ground? Are they related?

Can you also comment on rate of replenishment of the salt water 10-40 meters below ground and the rate of replenishment of the fresh water 70+ meters below ground?

The reason I ask: The local municipality does not give permits for deep wells(70+ meters). I want to see if I can make a plausible argument for a permit for the salt water that's available 10-40 meters below ground.

Does one have to pay attention to the rate of replenishment when deciding how much water to use from an aquifer? How do you calculate the rate of replenishment (if it's important) ?

Are under ground aquifers the water source of future for farming? how careful should one be in using them?
If I hit an aquifer where I can retrieve 200 liters per second should I use the entire amount or should I just use 30 liters per second?
In many areas governments are planning for water treatment programs and canals for directing sea waters to farms and rural areas. So for farming will we still need these under ground sources?

Answer
Amir:

I am assuming you are in the Middle East somewhere.  Okay, lots of questions so lets get started.

First, it depends on the geology of the subsurface whether the various aquifers are in "communication" with one another.

Generally the ground becomes saturated at a certain depth below ground and stays saturated.  That is all the rock or soil that is porous enough to hold water below that depth is filled with water.

Now, the subsurface is made up of soil, sediments and eventually rock.  Soil is near surface, then sediments are unconsolidated, then come the rocks.  Aquifers can exist in the latter two.  I will refer to them as formations.  Soils are stratified into horizons.

Okay, so some formations contain fine material with very little porosity and permeability. These are referred to as aquicludes or aquitards.  Since they prevent the movement of water up or down the formation column.

If a aquifer is bound by two aquicludes it is referred to as "confined" and is kind of line a pipe although the pipe might extend laterally in all directions over a large distance, the water cannot move up or down, only laterally and if recharged from an area higher than it is, becomes a "artesian spring" meaning it is under pressure and bubbles up.

Recharging takes place where porous rocks are exposed and can take in water, this can be under a river, a lake, or anywhere rainfall occurs, such as in the mountains.

Some water is thousands of years old, since it must make its way through the rock.  Only in areas of carbonate rock do "underground" streams occur.  Generally water moves through the interconnected voids in the rock, called porosity.  If the voids are well connected, it has high permeability.  

Think of a box of golf balls as a piece of sandstone or body of sand.  The box can still hold a lot of water between the balls in the void spaces.  Just as water moves through sand on the beach, buried sands a gravels can transmit water readily.

So question one. If the aquifers are connected, drawing down the top one will only affect the water level down to the bottom of the well.   It will not draw water up from below.  If there is an aquiclude between them, like a clay or shale, this would make it impossible.

If you have salt water at a shallow depth, the two cannot be connected, since the salt would spread by diffusion, molecular transfer, and turn the fresh water brackish or salty.

So you have a aquiclude between them preventing communication between them.

If your well is completed, that is set in the bottom of the salty aquifer, and does not penetrate through the aquiclude, you can pump at whatever the sustainable rate is for the salt water as it will pump dry and not affect the underlying fresh water.

A well will pull down water from a circular area around it called a cone of depression. Once the cone drops to the bottom of the well, it sucks dry, and pumping must stop for the water to rise in that area again to resume pumping.

If you have ever drunk a thick drink with a straw, you will know what I am talking about, as a dimple forms in the fluid as you suck on the straw.

Replenishment depends on the "transmissivity" of the rock.  There is a formula to determine it and you do a aquifer pumping test to determine the sustainability of the pumping rate.  You usually pump a well over a 24 hour period at a sustained rate, stepping up the rate to determine the maximum.

You can find documents on line detailing the technique.

http://www.aqtesolv.com/pumping-tests/pump-tests.htm

http://pubs.usgs.gov/twri/twri3-b1/pdf/twri_3-B1_a.pdf

Do a search for AQuifer pumping test to find more.


Ground water IS used in a lot of places for farming.  In the San Juaquin Valley of California, the huge agricultural food basket, ground water has been used for over 80 years.  The area was a great glacial lake at one time and is filled with sediment supported by hydrostatic pressure, that is it was saturated with water.  Pumping of the water over the last 80 years, has resulted in the ground settling over 40 feet over time!

http://www.watercalifornia.org/projects/enidbaxterblader_central.html

Each large field has its own well for use as a source for irrigation.  They do the same thing in Iran and other places.

You cannot use salt water for irrigation, even fresh ground water used for irrigation contains dissolved salts and over time will result in a build up of salts in the surface soil, degrading their fertility.  

Again, you want to pump at a rate, that matches the ability of the aquifer to recharge.  This will allow the well to retain a column of water and you pump will not pump dry.  

Using salt water for agriculture by desalination is a very expensive proposition and presumes that energy is cheap and readily available.  It also means that the agricultural products will be expensive as a result.  The use of salt water is a measure of last resort.  

California is now in the process of building desalination plants since their drought has caused critical shortages, shortages that everyone knew was coming.  For decades California had to bring snow melt water from the mountains down to Los Angeles, by aqueduct.  That is just not enough now.  

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