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Non-steady state conditions


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#1 ktulu

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Posted 15 October 2008 - 06:40 PM

I am wrapping up the WR depth questions in the OLD NCEES sample problems, and have become stuck on this question for awhile. I understand how they worked the problem, my only problem is why they have considered this as non-steady state conditions.

The problem is this: The 85 feet thick confined aquifer starts at a depth of 1450 feet. The water level is initially at a depth of 1400 feet, and after 100 days of pumping, is at a depth of 1420 feet. For a drawdown of 20 feet. They problem asks for the flow rate.

Is this non-steady state conditions because the water being pumped from the well comes from outside the confined aquifer?

Any insight would greatly appreciated.

ktulu

I have attached the question for those who better give insight from seeing the question.

#2 ktulu

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Posted 15 October 2008 - 08:15 PM

So after doing some online digging, it seems to be that a few pieces of information given in the problem tell you that the aquifer is in a non-steady state condition.

1. that a storativity value is given, and
2. the well fully penetrates the aquifer.

These two items are present in non-steady state conditions.

Any other opinions are appreciated.

ktulu

#3 jregieng

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Posted 15 October 2008 - 09:46 PM

QUOTE (ktulu @ Oct 15 2008, 04:15 PM) <{POST_SNAPBACK}>
So after doing some online digging, it seems to be that a few pieces of information given in the problem tell you that the aquifer is in a non-steady state condition.

1. that a storativity value is given, and
2. the well fully penetrates the aquifer.

These two items are present in non-steady state conditions.

Any other opinions are appreciated.

ktulu

Steady-State pumping utilizes the Thiem equation, Non-Steady-State utilizes the Theis equation. Storativity is a value that is calculated from the Non-Steady State (Theis) equation and therefore is a tell-tale sign the pumping was conducted under non-steady state conditions.

However, a fully penetrating well is the basic assumption for ANY pumping condition (steady or non-steady).

JR

#4 pinkpig

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Posted 15 October 2008 - 09:56 PM

QUOTE (ktulu @ Oct 15 2008, 08:15 PM) <{POST_SNAPBACK}>
So after doing some online digging, it seems to be that a few pieces of information given in the problem tell you that the aquifer is in a non-steady state condition.

1. that a storativity value is given, and
2. the well fully penetrates the aquifer.

These two items are present in non-steady state conditions.

Any other opinions are appreciated.

ktulu

Is the answer A? You can only reach a steady state after some time from initial pumping, and at that time, your Q-recharge=Q-pumpout. Apparently, this problem is a non steady state one: your original piezometric is EL=-1400' (assume EL of ground=0), and after 100 days, the piezometric is EL=-1420' @ the well. the drawdown is 20' after 100 days.

#5 ktulu

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Posted 15 October 2008 - 09:58 PM

Well, it's about time you chimed in, JR!! smile.gif

So, if you are given storativity, you have a non-steady state condition. Got it.

Thanks!!!

#6 ktulu

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Posted 15 October 2008 - 09:59 PM

QUOTE (pinkpig @ Oct 15 2008, 04:56 PM) <{POST_SNAPBACK}>
Is the answer A? You can only reach a steady state after some time from initial pumping, and at that time, your Q-recharge=Q-pumpout. Apparently, this problem is a non steady state one: your original piezometric is EL=-1400' (assume EL of ground=0), and after 100 days, the piezometric is EL=-1420' @ the well. the drawdown is 20' after 100 days.

Yes, the answer is A. But I went the way of storativity = unsteady state, as JR mentioned.

#7 pinkpig

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Posted 15 October 2008 - 10:00 PM

QUOTE (ktulu @ Oct 15 2008, 09:58 PM) <{POST_SNAPBACK}>
Well, it's about time you chimed in, JR!! smile.gif

So, if you are given storativity, you have a non-steady state condition. Got it.

Thanks!!!

BTW, if you take civil-water/environ PE, non-steady state is out of scope, as I remember.

#8 Jacob_PE

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Posted 25 September 2011 - 01:16 AM

I fell victim to lost time because of this problem. This is water resources depth problem 518 of the 2000 NCEES sample questions. It gives storativity which means unsteady flow which means ignore it. We're only gonna see steady flow well analysis on exam day.

#9 civilized_naah

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Posted 25 September 2011 - 03:01 AM

QUOTE (ktulu @ Oct 15 2008, 02:40 PM) <{POST_SNAPBACK}>
I am wrapping up the WR depth questions in the OLD NCEES sample problems, and have become stuck on this question for awhile. I understand how they worked the problem, my only problem is why they have considered this as non-steady state conditions.

The problem is this: The 85 feet thick confined aquifer starts at a depth of 1450 feet. The water level is initially at a depth of 1400 feet, and after 100 days of pumping, is at a depth of 1420 feet. For a drawdown of 20 feet. They problem asks for the flow rate.

Is this non-steady state conditions because the water being pumped from the well comes from outside the confined aquifer?

Any insight would greatly appreciated.

ktulu

I have attached the question for those who better give insight from seeing the question.

In my opinion, neither a given value of storativity, nor the fact that the well fully penetrates the aquifer are 'clues' that this is a unsteady drawdown problem. Storativity is an aquifer property (confined aquifers have much lower value of S than unconfined aquifers). It is the fact that the problem provides the drawdown after a SPECIFIC DURATION (100 days). In a steady state problem, the assumption is that 'enough' time has passed since pumping began and therefore does not matter. For very large values of time, the unsteady solution should conicide with the steady state solution.

And I agree, from the language of the exam specs, it seems like they say 'STEADY STATE ONLY'. So, applications of the Theis equation should not raise their ugly head in the currenty defined exam.




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