## Re: [gwmodel] Quantification of water in an unsaturated aquifer

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• Hi Frederic, First, you need information about existing pumping in the area. That would give you an idea and the range of pumping rate in the unsaturated
Message 1 of 6 , Jul 1 12:13 AM
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Hi Frederic,

First, you need information about existing pumping in the area. That would
give you an idea and the range of pumping rate in the unsaturated aquifer. If
you are not able to find this kind of information, you could try to find some
about trial pumping in the area. The trial pumping will give you data about
drawdown, exploration radius, permeability, and porosity. which would allow
you to estimate the rate. Yet, the rate will depend on your pumping well
geometry, casing, and also kind of pump.
You need to take care of existing pumping close to your area, in order to
keep the waterlevel high enough for the other wells.
In a productive aquifer, you can go up to 700 m3/h and perhaps a bit more
(this is just my experience), but you can also go down to 50 m3/h. this why
you need information about other pumpings in this aquifer.

ALexis
Alexis Valenza
SOL SYSTÈMES
Éden Parc "Les Cèdres" - Square Vilmorin
06160 Juan les Pins - France
alexisvalenza@...

[Non-text portions of this message have been removed]
• The first step is to estimate aquifer hydraulic properties and external stress conditions (as negatively suggested by dod). Second, develop a calibrated flow
Message 2 of 6 , Jul 1 6:12 AM
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The first step is to estimate aquifer hydraulic properties and external
stress conditions (as negatively suggested by dod).

Second, develop a calibrated flow model.

The third step is to formulate the 'hydraulic optimization problem'. This
involves three steps:
1.) define decision variables - in your case well pumping rate (and
maybe location) and water table elevation.
2.) define constraints - e.g., the water table cannot be lowered more
than some level at a number of predefined points (nodes of the mesh). In
addition, you can define a number of 'candidate' extraction well locations.
3.) define objective - e.g., what is the highest rate that can I pump a
well such that I do not lower the water table below the value defined at the
constraint points?

The forth step is to get an optimization package that can solve the problem
defined above. Basically, it systematically searches for the stress
configuration that meets the objective by sequentially setting well location
and rate, calling the flow model to solve for the resulting flow field, and
assessing whether the constraints are met. The algorithm stops when either
the objectives are met (feasible solution), or cannot be met for any
configuration (infeasible solution).

I know of two free optimization packages that can solve your problem. They
come with complete documentation that efficiently walk you through theory
and practice. The only issue is that they have been written to work with
MODFLOW (they have internal calls to MODFLOW to solve the flow problem for
each stress configuration). So you will have to develop a calibrated MODFLOW
model to represent the physical problem. You can find them at:
http://www.ecs.umass.edu/modofc/
http://www.geotransinc.com/modman.html

Joe Guarnaccia

-----Original Message-----
From: Daniel O'Donnell [mailto:dodonnell@...]
Sent: Saturday, June 29, 2002 8:55 AM
To: gwmodel@yahoogroups.com
Subject: Re: [gwmodel] Quantification of water in an unsaturated aquifer

Not much to go on Fred. Have you done the hydrogeologic work up of the
area? Have you done the aquifer testing? If not, hire a professional
with experience, http://home.att.net/~dodonnell/odonnell.html

If you have done the above and still don't know what to do, best of luck

Frederic Ghogomu wrote:
>
> Hi everyone,
> I am working on a project where I have to quantify
> water that can be pumped from an unsaturated aquifer.
> I am using FRAC3DVS. This model gives an option where
> you can place pumping well with specified rate. But
> the aim of our study is to determine this rate. I will
> apreciate any advice on the procedure to use or how to
> handle this problem.
> Thanks
>
> Fred Ghogomu
>
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• I am troubled by Fred s approach and agree with Daniel that a basic hydrogeologic understanding is essential BEFORE modeling. The wording of the original
Message 3 of 6 , Jul 1 4:37 PM
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I am troubled by Fred's approach and agree with Daniel that a basic
hydrogeologic understanding is essential BEFORE modeling. The wording
of the original question suggested little if any hydrogeologic
information was available.

--- In gwmodel@y..., Frederic Ghogomu <fred_ghogomu@y...> wrote:

> I am working on a project where I have to quantify
> water that can be pumped from an unsaturated aquifer.

I don't know if this statement concerns anyone else, but an aquifer
by definition is saturated. How can an aquifer be unsaturated? If one
were to accept a definition allowing for a portion to be unsaturated,
there would not be one value of hydraulic conductivity; there would
instead be characteristic curves for conductivity determined by
wetting or drying conditions.

> I am using FRAC3DVS. This model gives an option where
> you can place pumping well with specified rate. But
> the aim of our study is to determine this rate. >
Are there any wells in this aquifer? How deep are they? Are they
completed in the same kind of material as the proposed well? Will the
well fully penetrate the aquifer?
If this aquifer is "unsaturated", what portion of it is saturated and
do those conditions change with seasons? Are there barriers to flow
that could affect production? Are there surface water bodies that
could affect or be affected by production?

__________________________________________________
• I agree with Tom. A conceptual model needs to be constructed. FRAC3DVS is a finite-element variably-saturated flow and transport model. MODFLOW approach will
Message 4 of 6 , Jul 2 10:07 AM
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I agree with Tom. A conceptual model needs to be constructed. FRAC3DVS
is a finite-element variably-saturated flow and transport model. MODFLOW
approach will not work for this model. Unfortunately these days,
groundwater modeling implies applying MODFLOW. There are other types of
models besides MODFLOW. In FRAC3DVS, a well can be screened such that
the screen interesects both saturated and unsaturated zones. Obviously,
the saturated zone will contribute most of the well discharge.

It is essential that prior to model selection, one develops a
conceptual model. A conceptual model will lead to a thorough
understanding of hydrogeology, data gaps, processes taking place in the
flow domain, boundary and initial conditions, etc. Armed with a
conceptual model, one can then select an appropriate model. This
approach will lead to application of appropriate models and reliability
of model results. ASTM has good standards for application of groundwater
models.

If FRAC3DVS is approriate, there is a need to acquire a thorough
understanding of the physical problem and the model.

>>> tcudzilo@... 07/01/02 04:37PM >>>
I am troubled by Fred's approach and agree with Daniel that a basic
hydrogeologic understanding is essential BEFORE modeling. The wording
of the original question suggested little if any hydrogeologic
information was available.

--- In gwmodel@y..., Frederic Ghogomu <fred_ghogomu@y...> wrote:

> I am working on a project where I have to quantify
> water that can be pumped from an unsaturated aquifer.

I don't know if this statement concerns anyone else, but an aquifer
by definition is saturated. How can an aquifer be unsaturated? If one
were to accept a definition allowing for a portion to be unsaturated,
there would not be one value of hydraulic conductivity; there would
instead be characteristic curves for conductivity determined by
wetting or drying conditions.

> I am using FRAC3DVS. This model gives an option where
> you can place pumping well with specified rate. But
> the aim of our study is to determine this rate. >
Are there any wells in this aquifer? How deep are they? Are they
completed in the same kind of material as the proposed well? Will the
well fully penetrate the aquifer?
If this aquifer is "unsaturated", what portion of it is saturated and
do those conditions change with seasons? Are there barriers to flow
that could affect production? Are there surface water bodies that
could affect or be affected by production?

__________________________________________________

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