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From: Sandy Cruden on 3 May 2007 11:39

none เขียน:
> Bangkok gets that sinking feeling
>
> BangkokPost.com from Reuters reports
>
> Thailand's best known disaster prognosticator said on Wednesday global
> warming will put Bangkok a metre under water in less than 20 years,
> adding: "You will need a motorboat instead of a car."

>
> The problem, he says, is two-fold.
>
> The city is subsiding at a rate of 10cm per year, partly due to
> excessive pumping of underground water.
>

> Mr Smith, as usual, was scathing when asked about how authorities are
> facing the threat.
>
> "The government does not pay any attention at all."

*Some snipping*
*************************************

This pumping of water from under the city for industrial use was the
subject of discussion and controversy several (10?) years back, with
experts warning that if pumping continued at the rate it was, major
parts of the city must eventually subside, they provided the
government with evidence of areas and buildings in the city already
affected by subsidence. IIRC the government of the day vowed to put a
stop to it and compensate the industries involved.

Sandy
Huay Khwaang

From: maxwell on 3 May 2007 12:19
"Nisciuno" wrote ...
> maxwell ha scritto:
> > "Nisciuno" wrote ...
> >> barrister_rayong(a)yahoo.com ha scritto:
> >>
> >>> Most curious. Perhaps Khun Smith is correct. But I saw a story about
> >>> 2 weeks ago quoting some scientist from AIT who said that global
> >>> warming wouldn't affect the water tables in Thailand because the
> >>> glaciers are too far away from Thailand. Ah, Thai logic. I wonder
> >>> where that guy got his education.
> >> As far as I understand this matter, the scientist from AIT is right.
> >> The watertable, if I recall correctly, is not the same thing as
sealevel.
> >
> > Not as one gets inland/upland, or when sufficient outflow from land to
sea (high volume or higher elevation of land than sea).
> Actually, the watertable and the sealevel are *never* the same.

As one example, on low coral atolls, less-permeable Holocene sediments
overlie a highly
permeable Pleistocene karstic subsurface through which seawater can
infiltrate--then what do you have?
. . and don't forget the tides when you say 'never' ! ;~)

> Watertable is strongly affected by several factors, the most prominent
beeing: soil composition, nature and lay.
> Because of the different chemical potential, there exists an osmotic
equilibrium between the salty sea water and the fresh water usually

USUALLY.
Thank you.

> to be found in watertables. Even if the soil was porous and lacking any
bed of impermeable rock capable of holding back the seawater, it could be
that is is impermeable enough to the minerals of the sea water to let an
osmotic phenomenon to set in. In such a case this happens:
> * the waters in the sea and in the watertable seek a chemical potential
> equilibrium that results either in both having the same salinity, or in
> the two having a different salinity and a different gravitational
> potential to balance the chemical inequilibrium.
> * In order to reach it's chemical equilibrium, either fresh water must
> absorb some of the chemicals of the salty water, or it must seep into
> the salty water reservoir to dilute it's salinity and/or create a
> gavitational potential strong enough to create the equilibrium
> condition.
> * Sea water chemicals ususally can penetrate the land for a tiny depth:
> from a few meters to a kilometer or so (on sandy, highly porous land).
> * Artesian wells caused by geolocical strata and fed by highlands'
> freshwater also influence.
>
> What it happens is that a gradient both chemical and gravitational exist
> between the seawater and the watertable, more or less strong depending on
> the nature of the land that is separating the two water levels:
>
> ~~~~~~~~~~~____________________________________________ land
> /\ ^
> Seawater / \ |
> / \ | gravitational
> / \ | potential
> / <- \ |
> / <- \ |
> / <- osmotic \ |
> / <- pressure `____ |
> / <- \ v
> / <- ______________________ watertable
> /
>

> >> Watertables are water levels underneath the land. They receive their
water
> >> from sources of fresh water (lakes, rivers and, ultimately, melting
> >> glaciers), not from the sea.
> >
> > Not always. Sorry, but water is just a dumb liquid that 'seeks its own
> > level,' and if a body of standing water is higher than an adjacent area,
> > that's only temporary *unless* there's a barrier to motion.
> No. That is not even half the story. There are other factors to consider:
> chemical potentials (see the above about osmosis),

An osmotic gradient is a barrier of sorts--of course you previously knew
that--right?
;~)

>earth dymamics (the earth spinning causes the sealevel not to be "flat",
instead it tends to
> be higher near the equator and lower towards the poles),

Yes, and that oblate spheroidal eqautorial bulge has just how much to do
with local equilibriations between sea and land water, by comparison to the
tides?

astronomical
> gravitational effects invol ng primarily the moon and the sun (causing
> tidal effects),

Yeah, and what did I say about tides and waves?

>possible presence of rock beds or of a impermeable soil bed

BARRIERS!

> tilted by geolocycal forces in such a way to form a barrier to water
> flows within watertables or between watertables and other liquid
reservoirs.
>
> > The natural shape of a terrestial body of standing water, whether in the
> > open or under the ground, is equidistant from the terrestial center of
> > gravity (we see this as flat on top at short distances, and curving over
the
> > horizon, at sea).
> > Got that okay?
> No, that never happens in nature. In practice you can almost never find
> anywhere in the world a large liquid reservoir having a surface that �is
> equidistant from the terrestial center of gravity�.

The intentional oversimplification was offered in reply to your
non-attention to a great number of details.

> >> So, the glaciers melting are not a concern in the near future to
> > Thailand's water tables: their water does need to travel far too long a
> > distance from the Himalayas; before it could reach its watertables, it's
> > likely to be siphoned out of the soil by the intense agricultural use
other countries make of this water.
> >
> > Remember what I wrote just now about 'flat top water'?
> Yes, I do. And it is wrong. Again: http://en.wikipedia.org/wiki/Osmosis,
> http://en.wikipedia.org/wiki/Water_potential
>
Ummm, that's a reiteration of osmosis.
> [...]
>
> > That's a funny idea you have
> Evidently you never took a chemistry class.

No, not a one, LOL ! That's what you figure?

> >--and ignores that as ocean levels rise,
> > brackish (salty) water flows inland,
> Most of the times, almost always it does not.

OIC--so here the osmotic potential of the solutes (salt) does not move
toward equilibration ?

> > and yes, into the water table wherever
> > the hydraulic outflow pressure of inland subsurface water is less than
that
> > of the ocean's inflow. How else do you figure there are swamps with
> > subsurface brackish water kilometers inland from the coastline, in the
> > lowlands?
> Salty water inland is caused by the geological rise of the land.

OIC--neither osmotic nor hydraulic forces are material, but only the
geological rise--so therefore estuaries are constant in salinity, correct?
Hey--you wrote the conditions! ;~)

From: maxwell on 4 May 2007 17:06
"Nisciuno" wrote in ...
> Nisciuno ha scritto:
>> maxwell ha scritto:
>>> Not as one gets inland/upland, or when sufficient outflow from land to
>>> sea (high volume or higher elevation of land than sea).
>> Actually, the watertable and the sealevel are *never* the same.
> Well, they are *almost* never the same.
>
> I did however some more reading starting with what happened in the Gaza
> strip, where the increasing salinity of the watertable is causing less and
> less drinking water to be available to the local population. I knew it was
> caused by the drop in the watertable caused by the too strong pumping of
> fresh water out of it for irrigation and human drinking, and that it is a
> new phenomenon. And so I happend across this:
> http://en.wikipedia.org/wiki/Saltwater_intrusion
> Apparently osmosis is not a relevant force shaping the water levels
> inland, and the opposite of what i wrote actually happens:

Well, yes, and I also figured you'd reversed the arrows, which I took to
represent solutes (NaCl, mainly) in the seawater.

> �Saltwater intrusion is a natural process that occurs in virtually all
> coastal aquifers. It consists in salt water (from the sea) flowing inland
> in freshwater aquifers. This behaviour is caused by the fact that sea
> water has a higher density (which is because it carries more solutes) than
> freshwater. This higher density has the effect that the pressure beneath a
> column of saltwater is larger than that beneath a column of the same
> height of freshwater. If these columns were connected at the bottom, then
> the pressure difference would trigger a flow from the saltwater column to
> the freshwater column.
>
> �The flow of saltwater inland is limited to coastal areas. Inland the
> freshwater column gets higher and the pressure at the bottom also gets
> higher. This compensates for the higher density of the saltwater column.
> Where this happens, saltwater intrusion stops.�

That affirms what I'd initially posted, but there's one very significant
contributor to the water table you should not neglect, and if you were in
Thailand not many months from now, you'd find it *very* hard to ignore ;~)
```````````

> Other readings of intertest on the Wikipedia include:
> http://en.wikipedia.org/wiki/Water_table
> http://en.wikipedia.org/wiki/Aquiclude
> http://en.wikipedia.org/wiki/Sealevel
> http://en.wikipedia.org/wiki/Salinity
> http://en.wikipedia.org/wiki/Hydrogeology

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