A l'aval can mean downstream, downhill, downline, etc. What they mean, I think, is that the carpark is downstream of the piezometer (in the groundwater flow). Had they said simply "à l'aval", it might be thought that the carpark was downhill of the piezometer (a vertical line going up from the carpark would surface downhill of the top of the piezometer), the difference being that it could in fact be above the groundwater level and therefore not be strictly "downstream".

You haven't given the sentence, but I should think "à l'aplomb" would be vertically above (or below) something.

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Note added at 2003-05-18 22:48:12 (GMT)
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Re. \"aplomb\", since the school has to be at the surface, it must be ABOVE the piezometer.

Didn\'t understand why you were having a problem with this word until I looked in my dictionary. \"D\'aplomb\" is there, but no \"à l\'aplomb de\". Some 7000 Googles though, including \"Tracer la hauteur de la tringle à l\'aplomb de chaque côté de la fenêtre\" which clearly indicates the meaning.

Now did I answer that with aplomb, or did I not?

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Note added at 2003-05-18 23:50:36 (GMT)
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The piezometers might be sealed or unsealed (open standpipes). If the open type, and if the piezometer was drilled from the school, in Vincennes say, then it would communicate with the groundwater and substances from the subsurface (radon, say, having leaked from a photographic plant) could make its way to the surface.

Note that if the French \"piézomètre\" is not sealed off, it should be called a \"standpipe\", \"open standpipe\", \"standpipe piezometer\", or even \"open standpipe piezometer\". Piezometers are for measuring water pressure.

The following case, similar to yours apparently, has standpipes. These are used not to measure water pressure but to enable groundwater samples to be taken.

<<La pose des piézomètres (tubes d’une quinzaine ou vingtaine de mètres avec des trous lattéraux-crépines - sur presque toute la hauteur du tube) implique le creusement d’avant trous avec recueil des terres correspondantes (carottage). On a donc, si toutes les précautions sont prises, possibilité d’analyses de terre et de l’eau de la nappe qui pénètre dans le piézomètre.>>
[http://vigilancefranklin.ifrance.com/vigilancefranklin/doc/0...]


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Note added at 2003-05-19 16:05:24 (GMT) Post-grading
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Comments to comments:

\"below the level of\", to me, means vertically below. In the case in question, it is fair to assume that the groundwater into which the standpipes have been driven is moving. It will move downhill, yes, but the fact that traces of an element are found \"below\" another is pretty much irrelevant (unless you have a coal mine or something beneath the source of contamination). What matters is that the contamination is on the move pretty much horizontally as a result of the water movement (downhill, if \"downhill\" can be used for underground water) towards a river or something. So for \"à l\'aval hydraulique\", \"downstream\" is it (stream as in water, as in hydraulic). It seems to me that was my first comment.

A plumb line (plumb as in plomb as in lead, something heavy that when suspended from a string will hang as vertically as the Earth\'s rotation allows) serves to determine verticality, not level, so \"level with\" will not work. For the \"école à l\'aplomb du piézomètre\", you want something like \"school above the piezometer\", if the piezo does not surface, but is drilled from some subsurface space (tunnel, underground carpark, etc.), or \"school where the piezometer is located\" if it surfaces.