Re: A stab at the geology

From: Stewart Stansfield <stu_stansfield_at_...>
Date: Mon, 08 Mar 2004 11:02:33 -0000

HYDROGEOLOGY FOR DUMMIES (and volcanologists)

I pored (baddabum-tssshh!) through a few old textbooks I had't looked at since being a first year undergrad, and made a few notes on possible water notes. All this is depressingly mundane--Weirdify as you will! Though I'll make a few notes as I go through.

Water that passes trough soil, to percolate down into bedrock, passes through varying zones of saturation, from poorly saturated (with pores allowing water passage) through to fully saturated, where pores and fissures are all filled with the stuff. This typically occurs a few hundred metres below the surface.

Few rocks are completely impervious to water; those that are crystalline or well cemented are more so, but usually contain various fractures, fissures and faults that allow passage. Good transport systems are those which allow travel through their significant porosity--the pores spaces or interstices between minerals or grains. As an idea, porosity in a fairly typically packed sandstone is ~25%.

Limestones are a wee bit different. They have little natural porosity, but water dissolves the calcite, forming significant fissures and caverns (wa-hey!). Pot-holes, swallow-holes, sinkholes are all nice and probable for the speleologists out there. Extended erosion of above ground rocks forms those nice pinicular karst landscapes across the world, perhaps most pictorially in China. Whitewall? It also easily allows all those ravines and makes Lunar transport rather difficult.

So, rocks that store and transmit groundwater we call aquifers, and sandstones, chalks and limestones are big on this. Porosity is important for storage, but flow along fissures is also very important for transport and remobilisation of groundwater.

Rocks which are veritable hydrophobes we call aquicludes. Densely packed clays, metamorphic rocks, etc. Generally speaking all rocks possess enough permeability to transmit water to some extent, but often only in token amounts.

The rate at which water is transported depends on the hydraulic head-- the difference in elevation between the source and the point of measurement. The pressure of a given head relative to the distance over which it exists gives your driving force for water movement--the hydraulic gradient. Permeability, transmissibility (a function of the bulk permeability multiplied by the saturated thickness of the rock formation) and hydraulic gradient are basically analogous resistance, current and voltage.

When an aquifer crops out on the surface, it will develop its own water table conditions. Variations in the makeup of strata in the formation may allow several areas of saturation. Water tables broadly reflect topography, and where the table reaches the ground surface, outflow (in springs, standing lakes, whatever) will occur.

The nature of these will vary as the water table varies, which in turn will vary according to how caught short Heler is on a given day. So lots of ephemeral streams, or 'bournes'.

Aquifers and aquicludes can occcur as alternating strata in formations. If an aquifer is overlain by an relatively non-permeable layer at outcrop, it's 'confined'. Water can still enter the quifer by slow migration, but has to be got at by welling. Confined aquifers are usually fully saturated. If situated in an area where the water is forced below its natural pressure table, the reservoir is held at a pressure greater than atmosperic. Thus if drilled, it will rise through the confining layer to height where its pressure equilibrates itself. This is your artesian well.

So water wanders about until it reaches a point of outlet. Often this never occcurs, and it can store in areas below the level of natural discharge. This can be reached by mechanical means, but is often topped up by precipitation and further accumulation of water, leading to a renewal of outflow.


Depending on what you think Whitewall is composed of, the rock itself could be a natural aquifer, collecting water at perhaps a variety of levels. The water table and relative saturation may rise as a function of rain and snow, but there will still probably be deep reserves trapped underneath.

Sadly the human habitation of Whitewall will disturb much of the catching of precipitation, and I would suggest that the reservoirs underneath will be relatively undefilled relative to if Whitewall was a uninhabited lump of stone. Of there servoirs in Whitewall itself, many will find discharge out of the cliff face, in small springs. Oh, the Lunars have *plenty* of water!

There will be caves a plenty leading down into the depths, and no doubt some small local reserves there. The best hope I can see for an aquifer is from the local environment. An aquifer below whitewall, especially if confined (and *especially* if Whitewall itself is an aquiclude) can be tapped by wells. The efficacy of these will be greatly increased if Whitewall, although on a plateau, does lie within a relative bowl, where the water pressure rises the theoretical (piezometric) water table above the local valley/delve/dip surface.

All the best, and I hope this is useful,


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