 
Possible definition?: an ubiquitous formless inscrutable monster hiding just underground, waiting to suddenly, unexpectedly surface, terrorize, overrun and submerge everything within reach.
The term "water table" and "high water table" are buzzwords (pseudo techno-babble). The water table term is widely used to mean different things, depending on to whom you are talking, and about what. The various definitions of what a water table is are beyond the scope of this article However, for simplification a few things may be said: * A water table may exist where water gathers in a level body. * True water tables do not change much (one inch per century is a large change, ususlly a result of direct major human intervention. * Most true water tables are many feet down (usually 50 feet to hundreds of feet). * In developed locales where a true water table is "high" or not very far down, such as in many parts of southern coastal Florida, houses are, by law, not permitted to be built with basements --it is not possible.
There is another term called "ground water" that is different than a water table and is usually associated with water closer to the surface and distributed unevenly. Ground water, again, has different definitions depending on who you are talking to and about what. However, generally speaking, in common (non technical) terms ground water is more of a surface condition and does change sometimes dramatically, over time. Ground water commonly refers to the amount of moisture gathering in the earth from rain and snow melt. It can also be uneven, gathering more in some areas than others, depending on the varying density of the soil. This ground water changes seasonally, going up with wet weather and down with dry weather. Ground water is related to "soil moisture", another term relating more commonly to the amount of, or percentage of, moisture in a quantity of soil, indicating level of soil dampness. This gets complicated, since soil moisture and ground water interact, and affect each other. The mechanics of this interaction can be puzzling, for example, if the soil moisture is very low, ground water has to be low, but if a torrential downpour of rain occurs, the dryer ground will not easily absorb the water and surface flooding can result. If the soil moisture is high to the point of saturation, again, the soil cannot absorb more sudden increases of water especially during a rain storm, and flooding will occur. If the soil moisture is normal, the ground water will rise more slowly during a rainy period, and only show up as time passes. this sometimes causes leaky basements to start taking on water days after a period of heavy rain, and continue to seep and leak for days or weeks while no rain may occur.
If all this isn't confusing enough, let's add a little geology. There is a thing called bedrock, that is another somewhat ambiguous term and has various meanings. For the sake of discussion, generally speaking the highest, or initial bedrock strata, is usually covered with a layer of dirt again generically called "overburden", or "cover", this is the subsoil and mixed soil media above the "bedrock". There is often soil under the bedrock, that may be very course, a layer of sand, or broken rock, etc. This is often where a true water table exists that may be taped into with a well. The water in this area, if it exists, usually does not move up, unless (very rarely) it is under pressure, or coming from a much higher place, again causing it to be moving under pressure. This is also the place for so-called under ground streams, rivers, lakes and aquifers. And, the place of origin for "artesian wells" (another extreme rarity). The bottom line here is that this water almost never moves upward on its own. On the other hand, water in the soil above the bedrock can build up in the "overburden" to the point of surfacing, or causing excessively damp soil.
Water building up in the overburden is, non-specifically, generally, and improperly, what is commonly referred to as the water table. This water usually does not impinge upon and enter a basement, or does so with extreme rarity. This is true due to the design of basement foundations that by the process of experience, over the centuries and eons of time, have been adjusted to handle most of these variables most of the time. And to the fact that laws normally prevent construction work to enter a water table.
A normal foundation configuration is represented by a basement foundations dug into and placed in acceptable areas (defined by experience, codes of law and market forces), has a barrier of loose dirt, called back-fill that can handle through absorption, and other mechanical processes a great deal of moisture. Drainage beds or drains are normally placed outside of the base of the foundation to de-water or channel errant water away. Foundations are built to absorb, deflect water and/or redistribute water (masonry foundations are hollow, except poured concrete that is basically water proof). Finish soil grading, producing a properly contoured yard/lot surface will direct excessive water away from the foundation making it unlikely that the soil next to the foundation will gain more water or moisture that the surrounding soil. Finally, the foundation itself contains a cavity (the basement or crawl space), that compared to the surrounding soil surface has a much larger evaporative surface area, and a much lower vapor pressure, thus allowing a relatively larger amount of water to degas, or leave by osmosis and capillary action through the basement (in other words to travel gently but steadily through the wall and escape through evaporation).
It is my experience that water gets into the basement (and does other damage like cracking walls), when the outside ground contouring fails (swales fill up, curbs wear away or settle, window wells break down or leak), the original exterior foundation/footer drain fills up with silt and fails after becoming a working drain (due to the failure of above grade swales and soil curbs and broken window wells). Or when concrete or other pavement cracks go unsealed allowing water to flow through the cracks and enter the ground around the building under the pavement.
To clarify some points and address some common questions, I have included often asked questions as a part of the following text.
Q Almost everyone I have met in the neighborhood has a wet basement.
A This is usually true if the neighborhood is generally 20 years old or older, the older the neighborhood the worse the problem. This is due to general neglect, again due to general lack of knowlwdge of, or a misunderstanding of the above processes.
Q Our house is located on a hill where bedrock seems to be very close to the surface.
A Having the underlying rock strata close to the surface is a common but not typical situation, and yes, ground water will gather above and run (slowly through the loose upper layer of soil) across the top of the bedrock, even though the top of the rock strata is below grade. However, the normal foundation configuration described above allows for this and can usually handle the results easily.
Q We have only been in the house for eight months so our experience is rather limited. In the fall (same thing happens in the spring), our location experienced several heavy rain periods. The last one resulted in flooding across the neighborhood, including our house. Water came through cracks in the block foundation walls. Both at the base and a couple of feet up. Soon after that, it began to come up through cracks in the basement floor. There were some areas where the water actually bubbled up.
A For the purpose of verbal picturization, imagine a wooden or a Styrofoam box (your foundation), with minor holes and cracks in it, surrounded by a thick cotton blanket (the back-fill soil), pushed down into a bucket (a hole in the ground called a basement excavation), with a tube at the bottom of the bucket (the foundation drain) that punctures through the bucket and is able to slowly drain the bucket (a piping system that can drain the excavation). Now imagine taking a heavy rag such as an old shirt or an extra big sock, that you tightly twist and jamb between the bucket and the top of the box, above the cotton that is between the box and the bucket (this is the soil curbing over the foundation back-fill area). Now press down one or two places in the twisted rag so that there are low spots that will gather and hold some water. Pour water onto the low spot(s) on the twisted, jammed in rag, and keeping doing this for hours, or days, or even weeks until the entire rag, cotton blanket, and wooden box are saturated with water. Isn't it apparent that water will rise through cracks in the bottom of the box (basement floor), through cracks and holes in the walls? The undug, undisturbed soil (bottom and sides of the foundation excavation) are very dense and are generally impervious to water and do not absorb liquid water very fast in comparison to the more porous back-fill soil, or the relatively dryer foundation block. Isn't it obvious that if that relatively small drain at the bottom gets the least bit clogged, or was never there in first place, that water will inevitably enter the basement from the bottom of the foundation.
In essence (once again), proper exterior soil curbing answers to the challenges of a myriad of basement water problems.
Q In one corner of the basement, there is a 3'x3' pit where there is no concrete. This is where the sewer line and water line go to the street. When I placed one of our utility pumps in a depression in that area that was below the floor, all the basement except on corner (the 'up gradient' side) began to drain. After 8 hours of pumping I could see that the water was actually streaming into my makeshift sump from under one of the foundation walls.
A This is a de-facto or accidental sump (a small pit ), that should be protected with a pump for several reasons, not the least of which is the protection of the functional utility components located in the pit.
Lining the pit with treated wood, brick or concrete, would make the installation much "cleaner" and more professional. Again if the various outside protections are in place, water is unlikely to come in this fast. Damaged, leaking gutters and downspouts or damaged underground downspout leaders can also cause this much water to enter the foundation.
Q During two other rains, there was enough seepage through the base of the foundation wall that we had standing water but not enough to pump out with a utility pump. It seemed to be running back down into a crack in the floor at the center.
A Note that the water entry is directly related to the event of rain. This strongly suggests that the ultimate source of the water is surface effects. Rain hits ground, runs along surface until it settles into a depression or channel, where it does not have an opportunity to run away free. The water then sinks into ground. If that depression or channel is near your foundation, it enters the foundation back-fill area. If your outside original perimeter "French" drain tile are already filled up with mud and silt, WALLAH!, you have a wet basement.
Q However I am concerned about whether this concept of a 'high' water table is legit. If it is, won't I need some sort of drainage system in the basement? At least a sump on the up gradient side? Do I need to seal the cracks in the floor and foundation, or should that be non-issue as far as water is concerned if I get my exterior soil drainage right.
A Sealing cracks inside should be a functional non-issue, when we limit the discussion to water entry. By the way, sumps don't usually need the additional help of piping, or French Drains, the water will follow the path of least resistance to the lowest point under the floor.
Concerning a high water table, if there was truly such a thing, then an OUTSIDE serious business French Drain (8 inch pipe, 18 inch wide ditch 2 to 8 feet deep, pitched 1/4 to 1/2 inch per foot, packed with river gravel, replete with filter cloth, etc., drained to daylight or a storm sewer) placed ten (10) or more feet away from the house to interrupt the flow of water towards to house/building would then be appropriate. Do this only after all other previously mentioned remedies have failed to do a complete and reliable job.
It can not be emphasized too much, above grade work on, and around, the exterior of the building, manages most of the problems related to obtaining a dry foundation. Further, it would be difficult to obtain a completely satisfactory and permanent fix for damp/wet basement or crawl space if the exterior drainage was not in good order.
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