Environmental Geology - Hydrogeology basics
- Answers to questions.
What are the myriad uses
of water?
- drinking (for us, our pets, and our livestock).
- irrigation (from crops to lawns).
- power generation.
- cleaning (from streets to laundry to coal).
- sewage removal and disposal.
- industrial cooling.
- recreation: boating, swimming.
- fire protection.
- ecosystem maintenance.
- transportation.
- aesthetics (fountains and such).
- note that some of the uses may be in conflict with each
other, or that in a given area there is not enough water to go around.
How does the water get into
the ground?
An answer may be found in considering part of the hydrologic
cycle. The diagram below attempts to show some of the reservoirs linked
to groundwater, and the processes that transfer water between these reservoirs.
This leads to a second question. What determines how much
seeps into the ground?
- soil porosity/ permeability (what in turn determines
this?).
- slope.
- rainfall flux (rate of supply of the water).
How much water can the ground
hold (what is its porosity)?
- definition: percentage of void space within rock body
under consideration.
- two types:
- intergrain: space between grains, porosity typical of
sediment.
- fracture: space provided by fractures, typical of hard
rock.
- what attributes of the fracture system control fracture
porosity?
- determining factors on intergrain porosity -> sorting,
packing, and grain shape.
The above diagrams illustrate in 2-D how sorting and packing effect
porosity.
- how can porosity be measured?
- representative values for various geologic media: conglomerates
and sandstones - 20-40%, muds - 60%, shales (lithified muds) up to 10%.
What determines in what
pattern and how fast water flows in the ground?
- intergrain permeability: influencing factors -> porosity,
geometry of pore spaces, grain size.
- fracture permeability: common in crystalline rocks, anistropy
of permeability.
Does the equation for the flow of water through a rock
look like any other basic physical equations? It
is the same form of equation as for the flow of electrons through a wire.
Mathematical solutions good for the one can help with the other.
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