GEOL-1010 Lecture Notes - Lecture 15: Hard Water, Phreatic Zone, Surface 3

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5 Feb 2018
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Lecture 15: Groundwater- Guest Lecturer
Geology in the News
Alaskan volcano has erupted 36 times in the past 4 months
Result: the island it’s on has tripled in size
Groundwater
Water occurring within a zone in the subsurface where all open space is completely filled
with water…it soaked in (infiltrated) from the surface, and is being “stored”
Not moisture in soil, where water is attracted to roots, soil, etc. and “sticks” as a film
Groundwater fills pore spaces within rock or sediment, as well as fractures in rock (any
potential open space)
Key to defining is if it is groundwater it is occupying (something idk)
Earth’s outermost (just few kilometers) crust is like a sponge…everywhere even in
deserts
Source
Precipitation (rain, snow, ice) falling to the ground and soaking in is the source of
groundwater
In other words, all groundwater came from surface
Precipitation must soak in, and not immediately run off into surface channels
Occasional rain/ snow falls and soaks in here…long term (104 years)
And moves downward through pores and fractures in sediment/ rock via gravity, ending
up beneath the town
o Groundwater is thus “old” and represents stored accumulation of long periods of
precipitation… it takes a while to come back if overused
o You’re not just going to have one good storm and the people have a good source
of ground water
Groundwater is not subject to evaporation!!! This is why it survives in the desert
subsurface
In Cities
Land development can slow this recharge process
Hard surface, like parking lots, force rain water into channels before it soaks it in
Retention ponds store this water, and allow it to recharge
Leakage from water lines and sewers do as well- Barton Springs of Austin, Texas is a
good example
Big cities with lots of people has a tremendous demand of groundwater but also a lot of
covered surface
When you cover up ground surface you’re going to set in motion a sequence of events
that will impact our groundwater
Barton Springs Austin, Texas
Looks nice…
BUT chemical analysis indicates that most of the water is water main and sewer line
leakage!! (fortunately soil and bedrock naturally filter it)
How do we know? Chemical tracers (water itself is piped in from distant geologic units,
hormone molecules, etc.)
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