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