Presentation on theme: "Loess (wind-blown silt) Loess Hills (lower Mississippi basin) Terracotta soldiers of Xian, China (buried."— Presentation transcript:
Loess (wind-blown silt) Loess Hills (lower Mississippi basin) Terracotta soldiers of Xian, China (buried in loess from Gobi Desert)
Alluvial fan (Death Valley, CA) Very coarse, high K, high n, but water table often at great depth. (Water drains right through)
Braided river gravel deposits make excellent aquifers! Chitina River, Alaska (from Press & Siever, 1998) River gravel
Meandering river Near Anchorage, Alaska From Press and Siever (1998) sand bar silt and clay
Delta: mainly fine sand, silt and clay… low-med K units mostly
Glacial deposits (till) In front of the end moraine, glacial meltwater reworks the till and forms broad sheetlike deposits of sand & gravel (outwash plain). Excellent aquifer.
Glacial till forms a blanket-like deposit beneath a glacier. Till usually consists of boulders in a finer matrix of silt and clay. It will usually have low K unless the deposit has been reworked by wind or water.
Lateral and medial moraines (Press et al.) Moraines form at the edges and terminus of alpine or continental glaciers. They are rich in boulders and gravel, and if re-worked by rivers or glacial meltwater (or ocean currents) they can be very good aquifers.
End moraines Long Island, N.Y. is a good example of highly permeable sands & gravels left behind as a recessional end moraine near the end of the Wisconsin ice age The Oak Ridges Moraine (ORM) near Toronto is another end moraine. This gravel deposit has a unique habitat of wetlands and fresh lakes, and is also an important gw supply for metropolitan Toronto.
Press et al. Eskers are rivers of sand and gravel that formed when a meltwater channel beneath a continental glacier fills with sediment. They are excellent sources of gw, but are limited in size. Commonly mined for gravel.
Glacial lake sediments (glacio-lacustrine) Glacial lake sediments typically contain a lot of clay, but may be interlayered on a cm scale with sand and silt. The layers are called “varves”, and reflect alternating seasons. In winter the lakes and surrounding rivers are frozen, and clay is deposited. In summer, swollen rivers dump silt & sand into the lakes.
“Open-system pingos form when groundwater flows downhill and is trapped underneath the permafrost. Eventually the water forces itself up through cracks into the permafrost and freezes, pushing the soil above it up into a dome” Pingos
Prince Edward Island Bedrock = “redbed” sandstone and conglomerate (Carboniferous age) PEI gets 100% of its drinking water from gw in the fractured sandstone. They have abundant supplies. Total use is < 2% of total annual recharge. Main problems related to pollution.
Press et al., “Understanding Earth” Limestone karst features
tower karst in China (near Guilen) tower karst in Thailand
USGS hydrogeologist injecting rhodamine dye to see where water in a sinking stream turns up.
Sinkholes (collapsed cave roof) In rural areas, sinkholes are popular places to throw trash. Is this a good idea?
Floridan Aquifer Shows thickness of limestone aquifer. Brown = confined Blue = unconfined
Light blue is high purity water. Purple is saline. Red areas show areas where Floridan aquifer has been drawn down from withdrawels.
Edwards Aquifer (south Texas)
Igneous and metamorphic- rock aquifers at or near the land surface. Limit of continental glaciation. North of this line, glacial sand and gravel aquifers overlie bedrock aquifers in many places. Includes Columbia River and Snake River flood basalts Pacific NW basalt-rock aquifers
During cooling, basalt flows develop characteristic fracture patterns that increase K and n of rock. K is also increased by interflow sediments (loess, sand) and lava tubes/tunnels. A lava tube
Columnar joints in andesite lava flow (Patagonia)… …and in a sub-volcanic intrusion Devil’s Tower, Wyoming In general, lava flows and welded tuff have higher permeability than non-welded pyroclastic rock (tuff and ash). Much of the porosity is fracture or joint-controlled. Non-welded tuff doesn’t fracture easily, and can have a very high clay content. Layers of ash that fall 100’s of miles from the volcano can compact to form very low-K confining layers surrounded by otherwise hi-K sediments.
Welded tuff (fractures easily… good secondary permeability) Non-welded tuff (Very low K … this tilted outcrop is being mined for bentonite, or “swelling clay”) Pyroclastic volcanic rock
For plutonic and metamorphic rocks, gw storage and permeability is totally controlled by presence or absence of secondary fractures. Look for lineaments from air photos for location of major faults or fracture zones that have higher gw yield.
Humbug Spires (Butte, MT)
Permeability of regional metamorphic rocks can be strongly anisotropic, due to development of foliation (slaty cleavage, schistosity, gneissic banding).