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Aquifers Names based on geologic time period formed Geologic Time • Precambrian: 4 billion years 10.5 months • Paleozoic: 540 – 250 million years 1 month • Mesozoic: 250 – 65 million years 2 weeks • Cenozoic: 65 million years 1 week Devonian Fossil Gorge “When the 1993 flood abated, the eroded Gorge surface revealed a succession of 375 million year old bedding planes with diverse and abundant fossils commonly standing out in relief.” 1 U.S.G.S Groundwater ATLAS of the United States http://capp.water.usgs.gov/gwa/ 2 Major Types of Aquifers 1. unconsolidated sediments 2. semiconsolidated sediments 3. sandstone / carbonate aquifers 4. basaltic and other volcanic rock aquifers Some Example Aquifers • The High Plains Aquifer, Midwest U.S. • New Jersey Coastal Plain, East Coast U.S. • The Dakota Sandstone Aquifer, Midwest U.S. • Columbia Plateau Basalts, NW U.S. 3 High Plains Aquifer Ogallala Aquifer 4 Cambrian-Ordovician Aquifer Iowa Groundwater Information http://www.igsb.uiowa.edu 5 Bedrock Geologic Map of Iowa Silurian-Devonian Aquifer 6 Surficial Geologic Map of Iowa (Principal Aquifers of Iowa) Iowa City Aquifers *Glacial Drift aquifer *Silurian-Devonian aquifer Maquoketa aquiclude *Cambrian-Ordovician aquifer St. Lawrence aquifer Jordan sandstone aquifer Dresbach aquifer Two Important Aquifer Characteristics 1. to store ground water, and 2. to transmit water to well. Transmissivity: measure of how easily water moves through an aquifer. Similar to K, but applies over the thickness of a given layer in the aquifer. Ti = biKi Storativity: volume of water released or taken into an aquifer per unit head change per unit surface. S = volume of water (unit area)(unit head change) 7 Storativity S = Ss b Transmissivity T = Kb Water Storage is Different in Unconfined and Confined Aquifer 8 Why? First – Saturated conditions – Confined aquifer 1 - fluid compressibility, βw 2 - compression of porous media, βp Combined known as “elastic storage” Second – unconfined aquifer – partly saturated – now WT moves! Specific Yield (Sy) - ratio of volume of water that drains by gravity to total volume of rock in an unconfined aquifer. Specific Retention (Sr) - ratio of volume of water retained to total volume of rock in an unconfined aquifer. Specific yield known as “water table” or “phreatic” storage 9 Confined Aquifer Water released estimated using specific storage Ss = ρg (βp + nβw) Unconfined Aquifer Water released estimated using specific storage AND specific yield S = Sy + bSs Water Storage is Different in Unconfined and Confined Aquifer 10 Example – Water Storage in Unconfined Aquifer A glass funnel filled with a volume of 400 cm3 is filled with 763.2 g of soil (the soil solids have a density of 2.65 g/cm3). The soil is then saturated with water. Following saturation, the water is allowed to drain under gravity from the soil. The amount of water that has drained is 60 g. What is the porosity, specific yield, and specific retention of the sample? The aquifer underlies an area of 300 sq. miles and has a saturated thickness of 20 ft. Estimate how much water is in the aquifer in gallons and cubic ft. What is the total amount of obtainable water in gallons? If the water table in the aquifer was uniformly lowered over its entire area by 5 ft, how much water would the aquifer have yielded? 11 Example: Water Storage in a Confined Aquifer - Dakota Sandstone 1. Volume of water from fluid expansion? 12 2. Volume of water from compression of porous media? 13