Aquifers in Alluvial Sediment
• River valley draining glaciated area
• Rivers draining area in west with high Pleistocene rainfall
• Fault-bounded basins in west
• Partially dissected alluvial plain (High Plains)
Unconsolidated sands and gravels deposited by rivers. Must be large enough to produce significant rates and volumes of water
(b>5 to 10m)
Sea vs. Closed Basin as Deposition Site for Alluvial Sediments
Sea• Suspended load
possibly removed
• Salts possibly removed
• Sea level change important
Closed Basin
•Fine-grained seds in system
•Salts remain
•Isolated from effects of sea level change
•Affected by local climate
Alluvial aquifers in glacial deposits
Alluvial sediments in glaciated areas
• Glaciers advance, scour seds., modify river course. Sed comp. depends on location/source material. Large range of grn size. Till=clay-boulder beneath glacier.
• Sea-level drops as ice advances. Hydraulic gradient increase. Erosion, velocity, carrying capacity increase. Valleys incised into bedrock, older glacial sediments (cover earlier channel deposits)
• Glaciers recede. Discharge increases. Erosion. Braided rivers, large sediment capacity. Outwash plain (sands and gravels). Lakes in front of receding glaciers. Lacustrine=clay-silt (varved)
Alluvial sediments in glaciated areas, Cont
• Sea level rises, glaciers recede, hydraulic gradient diminishes, discharge diminishes, carrying capacity drops. Lakes.
• Coarse-grn seds deposited in incised valleys. Gravel on bottom, fining upward. Thickness depends on conditions during/following glaciation. Glacial landforms
• Region adjusts to interglacial. Discharge decreases. Sediments reworked.
• Important materials: Till, lacustrine, outwash, alluvial valley fill, diamicton, drift.
Gravel lens within a silty-clay till
AlluvialAquifer Systems
• Geometry
• Aquifer type
• Properties
• Recharge/Discharge
• Flow pattern
• Chemistry
• Examples
Geometry
• Channel deposits– Elongate, tabular bodies, sinuous
Length: many kmWidth: 0.1-several km Thickness: 0.01-0.1 km
• Outwash deposits, alluvial plain– planar sheets many km horizontally Thickness: 0.01-0.1 km
Aquifer Types• Unconfined
• Confined
• Both, unconfined with local confining unit
•Channel fill in modern valley•Buried channel•Outwash plain•Alluvial plain
Deposits
substratum
Idealized setting
Channel fill in modern valley
Sand and gravel,Primary aquifer
Confining unit where fine grained
substratum
•
Hydraulic conductivity of some major alluvial aquifers
Fining upward sequences in major alluvial aquifers
unconfined
Storativity of major alluvial aquifersconfined
Recharge to alluvial aquifersInfiltration through floodplain
Discharge from basement
Losing stream
including tributary
Irrigation return flow
Rise in river stage,
Bank storage
Rise in river stage,
Flood
Main channel losing due to pumping
Discharges from Alluvial Aquifers
1. To main channel or tributaries
2. Lakes on floodplain
3. Wetlands
4. Wells
Streambed conductance effects on gw/sw interaction
Fine-grained seds on streambed
Fine-grained seds in topstratum
Some examples
• Fox-Wolf River Basin, WI. Outwash
• Corning aquifer, NY. River valley
• Andruscoggin. ME. Alluvial valley once inundated by seawater
• Irondogenese, NY, Alluvial valley once filled with fresh water lake
• Others
140 miles
20 miles
Another major outwash deposit
•
Chemung river valley, Corning, NY
5 miles
1 mile
1:40 aspect ratio
4000
16 Mgpd
3000 ft1. Determine the horizontal head gradient at each
location
2. Estimate the ground water fluxes at each location
3. Estimate the average flow velocities
4. Estimate the volumetric rate per unit length of river that the aquifer is contributing to the rivers at each location.
5. Provide an explanation for the differences between the two locations
Corning Aquifer Exercise A.
B.
Glacial valley partially inundated by the sea
5000 ft
Hydraulic head in glacial outwash, Little Androscoggin Aquifer, Maine
7 Mgpd capacity
4 miles
Aquifer filling a valley once filled by fresh water glacial lake
4.3 Mgpd
Water Quality Summary
• TDS
• Hardness
• Major ions
Corning Aquifer. Ca, Mg, HCO3; Hardness: 225 ppm;
TDS: 212 ppm16 Mgpd
Little Androscoggin, Na, K, Ca, HCO3;
Hardness: 24-68ppm
TDS 67-128 ppm
Irondogenesee Aquifer, Ca, Na, HCO3, Cl, SO4; TDS 665, Hardness: 373
4 Mgpd
alluvium
bedrock
Some other alluvial aquifers
Dissolution of underlying evaporites forms deep troughs in Pecos River Basin
80 Mgpd
Water Quality: 1000+ mg/L common due to underlying evaporites and recharge from saline surface water and irrigation return flow where evaporation has increased salt content
100 miles
Relative sizes of example alluvial aquifers