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