Aquifers in Alluvial Sediment

• River valley draining glaciated area

• Fault bounded basins

• Partially dissected alluvial plain (High Plains)

• Mississippi embayment

Unconsolidated sands and gravels deposited by rivers. Must be large enough to produce significant rates and volumes of water from wells

Sea vs. Closed Basin as Drainage Destination 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

Large Glacial Lakes

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. Style changes from braided to meandering. 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. Complex facies distributions

Gravel lens within a silty-clay till

AlluvialAquifer Systems

• Geometry

• Aquifer type

• Properties

• Recharge/Discharge

• Flow pattern

• Chemistry

• Examples

1:100

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

10s km horizontally Thickness: 0.01-0.1 km

1:10

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

•

Hydraulic conductivity of some major alluvial aquifers

Storativity of major alluvial aquifersconfined unconfined

Fining upward sequences in major alluvial aquifers

Estimate how K varies with depth in alluvial aquifers?

Log(20)-Log(3)=0.82b=Slope=2/0.82=2.4

d50=C*Zb

Straight line on log*log plot

d50=C*Z2.4

Hazen method K=C1d102

Alluvial: K=C2Z4.8

Recharge to alluvial aquifersInfiltration through floodplain

Losing stream

including tributary

Stormflow off uplands

Irrigation return flow

Rise in river stage,

Bank storage

Rise in river stage,

Flood

Main channel losing due to pumping

Discharge from basement

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

10

9

10

9

10

9

Gaining reach

10

9

Stream-parallel flow,

Neither gain nor lose

Losing reach

Gaining losing

Preliminary interpretations of gw-sw interactions using

head contours

1.

2.3.

4.

Draw a Hydrogeologic Conceptual Model of Alluvial Aquifers

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

Wisconsin

Dome

140 miles

20 miles

Fox-Wolf River Basin

Buried pre-glacial valley, now covered by till and lacustrine deposits

What does this map tell you

about the Fox-Wolf River

aquifer?

30 miles

Regional GW flow patterns?

Where are thr recharge and discharge areas? What controls?

Expected fluxes?

GW discharge area?

Composition of GW and SW similar

Baseflow rate related

to T of surficial aquifer

Ground water flow through surficial aquifer, Paleozoic sandstones, and discharge to river

Flow-through lake

Another major outwash deposit

•

Cape Cod Bay

Atlantic Ocean

Nantucket Sound

3

6

9

1

12

2

15

18

5

0

3

1

99

3

6

2

2

N

State OutlineCape Cod OutlineStreams and RiversSurface Water BodiesGroundwater Flow Contours (3 m interval)

1 0 1 2 Kilometers

Conceptual Model

North South

Bedrock

Groundwater Flow Paths

Freshwater/Saltwater Interface

Saline Groundwater

Recharge Streams Cape Cod Bay

Fine-grainedSand/Silt

Glacial Till

Chemung river valley, Corning, NY

Limestone and shale bedrock on rounded hills 800 ft or more above the sand and gravel aquifer on the valley floor.

5 miles

1 mile

1:40 aspect ratio

4000

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.

Water Balance

• Info given in GW Atlas

ET=0.5 P

0.6Recharge is from uplands

• What is the total baseflow flux to streams?Water Balance from Conceptual Model

Recharge = Infiltration + Upland Runoff

I=0.5P

UR=0.6Re

Re=0.5P+0.6Re

Re=1.25P

From map, P = 40 inch/yr, so Re=50 in/yr

Water is magnesium bicarbonate type. Note the hardness. The region is underlain by limestone and shale

Hardness = 2.5 Ca(mg/l) + 4.1 Mg(mg/l)

<60 mg/l = soft

>150 mg/l = very hard

16 Mgpd

Fine-grained marine sediments underlie glacial outwash in the Little Androscoggin aquifer in Maine.

Glacial valley partially inundated by the sea

5000 ft

Water Balance• Info given in GW Atlas

P=43 in/yr, ET=23 in/yr (0.53), Ru=20in/yr (0.46)

Also given: Recharge as infiltration over 16 mi2 aquifer accounts for 16.4 cfs, overland from uplands 11.2 cfs, from river 1.4 cfs. 29 cfs total Re to aquiferArea of aquifer = 16 mi2

• Are these consistent? Demonstrate with water balances.

Watershed Balance: P+OU=ET+Ru different from above

Aquifer: Infilt+OU+RiverLoss=BaseflowInfiltration = 16.4 cfs; convert to flux over aquifer: 14 in/yrOverland from Upland= 11.2 cfs; 9 in/yrTotal Recharge=baseflow= 29 cfs: 24 in/yrRu=P+OU-ET=43+9-23=29 in/yr different from above

Ru=Base+Storm, So, stormflow must be 5 in/yr;

Ru=Baseflow+Storm=Recharge+Storm

Total Recharge=baseflow= 29 cfs: over 16 mi2= 24 in/yr

20 in/yr= 24 in/yr+Stormflow, Negative stormflow?? Problem

In general, the water flux values seem to be inconsistent. Always make certain

your water balances can be closed.

Hydraulic head in glacial outwash, Little Androscoggin Aquifer, Maine

7 Mgpd production

4 miles

Aquifer filling a valley once occupied by fresh water glacial lake

Structural Contours on Bedrock

4.3 Mgpd

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

100 miles

Relative sizes of example 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

Water Quality Summary

• TDS

• Hardness

• Major ions