Field Estimation of Wetland Soil Saturated Hydraulic Conductivity

Peter CaldwellSSC57012/2/03

Causeway Bay

Agenda Background The Bouwer-Rice Method Field Procedures Office Procedures Results Conclusions

BackgroundHydrology Modeling

Research project involves modeling the hydrology of three Carolina Bays

Each bay has 4-7 wells used to collect data for model calibration

Each well location will be modeled Model requires Ksat of all soil layers to 1 m

depth

BackgroundReference Bays

Charlie Long Bay

Tatum Millpond Bay

Causeway Bay

Bladen County, NC

The Bouwer-Rice Method Slug test method to determine Ksat in-situ

Only works at depths below water table

The Bouwer-Rice Method

Ground Surface

Impermeable Layer

Unsaturated Soil

Saturated Soil

Soil at Field Location

The Bouwer-Rice Method Bore a hole to desired depth

The Bouwer-Rice Method Insert well casing and screen

The Bouwer-Rice Method Drop in slug

The Bouwer-Rice Method Record height of water in tube vs. time

H

H vs. time

0.001

0.01

0.1

1

0 5 10 15 20 25 30

t (sec)

log

H (

m)

The Bouwer-Rice Method Calculate Ksat

t

o

e

wc

H

H

tLr

rK ln

1

2

Rln

e2

H

h Lw Le

2rc

2rw

Where,

A and B are dimensionless parameters based on Le/rw

Note: Use straight line portion of graph of H vs. time for t, Ho, Ht

1

eln

ln

1.1Rln

we

w

w

www

rL

rLhBA

rLr

Field ProceduresField Conditions

Thick

Field ProceduresField Conditions

Deep

Field ProceduresEquipment- Well Casing

4” PVC Pipe with holes drilled in lower 20 cm

Holes less likely to get clogged than slots Duct tape allowed fine tuning of screen

length

Field ProceduresEquipment- slug

Slug 2” PVC, 24” long Filled with gravel

Field ProceduresEquipment- Level-logger

Level-logger Made by Solinst Records height of

water above logger Records every 0.5

second

Field ProceduresProcedure

Wanted Ksat values for all soil horizons to a depth of 1 m

Had soil profile descriptions from previous study Depths/Thicknesses Texture

Field ProceduresProcedure- Augering Hole

Based on profile descriptions, tape screen to appropriate length

Auger through root mat Set well casing into hole created Auger inside casing Push casing down Repeat until screen is positioned at

desired depth

Field ProceduresProcedure- Insert Level-logger

Wait until water inside casing equalizes with water table

Drop level-logger inside casing

Field ProceduresProcedure- Inserting Slug

Wait 2-3 minutes for water to equilibrate from level-logger drop

Insert slug smoothly but quickly

Field ProceduresProcedure- Wait

Allow time for water level to lower (usually 10-15 minutes)

Approximately long enough for one game of crazy eights

Field ProceduresProcedure- Download data

Field ProceduresProcedure

Performed procedure 3 times for each horizon

Auger down to next horizon Repeat procedure for remaining horizons Repeat for each well location

Office Procedures

Compile H vs. time data for each horizon Fit straight line for points on curve Calculate Ksat

Office ProceduresH vs. Time data

CL1 Horizon 3Falling Water Table

0.1

1

10

100

0 10 20 30 40 50 60

Time (sec)

Wa

ter

Le

ve

l (c

m)

Trial 1Trial 2

ResultsKsat Values for Various Textures

fibric (8)51 172 479

hemic (5)41 76 125

sapric (9)6 81 460

mucky sandy loam (5)11 24 49

mucky loamy sand (6)64 147 263

mucky sand (1)20

sandy loam (1)10

loamy sand (3)58 83 109

sand (9)14 195 245

So

il H

ori

zon

Typ

e

450 500

Ksat (cm/hr)

50 100 150 200 250 300 350 400

Ksat Values for Various Soil Types in Reference Carolina Bays

ResultsKsat Vs. Depth by Soil Type

MineralOrg Depth < 20 cm

0

100

200

300

400

500

0 20 40 60 80 100

Depth (cm)

Ks

at

(cm

/hr)

CL1

TM1A

CW1

HisticOrg Depth = 20-40 cm

0

100

200

300

400

500

0 20 40 60 80 100

Depth (cm)

Ks

at

(cm

/hr) CL2

CL3

TM2

CW2

Shallow OrgOrg Depth = 40-80 cm

0

100

200

300

400

500

0 20 40 60 80 100

Depth (cm)

Ks

at

(cm

/hr) TM2A

TM3

CL4

CW3

Deep OrgOrg Depth > 80 cm

0

100

200

300

400

500

0 20 40 60 80 100

Depth (cm)

Ks

at

(cm

/hr) TM4

TM5

TM6

CW4

Conclusions

Slug test method was a relatively simple and straightforward method to estimate Ksat

Ksat values were reasonable, though slightly high

Will run further tests to validate the results