Outline Isotope hydrology I. Introduction hydrology C. Maule Dept of Agricultural & Bioresource Engineering Outline I. Introduction – Hydrologic cycle – Kinds of water isotopes

Isotope hydrology

C. MauleDept of Agricultural & Bioresource

Engineering

Outline

I. Introduction– Hydrologic cycle– Kinds of water isotopes– Isotope effect– Methods of analysis

II. Case Studies– Runoff separation– Evaporation– Recharge

III. Exercise; - Flow separation- Recharge rate

I. Hydrologic Cycle

Groundwater flow

E = 71

P=111

Q = 40E = 425

Q = 40P = 385

Global water balance components (km3)

Lee, 1980

I. Kinds of Isotopes

Isotope

HydrogenDeuteriumTritium

Oxygen 16Oxygen 17Oxygen 18

Protons

111

888

Atomic weight

123

161718

Neutrons

012

89

10

Symbol

H2H, D3H, T

16O17O18O

Abundance%99.9840.0160.00005

99.760.040.20

I. Tritium

1 Tritium Unit (TU) = [T/H] = 10-18

Natural production of tritium is 5 TU/yrfrom interaction of cosmic rays in upper atmosphere with N:

14N + n → 15N → 12C + 3H, 3H → 3He + -β

Human production: nuclear bomb testing 1952- 1970sPeak in 1963 with highest monthly of 10,000 TU

Half life of 12.3 years so for 1963 Ottawa 2900 TU1963 - 100; 1975 - 50%; 1988 - 25%; 2000 - 12.5%; 2012 - 6.3%

I. Tritium in Precipitation

0

1000

2000

3000

4000

5000

6000

7000

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Triti

um U

nits Monthly data, Ottawa, Canada

1963; precipitation weighted: 2900 TU

(18O/16O)sample

They are expressed as a ratio of the ‘abundant’ fraction, e.g. R = [18O] / [16O]

- (18O/16O)SMOW

(18O/16O)SMOW

I. Stable Isotopes of H2O2H and 18O are the ‘rare’ fractions.

δ18O ‰ = x 1000

They are also expressed in per mil, ‰

They are measured and also expressed relative to a standard (e.g. SMOW, Stand Mean Ocean Water)

I. δ18O Global Variations

http://www.iaea.or.at/programmes/ripc/ih/publications_home.html

I. The Meteoric Isotope Line

Craig, H. 1961. Isotopic variations in meteoric waters. Science 133, 1702-1703

Deviations in isotopic compositions away from meteoric water line as a consequence of various processes (from IAEA Report No. 288, 1983)

I. δ18O NA Seasonal Variations

JULY

JAN

http://www.iaea.or.at/programmes/ripc/ih/publications_home.html

I. Fractionation

HH16O 182HH16O 19HH18O 20

g/mole

Evaporation: less energy is required to vaporize lighter molecules

Liquid

δD = -100 ‰δ18O = -13 ‰

values

I. Fractionation

HH16O 182HH16O 19HH18O 20

δD = - 90 ‰δ18O = - 10 ‰

g/mole

Liquid becomes enriched in heavy isotopesVapour becomes depleted in heavy isotopes

Vapour

Liquidvalue

I. Isotopes, rainout effect

Values are d Oxygen-18 for vapour (clouds) and precipitation

Lee, 1980

0 ‰

-2 ‰

-13 ‰-18 ‰

-7 ‰

-23 ‰

-12 ‰

-28 ‰

-17 ‰

Victoria; -10 ‰Edmonton; -17 ‰

I. Temperature Effect

-30

-25

-20

-15

-10

-5

0

-30 -20 -10 0 10 20 30

Wynard, Sk

Ottawa

Edmonton

δO

xyge

n-18

(‰)

Monthly Temperature

FIGURE 4. Fractionation effects in melting snow columns revealed by a cold room experiment (adapted from Hermann et al., 1978).

I. Fractionation during melt

I. Extraction methods

• Centrifugation• Squeezing• Azeotropic distillation with toluene• Vacuum distillation• Direct equilibration

Problem with unsaturated soils is to obtain a water sample without fractionation

I. Azeotropic Distillation

Heater

Boiling toluene

Cellulose thimblewith wet soil sample

Water cooled condensor Cooling water

x

Condensed toluene (lighter than water)

Condensed water

valve

II. Case Studies

Streamflow

Soil water evaporation

Groundwater recharge

Streamflow Separation

Seasonal or storm differences in isotopes can be used to determine proportion of surface runoff from groundwater contributions to stream flow.

II. Streamflow separation

Runoff and Infiltration

Stream channel

Precipitation (event water or new water)

NEW WATER

OLD WATERGroundwater (pre-event water)

II. Streamflow Separation

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10

Time since start of storm, hours

Stre

amflo

w(m

3 /s)

Base Flow = Old Water

Surface Flow = New Water

Streamflow Separation

Maule & Stein1990. WRR

30

20

10

0

mm

/d

rainfall

snowmelt lysimeter

23-Mar 1-Apr 10-Apr 19-Apr 28-Apr 7-May 16-May 24-May

30

20

10

0

mm

/d

interflow stream

Streamflow separation

Snowmelt from Lysimeter

Groundwater

Maule & Stein1990. WRR

Streamflow Separation

Percent snowmelt in groundwater

Percent snowmelt in streamwaters

Maule & Stein1990. WRR

Evaporation

Of remaining water;- evaporation results in isotopic enrichment- transpiration has no effects

Evaporation

Dep

th (m

)

0

0

0.6

9

-20 0 +20 -40 -20 0 +20

δD

Barnes and Allison. 1988. J of Hydrology 100:143-176

Evaporation

Allison. 1982. JH 55:163-169

Evaporation

-160

-150

-140

-130

-120

-110

-20 -19 -18 -17 -16 -15 -14 -13 -12

0.1 - 0.9 m0.9 - 2.0 m2.0 - 4.0 m

Edmonton meteoric line

soil water line for 0.1 - 0.9 mD

eute

rium

(‰)

δ Oxygen-18 (‰)

Vadose zone deuterium and oxygen-18 values for an agricultural field near Edmonton, Alberta

Maule et al 1994. J H

Isotopic analysis indicates shift is result of 8% of soil water being evaporated

Snowmelt recharge

What is the contribution of snowmelt to soil and groundwaters?

I. Snowmelt Recharge

-30

-25

-20

-15

-10

-5

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Edmonton δ 18O values: winter precipitation: -26 ‰summer precipitation: -14 ‰Wynard, Sk

Ottawa

Edmonton

δO

xyge

n-18

(‰)

Snowmelt recharge

Bengtsson et al., 1987. J Hydr Sciences 32:497

Snowmelt recharge

-160

-150

-140

-130

-120

-110

-20 -19 -18 -17 -16 -15 -14 -13 -12

0.1 - 0.9 m0.9 - 2.0 m2.0 - 4.0 m

Edmonton meteoric line

soil water line for 0.1 - 0.9 mD

eute

rium

(‰)

δ Oxygen-18 (‰)

Vadose zone deuterium and oxygen-18 values for an agricultural field near Edmonton, Alberta

Maule et al 1994. J H

Average annual isotope value of precipitation (weighted)

Edmonton δ 18O values: winter precipitation: -26 ‰summer precipitation: -14 ‰

Snowmelt Recharge

Possible explanation for distribution of snowmelt recharge in some prairie soils.

Rain Snow

Snowmelt runoff

Snowmelt waters

Groundwater moundingLateral flow

Capillary rise

Infiltration,macropore flow

mixing

Snow make upPrecipitation: 19% of totalSoil water (0-0.9m): 26-31%Groundwater (3-4 m): 44%

Rate of recharge

In dry climates groundwater recharge is too slow for conventional methods of measurement. Environmental tracers offer greater accuracy

Recharge

Water table(3-10 m)

~1.2 m

Root zone

Groundwater

Recharge

Recharge is the downward flux of water from the root zone to the groundwater zone.

Groundwater is beyond the reach of plant roots.

• Three externally draining watersheds

• Each 4 to 7 ha in area

• Average slopes 3°

• SiL surface texture

• Measurements since 1998

Site Description

Bret Ward. 2003. MSc

Soil Moisture ProfilesSpring

Fall

0

1

2

3

0 0.2 0.4

Dep

th (m

)

θ(m3/m3)

0

1

2

3

0 0.2 0.4θ(m3/m3)

‘Active zone’ = regional of seasonal water difference due to springmeltrecharge and plant uptake, ∆θ of 0.03 m3/m3 (O’Brien et al., 1996)

Upper slopeBottom slope

Meteoric Tritium (Ottawa)

0

1000

2000

3000

4000

5000

6000

7000

1953

1956

1959

1961

1964

1967

1970

1972

1975

1978

1981

1983

1986

1989

1992

1994

1997

Year

Triti

um (T

U)

Recharge estimation: Profile methods

2. Curve Area MethodArea under the curve attributable to above background tritium inputs divided by annual tritium weighted precipitation.

0

1

2

3

4

5

Dep

th (m

)

Tritium Units (TU)

0 20 40 1. Peak MethodDistance peak has travelledbeneath the root zone bottom since 1963.

Soil Tritium Profiles

0

1

2

3

4

5

0 10 20 30 40 50

Upper

Bottom

0

1

2

3

4

5

0 10 20 30 40 50

Dep

th (m

)

TU TU

Watershed A Watershed C

Recharge Rate (mm/yr)

Watershed

Slope

Upper

Bottom

A

2.9

4.5

C

1.4

2.9

Peak MethodWatershed

Slope

Upper

Bottom

A

2.9

4.5

C

1.4

2.9

Peak Method Mass MethodWatershed

Slope

Upper

Bottom

A

5.3

10.2

C

8.4

7.5

Mass MethodWatershed

Slope

Upper

Bottom

A

5.3

10.2

C

8.4

7.5

Monthly stable isotope values of Saskatoon Precipitation, 1991-2002

-250

-200

-150

-100

-50

0-35 -30 -25 -20 -15 -10 -5 0

April-OctNov-March

δ oxygen-18 (‰)

δ deuterium(‰)

(Len Wassenaar, Environment Canada, unpublished data, 2002)

Monthly stable isotope values of Saskatoon Precipitation, 1991-2002

-250

-200

-150

-100

-50

0-35 -30 -25 -20 -15 -10 -5 0

April-OctNov-March

δ oxygen-18 (‰)

δ deuterium(‰)

(Len Wassenaar, Environment Canada, unpublished data, 2002)

Period

April-OctNov-MarYear

Precip(mm)27775

353

δ 18O(‰)

-13.1-21.7-14.9

Seasonal Recharge

Dep

th (m

)

δ oxygen-18 (‰)

0

1

2

3

4

5

6

7

-25 -15 -5

UpperBottom

Annual average δ 18O, weighted to monthly precipitation

Proportion winter (Nov-Mar)

Upper 23 %*Bottom 82 %*Annual precip 21 %

*Only applies to below 2 m

Rate of Recharge, mm/yr

Ele

vatio

n ab

ove

loca

l ben

ch m

ark

(m)

95

100

105

110

Upper slope

Lower slopeMiddle slope

Distance from slough (m)

0 40 60 80 100 120 140 160 18020

Distance from slough (m)

0 40 60 80 100 120 140 160 180200 40 60 80 100 120 140 160 18020

watertable

Active zone

12: Darcian10: Cl peakna: 3H peak11: NO3 peak

0.7: Cl mass10: 3H mass

na: Darcian17: Cl peakna: 3H peak25: NO3 peak

1: Cl massna: 3H mass

57: Darcian42: Cl peak15: 3H peak13: NO3 peak0.8: Cl mass

15: 3H mass

Joshi and Maule 2000 Hyd Pro 14:1503

Useful Reading

UNESCO/IAEA Series on Environmental Isotopes in the HydrologicalCycle Principles and Applications Edited by W.G. Mook

http://www.iaea.or.at/programmes/ripc/ih/volumes/volumes.htm *****