Isotope hydrology. Outline. C. Maule Dept of Agricultural & Bioresource Engineering. I. Hydrologic Cycle. I. Kinds of Isotopes

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1 Lee, 98 Isotope hydrology C. Maule Dept 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 I. Kinds of Isotopes P= Groundwater flow E = 7 Q = 4 Q = 4 P = 85 E = 45 Isotope Hydrogen Deuterium Tritium Oxygen 6 Oxygen 7 Oxygen 8 Symbol H H, D H, T 6 O 7 O 8 O ProtonsNeutrons Atomic weight Abundance % Global water balance components (km )

2 I. Tritium I. Tritium in Precipitation Tritium Unit (TU) = [T/H] = -8 Natural production of tritium is 5 TU/yr from interaction of cosmic rays in upper atmosphere with N: 4 N + n 5 N C + H, H He + - β Human production: nuclear bomb testing 95-97s Peak in 96 with highest monthly of, TU Tritium Units Monthly data, Ottawa, Canada 96; precipitation weighted: 9 TU Half life of. years so for 96 Ottawa 9 TU 96 - ; 975-5%; 988-5%; -.5%; - 6.% I. Stable Isotopes of H O I. δ 8 O Global Variations H and 8 O are the rare fractions. They are expressed as a ratio of the abundant fraction, e.g. R = [ 8 O] / [ 6 O] They are measured and also expressed relative to a standard (e.g. SMOW, Stand Mean Ocean Water) They are also expressed in per mil, ( 8 O/ 6 O) sample -( 8 O/ 6 O) δ 8 SMOW O = x ( 8 O/ 6 O) SMOW

3 I. The Meteoric Isotope Line Deviations in isotopic compositions away from meteoric water line as a consequence of various processes (from IAEA Report No. 88, 98) Craig, H. 96. Isotopic variations in meteoric waters. Science, 7-7 I. δ 8 O NA Seasonal Variations I. Fractionation Evaporation: less energy is required to vaporize lighter molecules JAN JULY δd = - δ 8 O = - HH 6 O 8 HH 6 O9 HH 8 O values g/mole Liquid

4 I. Fractionation I. Isotopes, rainout effect Liquid becomes enriched in heavy isotopes Vapour becomes depleted in heavy isotopes δd = - 9 δ 8 O = - HH 6 O 8 HH 6 O9 HH 8 O Vapour -7 Edmonton; Victoria; - - value g/mole Liquid Values are d Oxygen-8 for vapour (clouds) and precipitation Lee, 98 I. Temperature Effect I. Fractionation during melt Ottawa -5 Wynard, Sk δ Oxygen-8 ( ) Edmonton FIGURE 4. Fractionation effects in melting snow columns revealed by a cold room experiment (adapted from Hermann et al., 978) Monthly Temperature

5 I. Extraction methods I. Azeotropic Distillation Problem with unsaturated soils is to obtain a water sample without fractionation Centrifugation Squeezing Azeotropic distillation with toluene Vacuum distillation Direct equilibration Water cooled condensor Cellulose thimble with wet soil sample Boiling toluene x Cooling water Condensed toluene (lighter than wate Condensed water valve Heater II. Case Studies Streamflow Separation Streamflow Soil water evaporation Groundwater recharge Seasonal or storm differences in isotopes can be used to determine proportion of surface runoff from groundwater contributions to stream flow.

6 II. Streamflow separation II. Streamflow Separation Precipitation (event water or new water) Groundwater (pre-event water) Runoff and Infiltration Stream channel NEW WATER OLD WATER Streamflow(m /s) Surface Flow = New Water Base Flow = Old Water Time since start of storm, hours Streamflow Separation mm/d rainfall snowmelt lysimeter Streamflow separation Snowmelt from Lysimeter Maule & Stein 99. WRR mm/d interflow stream Groundwater Maule & Stein

7 Streamflow Separation Evaporation Percent snowmelt in groundwater Of remaining water; - evaporation results in isotopic enrichment - transpiration has no effects Percent snowmelt in streamwaters Maule & Stein 99. WRR Evaporation δd Evaporation Depth (m).6 Barnes and Allison J of Hydrology :4-76

8 Evaporation - Isotopic analysis indicates shift is result of 8% of soil water being evaporated - Edmonton meteoric line Deuterium ( ) soil water line for. -.9 m. -.9 m.9 -. m m δ Oxygen-8 ( ) Vadose zone deuterium and oxygen-8 values for an agricultural field near Edmonton, Alberta Maule et al 994. J H Snowmelt recharge I. Snowmelt Recharge δ Oxygen-8 ( ) Ottawa Wynard, Sk Edmonton Edmonton δ 8O values: winter precipitation: -6 summer precipitation: -4-5 What is the contribution of snowmelt to soil and groundwaters? - Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

9 Snowmelt recharge Snowmelt recharge Deuterium ( ) Edmonton δ 8O values: winter precipitation: -6 summer precipitation: -4 Average annual isotope value of precipitation (weighted) Edmonton meteoric line soil water line for. -.9 m. -.9 m.9 -. m m δ Oxygen-8 ( ) Bengtsson et al., 987. J Hydr Sciences :497 Vadose zone deuterium and oxygen-8 values for an agricultural field near Edmonton, Alberta Maule et al 994. J H Snowmelt Recharge Rate of recharge Rain Infiltration, macropore flow mixing Snow Snowmelt runoff Snow make up Precipitation: 9% of total Soil water (-.9m): 6-% Groundwater (-4 m): 44% Snowmelt waters Capillary rise Lateral flow Groundwater mounding Possible explanation for distribution of snowmelt recharge in some prairie soils. In dry climates groundwater recharge is too slow for conventional methods of measurement. Environmental tracers offer greater accuracy

10 Recharge Site Description Recharge Water table (- m) Root zone ~. m Ground water 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 SiL surface texture Measurements since 998 Bret Ward.. MSc Soil Moisture Profiles θ(m /m )..4 Spring Fall θ(m /m )..4 Meteoric Tritium (Ottawa) Depth (m) Tritium (TU) 4 Bottom slope Upper slope Active zone = regional of seasonal water difference due to springmelt Year

11 Recharge estimation: Profile methods Soil Tritium Profiles Tritium Units (TU) 4. Peak Method Distance peak has travelled beneath the root zone bottom since 96. TU 4 5 TU 4 5 Depth (m) 4 5. Curve Area Method Area under the curve attributable to above background tritium inputs divided by annual tritium weighted precipitation. Depth (m) 4 5 Upper Bottom Watershed A 4 5 Watershed C Recharge Rate (mm/yr) Monthly stable isotope values of Saskatoon Precipitation, 99- Peak Method Watershed Slope Upper A.9 C.4 Bottom Mass Method Watershed Slope Upper A 5. C 8.4 Bottom. 7.5 δ deuterium ( ) δ oxygen-8 ( ) April-Oct Nov-March (Len Wassenaar, Environment Canada, unpublished data, )

12 Monthly stable isotope values of Saskatoon Precipitation, 99- Period April-Oct Nov-Mar Year δ deuterium ( ) -5 - Precip (mm) δ oxygen-8 ( ) δ 8 O ( ) April-Oct Nov-March (Len Wassenaar, Environment Canada, unpublished data, ) Depth (m) Seasonal Recharge δ oxygen-8 ( ) Upper Bottom Annual average δ 8 O, weighted to monthly precipitation Proportion winter (Nov-Mar) Upper %* Bottom 8 %* Annual precip % *Only applies to below m Rate of Recharge, mm/yr Useful Reading Elevation above local bench mark (m) 5 95 Lower slope 57: Darcian 4: Cl peak 5: H peak : NO peak.8: Cl mass 5: H mass watertable Middle slope na: Darcian 7: Cl peak na: H peak 5: NO peak : Cl mass na: H mass Active zone Upper slope : Darcian : Cl peak na: H peak : NO peak.7: Cl mass : H mass UNESCO/IAEA Series on Environmental Isotopes in the Hydrological Cycle Principles and Applications Edited by W.G. Mook *****