Departments of Geology & Geophysics and Physics & Astronomy, University of Calgary, Calgary, Alberta, Canada T2N 1N4
The isotoic composition of dissolved sulfate (δ34S and δ18O) and nitrate (δ15N and δ18O) in surface water and groundwater often provides information about the sources of these solutes (Kendall 1998, Krouse & Mayer 2000). Additionally, spatial or temporal patterns of increasing or decresing sulfate and nitrate concentrations in conjunction with changing isotope ratios may reveal biogeochemical processes occurring in the hydrosphere. This contribution summarizes historical and contemporary case studies, in which the isotopic compositions of sulfate and nitrate have been successfully used to determine sources and transformations of these solutes in surface water and groundwater.
The isotopic composition of dissolved sulfate in surface water and groundwater can provide information about sulfur sources, particularly if both 34S/32S and 18O/16O ratios are determined. The following three major sources of sulfate can often be distinguished isotopically:
• sulfate derived from dissolution of primary evaporites with relatively high δ34S and δ18O values;
• sulfate derived from pyrite oxidation usually with comparatively low δ34S and δ18O values;
• sulfate derived from atmospheric deposition with δ34S often between 0 and +6 ‰ and high δ18O values.
Additionally, anthropogenic point sources may contribute sulfate with characteristic isotope compsotions. Simultaneous monitoring of spatial or temporal trends in concentration and isotopic composition of sulfate is an effective approach for revealing sulfur sources and transformation processes in aqueous systems. Decreasing sulfate concentrations with increasing δ34Ssulfate and δ18Osulfate values are indicative for dissimilatory bacterial sulfate reduction. Increasing sulfate concentrations accompanied by increasing δ34Ssulfate values are often typical for admixture of sulfate from evaporite dissolution. In contrast, increasing sulfate concentrations with decreasing δ34Ssulfate values may point to pyrite oxidation as a potential sulfate source. Finally, evaporation increases the concentration of dissolved sulfate without causing major shifts in its δ34Ssulfate and δ18Osulfate values. Eventually, gypsum or anhydrite may form with δ34S and δ18O values similar to those of the dissolved sulfate, since isotope fractionation during precipitation of sulfate minerals is small for both sulfur (< 2 ‰) and oxygen isotope ratios (< 4 ‰).
The isotopic composition of nitrate in surface water and groundwater can provide information about nitrogen sources, particularly if both 15N/14N and 18O/16O ratios are determined. The following four major sources of nitrate can often be distinguished isotopically:
• nitrate from atmospheric deposition with variable δ15N values and high δ18O values (+25 to +80‰);
• nitrate from synthetic fertilizers with δ15N values often around 0‰ and δ18O values near +23‰;
• nitrate from soil nitrification with δ15N values < +5‰ and δ18O values < +15‰, and
• nitrate from sewage and manure with δ15N values > +7‰ and δ18O values below +15‰.
Simultaneous monitoring of spatial or temporal trends in concentration and isotopic composition of nitrate is an effective approach for revealing nitrogen sources and transformation processes in aqueous systems. Decreasing nitrate concentrations with increasing δ15Nnitrate and δ18Onitrate values are indicative for the process of denitrification. In contrast, increasing nitrate concentrations accompanied by relatively high δ15Nnitrate values (> +7‰) are often typical for admixture of nitrate from sewage or manure. Concentration and isotope patterns indicative of other processes such as nitrate assimilation will also be discussed.
REFERENCE:
[1] EN.REFLIST
IAEA-CN-104/109 ISOTOPE TRACING OF WATER BALANCE AND CLIMATIC VARIABILITY ALONG THE MACKENZIE RIVER
J.J. GIBSON, T.D. PROWSE
National Water Research Institute, Water & Climate Impacts Research Centre, University of Victoria, Victoria BC Canada
A. PIETRONIRO, L. WASSENAAR, G. KOEHLER
National Water Research Institute, NHRC, 11 Innovation Blvd., Saskatoon SK Canada
The Mackenzie River, draining an area of 1.7 million km2, incorporates a diverse range of geographic source regions, including 8 of the 15 distinct ecoclimatic regions identified in Canada [1]. The basin is mountainous in the west and relatively flat-lying in the east with strong north-south climatic gradients, and generally cold, dry climate conditions compared to other large river basins in the world. As a major contributor of freshwater discharge to the Arctic Ocean, the river is also distinct due to the occurrence of several large lakes (Lesser Slave, Athabasca, Great Slave, Great Bear) which naturally act as flow, sedimentation, and biogeochemical regulators along its main drainage network.
Evolution of the isotopic composition of river discharge from the headwaters of the Mackenzie River to the mouth also reflects many of these complexities, particularly the mixing of tributary inflows and the buffering effect of large lakes. A map of the oxygen-18 composition of summer runoff in tributaries (Fig. 1) reveals distinct spatial patterns across the basin. Notably, the most depleted isotope signatures (<-20‰ in δ18O) are observed for tributaries of the Western Cordillera, especially the Mackenzie Mountains (min. of -22.9‰ in δ18O), which are characterized by higher altitude precipitation, greater snowfall, and higher runoff/precipitation ratios than other parts of the basin [2]. In shield-dominated areas to the east of Great Slave Lake and Lake Athabasca, and to a lesser extent in the central boreal-taiga plains, tributary runoff is typically enriched by lake and wetland evaporation in low-relief areas where rivers traverse extensive string-of-lakes and bog-fen drainage networks. Oxygen- 18 values in major tributaries typically range between –16 to –14 ‰ in shield areas with peak enrichment observed in wetland dominated drainage of the south-central Boreal Plain (Wabasca R. ~-13.9‰).
A synoptic survey along the main stem of the Mackenzie-Athabasca River (Fig. 2) reveals the periodic fluctuation of oxygen-18 from headwaters to mouth due to interaction of tributaries draining both western alpine regions (with depleted isotope signatures) and eastern lowlands (with enriched isotope signatures), overprinted by lake storage effects. In general, lakes serve a regulatory role in the runoff regime by reducing seasonality of discharge and amplitude of isotope variations. The 2-3 ‰ overall enrichment of oxygen-18 from headwaters to mouth, despite the north-flowing drainage network and northeastward decrease in oxygen-18 in precipitation across the region (see [3]) emphasizes the cumulative importance of open-water evaporation losses in the basin water budget (~15%).
Seasonality in the isotope composition of discharge, as measured near the river mouth upstream from the Mackenzie Delta is pronounced (Fig. 3). Peak flow produced by snowmelt typically occurs in April (~Day 150) at the basin outlet, and conincided with roughly a 2‰
depletion in δ18O during the early 1980s. Significantly higher depletions during freshet are
often observed in smaller tributaries,with similar recessions to higher δ18O values in summer and late fall. Reduced isotope variability is generally observed during extended winter periods with thick ice cover, and reflect the predominance of groundwater sources (~19.0‰ in δ18O) although the signature of water derived from lake storage is also evident during some years (max of –17.5‰ in δ18O).
This paper presents an overview of isotope datasets collected within the Mackenzie Basin since 1969 (Figs. 1-3) as well as more recent hydrograph separation, water balance and evaporation-transpiration partitioning studies conducted under the auspices of the Global Energy and Water Cycle Experiment, Mackenzie Study (GEWEX-MAGS) and the IAEA Coordinated Research Project “Isotope tracing of hydrological processes in large river basins”. One long-term objective of the research is to incorporate tracers in the suite of models currently being implemented and tested with the MAGS study. Coupled simulation of isotopic and hydrologic fluxes and storages, particularly on the land surface and in streamflow will be used to evaluate and refine these models so that they more realistically depict processes, and so that they evolve to become more relevant for evaluating and predicting both water quantity and quality impacts, especially in vast ungauged regions of the basin and other parts of northern Canada.
REFERENCES:
[1] Ecoregions Working Group, Ecoclimatic regions of Canada, first approximation.
Ecological Land Classification Series, No. 23. Sustainable Development Branch, Conservation and Protection, Environment Canada, Ottawa, 199 pp., 1989.
[2] HITCHON, B., KROUSE, H.R. Hydrogeochemistry of surface waters of the Mackenzie River drainage basin, Canada – III. Stable isotopes of oxygen, carbon and sulphur, Geochim. Cosmochim. Acta 36, 1337-1357, 1972.
[3] BIRKS, S.J., GIBSON, J.J., GOURCY, L., AGGARWAL, P.K., EDWARDS, T.W.D., Maps and Animations Offer New Opportunities for Studying the Global Water Cycle.
EOS Vol. 83, No. 37, 10 September 2002 + electronic supplement (http://www.agu.org/eos_elec/eeshome.html).
Fig. 1 Map of the Mackenzie Basin showing spatial distribution of δ18O in summer runoff during 1969 (modified after [2]).
Distance from mouth (km)
0 1000
2000 3000
4000 δ18O (‰)
-22 -21 -20 -19 -18 -17
Inflow from Great Bear Lake Great
Slave Lake
Inflow from Liard R.
Inflow from Peace R./
Lake Athabasca Inflow
fromLesser Slave
Lake Inflow from Clearwater R.
seasonal range
Fig. 2 Synoptic δ18O survey along the main stem of the Mackenzie-Athabasca River system. Data are generated from the above survey. Approximate seasonal range is from Fig. 3.
Day of Year (1981-1983)
0 100 200 300
Discharge (m3/s)
1 10 100 1000
-21 -20 -19 -18 -17
Fig. 3 Composite time-series of river discharge and δ18O content (solid circles) sampled near the
IAEA-CN-104/111 TRACING NUTRIENT SOURCES IN THE MISSISSIPPI RIVER BASIN, U.S.A.
C. KENDALL, S.R. SILVA, C.C.Y. CHANG, S.D. WANKEL United States Geological Survey, Menlo Park, California, U.S.A.