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Tracing ground water input to base flow using sulfate (S, O) isotopes
Ailiang Gu1, Floyd Gray, Christopher J Eastoe
1Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA. AGu@burnip.com
Ground Water
|March 12, 2008
Summary
Sulfate isotopes trace groundwater sources in base flow, especially in arid regions like Arizona. This method distinguished three distinct groundwater types contributing to Sonoita Creek, with proportions varying by season and location.
Area of Science:
- Environmental Science
- Hydrogeology
- Isotope Geochemistry
Background:
- Groundwater contributions to base flow are crucial for stream ecosystems, particularly in arid environments.
- Distinguishing between multiple groundwater sources can be challenging using traditional water chemistry or stable isotopes (H and O).
- Sulfur (S) and oxygen (O) isotopes in sulfate offer a powerful tracer for identifying groundwater origins, especially when contrasting sulfur sources are present.
Purpose of the Study:
- To demonstrate the utility of sulfate (S and O) isotopes as a tracer for groundwater contributions to base flow.
- To identify and quantify the contributions of different groundwater sources to base flow in the Sonoita Creek basin, Arizona.
- To investigate how groundwater source contributions vary with seasonal conditions and downstream location.
Main Methods:
- Analysis of sulfate concentrations and stable isotopes (S and O) in groundwater samples.
- Application of isotopic mixing models to determine the proportions of different groundwater sources.
- Comparison of isotopic signatures with known geological sources (Permian evaporite, acid rock drainage).
Main Results:
- Base flow in Sonoita Creek is a mixture of three distinct groundwater sources: Permian evaporite-like (Source A), Patagonia Mountains-like (Source B), and Temporal Gulch-associated (Source C).
- Source A (evaporite-like) contributes 50% to 70% of the base flow, with Sources B and C (acid rock drainage-like) contributing the remainder equally.
- The proportion of Source B increases downstream, and Source A's contribution is highest during drought conditions.
Conclusions:
- Sulfate isotopes, combined with concentration data, effectively differentiate groundwater sources contributing to base flow in complex geological settings.
- The study successfully quantified the contributions of three distinct groundwater sources to Sonoita Creek base flow, highlighting the influence of evaporite and acid rock drainage.
- Understanding these contributions is vital for water resource management, especially in arid regions where groundwater is a primary water source.
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