Carbon Sources and Water-Rock Interactions in the Allier River, France

AbstractThe Allier River is an important tributary of the Loire River, one of the major rivers in France. The Allier River presents both a natural environment upstream and a zone deeply impacted by mines and human activities. The δ13C and δ7Li combination show that the Allier River DIC is due to mixing of carbon from organic decay produced in a natural environment upstream, progressively enriched in DIC of anthropogenic origin downstream, and magmatic carbon inputs often associated with hydrothermal contributions

Improved method for isotopic and quantitative analysis of dissolved inorganic carbon in natural water samples

We present here an improved and reliable method for measuring the concentration of dissolved inorganic carbon (DIC) and its isotope composition (13CDIC) in natural water samples. Our apparatus, a gas chromatograph coupled to an isotope ratio mass spectrometer (GCIRMS), runs in a quasi-automated mode and is able to analyze about 50 water samples per day. The whole procedure (sample preparation, CO2(g)-CO2(aq) equilibration time and GCIRMS analysis) requires 2 days. It consists of injecting an aliquot of water into a H3PO4-loaded and He-flushed 12 mL glass tube. The H3PO4 reacts with the water and converts the DIC into aqueous and gaseous CO2. After a CO2(g)-CO2(aq) equilibration time of between 15 and 24 h, a portion of the headspace gas (mainly CO2+He) is introduced into the GCIRMS, to measure the carbon isotope ratio of the released CO2(g), from which the 13CDIC is determined via a calibration procedure. For standard solutions with DIC concentrations ranging from 1 to 25 mmol · L-1 and solution volume of 1 mL (high DIC concentration samples) or 5 mL (low DIC concentration samples), 13CDIC values are determined with a precision (1) better than 0.1. Compared with previously published headspace equilibration methods, the major improvement presented here is the development of a calibration procedure which takes the carbon isotope fractionation associated with the CO2(g)-CO2(aq) partition into account: the set of standard solutions and samples has to be prepared and analyzed with the same gas/liquid and H3PO4/water volume ratios. A set of natural water samples (lake, river and hydrothermal springs) was analyzed to demonstrate the utility of this new method. Copyrig