R. V. Krishnamurthy

Seasonal variations of dissolved inorganic carbon and δ13c of surface waters: Application of a modified gas evolution technique

Seasonal concentrations and δ13C of dissolved inorganic carbon (DIC) in a river-tributary system in Kalamazoo, southwest Michigan, USA, have been measured using a modified gas evolution technique. The technique makes use of evacuated glass septum tubes pre-loaded with phosphoric acid and a magnetic stir bar. Water samples are injected into these septum tubes in the field, which eliminates problems associated with CO2 loss/gain during sample storage and transfer to the vacuum line during DIC extraction. Using this technique, a precision of 1% and 0.1‰ can be achieved for DIC concentrations and δ13CDIC measurements, respectively. As this technique provides reliable measurements of DIC concentrations and carbon isotope ratios, it was used to evaluate the processes that control DIC in the river-tributary system. Results of DIC concentration and δ13CDIC measurements of water samples from the river-tributary system show that the DIC pool is mostly dominated by groundwater. The DIC concentrations and δ13CDIC are within the ranges measured for the most isotopically evolved groundwater in this region. Seasonal variations superimposed on the baseline values are attributed to secondary processes such as CO2 invasion from the atmosphere, enhanced recharge from lakes and biological activities of photosynthesis, respiration, and decay. With the onset of spring, there is a concurrent increase in the DIC concentration and δ13CDIC of these streams. A simultaneous increase in concentration and 13C enrichment of the riverine DIC pool is consistent with CO2 invasion and recharge from lakes. During the summer, biological activity is the predominant control on shifts in the DIC pool. Although photosynthesis, respiration and decay occur during this time, decreases in the DIC concentration and increases in the 13CDIC indicates CO2 removal from the pool by photosynthesis. In the late summer-early fall, photosynthesis declines and respiration and decay cause an increase in the DIC concentration and a decreases in the δ13CDIC. When biological activity decreases significantly during the late fall and winter months, the river and most of its tributaries approach a baseline DIC concentration and δ13CDIC similar to that of isotopically fully evolved groundwater in the Kalamazoo area. Although this holds true for tributaries and the main river, the timing and magnitude of shifts from background DIC is different for individual streams. The magnitude of shifts in the DIC concentration and δ13CDIC is most pronounced in tributary streams because of the short residence time of water in these streams. This study shows that DIC concentration and δ13CDIC measurements can be successfully used to evaluate the timing and dominance of the major processes that influence DIC in a riverine system.

Stable carbon isotope biogeochemistry of a shallow sand aquifer contaminated with fuel hydrocarbons

Abstract Ground-water chemistry and the stable C isotope composition (δ 13 C DIC ) of dissolved inorganic C (DIC) were measured in a sand aquifer contaminated with JP–4 fuel hydrocarbons. Results show that ground water in the upgradient zone was characterized by DIC content of 14–20 mg C/L and δ 13 C DIC values of −11.3‰ to −13.0‰. The contaminant source zone was characterized by an increase in DIC content (12.5 mg C/L to 54 mg C/L), Ca, and alkalinity, with a significant depletion of 13 C in δ 13 C DIC (−11.9‰ to −19.2‰). The source zone of the contaminant plume was also characterized by elevated levels of aromatic hydrocarbons (0 μg/L to 1490 μg/L) and microbial metabolites (aromatic acids, 0 μg/L to 2277 μg/L), non-detectable dissolved O 2 , NO 3 and SO 4 . Phospholipid ester-linked fatty acid analyses suggest the presence of viable SO 4 -reducing bacteria in ground water at the time of sampling. The ground-water chemistry and stable C isotope composition of ground-water DIC are interpreted using a chemical reaction model involving rainwater recharge, contributions of CO 2 from soil gas and biodegradation of hydrocarbons, and carbonate dissolution. The major-ion chemistry and δ 13 C DIC were reconciled, and the model predictions were in good agreement with field measurements. It was concluded that stable C isotope measurements, combined with other biogeochemical measures can be a useful tool to monitor the dominant terminal electron-accepting processes in contaminated aquifers and to identify mineralogical, hydrological, and microbiological factors that affect δ 13 C of dissolved inorganic C.

Isotopic investigations of carbonate growth on concrete structures

Stable C andO isotope ratios were measuredin carbonate minerals, growing und er concrete structures from two locations in the United States. These locations were under a bridge in Michigan and under an overpass in New York. The d 13 C of the carbonate samples rangedfrom � 21.6 to � 31.4% (with respect to V-PDB) andclearly ind icatedprecipitation under non-equilibrium conditions. Indeed, the values in some cases were more negative than could be accountedfor by existing mod els that invoke 4 stages of kinetic fractionation. There have been suggestions that microbial activity involving C from gasoline andother fossil fuel sources might be responsible for the relatively low C isotope ratios measuredin these carbonates. To explore this possibility, 14 C measurements were made in some of the samples. All samples measuredfor 14 C containedbomb C. The range of 14 C concentrations suggesteda non-uniform growth rate, although possible fossil fuel-derived carbon in the system needs future investigation. The d 18 O values of the carbonates analyzedfrom Michigan range from 12.5 to 15.7 % (with respect to V-SMOW), with a mean value of 13.7%. The d 18 O values of the NY samples range from 11.8 to 15.2%, with a mean value of 13.1%. The nearly identical mean values at both locations favors incorporation of O from atmospheric CO2 in carbonate precipitation. Additionally, the 210 Pb radiometric technique was also attempted to explore the applicability of this technique in dating concrete derived carbonates as well as recent carbonates forming in a wide variety of environments. The results gave ages between 64 and 3.8 a and are consistent when compared with the date the bridge was constructed. # 2002 Elsevier Science Ltd. All rights reserved.