SEASONAL-VARIATIONS OF DISSOLVED INORGANIC CARBON AND DELTA-C-13 OF SURFACE WATERS - APPLICATION OF A MODIFIED GAS EVOLUTION TECHNIQUE

Seasonal concentrations and delta(13)C 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 tec hnique makes use of evacuated glass septum tubes pre-loaded with phosp horic acid and a magnetic stir bar. Water samples are injected into th ese septum tubes in the field, which eliminates problems associated wi th 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 parts per thousand can be achieved for DIC concentrations and delta( 13)C(DIC) measurements, respectively. As this technique provides relia ble measurements of DIC concentrations and carbon isotope ratios, it w as used to evaluate the processes that control DIC in the river-tribut ary system. Results of DIC concentration and delta(13)C(DIC) measureme nts of water samples from the river-tributary system show that the DIC pool is mostly dominated by groundwater. The DIC concentrations and d elta(13)C(DIC) are within the ranges measured for the most isotopicall y 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 bio logical activities of photosynthesis, respiration, and decay. With the onset of spring, there is a concurrent increase in the DIC concentrat ion and delta(13)C(DIC) Of these streams. A simultaneous increase in c oncentration and C-13 enrichment of the riverine DIC pool is consisten t with CO2 invasion and recharge from lakes. During the summer, biolog ical activity is the predominant control on shifts in the DIC pool. Al though photosynthesis, respiration and decay occur during this time, d ecreases in the DIC concentration and increases in the delta(13)C(DIC) indicates CO2 removal from the pool by photosynthesis. In the late su mmer-early fall, photosynthesis declines and respiration and decay cau se an increase in the DIC concentration and a decreases in the delta(1 3)C(DIC). When biological activity decreases significantly during the late fall and winter months, the river and most of its tributaries app roach a baseline DIC concentration and delta(13)C(DIC) Similar to that of isotopically fully evolved groundwater in the Kalamazoo area. Alth ough this holds true for tributaries and the main river, the timing an d magnitude of shifts from background DIC is different for individual streams. The magnitude of shifts in the DIC concentration and delta(13 )C(DIC) is most pronounced in tributary streams because of the short r esidence time of water in these streams. This study shows that DIC con centration and delta(13)C(DIC) measurements can be successfully used t o evaluate the timing and dominance of the major processes that influe nce DIC in a riverine system. (C) 1998 Elsevier Science B.V. All right s reserved.