Tracing the Carbon Cycle in a Small Boreal Catchment of a Groundwater Dominated River Using the Isotopic Composition of Dissolved Inorganic Carbon

Understanding the carbon cycle in river systems is particularly important in fragile catchments with agriculture, urbanization, water purification facilities and other possible contamination sources. The isotopic composition and concentration of dissolved inorganic carbon (DIC) has been used to determine carbon sinks and sources in river systems. The Vantaanjoki River, in southern Finland, is located in one of the most densely populated areas in Finland. Previous studies have shown the river having a considerable amount of groundwater - surface water interaction which leads to local groundwater being vulnerable to any contaminants released into the river. The catchment of the river has six water purification facilities, and during times of high discharge some of the waste water is released into the river without treatment. Other possible sources of contamination are urban areas, agriculture and a saw mill. In this study the isotopic composition of DIC was studied, along with the concentration of DIC in the river water, to determine the major influences in carbon balance in the river water, to see if human induced changes in the environment are affecting the carbon cycle. The highest δ13CDIC values were found in the summer, and the lowest ones in the spring. Locations of the water purification facilities or fields along the flow path did not show on the δ13CDIC values, nor in the DIC contents of the water. Similar trends in δ13CDIC values related to the variations between warm and cold seasons have been reported in other studies as well and are likely due to organic material forming and decaying in and around the river channel.