Dissolved Gases as Indicators for Stream-Ground Water Interactions

The connection between groundwater and surface water varies along stream corridors, but these hydrologic changes are typically difficult to observe or measure. Many dissolved gases entering streams via groundwater discharge are either undersaturated or supersaturated with respect to atmospheric equilibrium due to physical or biological mechanisms. Because such gases behave non-conservatively (e.g., via losses to the atmosphere) within the stream channel, their longitudinal patterns can potentially help identify where groundwater enters or exits a stream system. Such information can be very useful for understanding stream water quality and the impacts of land management. Unfortunately, dissolved gases (other than oxygen) have not been frequently employed in studies of stream systems, and their full potential as hydrologic tools has not been established. A better understanding of how dissolved gases can be used to study the groundwater/surface water connection is needed. In this study we present and interpret longitudinal patterns of several gases along an 8 km stretch of a baseflow dominated stream located in a predominantly agricultural sand plain watershed of central Wisconsin. Dissolved gas measurements included oxygen, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and noble gases. Major and minor ions were also measured. Sampling locations were sited at 350-m intervals along the thalweg of the stream into headwater tributaries. Losing stream sections had CFCs, nitrous oxide, and methane concentrations near atmospheric equilibrium. Gaining stream sections were supersaturated with nitrous oxide, methane, and carbon dioxide and undersaturated with CFCs and oxygen. High concentrations of nitrous oxide accompanied nitrate entering the stream.