Tracer studies of transport processes in hydrological systems

This thesis is an examination of the use of deliberately injected and transient tracers for investigations of transport processes in natural water systems. First, the influence of rainfall on air-water gas exchange was investigated at NASA's Rain-Sea Interaction Facility (RSIF) using deliberately injected tracers (He, N₂O, SF₆) in gas evasion experiments. The results from experiments with a range of rain rates and drop sizes demonstrate a significant and systematic enhancement of air-water gas exchange by rainfall. The gas transfer velocity k(600) (cm h−1) was found to be related to rain rate R_n (mm h −1) by the following empirical relationship: k(600) = 0.663 + 0.485R_n − 0.001$R2n$.; Furthermore, the physical mechanisms influencing rain-induced air-water gas exchange were examined. In addition to the tracer data, rain-generated bubbles and surface waves were characterized using a video-microscope technique and a capacitance probe, respectively. Rain-generated turbulence was inferred from friction velocities u_*w, calculated from kinetic energy flux. Together, these data suggest that rain-induced air-water gas exchange is mainly caused by turbulence-driven exchange processes, with bubbles contributing from 0 to 20%, depending on rain rate, drop size, and the solubility of the gas tracer.; Second, to assess the potential impact of elevated atmospheric CFC and SF₆ concentrations on tracer-based groundwater ages near a large coastal urban center, atmospheric mixing ratios of these tracers were measured in Palisades, New York, 25 km north of New York City. A 16-month record of atmospheric CCl₃F (CFC-11), and CCl₂F₂ (CFC-12), and a 12-month record of atmospheric SF₆ were analyzed. Nearly all measured values are in excess of remote Northern Hemisphere (NH) atmosphere mixing ratios. The mean mixing ratios of CFC-11, CFC-12, and SF₆ are 6, 13, and 87% higher, respectively, than those measured at a remote NH location during the same time period (Niwot Ridge, CO). Diurnal, weekly, and seasonal patterns are evident in the CFC distributions, in addition to variations resulting from regional meteorological conditions. Only a diurnal pattern is seen in the SF₆ data. These observations indicate that, in order to use these tracers effectively as groundwater dating tools near local or regional sources, their local atmospheric tracer concentrations must be explicitly defined.; Finally, a fully automated gas chromatographic system for analysis of SF₆ in groundwater is described. The system is capable of rapidly analyzing large volume groundwater samples (1.51) with a concentration range (0.01 to 10 femtomoles l−1) covering the entire range of SF₆ values likely to be found for young groundwater (recharged during the past 20–30 years).