Research
Transport, Mixing and Gas Exchange
Quantitative assessments of how natural or human disturbances such as freshwater runoff changes or contaminant spills affect the health of the Hudson River, as well as successful modeling of water flow and contaminant transport require an understanding of physical processes such as mean advection (transport), dispersive mixing and air-water gas exchange. These processes determine the movement and fate of substances discharged into the river. As a result, they guide the development of methods and strategies for detection, remediation and treatment. Knowledge of these processes will also aid in understanding the movement of sediment through a river, enabling an evaluation of the role of sediments in the transport of toxic substances through the system.
Advection rates in rivers can sometimes be estimated from the freshwater discharge rate and river channel geometry. However, dispersion across the river channel and air-water gas exchange are more difficult to determine. Moreover, large disparities exist between idealized and real systems and also among different streams and rivers Because advection, dispersion and gas exchange are fundamental variables in evaluating the water quality of aquatic systems through the use of conceptual or numerical models, better understanding of these processes is necessary to continue improving these models. For this reason, field experiments are necessary to further our understanding of these processes.
In the past, advection and dispersion rates in the Hudson have been determined by releasing dyes such as Rhodamine. Gas exchange rates were determined on the basis of several methods, including the helmet method or by examining the balances of natural or anthropogenic gas tracers.
Most, if not all, of these methods are problematic in their application. Dyes break down, which makes them ill-suited to studies lasting longer than a few days, and some of the by-products are toxic. Also, because fluorescent dyes break down relatively quickly, dye experiments, while useful in examining near-field mixing and fine flow structure, are not useful in examining the footprint of a contaminant plume, which can be quite extensive.
Figure 1: SF6 distribution across the Hudson River 8 and 12 days after injection during the 1994 experiment [from Clark, et al., 1996].
Figure 2:SF6 distribution along the Hudson River 10, 12, and 14 days after injection during the 2001 experiment.
Gas exchange experiments based on the helmet method are influenced by several physical processes, including disturbance of the surface turbulence regime, heating and cooling effects, or changes in the partial pressure of the gases accumulating under the collection dome. These can lead to a significant bias in the results. The mass balance technique requires a thorough knowledge about sources and sinks of the tracers under observation.
Tracer experiments
During the past decade, the Environmental Tracer Group at Lamont-Doherty has applied and further developed new tracer methods to study advection, dispersion and gas exchange in the Hudson River. In a first series of experiments, the stable, non-toxic gases helium-3 (3He) and sulfur hexafluoride (SF6) were bubbled into the river. Afterward, the river water was monitored to obtain quantitative information on advection, dispersion and gas exchange. While this proved to be a methodological advance, this first set of tracer experiments suffered from the fact that discrete samples of SF6 and 3He analysis had to be collected, which significantly reduced the spatial resolution of the tracer surveys (Figure 1).
Recently, an automated, high-resolution SF6 measuring system was developed for several projects on the Hudson River, which dramatically improved the spatial and temporal resolution of the measurements, as well as the overall extent of the river that could be surveyed. With the new system, more than 60 miles (100km) compared to just 12 to 18 miles ( 20-30km) in the pilot studies (Figure 1b). The high-resolution system samples river water approximately once every minute and is capable of detecting tracer concentrations as low as one part per trillion. In these later experiments, a tracer mass balance was obtained, and only a single tracer (SF6) was used to simultaneously determine net advection, longitudinal dispersion, and gas exchange in the Hudson.