Formatted Title:

Evaluation of a Continuous Sediment Ebullition Monitor

Background/Objectives:

Understanding occurrence, frequency and distribution of gasses released from subaqueous sediments into the overlying water column (ebullition) can play an important role in contaminant flux and transport to the surface. Gasses are produced by microorganisms decomposing organic matter under anoxic conditions in the sediment. These processes occur naturally in the environment, in addition to contributions from degradation of organic contaminants that may contribute to the overall gas release from aquatic ecosystems. Hydrophobic contaminants located within the sediment bed can adhere to the surface of the gas bubble and may result in transport and redistribution to the surface water column. Accurate monitoring of gas flux from the sediments is used to support risk evaluations, remedy decisions and remedy design. Various methods have been employed to monitor this phenomenon, each with unique advantages and limitations. As the production and ebullition of gasses is driven by many dynamic factors, a continuous in situ method is ideal for developing a process-based understanding of ebullition to be included in site-specific or global models. An autonomous sediment ebullition monitor was developed by Eosense, designed to continuously monitor gas flux from sediment and capture temporal variability.

Approach/Activities:

The sediment ebullition monitor was designed and developed based on previous work by Varadharajan et al. (2010). The main instrument components consist of an inverted funnel and gas collection column, stabilized by a tripod system that rests on the sediment bed. There is an auto-siphon in the gas column to allow for continuous operation of the instrument. Sensors to measure the gas volume inside the chamber are included along the length of the column, along with peripheral sensors to measure water temperature and water depth. The system is supported by a datalogger and solar power at the water’s surface. Two different iterations of the design were deployed as pilot projects in diverse environments, including 1) a rain basin located within an industrial facility, and 2) a publicly owned waterway. The pilots were designed to test the operation and instrument performance over a continuous and extended monitoring duration, and to evaluate variable influences (such as air temperature, water temperature, barometric pressure, water levels) on measured ebullition rates.

Results/Lessons Learned:

The performance of the sediment ebullition monitors for each field pilot will be presented. Design improvements that were made from the first iteration to the second, based on challenges encountered in the pilots will be reviewed for efficacy. A continuous ebullition flux dataset will be presented, and a comparison of ebullition fluxes measured by the instrument over an extended period relative to historical manual short-term measurements. These data will be compared to previous flux data from similar waterways, as an alternative approach to traditional ebullition measurement tools and practices. Recommendations will be given for best practices in continuous sediment ebullition monitoring.

Nick Nickerson

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Evaluation of a Continuous Sediment Ebullition Monitor