Formatted Title
Use of Unmanned Aerial Vehicles to Monitor Ebullition-Facilitated NAPL Transport
Background/Objectives
Ebullition is ubiquitous in sediment environments. Where ebullition occurs in the presence of NAPL contaminated sediment, it can provide a transport mechanism for NAPL to enter the surface water body and be observed as sheens on the water surface. This study monitored the occurrence of ebullition-facilitated NAPL transport in lagoons that were formerly used for waste water from a former petroleum site.
Approach/Activities
A qualitative visual survey was performed in order to identify areas where ebullition-facilitated NAPL transport was occurring. Observations were made from a boat to note the location, qualitative bubbling rate, occurrence of sheens, and size of the sheen blossoms. Based on the qualitative survey, an area with high potential for ebullition-facilitated NAPL transport was identified for ongoing evaluation by observations from an unmanned aerial vehicle (UAV).
The UAV flights occurred approximately monthly from October 2019 through July 2020. The UAV was flown in a regular grid, 3 columns by 7 rows in two areas of the lagoons. The flight elevation was calibrated for a consistent field of view (approximately 1800 square feet) for each video and to have sufficient resolution to visualize individual bubbles and sheens in the videos. Temperature and pressure sensors were installed in the water and sediment column to explore the relationship between ebullition observations and site conditions.
The videos were visually processed to identify the number of bubbles observed and estimate an ebullition rate across the entire monitoring area. The occurrence and area of sheens was estimated for a subset of the monitoring events in a single area. The thickness of the sheen was estimated based on the sheen color and standard assumptions for sheen thickness for aerial monitoring from the Bonn Agreement Aerial Operations Handbook.
Results/Lessons Learned
UAV surveys are useful for estimating ebullition-facilitated NAPL transport over a large area. A linear relationship was observed between ebullition rate and the surface water temperature at the sediment/water interface. However, the NAPL sheen rate was largely consistent across seasons and did not show a relationship to temperature.
Ebullition and sheen rate data was utilized to inform remedy design. At the study site, a submerged cap was proposed for the remedy. However, extensive amounts of amendments would be required for a submerged cap making it practically and financially infeasible. This study was a critical component of the decision to change the remedial approach.