Sea Surface Oil Slick Light Component Vaporization and Heavy Residue Sinking: Binary Mixture Theory and Experimental Proof of Concept

Abstract

This is a first of its kind study demonstrating that oil weathering can result in its sinking. Although a controversial proposition, oil's appearance on the sea floor following the 2010 Macondo 252 spill in the Gulf of Mexico and its large unaccountable volume has raised awareness of possible alternative explanations. Drag-down by settling particles is one known process. Another possible cause is heavy residue sinking due to density increase that follows the evaporation of the light constituents from the mixture. The study outcome impacts the field of oil spill modeling because confirmed mechanism-based process algorithms are required in the oil fate models. Theoretical and experimental studies on the proof of concept for the evaporation/sinking [EVAPO-SINK] process were undertaken using binary chemical mixtures representing “model oils.” Laboratory macrocosm-scale experiments with surface spills were performed and a theoretical, binary-component mathematical model was developed. Direct visual observations and physical/chemical measurements during both evaporation to air and heavy droplet sinking in the water column confirmed the process. Data obtained on oil component and bulk density concentrations tracked the time-series oil constituent chemodynamics within the slick and droplets. At the critical density the slick exceeded neutral buoyancy in water, after which a droplet formed underneath, broke away from the slick, and settled to the bottom. The drop was collected and physical/chemical measurements performed. Independent of the initial slick composition, the measured critical densities were 1.04 g/mL (±0.006) for fresh water and 1.07 for seawater. Dependent on initial composition, evaporation time to droplet formation varied from 200 to 2400 s. Data yielded evaporation rate kinetics required in the theoretical model. This light component mass transport coefficient ranged from 120 to 300 cm/h. The model correctly mimicked slick and droplet composition chemodynamic behavior patterns and density and drop-time measurements. Based on these positive outcomes the proof of concept was achieved.