Radium isotopes released into the aqueous phase during oil degradation: Deepwater Horizon oil incubations

During March and April 2018, we performed a series of hydrocarbon incubations in Conway, SC using Macondo oil recovered during the Deepwater Horizon blowout to assess the degree of radium isotope release as a function of various degradation processes. We incubated 1 g of oil with 10 L of radium-free seawater to replicate surface sheen ('Surface') and submerged plume ('Deep') degradation. The surface sheen incubations were stored outside in ambient sunlight, with one set left unamended to allow both photodegradation and microbial degradation, and another set serving as a kill control (with 10 mM ZnCl2) to only allow photodegradation. The submerged plume simulations were stored in a dark refrigerator (2 deg. C), with one set left unamended (to allow microbial degradation) and one treatment serving as a kill control (with 10 mM ZnCl2) to act as a control. Each of the treatments underwent destructive analysis at various time points (t=0 day, 1 day, 3 days, 7 days, 11 days, 14 days, and 21 days) to determine the radium isotope (Ra-224, Ra-226, and Ra-228) activities in both the aqueous phase and on the oil phase through time. These incubations were replicated during July 2018 using 2 mM CdCl2 as the kill control agent and terminated at time points of t=0 day, 1 day, 2 days, 4 days, 7 days, 11 days, 14 days, and 21 days. During the July incubations, we placed 10-L blanks for radium-free seawater (no oil) alongside the surface (i.e., outside) incubations. Any radium detected in these blanks would be due to atmospheric contamination and should be subtracted from the measured activity for the surface incubations.