Sun­shine mak­ing Deep­wa­ter Hori­zon oil stick around

Iran Daily - - Cultural Heritage & Environment -

Sun­light shapes oil spills’ long-term lega­cies.

In the days and weeks af­ter the 2010 Deep­wa­ter Hori­zon oil spill in the Gulf of Mexico, sun­light hit the oil slicks on the sur­face of the wa­ter, sci­ence­ wrote.

That trig­gered chem­i­cal re­ac­tions that added oxy­gen to oil mol­e­cules that once were just chains of car­bon and hy­dro­gen atoms.

Th­ese oxy­genated hy­dro­car­bons are still stick­ing around eight years later with lit­tle ev­i­dence of degra­da­tion, re­searchers re­ported in En­vi­ron­men­tal Science and Tech­nol­ogy.

Chemist Christo­pher Reddy of Woods Hole Oceano­graphic In­sti­tu­tion in Mas­sachusetts and col­leagues an­a­lyzed the oily soup of mol­e­cules float­ing in the Gulf post-dis­as­ter. (The Deep­wa­ter Hori­zon spill was the largest marine oil spill in US his­tory, leak­ing more than three mil­lion bar­rels.)

While in­ves­ti­gat­ing how the leaked hy­dro­car­bons broke down over time, the team got a sur­prise: More than half of the de­grad­ing oil by-prod­ucts found in oil slicks from the spill were th­ese oxy­genated hy­dro­car­bons, the re­searchers re­ported in 2012.

The by-prod­ucts had gone rel­a­tively un­no­ticed af­ter pre­vi­ous oil spills, and so were mostly un­stud­ied in that con­text.

Now the team has ev­i­dence that th­ese oxy­genated hy­dro­car­bons aren’t just a ma­jor by-prod­uct of the Deep­wa­ter Hori­zon oil spill, but a par­tic­u­larly per­sis­tent one.

The sci­en­tists an­a­lyzed more hy­dro­car­bon sam­ples col­lected from the wa­ter sur­face and from sandy beaches in the area in the years since the spill to see how the mol­e­cules have fared.

All of the sand sam­ples had roughly the same pro­por­tion of oxy­genated hy­dro­car­bons be­tween years, sug­gest­ing that in the eight years since the dis­as­ter, th­ese mol­e­cules still haven’t bro­ken down.

Reddy said, “A nat­u­ral process took what was re­leased from the [spill] and made some­thing ei­ther as tough or tougher.”

The team did iden­tify one rel­a­tively small cat­e­gory of oxy­genated hy­dro­car­bons that ap­pear to be more sol­u­ble in wa­ter, though, sug­gest­ing that some of the mol­e­cules aren’t en­tirely resistant to break­ing down.

Zhan­fei Liu, an or­ganic geo­chemist at the Uni­ver­sity of Texas at Austin, who wasn’t part of the study, said, “One big, unan­swered ques­tion is just how toxic this stuff is.”

Reddy said, “But pin­point­ing tox­i­c­ity is chal­leng­ing.”

The oil residue is a com­plex mix­ture of dif­fer­ent mol­e­cules that varies from place to place, not a sin­gle com­pound that one could order from a chem­i­cal com­pany and test in a con­trolled study.

Reddy said, “It’s also too soon to say what the find­ing means for wildlife.”

The per­sis­tence of the oil by-prod­ucts means that they’re still hang­ing out in the en­vi­ron­ment, able to coat birds’ feathers and ot­ters’ fur.

But since most of th­ese mol­e­cules seem rel­a­tively reluctant to dis­solve into the wa­ter, it’s un­clear whether aquatic or­gan­isms are tak­ing the pol­lu­tion in at a level that could im­pact their health.

Liu said, “Sun­light prob­a­bly af­fects the chem­istry of how other oil spills break down in the en­vi­ron­ment, too, but per­haps less dra­mat­i­cally.”

That’s be­cause Deep­wa­ter Hori­zon may have had the per­fect com­bi­na­tion of fac­tors for sun­light to star: The hy­dro­car­bon mol­e­cules that leaked tended to be smaller in size than in some other oil spills, mean­ing the oil spread over a larger sur­face area and had more ex­po­sure to the sun­rays.

Plus, the Gulf of Mexico re­ceives brighter, more di­rect light than more tem­per­ate re­gions.

sci­ence­ Oil swirls at the wa­ter’s sur­face af­ter the 2010 Deep­wa­ter Hori­zon oil spill. Sun­light trans­formed the oil mol­e­cules on the sur­face into new mol­e­cules that are still stick­ing around.

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