During the Deepwater Horizon oil spill, many factors influenced the pathways of oil and dispersant as they traveled to the coast, including the input of freshwater from the many rivers that empty into the Gulf. CONCORDE scientists are exploring how these mechanisms work to get an idea of how they might move oil and dispersant through the ecosystem should it enter again.
According to CONCORDE scientist Stephan Howden, Associate Professor of Marine Science at University of Southern Mississippi, the large number of rivers that empty into the northern Gulf of Mexico complicates what we know about how oil moved through the area in 2010. The Mississippi River drains freshwater from 31 states and two provinces into the Gulf. A little over one hundred miles to the east, the Mobile Bay area has the fourth largest freshwater output in the country. To a certain extent these rivers’ outputs helped protect this portion of the Gulf during the months following the spill, however oil did find a way onshore to the barrier islands and beyond in some cases.
How could a river impact a deep water marine oil spill? As river water moves into the marine environment it has a different density than the water already there. When the currents of freshwater moving offshore meet the currents of salt water moving inshore, layers of different densities form beneath the surface. Simultaneously biological layers of microscopic sea creatures form as they move to one type of water or the other.
According to Howden, questioning how oil made its way to the coast from the site of the spill is at the core of CONCORDE’s mission: “Under so-called ‘normal’ conditions, what are the pathways of water from offshore to the coast? What vertical mixing occurs in the layers of water that would bring oil in contact with marine life in the biological layers?”
To better understand these processes, a group of CONCORDE scientists will collect data on waves, currents, water and air temperature, salinity, carbon dioxide levels, and other factors to shed light on this unique nearcoastal riverine environment. The project kicked off with the AUV Jubilee in July 2015, led by USM’s Ocean Weather Lab under Bob Arnone and Oscar Schofield of Rutgers University. It will continue with intensive data collection coinciding with project cruises in fall 2015 and spring 2016. Highly specialized buoys, such as the CENGOOS buoy, will constantly gather data and distribute it online in real-time. Alan Weidemann of the Naval Research Lab (NRL) will mount a laser scanning system to the vessel and tow a suite of bio-optical sensors to locate biological formations. The team will deploy moorings designed by Jeff Book of the NRL and Jim Moum of Oregon State University. Brian Dzwonkowski of the University of South Alabama will launch drifters from small vessels to measure currents and freshwater plumes in the Mississippi Sound. The vessels, and Rutgers’ underwater autonomous gliders, will be guided by views of the offshore river plume development from the Ocean Weather Laboratory that synthesize satellite data and numerical ocean model output from Gregg Jacobs’ group at NRL. Collaborative projects will include moorings at the shelf break and autonomous Saildrones.
The goal is to improve readiness and response in the wake of another catastrophic event. “In previous oil spills, we really had no idea how oil was going to mix with biology, particularly in this part of the Gulf,” said Howden. “If we have another subsurface spill, this research will give us a better idea of where to deploy instruments to monitor the spill and how best to target our response.”