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Compared to What?

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Scientific detective work is essential to determining the impacts of the Deepwater Horizon Oil Spill on coastal habitats in the Gulf of Mexico. Just after the beginning of the spill, but before any oil had reached land, U.S. Geological Survey scientists conducted a rapid response sampling of 70 coastal sites along the Gulf and on the Atlantic coast of Florida.


The scientists gathered samples for water quality analysis and bottom sediment composition that are providing a valuable starting point for understanding the impacts of the oil spill.

Dennis Demcheck , USGS, Hydrologist

We were able to get the true background conditions before there were any effects... and a lot of spills , like the Exxon Valdez one of the major shortcomings was that there was no pre event data to compare the post event data to. So we were very lucky, even though we mobilized in basically the early part of May, we were able to get our samples before the oil made landfall along the Northern Gulf Coast.


The scientists used a rigorous, methodical approach to sample the wide variety of ecological types represented along the Gulf Coast. From beaches to cypress swamps to marshes and wetlands.

Dennis Demcheck

We wanted to collect from the 50 sites that were most likely to have been hit by oil but we also at the same time wanted to make sure that our pre-landfall and our post-landfall sites were the same. We weren't just hunting oil. We wanted to have a pre and a post that we can compare.

Dennis Demcheck

And obviously, I mean it's certainly obvious to anyone that a beach site is very different than a marsh site. A beach site is almost pure sand where as a marsh site has a lot of organic material. A lot of peat, a lot of decomposing vegetation. And contaminants are going to react with the sand much differently than they are going to react with the peat and organic material.

Dennis Demcheck

But we want to make sure that our collection process is identical so that when we are comparing beach sand to mucky marsh mud we will know that the differences are truly differences between sand and marsh mud and not a difference in our collection method.


Sampling teams from five USGS water science centers in Louisiana, Mississippi, Alabama, Florida and Texas conducted both the pre and post- landfall studies. Each team used the same set of sampling gear and collected the same suite of samples covering water quality and the make-up of the bottom.

Dennis Demcheck

Once we get these samples there's going to be a huge volume of data coming in and the data management is very complex and very detailed and ... that's why there's a lot of paper work in the field, our 3 man crews are seemingly doing a lot of paper work but that's to insure that the data flow is correct and timely.


The sampling teams can return to these same sites in the future to help determine the long-term impacts of the spilled oil on the beaches, marshes, swamps and wetlands of the northern Gulf Coast.

Dennis Demcheck

I think this is important because..... when you have your post landfall samples or the samples from any major hazardous waste event, spill is that you have nothing to compare it to. It's always the question "compared to what?". And so this is one of the rare times where someone says well these are the numbers you got after the oil made landfall, compared to what ?, we can say, compared to this...We have a good data set of what the northern Gulf was like before the oil made landfall.


In sampling conducted in the fall of 2010, oil from the Deepwater Horizon spill was identified in coastal sediment samples from 11 of the sampling sites and in 17 of 20 tar balls found during sampling. Details of the sample analyses can be found at



Title: Compared to What?


The video relates how a team of scientists conducted rapid response sampling of coastal environments before any oil had reached land, following up in October 2010 with post-landfall sampling for comparison. USGS hydrologist Dennis Demcheck describes details of the work and explains the importance and value of having pre-landfall data for assessing impacts of the oil spill.

Location: , Gulf of Mexico, USA

Date Taken: 10/5/2010

Length: 5:37

Video Producer: Stephen Wessells , U.S. Geological Survey

Note: This video has been released into the public domain by the U.S. Geological Survey for use in its entirety. Some videos may contain pieces of copyrighted material. If you wish to use a portion of the video for any purpose, other than for resharing/reposting the video in its entirety, please contact the Video Producer/Videographer listed with this video. Please refer to the USGS Copyright section for how to credit this video.

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