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National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling

Natural Resource Damage Assessment: 

Evolution, Current Practice, and Preliminary Findings Related 

to the Deepwater Horizon Oil Spill

Staff Working Paper No. 17

Staff Working Papers are written by the staff of the BP Deepwater Horizon Oil Spill Commission for the 

use of the members of the Commission. They do not necessarily reflect the views of the Commission as a 

whole or any of its members. In addition, they may be based in part on confidential interviews with 

government and non-government personnel.

Six months after the oil has stopped flowing from BP’s damaged Macondo well, the amount of 

environmental harm caused by the spill is uncertain, as is the adequacy of existing legal, regulatory, and 

policy mechanisms to ensure that restoration needed to redress the damage will be fully implemented by 

government and paid for by responsible parties. This background paper describes the process that was 

established under the Oil Pollution Act of 1990 for assessing natural resource damages caused by the spill 

and restoring damaged resources to their pre-spill condition. Known as Natural Resource Damage 

Assessment (NRDA), this process is still in the early phases of being applied to the BP spill and 

conclusions about its efficacy or success in this instance will be impossible to draw for a number of years, 

possibly decades. This background paper describes the history and purpose of the NRDA, reviews the 

main steps in the NRDA process, and reports on the status of current damage assessment efforts in the 

Gulf. 

Natural Resource Damage Assessment: History and Purpose

In the wake of the Exxon Valdez disaster in 1989, Congress passed legislation specifically aimed 

at responding to and addressing environmental and economic damages from oil spills. As part of the Oil 

Pollution Act of 1990 (OPA), 33 U.S.C. §§ 2701 et seq., “responsible parties”1

 1 In the case of offshore facilities, “responsible party” is defined as the “lessee or permittee of the area in which the 

facility is located or the holder of the right of use and easement granted under applicable State law or the Outer 

Continental Shelf Lands Act for the area in which the facility is located (if the holder is a different person than the 

lessee or permittee) . . .”). 33 U.S.C. § 2701(32).

were made liable for the 

removal costs and damages resulting from discharges of oil from vessels or facilities. Among other 

things, this liability extends to:
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Damages for injury to, destruction of, loss of, or loss of use of, natural resources, including the 

reasonable costs of assessing the damage, which shall be recoverable by a United States trustee, a 

State trustee, an Indian tribe trustee, or a foreign trustee.

2

The measure of damages under OPA is:

(A) The cost of restoring, rehabilitating, replacing or acquiring the equivalent of, the damaged natural 

resources; 

(B) The diminution in value of those natural resources pending restoration; plus

(C) The reasonable cost of assessing those damages.3

Under OPA, responsibility for promulgating regulations to guide the assessment of natural 

resource damages fell to the National Atmospheric and Oceanic Administration (NOAA).4

 NOAA 

completed this task in 1996 and NRDA regulations became effective on February 5, 1996.5

Prior to 1990, natural resource damage assessments and the associated cost recovery for oil spills 

were governed by the Comprehensive Environmental Response, Compensation and Liability Act of 1980,

or CERCLA, which imposes liability for damages resulting from releases of “hazardous substances” as 

defined by the statute.

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In its OPA regulations, NOAA seeks to promote cooperation between the trustees and the 

responsible party in carrying out the natural resource damage assessment. This process, referred to as a 

cooperative assessment, is being used in the Deepwater Horizon case, where BP (a “responsible party”

 CERCLA regulations provided the model for the natural resource damage 

assessment authority set forth in OPA and continued to govern damage assessments for oil spills between

1990 and 1996, when NOAA was developing new regulations under OPA. 

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) 

is working with government agencies (i.e., the “trustees”) to identify and quantify damages.8

 2 33 U.S.C. § 2702(b)(2)(A). Trustees act “on behalf of the public” as trustees for natural resources. Federal trustees 

are designated by the President. State trustees are designated by their Governors. Affected Tribal and foreign nations 

can also claim trustee authority. 33 U.S.C. § 2706(b).

 NOAA 

guidance documents set forth the specifics of the cooperative process, including level of participation, 

3 33 U.S.C. § 2706 (d)(1).

4 Ibid. at § 2706(d). 5 15 C.F.R. part 990. 6

 42 U.S.C. §§ 9601 et seq. CERCLA, in turn, built on provisions in the Clean Water Act Amendments of 1977, 

which first codified federal authority to recover damages for natural resources. Specifically, CERCLA provided 

additional direction concerning the measure of damages, the use and effect of natural resource damage assessments, 

and the designation of trustees.

7 BP is a responsible party, but other companies involved may be named also once liability issues are established. 

For the time being, BP has entered into a funding agreement with the Natural Resource Damage Trustees, and is 

currently reimbursing their costs. If, ultimately, other responsible parties are named, the Trustees must find them 

jointly and severally liable under the law. 

8 Under CERCLA, by contrast, damage assessments were carried out, for the most part, in a non-cooperative and 

adversarial manner. Since the trustees could essentially dictate how damages would be determined, responsible 

parties typically opted to conduct their own assessment in preparation for a court defense should the case end up in 

litigation. CERCLA regulations (and OPA itself) impose a “rebuttable presumption” in favor of the trustee’s damage 

assessment: If the responsible party disagrees with the trustee’s assessment, it bears the burden of proving that the 

assessment was wrong. This regulatory arrangement often set the stage for parallel and dueling assessments, as 

emerged in the aftermath of the Exxon Valdez incident.
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dispute resolution, agreement on scientific methods, sharing of equipment and experts, and funding.

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 As 

the guidance suggests, these issues are generally laid out in a memorandum of agreement between the 

trustees and the responsible party.10 While past attempts to use the cooperative assessment process did

not measurably shorten the time or administrative costs incurred between the event and final settlement, 

trustees are quick to point out that the cooperative assessment process provides other advantages.11 In 

particular, states that do not have dedicated natural resource damage assessment programs12

Natural Resource Damage Assessment versus Civil and Criminal Penalties

maintain that 

they would not have the budget or resources to carry out damage assessments if not for the cooperative 

agreement that allows for periodic funding and the sharing of equipment and experts. Further, one trustee 

also pointed out that the more recent emphasis on data collection – as opposed to making assumptions 

based on past spills or existing scientific knowledge – to quantify damages and reach settlement has 

lengthened the time to settlement but strengthened the process and its outcome.

It is important to highlight the distinction between legal action to recover costs for damages to 

natural resources, and enforcement actions aimed at imposing civil or criminal penalties on the 

responsible party under an environmental statute. Both actions may be pursued, under separate authority, 

by states and the federal government in response to an event such as an oil spill. In bringing an 

enforcement action for civil or criminal penalties, the Department of Justice – on behalf of EPA, the 

Coast Guard, or another agency – acts in the role of prosecutor. By contrast, when the Department of 

Justice sues to recover natural resource damages, it is acting on behalf of the trustees with jurisdiction 

over the injured resources and the action is in many ways similar to a tort action.

13 As a general rule, 

funds recovered as a result of civil or criminal enforcement actions under federal environmental statutes 

are deposited in the federal treasury and may not be used to redress the harms caused by the pollution 

event or incident.14 The authority to recover costs for damages to natural resources, conversely, is unique 

in that the funds recovered from responsible parties must be used to restore the specific resources injured 

by the event.15

Understanding the NRDA Process

 

NRDA is the regulatory process used by designated natural resource trustees to identify, assess 

and restore damages to: (1) public natural resources, (2) the ecosystem services they provide (e.g. oysters 

 9 NOAA, Preassessment Phase: Guidance Document for Natural Resource Damage Assessment Under the Oil 

Pollution Act of 1990 (August 1996), http://www.darrp.noaa.gov/library/pdf/PPD_COV.PDF. 10 Ibid. 11 Confidential Commission staff interviews with state and federal trustees. 12 With the exception of a few coastal states, damage assessment training, resources, and staff are often gathered onthe-fly when a spill occurs. Expertise is pulled from within state agencies. 13 The same distinction applies in the case of actions brought by state attorneys general on behalf of state agencies. 14 There are some exceptions to this general rule. A description of the sources and uses of penalties and fines 

resulting from oil spills is provided in the Oil Spill Commission’s Staff Working Paper 14: “Unlawful Discharges of 

Oil: Legal Authorities for Civil and Criminal Enforcement and Damage Recovery.” 

15 In the Exxon Valdez case, of the $900 million recovered from Exxon in a civil settlement, roughly one-fourth 

($213.1 million) was used to reimburse the federal government and the State of Alaska for costs incurred in damage 

assessment and spill response. The remaining $686.9 million was spent in Alaska on efforts to restore resources that 

were directly harmed by the spill (e.g., sea birds, sea otters, whales and their habitat, etc.). If the State and federal 

government had brought suit solely under criminal fine or civil penalty authority, only a small portion of the funds 

recovered from Exxon could have been used to restore resources damaged by the spill.
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provide water filtration) and (3) the public’s lost use of those resources. Based on the damage assessment, 

the trustees either bring a lawsuit against the responsible party to recover the damages (which may be 

settled), or enter into a settlement with the responsible party without filing a lawsuit. 

When an oil spill occurs, the trustees must work through three phases to determine the 

appropriate type and amount of restoration required to compensate the public: 

1. Preliminary Assessment (also referred to as pre-assessment). In the aftermath of the release, the 

trustees collect time-sensitive data and observations and conduct research to determine if damage

to a particular resource has occurred or is likely to occur. Did damage likely occur? If so, the 

trustees move to the next phase.

2. Restoration Planning (including injury assessment). In this phase, the trustees conduct scientific 

and economic studies to quantify damages and use local knowledge and expertise to identify 

potential restoration projects. A draft restoration plan describing potential compensatory 

restoration projects and recommending preferred projects based on applicable regulatory criteria 

is made available to the public for review and comment. The National Environmental Policy Act 

requires that any potentially significant environmental impacts of the proposed restoration 

activities be considering during the process of reviewing the draft plan.16

3. Restoration Implementation. At this point, the restoration plan as proposed by the trustees and 

reviewed by the public is implemented and monitored to ensure its success. 

Though the logic of this progression is straightforward, its implementation is anything but. 

Identifying and quantifying damages, particularly where complex ecosystems are involved, presents 

enormous challenges. Developing sound sampling protocols that cover adequate time scales, teasing out 

other environmental disturbances, and scaling the damages to the appropriate restoration project often 

takes considerable time; in fact, a typical damage assessment can take years. Two sets of determinations –

one concerning the baseline conditions against which damages will be assessed and the second 

concerning the quantification of those damages – are particularly difficult and consequential in terms of 

the overall assessment results.

Determining Baseline Conditions

Natural resource damages must be measured against baseline conditions. The Oil Pollution Act

defines the baseline against which damages are to be measured as “. . . the condition of the natural 

resources and services that would have existed had the incident not occurred.”17 Making this 

determination, however, is inherently difficult and often highly contentious. Baseline conditions may be 

estimated, according to the OPA regulations, “using historical data, reference data, control data, or data 

on incremental changes (e.g., number of dead animals), alone or in combination, as appropriate.”18

 16 42 U.S.C. §§ 4321-4347. 

 

Without a well-established and agreed-upon definition of baseline conditions, there can be no agreement 

about a subsequent assessment of damages or quantification of required restoration. Given that the 

17 15 C.F.R. § 990.30 (emphasis added). 18 Ibid.
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ecological baseline can vary seasonally, annually, and over much longer time scales, it can be difficult for 

all parties to agree upon the exact condition of an ecosystem prior a spill. Since long-term historical data 

sets are often non-existent or discontinuous in many areas of the country, natural resource trustees are 

likely to be disadvantaged by a lack of sufficient data to fully characterize the condition and trends of 

relevant ecosystems prior to the incident in question.

Quantifying Damages

Once baseline conditions have been established, an effort is made to quantify damages. 

Notwithstanding its inherent difficulty, quantification is needed to determine the appropriate amount of 

restoration required to compensate for the natural resource damages that have been incurred. Scientists 

use various methods to measure a reduction in ecological resources and the services they provide, as well 

as, the public’s reduced enjoyment of the resources. These methods are highly dependent on the resource 

being assessed and on the proxies available for measuring the ecological function. For example, one study 

may use measured reductions in nutrient filtration to determine relative impact on a wetland while another 

may use decreased fish populations in future years to determine the relative impact the spill may have had 

on juvenile fish or larvae. To measure the public’s reduced enjoyment of a resource, say recreational 

boating, a survey might to conducted to determine if the spill forced boaters to change plans, travel 

further, or forego trips altogether.

Typically, there is a fluctuating ecological baseline at the time of the spill. Once the spill occurs 

there is some decrease in the function of the resources (e.g., decrease in nutrient filtration, decline in an 

animal population, or loss of boating opportunities). If the ecosystem is left to recover naturally, it may 

eventually return to baseline conditions.19

There are three types of restoration options: emergency restoration, primary restoration and 

compensatory restoration. 

However, restoration efforts must be implemented to 

compensate for the damages that occur (relative to baseline) during this period of natural recovery. 

Emergency Restoration. Emergency restoration is invoked when time is of the essence to curtail a 

threat or reverse the damage to a particularly sensitive resource. For instance, emergency 

restoration would include the immediate planting of seagrass before the planting season ends for 

the purpose of mending damage left by response vessels. Time would be of the essence in that 

circumstance because the damage may worsen while waiting for the next planting season. 

Primary Restoration. Primary restoration is intended to return a damaged natural resource to 

baseline after an oil spill. For instance, if a spill is killing marsh grasses along a wetland edge and 

exacerbating marsh erosion, the trustees may opt to plant new vegetation at the site of impact to 

curtail or reverse the inevitable land loss. 

Compensatory Restoration. Once emergency and/or primary restoration (if implemented) and 

cleanup activities are completed, the trustees then quantify the remaining, or interim, natural 

resource damage when compared to baseline. In the end, the amount of compensatory restoration 

required should equal the full amount of interim natural resource damages incurred. For example, 

 19 There are times when an ecosystem experiences a total loss in services and cannot recover naturally. In this case, 

the services are considered to be lost in perpetuity and compensatory restoration is calculated accordingly.
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if 5000 adults birds were killed due to the spill, the responsible party must not only implement 

restoration to replace 5000 adult birds (e.g., buying prairie pothole land in the Midwest to 

dedicate to nesting habitat), it must also provide for the “production foregone,” or the impact on 

future generations due to the loss of the adults (which would ultimately increase the number of 

animals required to be restored). With compensatory restoration, there is also a matter of 

compensating for the period of time for which the public has lost the resource. For simplicity, 

assume that 500 acres of marsh is lost in 2010 due to the spill. If restoration to restore the 500 

acres is not completed until 2015, then the responsible party must pay interest, in the form of 

additional marsh acres, on the five years that the public did not have those 500 acres. The 

traditional interest rate in natural resource damage assessment is three percent.

Applying the NRDA Process to the Deepwater Horizon Oil Spill

Initial Response and Organization of Damage Assessment Activities

When the Deepwater Horizon explosion occurred on April 20, 2010, NOAA’s Assessment and 

Restoration Division was already extremely busy conducting a number of oil spill and waste damage 

assessments and training activities. Ironically, the Division had just participated in a drill aimed at testing 

preparations for a “spill of national significance” (the drill took place in Portland, Maine in late March

2010). While such a spill had never been declared before, the Division was focused on evaluating and 

developing lessons learned from the drill, in coordination with its sister division, the NOAA Emergency 

Response Division, participating co-trustees, and the acting responsible party and sponsor for the drill, 

Shell Oil Company. 

The Assessment and Restoration Division was first notified of the Deepwater Horizon explosion 

via a hotline report generated by NOAA’s Chief Scientific Support Coordinator, Charlie Henry, in early 

morning hours of April 21, 2010. The initial report indicated that there were fatalities, there was oil on 

board the rig, and the rig was burning. At this point, the standard procedure for the Division is to stand by 

and initiate contact with other federal and potentially affected state and tribal trustees. By April 25, the 

Assessment and Restoration Division was on-scene in Houma, Louisiana and ready to begin collecting 

time-sensitive data that would help establish the toxicity of the oil and the baseline condition of 

potentially affected resources. By late April, the trustees were holding daily conference calls to provide 

situational updates and to construct and adapt future field sampling plans. By Saturday, May 1,

experienced environmental and contaminant scientists from Florida, Alabama, and Mississippi, along 

with various academic institutions, began collecting coastal baseline data in anticipation of the oil 

reaching their respective coastlines. In Houma, NOAA, the State of Louisiana, and the Department of the 

Interior began organizing technical working groups and collecting baseline data along the Louisiana 

coast. Texas, with an experienced damage assessment program in place, was also engaged and was 

monitoring the movement of oil from the spill. The delay between the day of the explosion and the day 

the oil finally reached the coastline allowed the trustees to organize and collect vital baseline data over a 

large portion of the Gulf coastline and intermediate waters. Figure 1 lists the trustee agencies currently 

involved in the Deepwater Horizon damage assessment.
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Figure 1: With two federal and 12 state agencies participating in the Deepwater Horizon NRDA process, 

coordinating schedules, reviewing documents, and communicating effectively across so many trustee 

agencies is complicated.

In the days following the Deepwater Horizon explosion, it became clear to NOAA’s Assessment 

and Restoration Division that this was a generational spill that would require the majority of Division 

staff and resources to conduct a comprehensive damage assessment. State agencies with ad hoc natural 

resource damage assessment programs scrambled to provide staff that would be dedicated to the spill 

long-term. Two week rotational staffing assignments were quickly put in place and emergency contracts 

for technical support staff were activated. Charlie Henry, NOAA’s Chief Scientific Support Coordinator 

for the spill warned that the response and impact assessment of this release would not be a sprint; it would 

be a marathon.

By early May, BP and the trustees agreed to work under the cooperative assessment framework to 

assess potential damages from the spill. Whether this process will work as intended under the OPA 

guidance remains uncertain at this time. In September, the trustees requested that BP fund several

emergency and/or primary restoration projects to curtail impacts to migratory birds, submerged aquatic 
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vegetation and wetlands. BP agreed to fund these projects and a migratory bird emergency restoration 

project has begun in Mississippi.

20

At present, damage assessment activities being carried out across the Gulf are being managed 

from a central “war room” at the Incident Command Center in Houma, Louisiana. Needless to say, the 

scale of the undertaking represents new challenges for everyone involved – not only in terms of the

geographic scale of the area being studied (both horizontally and vertically), but also in terms of the 

sophistication of the oceanographic equipment and the breadth of expertise being tapped to conduct the 

assessment. Scientists are observing oiled shorelines, tracking marine mammals, assessing fisheries 

impacts, and collecting water, oil, and sediment samples. BP and the trustees also have a handful of

dedicated research vessels collecting data throughout the water column at various intervals. 

 

Status and Early Results of the Damage Assessment Effort

As the language of the OPA regulations indicate, “baseline” for purposes of damage assessment 

in the NRDA context is generally considered to be the condition of the resource just prior to the spill. The 

precise application of this definition has particular importance in the Gulf of Mexico context, where many 

coastal habitats have been substantially degraded over decades – even centuries – under the pressure of 

ever-expanding industrial, commercial, and residential development. The NRDA regulations, as generally 

applied, require that BP restore Gulf resources to their functioning level as of April 19, 2010. However, 

the Gulf ecosystem in April 2010 was already weakened. Every year in the Gulf, for example, nutrient 

runoff from farms throughout the Mississippi River watershed creates a “dead zone” of extremely low 

oxygen levels in which few water organisms can survive. In some years, the area affected by this dead 

zone is as large as New Jersey. Throughout the region, erosion and destabilization of wetlands has been 

accelerated by the patchwork of canals carved out by the oil and gas industry. Cut off from natural 

deposits of sediment from the Mississippi River, delta wetlands have been unable to keep pace with rising 

sea levels and are sinking into the Gulf. These are only a few of the factors contributing to an imbalanced 

and already degraded Gulf ecosystem. In this context, effective long-term restoration will require the 

stabilization and eventual reversal of a number of long-standing, damaging trends.

As a first step in assessing damages from the Deepwater Horizon spill, state and federal trustees 

identified numerous categories of resources that might be at risk of adverse impacts, and began 

developing and carrying out preliminary assessment plans. Table 1 lists the specific resources being 

studied by the trustees and BP through their technical representative, Entrix, as part of the damage 

assessment process. 

 20 Email from Cynthia Dohner, U.S. Fish and Wildlife Service, January 20, 2011; “First project to restore waterfowl 

habitat since oil spill is underway,” WLBT-TV [Jackson, MS], January 21, 2011
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Table 1: The trustees are currently assessing potential damages to the following resource. If there is a 

determination that an injury occurred the magnitude of the injury will be quantified as part of the 

restoration planning phase. 

Resource Focus Studies Status

All Resources • Review historical information to help document pre-spill conditions. • Ongoing

Water Column

and Sediment

Water

Oil

Sediment

Document the amount of oil in the water, and determine how and where the oil is 

moving.

• Various types of Water quality surveys document the presence of oil at various 

depths.

• Transect surveys and sentinel stations detect submerged oil. 

• Plume modeling and other studies provide detail about the type of oil and how 

it moves in water.

• Sediment sampling documents the presence of oil across habitats.

• Ongoing

• Ongoing

• Ongoing

• Ongoing

Shorelines

Beaches

Wetlands

Mudflats

Mangroves

Document the extent and amount of oil on shoreline habitats.

• Aerial surveys provide a bird’s eye view of coastlines to determine the extent 

of oil; the resulting maps and data help target ground surveys.

• Ground surveys allow scientists to collect more detailed data on the 

degree of oiling (e.g. light vs. heavy) and focus future data collection efforts.

• Ongoing

• Ongoing

Aquatic 

Vegetation

Seagrasses

Sargassum

Document the presence/diversity of aquatic vegetation, and determine if it has been 

oiled.

• Aerial surveys help identify where and to what extent aquatic vegetation may 

be oiled.

• Ground surveys help identify location and extent of oiled aquatic vegetation.

• Ongoing

• Ongoing

Fisheries

Plankton

Fish larvae

Nearshore fish

Offshore fish

Document the presence/diversity of fish and plankton, and determine if they have 

been oiled.

• Plankton, invertebrate, fish, and fish larvae surveys help determine the 

presence and/or abundance of these resources in oiled and non-oiled open 

water areas. 

• Ongoing

Shellfish

Oysters

Mussels

Shrimp

Crabs

Document the presence/diversity of shellfish, and determine if they have been 

oiled.

• Oyster surveys document the presence and/or abundance of oysters in 

affected areas, and provide scientists with tissue for lab analysis.

• Mussel collections at monitoring stations help identify if mussels have been 

oiled, and if so, provide data for future studies.

• Shrimp collections help document the presence and abundance of shrimp in 

the open water and in oil plumes.

• Ongoing

• Ongoing

• Ongoing

Corals

Shallow water 

corals

Deep water 

corals

Document the presence/diversity of corals, and determine if they have been oiled.

• Shallow-water coral surveys and tissue collection help identify and evaluate 

exposure to existing communities.

• Deep-water coral surveys and tissue collection help identify and evaluate 

exposure to existing communities.

• Monitoring devices are installed in coral communities

to determine exposure to oil.

• Ongoing

• Ongoing

• Ongoing
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Resource Focus Studies Status

Marine Mammals

and Turtles

Whales

Manatees

Dolphins

Sea Turtles

Document the presence/diversity of marine mammals and turtles, and determine if 

they have been oiled.

• Aerial surveys document the location of marine mammals and turtles before 

they have been impacted by oil, and document the location and number of 

marine mammals and turtles that may be oiled, distressed, or dead; these 

surveys also document the potential changes in marine mammal behavior and 

distribution.

• Tissue sampling from live and dead sea turtles and marine mammals helps 

assess oil exposure.

• Acoustic technology and satellite tags help scientists

assess the behavior and movement of marine mammals.

• Ongoing

• Ongoing

• Ongoing

Birds

Shorebirds

Colonial 

seabirds

Pelagic seabirds

Secretive/marsh 

birds

Document the presence/diversity of birds, and determine if they have been oiled.

• Ground surveys identify injured, dead, or oiled birds on shorelines.

• Aerial and photograph surveys of open sea, shorelines, and islands help 

identify the location and abundance of birds, and determine if they and/or 

their habitats have been oiled.

• Ground and boat surveys in marshes document the abundance and degree of 

oil affecting marsh birds; radio transmitters provide for the assessment of bird 

movement and mortality.

• Point and transect boat surveys help scientists monitor pelagic birds.

• Ongoing

• Ongoing

• Ongoing

• Ongoing

Terrestrial Species

Terrapins

Crocodiles

Small Mammals

Document the presence/diversity of terrestrial species, and determine if they have 

been oiled.

• Ground surveys help identify and quantify oiled animals and/or habitats. • Ongoing

Human Use

Public beaches 

and parks

Public facilities

Cultural uses

Document the many ways humans recreationally use and enjoy the natural 

resources of the Gulf, if these uses or enjoyment have been impacted by the spill, 

and if so, to what extent.

• Overflight Surveys identify public beach use.

• Intercept Surveys identify public boat ramp use.

• Information Surveys to assess cultural uses.

• Ongoing

• Ongoing

• Ongoing

On September 29, the Trustees announced that they had found sufficient evidence of natural 

resource damage to file a Notice of Intent to Conduct Restoration Planning. This legal document signals a 

move from pre-assessment activities to “identify and document impacts to the Gulf’s natural resources, 

and the public’s loss of use and enjoyment of these resources, as the first stage under the regulations for 

developing a restoration strategy.”21

 21 Notice of Intent to Conduct Restoration Planning (pursuant to 15 C.F.R. Section 990.44) -DISCHARGE OF OIL 

FROM THE DEEPWATER HORIZON MOBILE OFFSHORE DRILLING UNIT AND THE SUBSEA 

MACONDO WELL INTO THE GULF OF MEXICO, APRIL 20, 2010,

http://www.darrp.noaa.gov/southeast/deepwater_horizon/pdf/Deepwater_Horizon_Final_NOI.pdf. 
11

As of mid-January 2011 over 28,000 water, sediment, tissue and tar ball samples have been 

collected and over 52,000 analyses completed.22 These samples were collected during 89 off-shore 

research cruises and along approximately 4,215 linear miles of coastal shoreline.23

As of November 1, 2010 wildlife responders had collected 8,183 birds, 1,144 sea turtles and 109

marine mammals, alive and dead; oiled and unoiled. These numbers may have increased since then, but 

presumably, because the oil stopped flowing several months ago, the numbers will soon plateau if they 

have not already done so. Given that collection efforts are bound to miss some number of affected 

animals, many of which will never be found because of the effects of hiding, scavenging, sinking, 

decomposition, or the sheer size of the search area, the trustees will have to make assumptions to quantify 

impacts on wildlife. A common practice is to assign a multiplier to the final observed number of affected

animals. The multiplier will vary depending on the species, its behavior, and its habitat. The multiplier is 

then used to estimate the total number of animals impacted.

Results of the assessment effort to date indicate that more than 650 miles of Gulf coastal 

habitats—salt marsh, mudflat, mangroves, and sand beaches—were oiled; more than 130 miles 

have been designated as moderately to heavily oiled. Oiled birds and beaches are often the most 

visually disturbing and widely disseminated images associated with a major oil spill, however, public and 

scientific concern in the Deepwater Horizon case has for some time focused on the impacts of an invisible 

sub-surface “plume” or “cloud” of oil. As part of the response and damage assessment effort being 

coordinated by BP and the trustees, 23 research vessels have been working to collect thousands of data 

points over 5000 feet of water column to assess potential impacts on subsurface biota, both from the oil 

and from the use of dispersants. 

While the biological impacts are not fully yet understood, the National Incident Command’s Joint 

Analysis Group, an inter-agency workgroup that was set up to analyze sub-surface data collected by 

scientists from federal, private, and academic institutions, released a report that described the chemical 

behavior of the subsurface oil.24

The Contribution from Non-Governmental Scientists

 The report summarizes 419 data points collected from 9 different vessels 

between May 8, 2010 and August 9, 2010. According to these data, depressed oxygen levels have been 

detected more than 80 km from the wellhead. The report concludes that while oxygen levels are depressed 

in the subsurface plume as a result of biodegradation (referred to in the report as biochemical oxygen 

demand), oxygen levels that would be detrimental to water column organisms have not been found and 

are not expected.

Three peer-reviewed studies have been published to date in Science related to the behavior of oil 

from the Deepwater Horizon spill in the deep sea environment. Another publication focuses on the 

potential toxicity of oil in the deep sea. Camilli et al. discovered, tracked and sampled a deepwater plume 

of dispersed hydrocarbons measuring at least 35 km long by 2 km wide and 200 m high at a depth of 

 22 Once validated, the data are being made public at www.geoplatform.gov and www.data.gov. 23 NOAA, Deepwater BP Oil Spill, Natural Resource Damage Assessment: NRDA By the Numbers, January 2011, 

http://www.gulfspillrestoration.noaa.gov/wp-content/uploads/2011/01/Final-NRDA-by-the-Numbers-Jan-20111.pdf 24 National Incident Command Joint Analysis Group, Review of Preliminary Data to Examine Oxygen Levels In the 

Vicinity of MC252#1: May 8 to August 9, 2010 (August 16, 2010).
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about 1100 m below the ocean surface from June 19 to June 28, 2010.25 Interested, in part, as to whether 

microbial degradation of the plume would result in lethal depletion of oxygen in the water column26 – or a 

“hypoxic zone” – the research team found no significant drawdown of oxygen inside the plume. 

However, they do note that a hypoxic zone could ultimately develop over a period of time, as the 

microbes continue to degrade the oil therein: “[I]f the hydrocarbons are indeed susceptible to 

biodegradation, then it may require many months before microbes significantly attenuate the hydrocarbon 

plume to the point that oxygen minimum zones develop that are intense enough to threaten Gulf 

fisheries.”27

Hazen et al. measured physical, chemical and microbiological properties of water samples taken 

from the same research area as Camilli, et al. from May 25 to June 2, 2010.28

Valentine et al. investigated dissolved hydrocarbon gases (methane, ethane, and propane) in the 

Gulf of Mexico water column from June 11 to 21, 2010.

They report similar findings 

of only slight oxygen drawdown, and contend that the rate of biodegradation inside the plume is much 

faster than reported by Camilli et al. 

29

As of mid-January, one scientific study on the potential toxicity of this deep sea plume has been 

published.

 This study again confirms the presence of the 

southwest plume at an average depth of 1100 m and identifies additional plumes, defined by elevated 

levels of methane, to the north and east of the well head, which probably were formed earlier when 

currents flowed in a different direction. The study suggests that the microbes in the plume have a 

preference for the lighter petroleum constituents (ethane and propane). They conclude, therefore, that the 

aging plume, once devoid of the lighter constituents, have bacterial populations that are primed for 

degradation of other hydrocarbons, but at a slower rate. 

30

Taken together, these studies show the presence of deepwater plumes of highly dispersed oil 

droplets and dissolved gases between at 1000 and 1300 meters deep. Bacterial decomposition begins 

quickly for the light hydrocarbon gases propane and ethane but more slowly for the heavier hydrocarbons 

typically present in a liquid form and for the predominant gas, methane. The degradation rates are 

sufficient to reduce the dissolved oxygen concentrations, but not to harmfully low levels associated with 

“dead zones.” Subsequent dilution with well-oxygenated, uncontaminated waters is sufficient to prevent 

any further drawn down of dissolved oxygen in the aging plumes. While oxygen levels do not appear to 

 Water samples collected from the discovered plume (discussed above) in mid-May 

contained levels of polycyclic aromatic hydrocarbon (PAH) that are high enough to be acutely toxic for 

some marine organisms. The PAH levels decreased with distance from the wellhead and persisted as far 

as 13 km from the source. 

 25 Richard Camilli et al., “Tracking Hydrocarbon Plume Transport and Biodegradation at Deepwater Horizon,” 

Science 330, no. 6001 (2010): 201–204. 26 Microbes give off carbon dioxide as part of the biodegradation process which, in turn, depletes oxygen in the 

water column.

27 Ibid. 28 Terry Hazen et al., “Deep-Sea Oil Plume Enriches Indigenous Oil-Degrading Bacteria”, Science 330, no. 6001 

(2010): 208–211.

29 David Valentine et al., “Propane Respiration Jump-Starts Microbial Response to a Deep Oil Spill,” Science 330, 

no. 6001 (2010): 204–208. 30 Arne-R. Diercks et al., “Characterization of Subsurface Polycyclic Aromatic Hydrocarbons at the Deepwater 

Horizon Site,” Geophysical Research Letters 37 (2010).
13

be lethal and the plume will dissipate with time, the PAH levels that accumulated in the deep sea plume 

likely had adverse and potentially acute effects on marine organisms that need to be further assessed and 

quantified. 

A Historical Comparison

Human nature cannot resist the temptation for disaster comparison. Hence, the question is often 

asked, how does this spill compare to a previous oil spill volume record holder, the Exxon Valdez oil 

spill? At this point, because data are still being collected on water column and fisheries impacts, it is too 

soon to tell whether the immediate effects of the Deepwater Horizon oil spill on coastal areas and wildlife 

will turn out to be smaller in scale than those associated with the Exxon Valdez oil spill. Based on current 

information on marine life fatalities, that remains a possibility. In the aftermath of the Exxon Valdez, for 

example, more than 35,000 dead birds and 1,000 dead sea otters were recovered. Additionally, that spill 

oiled 1500 miles of Alaska coastline, of which 350 miles were heavily oiled. Those Exxon Valdez 

numbers are higher than currently known numbers for the Deepwater Horizon spill. At the end of their 

assessment, the Exxon Valdez Trustee Council estimated the final wildlife death toll to be “250,000 

seabirds, 2,800 sea otters, 300 harbor seals, 250 bald eagles, up to 22 killer whales, and billions of salmon 

and herring eggs.”31

Next Steps in the Damage Assessment and Restoration Process

 Because, however, the Deepwater Horizon spill has not been fully assessed and was 

of a very different character, most notably occurring in the subsea, a focus on readily discernible surface 

expressions of harm measured by known marine life impacts may not ultimately prove to be a fair basis 

for comparison.

The data collected as part of the damage assessment process will at some point be evaluated by 

resource specialists for both the trustees and BP. Given that there is no way to exactly quantify the extent 

of shoreline oiling or the number of birds or other wildlife impacted, the final damage assessment will 

inevitably consist of estimates developed on the basis of careful examination of the field data (including 

on-going studies), comparisons to existing baseline data, reviews of the relevant literature, and much 

debate among the parties involved. Best professional judgment will be needed where data gaps or 

uncertainty exist. 

When the trustees reach a conclusion as to the extent and nature of the damages that occurred and 

the appropriate amount of restoration required to compensate for the damages, then the matter may 

proceed to litigation, and be resolved by either court order or settlement. Or, in the spirit of cooperative 

assessment, the parties may reach a settlement without any litigation. Once a settlement is reached, 

depending on the terms of the settlement, the responsible party may have two choices. It can opt to 

implement the required amount of restoration with trustee oversight, or it can pay the trustees to 

implement the required restoration. Either way, the terms of the agreement are memorialized through a 

consent decree which must be approved by the Department of Justice. 

 31 Exxon Valdez Oil Spill Trustee Council, Exxon Valdez Oil Spill Restoration Plan: Update on Injured Resources 

and Services, September 1996.
14

Finally, with numerous studies ongoing, both under the auspices of the formal damage 

assessment process and outside it, the published literature regarding environmental impacts from the 

Deepwater Horizon spill can be expected to grow substantially in the months and years ahead.

Sources of additional information on the NRDA process:

• NOAA’s Damage Assessment Remediation and Restoration Program: 

http://www.gulfspillrestoration.noaa.gov/

• NOAA’s Office of Response and Restoration: www.deepwaterhorizon.noaa.gov

• US Fish and Wildlife Service: http://www.fws.gov/home/dhoilspill/restoration.html

• Online tool that provides you with near-real time information about the response effort. 

Developed by NOAA with the Environmental Protection Agency, U.S. Coast Guard, and the 

Department of the Interior, the site offers you a “one-stop shop” for spill response information: 

www.geoplatform.gov

• Data from the Department of Energy, the Environmental Protection Agency, the National 

Oceanic and Atmospheric Administration, the Department of the Interior, and the states of 

Florida and Louisiana related to the spill, its effects, and the cleanup effort: 

www.data.gov/restorethegulf