OCS Study 

MMS 2005-044 

 

Relative Contribution of Produced Water 

Discharge in the Development of Hypoxia 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 2005 – 0044 

Gulf of Mexico OCS Region 

Relative Contribution of Produced Water Discharge in the 

Development of Hypoxia 

Author 

Nancy N. Rabalais 

Prepared under MMS contract 

No. 0104P036634 

by 

Nancy N. Rabalais 

Baton Rouge, LA 70810 

Published by 

U.S. Department of the Interior 

Minerals Management Service New Orleans 

Gulf of Mexico OCS Region July 2005 
DISCLAIMER 

This report was prepared under contract between the Minerals Management Service and 

Nancy N. Rabalais. This report has been technically reviewed by the Minerals Management 

Service (MMS), and it has been approved for publication. Approval does not signify that the 

contents necessarily reflect the views and policies of MMS, nor does mention of trade names or 

commercial products constitute endorsement or recommendation for use. It is, however, exempt 

from review and compliance with MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office (Mail Stop 5034) 

at the following address: 

 U.S. Department of the Interior 

 Minerals Management Service 

 Gulf of Mexico OCS Region 

 Public Information Office (MS 5034) 

 1201 Elmwood Park Boulevard 

 New Orleans, Louisiana 70123-2394 

 Telephone: (504) 736-2619 or 

 1-800-200-GULF 

CITATION 

Suggested citation: 

Rabalais, N. N. 2005. Relative Contribution of Produced Water Discharge in the Development 

of Hypoxia. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, LA. OCS Study MMS 2005-044. 56 pp. 

ACKNOWLEDGMENT 

Thanks are extended to Adam Sapp for help with graphics and compilation of data. 

ABOUT THE COVER 

The inner continental shelf of central Louisiana with OCS blocks, total produced water volume 

for 2001, and contours of frequency of mid-summer, bottom-water hypoxia for the period 1985-

2002. Contours from station C6 to the southeast represent the presence of hypoxia at greater 

than 75 percent, greater than 50 percent, greater than 25 percent, and less than 25 percent of the 

sampling events. 
EXECUTIVE SUMMARY 

The northern Gulf of Mexico is the site of the world’s second largest zone of anthropogenic coastal 

hypoxia (water with dissolved oxygen less than 2 mg l-1, ppm). It is also an area of extensive oil and gas

exploration and production activity. The distribution, severity and temporal variability in hypoxia and its

historic worsening is most closely related to the flux of nitrate-nitrogen from the Mississippi River system 

and its increase over the last half of the 20th century. The discharge of the Mississippi River, which 

directly influences the formation and dynamics of hypoxia, has made it difficult to isolate the influences 

of the river from those related to offshore production activities. Examination of these relationships has 

mostly focused on river-coastal ecosystem interactions and their influence on benthic communities that 

are also exposed to contaminants from oil and gas production activities. More recently, questions have

been raised regarding the relative contribution of riverine organic carbon and nitrogen loads to those from 

produced water discharge in the formation of continental shelf hypoxia. 

Management options are being considered for implementation within the Mississippi River watershed to

reduce the nutrient inputs to the Gulf of Mexico in order to reduce the size, severity and duration of 

hypoxia. If produced water discharges were to contribute substantially, either directly or indirectly, to the 

respiratory consumption of dissolved oxygen, then a case could be made under the U.S. Environmental

Protection Agency ocean discharge criteria to impose effluent limitations on offshore produced water

discharges to prevent unreasonable degradation of the marine environment. 

The objective of this analysis is to evaluate the relative contribution of produced water organic and 

nutrient compounds in relation to those of the Mississippi and Atchafalaya discharges in the context of 

hypoxia formation on the continental shelf. Relevant information includes: 

(1) geographic extent, severity and duration of the hypoxic zone; 

(2) causes of hypoxia; 

(3) produced water constituents, volumes, and geographic location; 

(4) relative inputs of organic matter and nitrogen from produced water and river

discharge; 

(5) oxygen utilization as a result of chemical constituents of produced water; and 

(6) potential for nutrient-enhanced production related to produced water nutrients. 

Produced water generated during the production of oil and gas is a mixture of hypersaline water with

volatile, water soluble and more recalcitrant hydrocarbons, reduced metals, radionuclides, occasional pipe 

scale particulates, and sulfur and nitrogen compounds. The organic content of produced waters has the 

potential to be degraded by aerobic microbes, the activity of which may reduce dissolved oxygen in the

ambient water. The nitrogen, mostly in the form of ammonium, has the potential to stimulate 

phytoplankton production, some of which may flux to the lower water column to be decomposed and 

contribute to the respiratory demand in the bottom waters. 

The relative amounts of organic carbon and ammonium in produced water discharges were compared to

those delivered by the Mississippi and Atchafalaya Rivers to the adjacent continental shelf. Estimates

were made both across the large area where hypoxia occurs and in the area of South Timbalier Blocks 52 

and 53 where more detailed data on oxygen and carbon dynamics are available for composite station C6*

in 20-m water depth in the core of the hypoxic zone. The inputs from the Mississippi River are well 

characterized and modeled as to their effects on the continental shelf, including the formation of hypoxia 

on fine temporal scales at composite station C6* and over a broad spatial scale from the Mississippi River 

to the upper Texas coast. 

v 

This project required the comparison of a minimal amount of produced water constituent load data to

more thoroughly characterized Mississippi River data. The relationships between multiple forms of 
organic carbon measurement to each other and to oil and grease measurements have not been established.

Additionally, the concentration of nitrogen compounds in produced water from differing locations or

hydrocarbon extraction activities is not known. Therefore, numerous assumptions were made and the first

order estimations presented lean towards worst-case scenarios for both the produced water carbon and 

nitrogen inputs. However, the expected dilutions and dispersions correspond to available models. 

The oil and grease loadings do not track well with total organic carbon content of produced water nor do 

they distinguish among the hydrocarbon compounds that vary with regard to volatility, solubility, photooxidation, likelihood of adhering to sedimenting particles and susceptibility to microbial degradation. All

hydrocarbons included in the oil and grease content and loadings were included in calculations of organic

carbon that would reach the lower water column and sediments and contribute to respiratory reduction of

dissolved oxygen. This, of course, is not the case. Estimates of produced water ammonium were

compared to river ammonium, nitrate and nitrite (dissolved inorganic nitrogen) and total nitrogen that 

includes both dissolved and particulate organic nitrogen, because all can stimulate phytoplankton 

production. The role of dissolved inorganic nitrogen in the dynamics of hypoxia is better understood than 

the organic fraction of total nitrogen. 

The first order estimations of the contribution of carbon and nitrogen in produced water discharges are 

minimal compared to those of the river. The produced water ammonium discharged within the area

subject to hypoxia is estimated to be 0.013 percent of the total nitrogen delivered by the Mississippi River 

system, 0.008 percent of the total dissolved inorganic nitrogen, and 0.002 percent of the total ammonium. 

Up to 4 percent of the organic carbon delivered to the Gulf of Mexico, which includes terrestrial material 

along with oil and grease from nonpoint source runoff, is estimated to contribute to respiratory demand in 

water below the pycnocline in the area of hypoxia. The produced water oil and grease load for the Gulf

of Mexico is 10.9 percent of the oil and grease load from the Mississippi River that might contribute to 

hypoxia. Ideally, there would be TOC, DOC, and POC data for direct comparison of produced water and 

riverine discharges. With the data currently available it would be inappropriate to compare produced

water oil and grease loads to riverine TOC data. However, considering that the Gulf produced water oil 

and grease load is a relatively small percent of the Mississippi River load that might contribute to 

hypoxia, it is reasonable to expect that this small percentage would also be overwhelmed by the nutrientenhanced surface in situ production offshore. The in situ production is estimated to be 96 percent of the 

organic matter loading that drives the oxygen and carbon budgets of the continental shelf where hypoxia 

occurs, compared with the up to 4 percent riverine organic loading that might contribute. The produced 

water contribution of organic carbon in the total loading of organic carbon within the hypoxic area 

appears to be insignificant given the present body of information. 

vi 
TABLE OF CONTENTS 

Page 

FIGURES.....................................................................................................................................................ix

TABLES ......................................................................................................................................................xi

ABBREVIATIONS, ACRONYMS, AND SYMBOLS ............................................................................xiii

BACKGROUND ..........................................................................................................................................1

OBJECTIVES...............................................................................................................................................2

HYPOXIA IN THE NORTHWESTERN GULF OF MEXICO...................................................................2

Distribution and Dynamics.....................................................................................................................2

Causes of Hypoxia..................................................................................................................................6

Historical Changes .................................................................................................................................9

Other Factors........................................................................................................................................10

HYDROCARBONS IN THE GULF OF MEXICO ...................................................................................12

History of the Offshore Petroleum Industry in the Gulf of Mexico .....................................................12

Hydrocarbon Inputs to the Gulf of Mexico ..........................................................................................12

Produced Water Constituents...............................................................................................................14

Produced Water Fate ............................................................................................................................16

Produced Water Discharges – Volume and Geographic Distribution..................................................20

INTERACTION OF HYPOXIA AND PRODUCED WATERS ...............................................................23

Louisiana Shelf.....................................................................................................................................23

Gulf of Mexico Offshore Operations Monitoring Experiment (GOOMEX)........................................24

Oxygen and Carbon Budgets................................................................................................................25

Relative Contribution of Inputs of Produced Water Versus Those of the Mississippi River 

(direct and indirect) in the Dynamics of Hypoxia.....................................................................26

Nitrogen ....................................................................................................................................26

Hydrocarbons............................................................................................................................29

IMPLICATIONS ........................................................................................................................................30

SUMMARY................................................................................................................................................31

LITERATURE CITED ...............................................................................................................................32

APPENDIX A COMPARATIVE CTD PROFILES.........................................................................A-1

APPENDIX B AVERAGE PRODUCED WATER VOLUME IN A 5-, 10-, AND 20-MILE 

RADIUS OF COMPOSITE STATION C6* (RADIUS MEASURED FROM 

C6C LOCATION) (BBL MO-1)...............................................................................B-1

vii 
FIGURES 

Page 

Figure 1. Estimated size of the bottom area encompassed by dissolved oxygen values less than 2 

mg l-1 during a 5- to 6-day mapping cruise in mid-summer. ...................................................3

Figure 2. Frequency of occurrence of mid-summer, bottom-water hypoxia in the Gulf of Mexico 

from 1985-2002.......................................................................................................................3

Figure 3. Seasonal changes in average surface water salinity (psu) and average bottom water 

dissolved oxygen (mg l-1) along transect C by month January through December 1985-

2002.........................................................................................................................................4

Figure 4. Seasonal changes in average surface water dissolved inorganic nitrogen (NO3

-

+ NO2

-

 + 

NH4

+

 in μM) and average surface water chlorophyll biomass (μg l-1) along transect C by 

month January through December 1985-2002........................................................................5

Figure 5. Bottom water dissolved oxygen concentration (mg l-1) within 1 m of the bottom in 20 m

depth at station C6B, from April through November 1993.....................................................6

Figure 6. Contoured daily samples of low-pass filtered currents (cm s-1) after rotation into

alongshore and cross-shore directions. ...................................................................................7

Figure 7. First and second empirical orthogonal functions from the acoustic Doppler current 

profiler (ADCP) data...............................................................................................................7

Figure 8. Examples of pycnoclines and oxyclines at station C6B..........................................................8

Figure 9. Potential contribution of labile allochthonous carbon to the continental shelf from the 

Mississippi River ..................................................................................................................11

Figure 10. Conceptual model of transport in stratified receiving water.................................................18

Figure 11. Model prediction of plume behavior for strong stratification, based on data for dissolved 

oxygen, salinity, and temperature reported in Rabalais et al. (1993)....................................18

Figure 12. Histogram of fraction of water depth between discharge pipe and sea bottom for 

produced water discharges in the hypoxic zone. .................................................................19

Figure 13. Distribution of produced water discharge volumes by OCS lease block for a portion of 

the Central and Western OCS superimposed on the frequency of mid-summer, bottomwater hypoxia........................................................................................................................20

Figure 14. Number of oil completions and oil and water from oil completions within the area of 

hypoxia (Figure 13) for the period 1996-2003......................................................................21

Figure 15. Time course of produced water volumes for all platforms within a 5-, 10-, and 20-mi

radius of composite station C6* and size of the bottom water hypoxic area based on a 5-

day mid-summer mapping cruise ..........................................................................................22

Figure 16. Dissolved oxygen (mg l-1) and salinity (psu) profiles for stations C6, C6B, and C6C for 

dates indicated.......................................................................................................................24

ix 
Figure 17. Coupled physical-biological model of oxygen and carbon dynamics at representative 

composite station C6* in the core of the hypoxic zone.........................................................26

Figure 18. Produced water discharge volume in April (during high Mississippi River discharge) 

and October (during low Mississippi River discharge) by year for all discharging 

platforms in 5-mi, 10-mi, and 20-mi radius circles around composite station C6*..............28

Figure 19. Bottom water concentration of NH4

+

 and NO3

-

 (μM) at station C6* compared to bottom

water dissolved oxygen (mg l-1) for period 1985-2002.........................................................29

x 
TABLES 

Page 

Table 1 Estimates of Oil Discharges to the Marine Environment from Produced Water 

Discharges.............................................................................................................................13

Table 2 Estimates of Land-Based Contributions of Hydrocarbons and Polycyclic Aromatic 

Hydrocarbons (PAH) to Marine Waters in the Northwestern Gulf of Mexico .....................14

Table 3 Maximal Concentrations of Representative Compounds in OCS Produced Waters from

Louisiana...............................................................................................................................16

Table 4 Effect of Biodegradation on Selected Organics in Produced Water Collected from North 

Sea Fields..............................................................................................................................17

Table 5 Comparative Values of Produced Water Constituents in Sediments within 100 or 250 m 

at Production Platforms in June 1990 ...................................................................................23

Table 6 Mean Annual Flux of Nutrients from the Mississippi-Atchafalaya River Basin to the 

Gulf of Mexico, 1980-1996 ..................................................................................................27

xi 
ABBREVIATIONS, ACRONYMS, AND SYMBOLS 

NH4

+ ADCP acoustic Doppler current profiler ammonium

NO3

- AEC acid-extractable compounds nitrate nitrogen 

NO2

- CTD conductivity, temperature, and depth nitrite nitrogen 

C6* combination of stations in South 

Timbalier Blocks 52 and 53

NPDES National Pollutant Discharge 

Elimination System 

bbl barrel (1 bbl = 42 gallon) NRC National Research Council 

B/N base/neutral fraction O2 oxygen 

BOD biochemical oxygen demand OC organic carbon 

BOD5 biochemical oxygen demand (5 day) OCS Outer Continental Shelf 

BOD21 biochemical oxygen demand (21 day) OOC Offshore Operators Committee 

BSi biologically bound silica POC particulate organic carbon 

BTEX benzene, toluene, ethyl benzene, xylene P phosphorus 

CENR Committee on Environment and Natural 

Resources 

PAH polycyclic aromatic hydrocarbon 

ppm parts per million 

DOC dissolved organic carbon ppt parts per thousand 

DOI Department of the Interior psu practical salinity units 

GOOMEX Gulf of Mexico Offshore 

Operations Monitoring Experiment Ra radium

SBF synthetic-based fluid HC hydrocarbon 

Si silicon 

μM micromoles

ST South Timbalier 

mg milligram

TKN total Kjeldahl nitrogen (TKN = organic 

nitrogen + ammonium nitrogen) MMS Minerals Management Service 

N nitrogen TOC total organic carbon 

organic nitrogen (organic nitrogen = TKN - 

NH4

+

) USEPA United States Environmental 

Protection Agency

inorganic nitrogen (inorganic nitrogen = NO3

- 

+ NO2

- + NH4+

) VFA volatile fraction (all) 

WD West Delta 

xiii 
BACKGROUND 

The northern Gulf of Mexico is the site of the world’s second largest zone of anthropogenic coastal 

hypoxia (water with dissolved oxygen less than 2 mg l-1, ppm). The northern Gulf of Mexico is also an 

area of extensive oil and gas exploration and production activity. The United States Environmental 

Protection Agency (USEPA) is tasked with protecting the nation’s water quality while the Department of

the Interior, Minerals Management Service (DOI, MMS) is tasked with managing the development of 

hydrocarbon resources while protecting the environment. In this overlapping area of concern on the Gulf 

of Mexico Outer Continental Shelf (OCS) is the large area of hypoxia off the Louisiana and Texas coasts

that occurs each spring through summer. Many studies of the cause and effects of the hypoxic zone have 

been published. Similarly, many studies have documented the impacts of OCS produced water discharges 

on sediment, water, and biota. The discharge of the Mississippi River, which directly influences the

formation and dynamics of hypoxia, has made it difficult to isolate the influences of the river from those 

related to offshore production activities (Bedinger 1981; Rabalais et al. 1993; Kennicutt 1995). 

Produced water is a waste product generated during the production of oil and gas. It is also called

formation water or brine. It is a mixture of hypersaline water with varying volatile, water soluble and 

more recalcitrant hydrocarbons, reduced metals, radionuclides, occasional pipe scale particulates, and 

sulfur and nitrogen compounds, as well as residual chemicals added to assist in production or to treat the

produced water prior to discharge. 

The USEPA Best Available Treatment Technology Economically Achievable for the National Pollutant

Discharge Elimination System (NPDES) permit restricts the concentration of oil and grease in produced

water destined for ocean disposal to a monthly average of 29 mg l-1 (USEPA 1993). The produced water 

also must meet an acceptable toxicity level. The majority of research efforts on produced waters have

focused on the directly toxic or persistent, bioaccumulative constituents in produced water, such as the

volatile hydrocarbons, polycyclic aromatic hydrocarbons, and heavy metals. 

A toxic endpoint resulting from oxygen depletion has not been a part of prior MMS studies or USEPA 

NPDES regulations. The potential for produced water constituents to contribute to the biochemical

oxygen demand (BOD) in offshore waters has not been studied. The BOD of produced waters may be a 

feature of concern where seasonal hypoxic waters form yearly on the northwestern Gulf of Mexico 

continental shelf. The nitrogen in produced water, primarily ammonium, may also be a concern in as far

as it contributes to enhanced primary production in surface waters or BOD in bottom waters. There are 

no known analyses of phosphorus (either total or speciated) in produced water, published or otherwise (J.

P. Smith, ExxonMobil Upstream Research Company, pers. comm.). 

The upper limit for a consistent definition of hypoxic waters in the northern Gulf of Mexico is 2 mg l-1

(Rabalais et al. 2002a). Below this level, the catch of demersal fish and crustaceans in bottom trawls is 

minimal or zero (Renaud 1986; Pavela et al. 1983). Some fauna, such as sharks, may be affected at levels

of low oxygen at or below 3 mg l-1, whereas some benthic infauna may be resistant to extremely low 

levels of dissolved oxygen, approximately 0.5 mg l-1 (Rabalais et al. 2001). The loss of essential fish and 

crustacean habitat for extended periods in spring and summer, disruption of demersal organism migratory

patterns, loss of infaunal prey organisms, increase in susceptibility to predation, decrease in reproductive 

and growth indicators, and direct mortality are concerns for managers and fishers of the abundant living 

marine resources in the Gulf of Mexico. 

The USEPA published (October 2004) the Notice of Final NPDES General Permit for New and Existing 

Sources and New Discharges in the Offshore Subcategory of the Oil and Gas Extraction Category for the

Western Portion of the Outer Continental Shelf of the Gulf of Mexico (GMG290000). The Ocean

Discharge Criteria prevent unreasonable degradation of the marine environment and authorize the

imposition of effluent limitations, including prohibition of discharge, if necessary to ensure this goal. 

Unreasonable degradation is defined under the Clean Water Act Section 403(c) as “(1) significant adverse 

changes in the ecosystem diversity, productivity, and stability of the biological community within the area 

of discharge and surrounding biological communities, (2) threats to human health through direct exposure

1 
to pollutants or consumption of exposed aquatic organisms, or (3) loss of esthetic, recreational, scientific,

or economic values which are unreasonable in relation to the benefits derived from the discharge.” For 

produced water, the permit requires that the operator must meet technology standards so that the best 

available treatment technology is used to reduce oil and grease and toxicity before discharge. Although 

MMS and USEPA have funded studies, which assist them in their respective responsibilities, and the 

National Oceanic and Atmospheric Administration through the Coastal Ocean Program has funded

competitive research programs on the influence of the Mississippi River on the adjacent continental shelf, 

including the dynamics of hypoxia, none have researched the contribution of produced water discharge, if 

any, to the hypoxic zone. Relevant information exists in many studies to be summarized and synthesized 

to address this question. 

OBJECTIVES 

The objective of this analysis is to extract and utilize data from existing studies in order to describe the 

relationship of produced water discharges to the hypoxic zone. Relevant information includes: 

(1) geographic extent, severity and duration of the hypoxic zone; 

(2) causes of hypoxia; 

(3) produced water constituents, volumes, and geographic location; 

(4) relative inputs of organic matter and nitrogen from produced water and river

discharge; 

(5) oxygen utilization as a result of chemical constituents of produced water; and 

(6) potential for nutrient-enhanced production related to produced water nutrients. 

Rather than collect new data, this report summarizes published information, unpublished data relevant to

the question at hand, and existing summaries of industry or MMS data. These data allow insight into (1) 

the dynamics of hypoxia, (2) the characterization of produced waters and their discharge into the 

northwestern Gulf of Mexico, and (3) the relative contribution of oxygen demanding materials and 

nutrients from produced waters compared with those of the Mississippi River and in situ marine 

phytoplankton production.

A summary of the knowledge of hypoxia in the Gulf of Mexico was presented in the findings of the

Committee on Environment and Natural Resources (CENR) Integrated Assessment of Hypoxia in the 

Northern Gulf of Mexico (CENR 2000) and the six technical reports on which it was based (Rabalais et

al. 1999; Diaz and Solow 1999; Goolsby et al. 1999; Brezonik et al. 1999; Mitsch et al. 1999; Doering et

al. 1999). Portions of these reports and additional papers have since been published that complement or

refine these syntheses. 

The available produced water data were used to assess relationships among variables such as change in 

produced water through time versus change in hypoxic area through time. First-order calculations were 

made with regard to the amount of nitrogen and organic carbon in produced water discharges compared to

similar discharges from the Mississippi River, the Mississippi River nutrient-enhanced in situ production 

of carbon on the shelf, and ambient nutrient conditions offshore during the period of hypoxia formation. 

HYPOXIA IN THE NORTHWESTERN GULF OF MEXICO 

DISTRIBUTION AND DYNAMICS

A zone of hypoxia forms each spring and summer on the Louisiana-Texas shelf beneath a well-defined 

density stratification. The hypoxic zone increased from an average size of 8,300 km2

 in 1985-1992 to

over 16,000 km2

 in 1993-1997 (Rabalais et al. 1999, 2002a), and reached a record 22,000 km2 in 2002 

(Figure 1). The size of the hypoxic zone is directly correlated with the flux of nitrogen from the 

Mississippi River and river discharge (Scavia et al. 2003; Turner et al. 2005). The bottom area size

2 
relationship is strongest when the integrated nitrate-N flux over 75 days prior to the mapping cruise is the 

independent variable (R2

= 0.61, P < 0.001, n = 16); Justić et al. unpubl. data). The oceanographic 

conditions and weather prior to and during the time of the mid-summer cruise also dictate the

configuration. For example, the predicted size of the hypoxic area in 2003 was 20,000 km2

, but the

measured size was less than half that, because a series of tropical storms and hurricanes in the two weeks

preceding the mapping mixed and re-aerated the water column. 

0

5000

10000

15000

20000

19851986198719881989199019911992199319941995199619971998199920002001200220032004

Area (km2)

nd

0

5000

10000

15000

20000

19851986198719881989199019911992199319941995199619971998199920002001200220032004

Area (km2)

nd

Action

Plan

Goal

Long

Term

Average

5-year

Average

Action

Plan

Goal

Long

Term

Average

5-year

Average

Figure 1. Estimated size of the bottom area encompassed by dissolved

oxygen values less than 2 mg l-1 during a 5- to 6-day mapping

cruise in mid-summer (nd=no data) (modified from Rabalais et al. 

2002a). The Action Plan Goal of 5,000 km2

 running average by

2015 is denoted along with the 1985-2004 average size and the 5-yr 

running average.

Hypoxic waters are distributed from shallow depths near shore (4 to 5 m) and reach to 60 m water depth. 

They more typically occur between 5 and 35 m. They can impinge onto the barrier islands when 

upwelling-favorable conditions drive hypoxic waters towards shore. Hypoxia occurs mostly in the lower

water column but encompasses as much as the lower half to two-thirds of the water column. The

summertime frequency distribution of bottom water hypoxia is shown in Figure 2, and qualitatively 

depicts the relative, integrated down-current effects of Mississippi and Atchafalaya discharges.

-93.5 -92.5 -91.5 -90.5 -89.5

28.5

29.0

29.5

30.0

50 km

Sabine L. L. Calcasieu Atchafalya R.

Mississippi R.

Terrebonne

Bay

0

0.25

0.5

0.75

transect C

1 transect F

75%

50%

25%

25%

Figure 2. Frequency of occurrence of mid-summer, bottom-water hypoxia in the Gulf of Mexico

from 1985-2002 (modified from Rabalais et al. 2002a). 

3 
Data collection along transect C, off Terrebonne Bay, began in 1985 and has continued fairly consistently

since 1989. Initially, two production platforms, a non-production platform and an open shelf ‘control’—

all within the persistent summertime hypoxic zone on the southeastern Louisiana shelf—were

investigated to determine the response of benthic infauna to hydrocarbon contaminants in the context of 

natural, temporal variability as caused by hypoxic bottom waters (Rabalais et al. 1993). A platform in

West Delta Block 32 (20,000 bbl d-1 produced water discharge) and an open water site in the same block

were characterized by silty sediments and intermittent summertime hypoxia. A production platform in

South Timbalier Block 53 with a 5,500 bbl d-1 discharge [station C6 of Rabalais et al. (1999) hypoxia 

studies] and a non-discharging platform in the same block [station C6A of Rabalais et al. (1999)] were 

characterized by primarily silty sands and low dissolved oxygen levels for prolonged periods. After the 

non-discharging platform at C6A was dismantled and removed, another non-discharging platform in that 

block [station C6B] was used for hydrographic profiles and instrument moorings in subsequent hypoxia 

studies (Rabalais et al. 1995, 1999). Eventually the instrument mooring was moved to South Timbalier 

Block 52A [station C6C] where it is currently in operation in collaboration with Louisiana State 

University WAVCIS (http://wavcis.csi.lsu.edu/, select station CSI-6). The production platform in this 

block South Timbalier 52C (20,000 bbl d-1) was under consideration for the initial study by Rabalais et al. 

(1993), but was eliminated due to the extensive shell pad and the limitations to box coring due to the 

numerous pipelines that feed the platform. 

The location of station C6 moved several times in the past decade in response to changes in platform 

activity. When the data has been composited, the station location is referred to as composite station C6*. 

Composite station C6* denotes a combination of sampling stations in South Timbalier Blocks 52 and 53 

in 20-m depth (C6, C6A, C6B and C6C). 

Data collected from transect C indicate that hypoxic conditions begin in early to mid-spring but are most

prevalent and widespread from late spring through late summer (Figure 3). 

Bottom Dissolved Oxygen (mg l-1 Surface Salinity (psu) ) 0

10

20

40

50

30

C1

C3

C4

C5

C6B

C6

C7

C9

C8

J A F A M M J S J O N D J A F A M M J S J O N D

Bottom Dissolved Oxygen (mg l-1 Surface Salinity (psu) ) 0

10

20

40

50

30

0

10

20

40

50

30

0

10

20

40

50

30

C1

C3

C4

C5

C6B

C6

C7

C9

C8

C1

C3

C4

C5

C6B

C6

C7

C9

C8

J A F A M M J S J O N D J A F A M M J S J O N D

Figure 3. Seasonal changes in average surface water salinity (psu) and average bottom

water dissolved oxygen (mg l-1) along transect C by month January through

December 1985-2002 (modified from Rabalais et al. 2002c). Y axis is

distance from shore (km). Station C1 is in 5 m, C6* in 20 m and C9 in 30 m.

The seasonal progression of lower surface water salinity from winter through summer (as shown for

transect C in Figure 3) is dictated by higher river discharge in March-May (long-term average) when 

persistent winds from the southeast and Coriolis effect move most of the water from the Mississippi River

birdfoot delta to the west along the inner to mid continental shelf. In summer, winds more often from the 

south and southwest hold the river water (lower in volume) on the shelf for a continued stratified system.

Water from the Atchafalaya River in summer often moves eastward to the area of transect C and

contributes to lowered surface salinity and strengthened stratification. 

4 
The average bottom waters along transect C are well oxygenated from October through March (Figure 3). 

The nearshore waters are more likely to be oxygenated through the year because of the shallow depth and 

influence of wind-induced mixing, but hypoxia will occur there. The offshore waters are less likely to be

hypoxic compared to the stations in 10 to 25 m depth because they are affected by the advection of deeper 

oxygenated waters from offshore and stratification intensity is less. The majority of the transect is

depleted in oxygen for most of June through August.

There is a seasonal peak in dissolved inorganic nitrogen (DIN) (DIN = NO3

-

+ NO2

-

 + NH4

+

) in the spring

followed by an increase in surface water chlorophyll biomass (an indicator of phytoplankton production) 

(Figure 4). The concentration of bottom water phaeopigments (the degradation products of chlorophyll in

organic matter that fluxes from the surface waters) parallels the generation of phytoplankton biomass in

the spring and is intensified under strong stratification in the summer (Rabalais et al. 2002b).

Surface DIN (μM) Surface Chlorophyll (μg l

-1) 0

10

20

40

50

30

C1

C3

C4

C5

C6B

C6

C7

C9

C8

J A F A M M J S J O N D J A F A M M J S J O N D

Surface DIN (μM) Surface Chlorophyll (μg l

-1) 0

10

20

40

50

30

0

10

20

40

50

30

0

10

20

40

50

30

C1

C3

C4

C5

C6B

C6

C7

C9

C8

C1

C3

C4

C5

C6B

C6

C7

C9

C8

J A F A M M J S J O N D J A F A M M J S J O N D

Figure 4. Seasonal changes in average surface water dissolved inorganic nitrogen (NO3

-

+ NO2

-

 + NH4

+

 in μM) and average surface water chlorophyll biomass (μg l-1) 

along transect C by month January through December 1985-2002 (modified

from Rabalais et al. 2002c). Y axis is distance from shore (km). Station C1 is

in 5 m, C6* in 20 m and C9 in 30 m.

The mid-summer mapping and monthly transect data are supplemented by continuous data from a series 

of oxygen meters deployed at composite station C6* since 1989. As noted above, composite station C6*

denotes a combination of four sampling stations in South Timbalier Blocks 52 and 53 in 20-m depth (C6,

C6A, C6B and C6C). While spatially limited, these data provide the time series of coupled physical and

biological processes that affect the concentration of oxygen. An example of bottom water dissolved 

oxygen is illustrated in Figure 5. The features in this time series include: (1) A gradual decline of bottom

oxygen concentrations through the spring. Respiration during decomposition of organic matter in bottom 

waters occurs at a rate faster than dissolved oxygen can diffuse from surface waters across a strong 

pycnocline. (2) Reoxygenation of bottom water from mixing events. As stratification reforms, oxygen

consumption rates in the lower water column begin to outpace resupply of oxygen from the surface layer 

and hypoxia reforms. (3) Persistent hypoxia and often anoxia for extended periods in May-September. 

(4) A mixing event in mid-summer that reaerates the water column followed by a decline in oxygen

similar to that seen in the spring. This event resulted from a tropical storm that crossed the Gulf of 

Campeche and generated 3- to 4-m waves at the instrument deployment site. (4) An intrusion of more 

saline, higher oxygen content water from deeper offshore during an upwelling regime in September 

followed by a downwelling regime and movement of the low oxygen water mass back offshore. (5) Cold 

fronts in October and November that continue to mix the water column sufficiently to prevent prolonged

instances of hypoxic conditions (Rabalais and Turner 2001). 

The deployed instruments provide the detail often missed by monthly sampling cruises. The bottom 

oxygen conditions fall below 2 mg l-1 in October-November and February-March (not shown), but these 

5 
events are unlikely to be captured in a one-day cruise. Hypoxia during these months, however, seldom 

persists due to the high frequency of mixing events. Hypoxia has not been recorded in the months of 

December-January. 

Apr

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

May

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Jul

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Oct

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Day

Sep

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Aug

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Nov

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Day

Jun

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

cold front cold front

tropical storm

deep water intrusion

cold fronts

respiration respiration

respiration

Apr

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

May

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Jul

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Oct

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Day

Sep

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Aug

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Nov

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Day

Jun

0

1

2

3

4

5

6

7

8

9

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

cold front cold front

tropical storm

deep water intrusion

cold fronts

respiration respiration

respiration

Figure 5. Bottom water dissolved oxygen concentration (mg l-1) within 1 m of

the bottom in 20 m depth at station C6B, from April through

November 1993 (modified from Rabalais and Turner 2001). The

dashed line defines hypoxia as 2 mg l-1 (~20% oxygen saturation). 

CAUSES OF HYPOXIA

The currently accepted paradigm supported by peer-reviewed literature is that nutrients and fresh water

from the Mississippi River, coupled with physical stratification of Gulf of Mexico coastal waters, are the 

principal complementary causes of hypoxia (Rabalais et al. 1999, 2002b; CENR 2000). This paradigm is 

also supported by numerous studies from other coastal hypoxic areas worldwide (Boesch 2002). The 

relative influence of the physical features of the system and the progression of biological processes vary 

spatially and over an annual cycle, are complexly inter-related, and are directly linked with the dynamics

of the Mississippi and Atchafalaya discharges. 

Acoustic Doppler current profiler (ADCP) data from 14 March through 12 November 2002 at station

C6B, midway between the Mississippi River birdfoot delta and the Atchafalaya River delta illustrate the 

anticipated strong dominance of the alongshore currents (Figure 6) (Wiseman et al. 2004). The flow is

strongly sheared in the alongshore direction with weak lower layer flow and a weak indication of reversal. 

The cross-shore flow is consistent with a wind-driven downwelling regime as would be expected under

the predominantly southeasterly winds affecting the region. More surprising is the highly variable

structure of the flow. Periods of strong vertical shear (e.g., hours 3200-3500) are interspersed with long 

periods of very weakly sheared flow (e.g., hours 4800-5000). Long periods of flow contrary to the 

expected westward flow regime were also observed (e.g., hours 2700-3100). 

6 
strong

vertical

shear

weak

sheared

flow

unexpected

eastward

flow

strong

unidirectional

to bottom

Hurricane Isadore

upwelling

favorable

consistent with wind-driven downwelling expected from predominantly SE winds

strong

vertical

shear

weak

sheared

flow

unexpected

eastward

flow

strong

unidirectional

to bottom

Hurricane Isadore

upwelling

favorable

consistent with wind-driven downwelling expected from predominantly SE winds

Figure 6. Contoured daily samples of low-pass filtered 

currents (cm s

-1) after rotation into alongshore 

(top) and cross-shore (bottom) directions (from

Wiseman et al. 2004). Bins are 0.5 m thick and

begin 1.71 m above bottom.

Two-layered flows or flow confined to the upper layer were anticipated from both theory and prior 

isolated observations, particularly during the highly stratified summer season. Yet an empirical

orthogonal functions analysis of the ADCP data in Figure 6 (Wiseman et al. 2004) indicated that 82 

percent of the variance in the low-passed currents was accounted for by the first mode, which describes 

nearly unidirectional, vertically-sheared flow (Figure 7). The second mode accounted for only 13 percent 

of the variance and describes a two-layered flow field. The latter condition was most prevalent in spring 

and summer, when bottom currents were weakest, the lower water column was more stagnant, and 

hypoxia was more likely to form. 

7 

Figure 7. First (left) and second (right) empirical orthogonal

functions from the acoustic Doppler current profiler 

(ADCP) data (from Wiseman et al. 2004). Values 

associated with every second ADCP bin are plotted. 
Hypoxia is most evident when the water column is stratified as a result of high fresh water runoff,

warmed surface waters, and calm winds. The physical structure of the water column is defined by water

masses that differ in temperature, salinity, or both. Fresh water from rivers and seasonally warmed

surface waters reside above the saltier, cooler, and denser water near the bottom. The existence of a 

strong near-surface pycnocline, usually controlled by salinity differences, is a necessary condition for the 

occurrence of hypoxia, while a weaker, seasonal pycnocline, influenced by temperature differences, often

guides the morphology of the bottom water hypoxia (Wiseman et al. 1997) (Figure 8). 

Upper Water Column 

Halocline ------

Lower Water Column

Thermocline ------

Depth (m)

Salinity psu and Temperature °C

10

12

14

16

18

20

20 25 30 35 40

Temp.

Sal.

D.O.

0

2

4

6

8

0 2 4 6 8

0

4

6

8

10

12

14

16

18

20

20 24 28 32 36

0 2 4 6

Dissolved Oxygen (mg/L)

8

Dissolved Oxygen (mg/L)

Salinity psu and Temperature °C

Temp.

Sal.

D.O.

Upper Water Column 

Halocline ------

Lower Water Column

Thermocline ------

Depth (m)

Salinity psu and Temperature °C

10

12

14

16

18

20

20 25 30 35 40

Temp.

Sal.

D.O.

Temp.

Sal.

D.O.

0

2

4

6

8

0 2 4 6 8

0

2

4

6

8

0 2 4 6 8

0

4

6

8

10

12

14

16

18

20

20 24 28 32 36

0 2 4 6

Dissolved Oxygen (mg/L)

8

Dissolved Oxygen (mg/L)

Salinity psu and Temperature °C

Temp.

Sal.

D.O.

Temp.

Sal.

D.O.

Figure 8. Examples of pycnoclines and oxyclines at station C6B

(from Rabalais and Turner in press). Left panel: example of

upper water column halocline and oxycline. Right panel: 

example of upper water column halocline and lower water 

column thermocline and oxycline. 

The concentrations and total loads of nitrogen, phosphorus, and silica delivered to the coastal ocean

influence the productivity of the phytoplankton community, the types of phytoplankton that are most 

likely to grow, and the flux of phytoplankton-derived organic matter (Rabalais et al. 1996; Turner et al. 

1998; Dortch et al. 2001). Phytoplankton not incorporated into the food web, and fecal material 

generated via the food web, sink into bottom waters where they are decomposed by aerobic bacteria, 

which consume oxygen in the process. The source of the organic matter for this respiratory activity is 

mostly from phytoplankton growth stimulated by river-delivered nutrients, and not from the carbon in the 

Mississippi River (Eadie et al. 1994), although the relative amount of allochthonous carbon and the

lability of both has been questioned (see Other Factors). 

Primary production in shelf waters near the delta and to some distance from it is significantly correlated 

with nitrate and nitrite concentrations and loads (Lohrenz et al. 1997; Justić et al. 1993, 1997, 2002). The 

relationship with dissolved orthophosphate is linear but not significant (Lohrenz et al. 1997). There is a 

pattern of localized maxima of phytoplankton biomass and production in salinities of 15 to 25 psu where 

water clarity improves in the presence of relatively high nutrient levels (Lohrenz et al. 1997; Rabalais et 

8 
al. 2002b), similar to other major rivers including the Amazon (DeMaster et al. 1986; Smith and

DeMaster 1996), Huanghe (Turner et al. 1990) and Changjiang (Xiuren et al. 1988). 

The distribution of low dissolved oxygen in bottom waters is related to high surface net production, but 

this relationship is lagged in time (Justić et al. 1993; Rabalais et al. 1994). Phytoplankton and fecal

pellets in surface waters fall into the bottom layers quickly, perhaps in a day or less, but surface and

bottom currents are not traveling in the same direction or at the same speed (Figures 6 and 7). Most of the 

organic matter reaching the bottom is not consumed in hours, but many days (and probably weeks). The 

depletion of oxygen is cumulative, and depends also on the re-aeration rate (Justić et al. 1996). There is a 

downstream transition away from the discharges of the Mississippi and Atchafalaya Rivers from lower to 

higher salinities, higher to lower nutrients, shifts in nitrogen compounds, higher to lower surface 

chlorophyll a concentrations and accumulation of phaeopigments in the lower water column. This results

in a consistent transition away from the river discharges in production of carbon, flux of organic material, 

respiration rates, and incidence of bottom water hypoxia. 

The vertical export of particulate organic carbon (POC) should be high, given the high rates of primary

production, and should be roughly proportional to the quantity of carbon fixed in the surface waters, as 

predicted by Suess (1980). The net production rates for the upper water column, represented by 

composite station C6* (Justić et al. 1997) showed a well-defined seasonal cycle with a minimum of -0.2 g

C m2 d-1 in December and a maximum of 1.2 g C m2

 d-1 in April. The high particulate organic carbon flux

to a 15-m moored trap at station C6B in spring was sufficient to fuel hypoxia in the bottom waters below 

the seasonal pycnocline (Qureshi 1995; Justić et al. 1996). The flux of organic matter in summer, while it 

sustained hypoxia, was incremental to the majority flux of carbon in the spring (Qureshi 1995).

Despite the high nutrient inputs to the shelf, nutrients are depleted to low, and sometimes undetectable, 

concentrations within a short distance of the river mouth (Lohrenz et al. 1990, 1997, 1999; Bode and

Dortch 1996; Dortch and Whitledge 1992; Nelson and Dortch 1996; Rabalais et al. 1996, 2002b).

Patterns of nutrient depletion provide evidence that riverine inputs of nutrients and their pattern of

regeneration ultimately limit the extent of river-enhanced production and biomass. Ambient nutrient

concentrations and ratios, bioassay experiments, and other indicators of nutrient limitation (Rabalais et al. 

1999, 2002b) suggest that nitrogen (N), phosphorus (P), silicon (Si) or combinations of these may be

limiting at some times and places in the outflow of the Mississippi River, the details of which are not

completely known. Although limitation by both Si and P does occur, N limitation extends over a larger 

area. Consequently, the rate of N loading is considered to be a critical factor in regulating the overall

production of phytoplankton over the broad region influenced by the river and affected by bottom water

hypoxia. 

HISTORICAL CHANGES

Given the high volume of fresh water and nutrients delivered by the Mississippi River to this stratified

coastal system, one might expect a propensity for the ecosystem to develop hypoxia naturally and that

hypoxia has always been a natural feature of this system. While average river discharge over the last 150 

years has not changed, nutrient flux has. The annual flux of nitrate nitrogen from the Mississippi basin to

the Gulf almost tripled between the periods 1955-1970 and 1980-1996 from 0.33 to 1.6 million metric

tons per year. Nitrate inputs to the basin are primarily from nonpoint sources (90 percent) of which 74 

percent are agricultural in origin; 56 percent of the nitrate enters the Mississippi River system north of the

Ohio River. Organic nitrogen measurements were not regularly made before 1973 but show no trend

since then. Both stream flow and nitrate flux have become much more variable in the last 25 years 

(Goolsby et al. 1999). 

Because relevant offshore water column data do not exist before 1972 and systematic hydrographic data 

collection did not start until 1985, the sediment record was examined for palaeoindicators of long-term 

transitions related to eutrophication and oxygen conditions beneath the Mississippi River plume

(summarized in Rabalais et al. 2002a). These records reflect conditions at the time the sediment was 

deposited and thus provide clues to temporal changes in biogeochemical conditions. 

9 
Sediment core analyses (Eadie et al. 1994, Turner and Rabalais 1994) document increased recent

eutrophication and increased organic sedimentation in bottom waters, with the changes being more 

apparent in areas of chronic hypoxia and coincident with the increasing nitrogen loads beginning in the

1950s. The evidence is increased accumulation of diatom remains (biologically bound silica, BSi) and 

marine-origin carbon accumulation in the sediments. Because there have been no significant increases in 

either the riverine organic carbon or silica loads, it is reasonable to infer that these increases in the 

sediment record since the 1950s are due to in situ production of marine algae. 

A series of indicators serve as surrogates for oxygen conditions: glauconite abundance (a clay mineral 

formed under reducing conditions), benthic foraminiferans, and ostracods (Nelsen et al. 1994;

Blackwelder et al. 1996; Rabalais et al. 1996; Sen Gupta et al. 1996). These surrogates for oxygen 

conditions indicate an overall increase in continental shelf oxygen stress (in intensity or duration) in the

last 100 years that has accelerated since the 1950s. These indicators of worsening oxygen conditions 

parallel the increase in indicators of surface water primary production that accumulate in the sediments,

i.e., diatom remains and accumulated carbon. Examination of specific foraminiferans indicates that a 

hypoxia-tolerant species has increased markedly in recent decades and now dominates the present-day 

population within areas of chronic seasonal hypoxia. A hypoxia-intolerant foraminiferan was a

conspicuous member of the fauna from 1700 to 1900 but is no longer present, indicating that oxygen 

stress was not a problem prior to 1900. 

Hypoxia is not a natural feature of this continental shelf but has worsened consistent with landscape

changes and increased nitrogen load from the Mississippi River (Rabalais et al. 2002a; Turner and

Rabalais 2003). Hindcasting by models based on measured conditions beginning in the 1980s indicate 

that hypoxia, as a widespread phenomenon or as a frequently occurring condition, was not present before 

the early to mid-1970s (Justić et al. 2002, Scavia et al. 2003, Turner et al. 2005). 

OTHER FACTORS

While literature supports that only increased nitrogen loads from the Mississippi River system can

account for the changes in hypoxia since the 1950s, other factors may contribute to its formation, 

dynamics and historical change. These are not competing hypotheses, but rather a set of potential

contributing factors that were considered as to their relative importance by CENR (2000) and Rabalais et 

al. (2002a). 

With the release of a USEPA Region 4 paper (USEPA 2004), the importance of phosphorus in 

contributing to hypoxia in the northern Gulf of Mexico has resurfaced since the CENR (2000) assessment. 

The new focus calls for a consideration of phosphorus management along with that of nitrogen. With

regard to the issue of produced water and hypoxia, there are no known analyses of phosphorus in 

produced water (see Background). 

Earlier researchers (cited in Turner and Allen 1982) proposed that intrusion of a low-oxygen layer from

deeper, offshore waters onto the continental shelf was the source of shelf bottom water hypoxia. This

low-oxygen layer at 400-700 m was first documented in the Gulf of Mexico in the mid-1930s (Conseil 

Permanent International pour l’Exploration de la Mer 1936). There were no oxygen values in that data 

set, however, that were less than 4.3 mg l-1. Low oxygen conditions reported from the Gulf in the 1930s,

therefore, are clearly references to the oxygen minimum layer in deeper waters that are not connected to

the continental shelf hypoxia that has been measured beginning in the 1970s (Pokryfki and Randall 1987,

Rabalais et al. 1991a). 

Organic carbon supplied by the Mississippi River could contribute to the oxygen demanding materials in 

the hypoxic zone (Carey et al. 1999). Because the suspended sediment load has declined by about half 

since the 1950s, the particulate organic load that could settle on the shelf has likely decreased as well. 

Terrestrially derived carbon is deposited close to the birdfoot delta (Turner and Rabalais 1994). Stable 

carbon isotope data indicate that 80 percent of the carbon accumulated in the Mississippi River bight

hypoxic zone is of marine, not terrestrial, origin (Eadie et al. 1994). The distances to which particulate

10 
organic carbon would need to be transported to affect the large area of hypoxia are too great to provide a

significant load. A maximum deposition of labile carbon from the Mississippi River into the bottom layer 

of the hypoxic zone was estimated by Boesch (1999) to be 11 percent of the total oxygen demanding 

carbon load (Figure 9). This calculation was based on respiration rates, annual organic carbon 

consumption, and proportion of labile carbon (Dortch et al. 1994; Trefry et al. 1994; Ittekkot 1988;

respectively). If less than 35 percent of this organic matter is actually oxidizable within the time frame in 

which hypoxia develops, the contribution of terrigenous organic sources to hypoxia must be a small part

of the carbon respired in hypoxic waters (negligible up to 4 percent of the total terrestrial organic flux).

On the other hand, the production of plankton carbon in the surface waters overlying hypoxic waters 

(CENR 2000) is 9 to 32 times greater than the maximum amount of riverine labile carbon reaching

hypoxic zones, depending on assumptions about the lability of the terrestrial carbon. About 44 percent of 

the surface carbon production in the form of phytoplankton eventually reaches the seafloor, either as 

senescent cells, in fecal pellets, or in aggregates. 

Deposition/respiration in

Atchafalaya basin/delta

Near-field POC desposition

Off-shelf transport

Advection to shelf outside of

hypoxic zone

Respiration in surface layer 

of hypoxic zone

Maximum deposition into bottom 

layer of hypoxic zone 11%

5%

27%

25%

17%

15%

[Oxidizable portion of deposited

terrestrial carbon < 4%]

Deposition/respiration in

Atchafalaya basin/delta

Near-field POC desposition

Off-shelf transport

Advection to shelf outside of

hypoxic zone

Respiration in surface layer 

of hypoxic zone

Maximum deposition into bottom 

layer of hypoxic zone 11%

5%

27%

25%

17%

15%

[Oxidizable portion of deposited

terrestrial carbon < 4%]

deposition

Figure 9. Potential contribution of labile allochthonous carbon to

the continental shelf from the Mississippi River 

(modified from Boesch 1999).

Decreased over-bank flooding in the Mississippi delta and Louisiana wetland loss are not considered to be

relevant factors in the formation and historical change of hypoxia. Leveeing of the river, completed in 

1927 to reduce flooding, occurred well before the significant increase in nitrate loads and indicators of

hypoxia began in the 1950s. Mass flux calculations of organic matter from wetland loss clearly indicate

this is a relatively small source of organic carbon, and stable carbon isotope analyses indicate that wetland

organic carbon accumulation is confined to a narrow band next to shore. However, restoration of 

wetlands and river flood plains within the basin would contribute to an overall reduction in nutrients 

reaching the river terminus (Mitsch et al. 2001). 

The Mississippi and Atchafalaya Rivers contribute, by far, the major sources of nutrients to the northern 

Gulf of Mexico (Dunn 1996). Direct atmospheric deposition to the offshore surface waters is a small

amount of the total nitrogen budget (1 - 2 percent, Goolsby et al. 1999). Upwelled nutrients from deeper 

waters may be important at the shelf edge (100 m), but there are limited transport mechanisms for this 

source to enhance continental shelf processes. Groundwater discharges to the Gulf are unlikely to be 

important because of the lack of shallow aquifers along the Louisiana coast. The best current knowledge

11 
is that the outflows of the two rivers dominate the nutrient loads to the continental shelf where hypoxia is

likely to develop. 

River discharge, nitrate concentration, and sediment core data provide over 100 years of record for this

system. On that time scale, there is no indication that climate factors override the impacts of human 

activities in the basin. The long-term river discharge (150 years), while variable, shows no long-term 

statistical trend. Average annual flow in the Mississippi River increased 30 percent between 1955 -1970

and 1980-1996 (Bratkovich et al. 1994) compared to the 300 percent increase in nitrate flux over this

period (Goolsby et al. 1999). Using two different approaches, Donner et al. (2002) and Justić et al.

(2003) agreed that 20-25 percent of the increased nitrate flux between the mid-1960s to the mid-1990s 

was attributable to greater runoff and river discharge, with the rest (75-80 percent) due to increased

nitrogen loading on the landscape. 

Stow et al. (2004) analyzed summer data from 1982 to 2002 and found that the degree of stratification 

needed to induce hypoxia has decreased. Within their summertime data set, surface water temperature

increased and subsequently oxygen saturation decreased. Their results imply a long-term factor for

summer conditions that transcends flow-induced stratification differences. One explanation would be the 

increase in nutrient loads in summer during their period of record, i.e., the nutrient load increase has led to 

more hypoxia regardless of the stratification. An alternative explanation might be that surface water

temperatures have increased due to climate change (at least for the period of record of these data). There

is, however, no literature for the northern Gulf that identifies a long-term warming of surface waters. 

HYDROCARBONS IN THE GULF OF MEXICO 

HISTORY OF THE OFFSHORE PETROLEUM INDUSTRY IN THE GULF OF MEXICO

The first oil well drilled in the open Gulf in 1938 was the Creole field about 2.4 km from the Louisiana 

coastline (Bedinger 1981). Significant development to explore the offshore did not commence until 1947 

when the Ship Shoal Block 32 field was found 19 km from the coastline. Once the ownership and

jurisdiction of the OCS was defined by the Submerged Lands Act and the Outer Continental Shelf Lands

Act, leasing and development activities in the Gulf of Mexico accelerated. In 1955, there were 400 wells 

drilled, increasing to 5,702 wells in 1966, and 16,169 wells drilled in 1978. The proportion of the total 

U.S. production coming from the Gulf of Mexico increased over the same time period to a peak in 1972

of 389 million barrels of oil and gas condensate, which was 10.7 percent of the nation’s domestic 

production. Production has decreased since then, but is being supplemented from new fields in deeper 

Gulf waters (>300 m). A new forecast from the Department of the Interior, Minerals Management

Service, indicates that oil production in the Gulf may start to decline within the next six years (Holden 

2004). In the ‘ultradeep’ waters (>1500 m) and areas that are more than 9 km beneath the shallow sea

floor, production will temporarily surge. But that surge will be short-lived and Gulf production is likely 

to decline after 2011. 

HYDROCARBON INPUTS TO THE GULF OF MEXICO

Several sources of materials are released to the water column from exploration, drilling, and production of

oil and gas. These releases are intentional and permitted, or accidental. They include permitted deck 

runoff of rainfall which may contain some oil and grease, permitted treated sewage and domestic waste

disposal, permitted drilling and produced water discharges, occasional accidental releases of product,

diesel fuels or chemicals associated with production, and sloughing of organic matter from biofouling 

organisms on surfaces of the platform (Neff et al. 1987). It should be noted that oil seeps are a major

source of hydrocarbon input to the Gulf of Mexico and outweigh the sum of all inputs related to human

activities. The following discussion focuses on sources related to human activities rather than naturally

occurring sources. 

12 

Drilling discharges consist of formation solids that are separated from the circulating mud stream and 

bulk mud discharges (Ayers 1983). Over the time it takes to drill a well (3 - 6 mo), 1000 m3

 (2000 tons)

of material (dry weight basis) will be discharged for water-based muds. Mud additives, which are mostly
inert, account for roughly half of this amount, and formation solids, the other half. The muds are either

discharged beneath the surface through a large diameter (25 cm) shunt pipe or allowed to freefall from the 

rig to the ocean surface. The main plume sinks rapidly leaving only 5 -7 percent of the materials in the 

surface plume. Unlike the North Sea, where they were allowed until 1997, the discharge of oil-based 

drilling fluids or of cuttings drilled with these fluids is not permitted in the U.S. OCS and thus do not 

contribute to organic loading in the vicinity of the platform or subsequent, localized depletion of oxygen. 

Synthetic-based drill fluids (SBF) are increasingly used in difficult or deep-water drilling. The discharge

of SBFs is not permitted. However, drill cuttings that are wetted with SBF may be discharged following 

treatment to remove the SBF. The residual SBFs that adhere to the cuttings are biodegradable and thus 

contribute to the organic loading in the vicinity of the platform. At sites where SBF-wetted cuttings had 

been discharged, studies noted an increase in organic enrichment and suboxic conditions in sediment

samples (CSA 2004; Trefry et al., 2005). The concentrations of SBF decline over time in both laboratory

and field studies and degradation rates may take from months to several years (Roberts, in preparation;

CSA, 2004). The primary mechanism for the degradation of SBF is sulfate reduction (Nguyen, 2005). 

During the production of crude oil, condensates or natural gas, water that is trapped within permeable 

sedimentary rock may also be brought to the surface. This water is called formation water, produced

water or oil field brine. The water must be removed from the oil, condensate, or gas as completely as 

possible in order to transport and use the product. The separated produced water may be reinjected down 

a well, either for disposal or to enhance recovery of hydrocarbons, or as is the case of the vast majority of

production from the Federal waters of the northwestern Gulf of Mexico region, it is discharged into the 

sea. Produced waters generally have concentrations of dissolved salts much higher than sea water

(Rittenhouse et al. 1969). In addition, produced waters may contain various inorganic and organic 

substances and radionuclides. The constituents in produced water may contribute to biological oxygen 

demand either directly, through organic matter decomposition, or indirectly through the flux of carbon 

from nutrient-enriched phytoplankton production, or chemical oxygen demand in bottom waters. 

Petroleum hydrocarbons enter the marine environment from a number of sources including produced 

water. The most recent Oil in the Sea III (NRC 2003) report updated estimates of these sources for North 

American and global marine waters for natural seeps, and for petroleum extraction, transportation and

consumption. The estimates for produced water in the United States are given in Table 1. Of the total

petroleum sources to the marine environment in North America and the combined central and western 

Gulf of Mexico, produced water discharges comprise a little more than 1 percent and 2.5 percent of the 

total petroleum, respectively. 

Table 1 

 

Estimates of Oil Discharges to the Marine Environment from Produced Water Discharges 

(taken from NRC 2003) 

Area Produced Water 

(1,000 bbl y-1) 

Oil & Grease Content

(mg l-1) 

(min-max estimates) 

Oil & Grease 

Discharge 

(tonne y-1) 

(min-max estimates) 

Total U.S. 745,000 20 2,500 (2,000-3,600) 

Gulf of Mexico OCS 473,000 20 (15-29) 1,700 (1,300-2,500) 

Louisiana Territorial Seas 186,000 20 (15-29) 600 (450-860)

Texas Territorial Seas 4,300 6.6 4.5 

Total U.S. Gulf of Mexico 663,300 ~6.6-29 2,305 

Mexico 15,200 ~60 (29-100) 140 (66-230)

13 
Estimates of petroleum inputs to the marine environment attributable to the consumption of 

hydrocarbons, through river and urban runoff, oil spills from cargo ships, operational discharges from 

commercial vessels and recreational craft and atmospheric deposition are high (NRC 2003; Rabalais 

2003). Diffuse sources of petroleum on land contribute to runoff and rivers polluted with petroleum.

Although individual releases may be very small, the cumulative load from all land-based sources accounts 

for about half the total annual load of petroleum to the marine environment from human-related activities. 

Thus, in terms of volume, these sources far exceed the contribution of activities associated with the 

extraction and transportation of petroleum, combined. Based on the average annual flow of major rivers

for which oil and grease concentration data were available, the Mississippi River contributes 70 percent of 

the riverborne hydrocarbons to the northwestern Gulf of Mexico yearly (Table 2) (see also Oxygen and 

Carbon Budgets). 

Table 2 

 

Estimates of Land-Based Contributions of Hydrocarbons and Polycyclic Aromatic Hydrocarbons (PAH) to

Marine Waters in the Northwestern Gulf of Mexico (taken from NRC 2003) 

Coastal Zone

Description 

OC Flux

(tonne y-1) 

Oil & Grease 

(tonne y-1) 

Hydrocarbon

(tonne y-1) 

PAH 

(tonne y-1) 

Coastal Sources 85,345 1,416 14 

Gulf Coast Refineries 2,005 43 0 

Brazos River 16,309 271 3 

Colorado River 16,879 280 3 

Mississippi River DOC 3,477,000 525,638 7,885 79 

POC 3,209,536 (15% of DOC flux)

TOC 6,686,536

Rio Grande River 49,709 825 8 

Sabine River 7,896 131 1 

Trinity River 26,736 444 4 

Total 730,517 11,294 113

DOC – dissolved organic carbon 

OC – organic carbon

PAH – polycyclic aromatic hydrocarbon 

POC – particulate organic carbon 

TOC – total organic carbon 

PRODUCED WATER CONSTITUENTS

Produced waters vary in salinity from 3 to 300 psu, but are generally more concentrated in salinity than

normal seawater (Rittenhouse et al. 1969). Produced waters may contain various inorganics (ammonium,

sulfide and elemental sulfur) and elevated levels of several metals—barium, beryllium, cadmium, copper, 

iron, lead, nickel, silver, zinc, and mercury (Neff et al. 1989). Radionuclides also occur at elevated levels 

in produced water, primarily as 226

Ra and 228Ra (Reid 1983). Produced waters also contain high

concentrations of organic compounds, primarily petroleum hydrocarbons but also partially oxidized 

organics. The petroleum hydrocarbon fraction has been thoroughly analyzed (Neff et al. 1987, 1989;

Boesch and Rabalais 1989a, b; Rabalais et al. 1991b, Ray and Engelhardt 1992). 

The OCS-generated produced waters sampled by Rabalais et al. (1991b) ranged between 43 and 192 ppt 

salinity. There was little variation in the salinity through time for effluents sampled two to four times. 

The Offshore Operators Committee (OOC) (2004) reports mean salinity of produced water as 113 ppt

(median 95 ppt). Sulfide concentrations were high (113 to 134 μg as S l-1) in one discharge, elevated in

several others (3.6 to 48 μg as S l-1), while the remaining discharges contained trace or nondetectable 

sulfide (Rabalais et al. 1991b). Where elevated levels were detected, the analyses were consistent across 

time for those discharges sampled more than once. 

14 
The nature of the hydrocarbons in produced water differs depending on the product type, oil, condensate 

or gas (Melancon, Minerals Management Service, pers. comm., August 2004). Wells in the eastern

portion of the Central Planning Area of the Gulf of Mexico tend to produce more oil while those in the

western Gulf produce more gas. Knowing the properties of the formation that generate the produced 

water may help to predict the proportion of constituents in produced water on a Gulf-wide basis. 

The OCS-generated produced water discharges varied greatly in the concentrations of identified organic

compounds (Rabalais et al. 1991b). The variations result from differences in treatment processes, the 

product being treated, and efficiency of separation. The volatile fraction contains mainly benzene with

lower amounts of toluene, xylene, and ethyl benzene (BTEX). They likely contain other volatile 

compounds at low levels such as short chain normal alkanes, cyclic alkanes, other C3-benzenes, C4-

benzenes and ketones. The volatiles, while acutely toxic, are highly water soluble and likely to be diluted

and dispersed in the water column, thus not contributing to the BOD of the effluent. 

The acid-extractable compounds (AEC) were usually the largest component of the organics measured in

most OCS discharges. Although they are a small fraction of crude oil, these compounds have the highest

water solubility, which favors their incorporation in produced water at higher levels than the less soluble 

polycyclic aromatic hydrocarbon (PAH) fraction. The AEC fraction contained high amounts of aliphatic 

fatty acids, aromatic acids, benzoic acid, and alkyl-substituted benzoic acids. This is the most degradable 

fraction of produced waters. These compounds are less likely to be deposited in sediments, but are more

likely to be diluted in the water column and dispersed from the discharge point (see Produced Water

Fate). 

The saturated hydrocarbons were the organic compounds next highest in concentration after the AEC. 

This fraction is composed of normal straight-chain hydrocarbons (alkanes), isoprenoid hydrocarbons, 

branched-chain hydrocarbons and naphthenes (cycloalkanes). This unresolved class of petroleum 

components accounts for more than 50 percent of the total saturated hydrocarbons. The saturated

hydrocarbons are the least toxic fraction of crude oil, and are very susceptible to microbial degradation 

when deposited in the environment. This component of produced waters would contribute to the BOD of 

the effluent. 

The PAH class was the smallest organic component of the produced water discharges, consisting of 

naphthalene and its alkylated analogs, napththalenes, phenanthrenes, and fluorenes. This fraction is the 

heaviest, most toxic, and environmentally stable fraction of crude oil. The toxicity of crude oil is a 

reflection of its aromatic content, primarily the akyl-substituted naphthalenes and phenanthrenes (NRC 

1985). The PAHs are the most likely components of produced water to be incorporated into the 

sediments because of low water solubilities and high sorption coefficients. The compounds are longer

lasting than the AEC and less likely to contribute to the BOD of the effluent. 

For purposes of calculating the carbon load from representative produced water constituents, the worstcase scenario can be based on maximal concentrations from Louisiana OCS produced water discharges 

(Table 3). 

15 
Table 3 

 

Maximal Concentrations of Representative Compounds in OCS Produced Waters 

from Louisiana (Rabalais et al. 1991b) 

Constituent Concentration (μg l-1) 

Benzene ≤ 4,000 

Toluene ≤ 1,000 

Aromatic acids ≤ 18,000 

Aliphatic fatty acids ≤ 120,000 

Saturated HC ≤ 20,000 

Total HC ≤ 150,000 

PAH ≤ 500 

In the absence of adequate Gulf of Mexico data for total organic carbon (TOC), data from the North Sea 

fields has been used. Although there are differences between the North Sea and the Gulf of Mexico, 

findings from the North Sea generally indicate what may be expected to occur in the Gulf of Mexico. The

TOC for Gulf of Mexico produced waters averaged 340 mg l-1 (median of 193 mg l-1) (OOC 2004). These 

values are similar to North Sea produced water where the TOC averaged 185 mg l-1 (range 45-350 mg l-1) 

(Flynn et al. 1996). The OOC (2004) also reports BOD for produced water with a mean of 1,007 mg

O2 l

-1 (BOD21) (median of 580 mg O2 l-1). The BOD reflects the degradable materials in the produced 

waters, i.e., oxygen consumption potential. 

The ammonium-N content of produced waters from several North Sea fields ranged from 1-17 mg l-1 for

Murchison and 0-8.9 mg l-1 for Hutton, compared to literature values of 10-250 mg l-1 for the North Sea 

and 10-300 mg l-1 for other areas (Tibbetts et al. 1992). The OOC (2004) reports average total Kjeldahl

nitrogen (TKN) (ammonium + organic N) values from Gulf of Mexico produced waters as a mean of 150

mg l-1 (median 135 mg l-1). The nitrogen in the TKN is present primarily as ammonium. 

PRODUCED WATER FATE

Several studies have been conducted in the North Sea on the fate of produced waters upon release into 

ambient waters. Flynn et al. (1996) reviewed North Sea produced water discharges, three from oil

production and one from gas condensate production. Upon release, the constituents in the produced water

will volatilize, biodegrade, and disperse. 

Biodegradation tests indicated that the majority of organic materials in Clyde produced water (North Sea, 

Flynn et al. 1996) were readily biodegradable. The ultimate BOD was reached in less than 9 days.

Ninety percent of the total organic carbon (TOC) was biodegraded within 8 days and after 28 days the

proportion had risen to 96 percent (Table 4). The addition of inorganic nutrients to the BOD tests made

little difference, suggesting that the nutrient status of produced water is sufficient to support substantial 

biodegradation. Volatilization will occur rapidly for the lower molecular weight components, especially 

the phenols and BTEX. 

16 
Table 4 

 

Effect of Biodegradation on Selected Organics in Produced Water Collected from North Sea Fields 

 (from Flynn et al. 1996)

Time 

(days)

TOC 

(mg l-1) 

VFA 

(mg l-1) 

Phenols 

(μg l-1) 

BTEX 

(μg l-1) 

Total B/N 

(μg l-1) * 

PAH 

(μg l-1) 

0 45 25 8,810 4,800 9,210 1,460

8 1.42 6 4.7 <0.5 610 99

28 0.52 4 na <0.5 630 4.6

* – Total B/N Total Bases/Neutral Fraction, BTEX + non-volatile base neutrals 

BTEX – benzene, toluene, ethyl benzene, xylene 

na – not available 

PAH – Polycyclic aromatic hydrocarbon 

TOC – total organic carbon

VFA – volatile fraction (all) 

Flynn et al. (1996) also summarized the results of a particle-tracking model applied to the dilution of a

produced water plume after discharge. The model predicted that the center of the discharge plume would 

sink about 10 m due to the high salinity of the produced water and the initial momentum. As the plume 

entrains more sea water, it becomes neutrally buoyant and the center line of the plume stabilizes in the

water column. During a tidal cycle the plume becomes relatively long and narrow (approx. 50 m x 4 m in

cross section at 100 m (0.06 mi) from the platform) and rotates around the platform. The greatest 

proportion of the dilution effect is manifest in the first 100 m (0.06 mi) from the discharge. At 500 m (0.3

mi) from the platform the model predicts that the minimum dilution at any point in the plume will 

approach a value of 775 for a short period during the tidal cycle. The minimum dilution was predicted to

reach 9,000 at 20 km (12.4 mi). The conversion to miles is included here for comparison with other 

calculations based on 5-, 10-, and 20-mi distances for cumulative discharge volumes. Over this distance,

biodegradation would begin to have a significant impact. At 15°C the minimum dilution of 60,000 is

reached at 20 km (12.4 mi) from the platform. The initial PAH concentrations in produced water 

discharged from four platforms on the OCS off Louisiana and Texas were much lower (37 to 578 μg l-1) 

than those of the North Sea (1,460 μg l-1) and reached background concentrations (0.058 to 0.118 μg l-1) 2

km (1.2 mi) down-current from the discharge point, similar to ambient concentrations in the open Gulf of

Mexico (Neff and Sauer 1996). 

Brandsma and Smith (1996) used the OOC model to estimate concentrations of discharged produced 

water based on conditions applicable to the Gulf of Mexico, primarily to assess the concentrations where 

potential effects could occur. The information, however, proves useful to estimates of dilution from the

point of discharge. They used a median flow rate for the Gulf of Mexico (728 bbl d-1, 116 m3

 d-1), a slow 

current speed of 3.3 cm s-1, and an intermediate density gradient to predict a maximum concentration at 

100 m (0.06 mi) from the outfall in 27 m water depth of 0.04 percent. They used the maximum allowable

discharge of 25,000 bbl d-1 (3,978 m3

 d-1), under the permit then in effect, with field current data within a 

100-m vertical cylinder and predicted that the maximum concentration estimated at the edge of the 

cylinder during the 24-h period was 0.35 percent. It was noted by Scott Wilson (pers. Comm., USEPA 

Region 6) that the method the authors used to calculate the effluent concentration appears to overestimate

dispersion. Constantly changing current conditions and the effects of volatilization, biodegradation or 

precipitation may act to remove produced water components at a higher rate than just simple dispersion. 

The median density stratification used by Brandsma and Smith (1996) for the Gulf of Mexico was 0.15 kg 

m-3 m-1, but ranged from relatively weak to strong (0.04 to 0.2-0.7 kg m-3 m-1). Similar results were

obtained in a recent model simulation (OOC 2004) (Figures 10 and 11), where weak density gradients led

to ‘trapping’ of discharge plumes. 

17 
Density difference values for composite station C6* in the core of the hypoxic zone off Terrebonne Bay

in 20 m were higher than the value used by Brandsma and Smith (1996) for most of the stratified spring

and summer (0.25 to 0.55 kg m-3 m-1) but minimal in winter (0.02 kg m-3 m-1) (Justić et al. 1993). Station

C6* is a suite of stations in 20-m water in South Timbalier Blocks 52 and 53. Strong density stratification

would not affect the precipitation of particle adhered hydrocarbons to the sediments. Given that 

zooplankton fecal pellets sink to the bottom in less than 1 day in 20 m water depth, it would be expected

that particle adhered hydrocarbons would also reach the bottom. 

Transport by ambient currents

Low salinity surface water

Higher salinity bottom water

Rapid 

entrainment...

...leads to 

“trapping”

Transport by ambient currents

Low salinity surface water

Higher salinity bottom water

Rapid 

entrainment...

...leads to 

“trapping”

Figure 10. Conceptual model of transport in stratified 

receiving water (OOC 2004). 

0 50 100 150

Distance (m)

-15

-10

-5

Depth (m)

>1

>0.1

>0.01

<12

<6

<2

PW

%

D.O.

(mg l-1)

0 50 100 150

Distance (m)

-15

-10

-5

Depth (m)

>1

>0.1

>0.01

<12

<6

<2

PW

%

D.O.

(mg l-1)

Figure 11. Model prediction of plume behavior for strong stratification, based on

data for dissolved oxygen, salinity, and temperature reported in

Rabalais et al. (1993) (OOC 2004). 

Most of the produced water discharges are thought to be located near the surface in the Louisiana-Texas 

hypoxic zone (Figure 12) (OOC 2004). Therefore, the effluent would likely be entrained in the upper 

water column, and dispersed similar to the Brandsma and Smith (1996) model. If produced waters were

18 
discharged below a seasonal pycnocline when bottom current speeds are minimal (< 5 cm s-1) (W. J.

Wiseman, Jr. et al. unpubl. Data) and the plume were to contact the seabed, dispersion would be

minimized and a hypersaline produced water plume at the bottom could result (see Interaction of Hypoxia 

and Produced Water). 

Depth Available for Plume Trapping (%)

10 20 30 40 50 60 70 80 90 100

Number of Facilities

0

50

100

150

200

Figure 12. Histogram of fraction of water depth between discharge pipe 

and sea bottom for produced water discharges in the hypoxic 

zone. The overwhelming majority of outfalls are located

within 5% of the sea surface or above the sea surface 

entirely [developed by OOC (2004) from data in Shannon

1992]. 

19 
PRODUCED WATER DISCHARGES – VOLUME AND GEOGRAPHIC DISTRIBUTION

The discharge of produced waters into the offshore waters of Louisiana and Texas is extensive (Figure

13). Estimates of produced waters discharged into OCS waters of the northwestern Gulf were 494.4 x 106

bbl y-1 with the predominance of the discharges occurring off Louisiana (Rabalais et al. 1991b). A more 

recent estimate (NRC 2003) indicated 473 x 106

 bbl y

-1 for the OCS across the Gulf with an additional

186 x 106

 bbl y-1 for Louisiana territorial waters and 4.3 x 106 bbl y-1 for Texas territorial waters for a total

U.S. Gulf of Mexico of 663.3 x 106

 bbl y

-1 produced water effluent (Table 1). The OCS total volumes 

from the two estimates are quite similar. The MMS data indicate that there has been a slight decline in 

the volumes discharged into the hypoxia zone between 1996 and 2004 (Figure 14). 

Figure 13. Distribution of produced water discharge volumes by OCS lease block for a portion of the

Central and Western OCS superimposed on the frequency of mid-summer, bottom-water

hypoxia. The asterisk (*) indicates the general location of composite station C6* in 20-m water 

depth, which is the location of specific carbon, nitrogen, and oxygen calculations. [Map 

generated by the Minerals Management Service with input from Rabalais et al. (2002) for the

distribution of hypoxia frequency.] 

20 
0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

18,000,000

Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04

Barrels per Month

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

Number of Completions

Oil from Oil Completions Water from Oil Completions Number of Oil Completions

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

18,000,000

Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04

Barrels per Month

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

Number of Completions

Oil from Oil Completions Water from Oil Completions Number of Oil Completions

Figure 14. Number of oil completions and oil and water from oil completions within the area of hypoxia 

(Figure 13) for the period 1996-2003 (from Mike Melancon, Minerals Management Service, pers.

Comm., August 2004). 

The amount of produced water generated by any single well tends to go up as the oil or gas product is 

depleted from the formation, and may be as high as 95 percent of the product stream in older fields such

as coastal Louisiana and Texas (Neff et al. 1987). Thus, as a field ages the produced water effluents also

rise. However, once a formation is depleted, the well is taken off line and no longer generates produced

water. There is a general decline in the Gulf of Mexico (Figure 14) of the number of oil completions and 

the amount of oil from completions from January 1996 to January 2004, with a step-wise decrease in

water from oil completions during that period beginning 2001 and 2002. There has been a steady

decrease in gas production from gas completions from January 1996 to January 2004, a slight decrease in

gas completions during that period and no change in the produced water volume from gas depletions (M.

Melancon, Minerals Management Service, pers. Comm., August 2004). 

Volumes of produced water were calculated for the area surrounding composite station C6* in South

Timbalier Blocks 52 and 53, within 5-, 10-, and 20-mi radius circles of station C6C, for comparison with

estimated net production of carbon in surface waters. These values are illustrated in Figure 15 for 1984-

2001 and provided in Appendix 2. The average produced water discharge has risen fairly consistently for

this part of the Louisiana OCS where the amount of formation water may be increasing as the amount of

petroleum product decreases. Changes in produced water volume were compared with changes in the size 

of the hypoxic zone to see if there were any empirical relationships. While the relationship between area 

and produced volume within 5 mi of C6* is significant (R2

 = 0.24, P < 0.02), the relationships between

area and cumulative volume within 10 mi and 20 mi are not (R2

 = 0.30, P < 0.1 and R2 = 0.15, P < 0.12, 

respectively). None of the relationships of hypoxic area with produced water volume are particularly 

strong (all R2

 < 0.24), thus there is little evidence to link the increase of produced water volume with

changing size of the hypoxic zone. The size of the hypoxic area, however, is strongly correlated with the 

integrated flux of nitrate from the Mississippi River 75 days prior to the survey cruise (R2

 = 0.61, P < 

0.001). 

21 
1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Year

0

5000

10000

15000

20000

Area of Bottom-Water Hypoxia (km2) 25000

0

1000000

2000000

3000000

P

4000000

roduced Water Discharge (bbl mo-1)

Area

Ave5mi

Ave10mi

Ave20mi

Figure 15. Time course of produced water volumes for all platforms within a 5-, 10-, 

and 20-mi radius of composite station C6* and size of the bottom water

hypoxic area based on a 5-day mid-summer mapping cruise. Data for 

produced water volumes provided by the Minerals Management Service. 

Data for size of hypoxic zone modified from Rabalais et al. (2002) for all 

years except 1989, which is derived from Turner et al. (2005). 

22 
INTERACTION OF HYPOXIA AND PRODUCED WATERS 

LOUISIANA SHELF

The initial reconnaissance work on the four platforms in Rabalais et al. (1993) revealed varying levels of 

hydrocarbon contamination with distance from the produced water discharge. For comparison, the levels

of representative constituents of produced water in the sediments are presented in Table 5. In general 

(based on the complete data set), there were no consistent spatial or temporal trends in hydrocarbon

chemistry at the platforms. Hydrocarbon concentrations, in general, were low for most, even where the 

sediments were silty, or were characterized as weathered petrogenic or biogenic in nature. There were 

stations, however, where sediment hydrocarbons were elevated and distinctly petrogenic, but there were 

no clear patterns or gradients. The hydrocarbon characteristics, along with some sedimentary variability,

and differences in the hypoxic conditions from the West Delta area to the South Timbalier area made it

difficult to discern trends in benthic community changes due to hydrocarbons in sediments. Differences 

in benthic communities, however, could be attributed to sediment characteristics and oxygen 

environments (Rabalais et al. 1993). 

Table 5 

 

Comparative Values of Produced Water Constituents in Sediments within 100 or 250 m

at Production Platforms in June 1990 (Rabalais et al. 1993). 

(OCS block numbers, hypoxia station designations, and discharge volumes as indicated.)

Platform WD32 WD32 ST53 ST53 ST52 ST52

Station (C6) (C6) (C6C) (C6C)

(20,000 bbl day-1) (5500 bbl day-1) (20,000 bbl day-1) 

Distance (m) 100 250 100 250 100 250

Constituent

Zn (ppm) 92.1 89.4 48.2 98.6 88.3 133.4 

Pb (ppm) 17.0 0.0 6.0 11.8 0.0 1.8 

Total Saturated 

HC (ppb) 

7,659 6,052 3,197 2,183 3,257 1,3758

Alkylated PAH 

(ppb)

425 180 68 84 98 470

WD – West Delta 

ST – South Timbalier 

The Rabalais et al. (1993) study and further work in the hypoxic area off Terrebonne Bay provided many

hydrographic profiles for comparing temperature, salinity, dissolved oxygen, redox and pH between 

discharge and non-discharge sites. Two or sometimes three of the stations [C6 (discharge), C6B (no 

discharge) and C6C (discharge)] were profiled in the same day within an hour’s time. The comparative 

profiles of these data are illustrated in Appendix 1. The profiles of C6 and C6B were essentially the same 

for 2000-2004. There was a concerted effort to compare C6B and C6C before moving the instrument 

mooring to a fixed position at C6C for real-time data transmission. Ten of the 18 comparative profiles

were similar, while eight profiles indicated a thin, hypersaline, lower dissolved oxygen lens very near the 

bottom, within 0.05 to 1 m above the sea floor; the dissimilar profiles occur primarily at C6C in 2003-

2004. The near-bottom oxygen meter at C6C is positioned 1+ m above the bottom to avoid the 

hypersaline waters that can distort the dissolved oxygen values more representative of the broader area 

around the platform. Good coherence of conductivity, temperature, and depth (CTD) profiles for all three 

stations is illustrated in Figure 16 for 4/11/2003 and for two stations for 11/22/2004, but the bottom water 

lens of C6C is apparent in the 5/13/2003 and 5/5/2004 comparisons. These hypersaline waters are reduced 

in pH and redox; redox can fall below 0 mV. With the exception of these hypersaline plumes at C6C in 

23 
2003-2004, most profiles of dissolved oxygen and salinity near production platforms are similar to waters 

some distance from the platform or to platforms without a discharge. 

Figure 16. Dissolved oxygen (mg l

-1) and salinity (psu) profiles for stations C6 (triangles), C6B

(circles) and C6C (squares) for dates indicated. Solid shapes are salinity and open

shapes are dissolved oxygen (from Rabalais et al., unpublished data from hypoxia

studies). 

GULF OF MEXICO OFFSHORE OPERATIONS MONITORING EXPERIMENT 

(GOOMEX) 

In the GOOMEX studies of drilling and produced water discharges off the Texas coast (Kennicutt 1995),

the production platforms had little effect on most ambient water properties. The exception was that 

hypoxic or near hypoxic conditions were observed in the summer near the platforms at Matagorda Island

Block (MAI)-686 (29 m depth) and Mustang Island Block MU-A85 (85 m deep) during periods of high

stratification. Dissolved oxygen values determined at these two platforms during stratification (Cruises 2

and 4) were 1.76 and 0.32 mg l-1 for MAI-686 and 2.7 and 2.54 mg l-1 for MU-A85. The drilling 

discharges had been shunted to the bottom at MU-A85, but not at MAI-686. High dissolved nitrate,

phosphate, and silicate concentrations in the bottom water during hypoxia indicated that they were being

regenerated from the sediments. 

24 
Sediment texture was predominantly sand near the platforms as a result of disposal of drill cuttings. The 

organic carbon content decreased toward the platforms as the sand content increased. The inorganic 

carbon increased near the platform, most likely due to deposition of calcareous debris from platform

associated fauna and the disposal of carbonate-containing cuttings. Hydrocarbon contaminants were 

elevated in sediments close to the MU-A85 platform (85 m, summer, near-hypoxia) but rapidly decreased

over a distance of 100 to 200 m from the platform. Most hydrocarbons were degraded at MU-A85, but

those of several stations at MAI-686 (29 m, summer hypoxia) were rich in n-alkanes. PAH levels in 

sediments were well below levels known to be associated with toxic biological effects (< 4000 ppb). 

The localized bottom water hypoxia occurred only during stratified conditions. However, localized 

hypoxia was not observed during all episodes of stratified conditions. No clear conclusion can be drawn 

as to whether the localized hypoxia was a result of elevated hydrocarbons in the sediments, organic

loading from produced waters, degradation of biofouling materials from the platform legs, or a 

combination of these. 

OXYGEN AND CARBON BUDGETS

Oxygen profiles and calculations of oxygen anomalies for composite station C6* over the period 1985-

1992 were used to define oxygen and carbon budgets for an area that is consistently hypoxic on a seasonal 

basis (station C6* in Figure 13) (Justić et al. 1996, 1997). The surface-water layer oxygen content, above

the prevalent pycnocline at 10 m, is above the saturation level expected for the temperature and salinity 

conditions during February-July; the maximum in April and May coincides with mean peak Mississippi

River flow. Oxygen content above saturation indicates that phytoplankton photosynthesis rates are high 

resulting in high levels of organic carbon production that can be redistributed within the system; much of 

this will eventually reach the lower water column and sediments. The bottom layer oxygen content, 

below the pycnocline to 20 m, is less than the saturation level expected for salinity and temperature

conditions, meaning that respiration rates are exceeding any oxygen production rates. The oxygen

content of bottom waters is lowest in July when surface-to-bottom density differences are greatest. 

The integrated annual net productivity (NP) of the upper water column (0-10 m) at composite station C6*

is 423 g O2 m-2 (Figure 17), however, 90 percent occurs between February and June. Conversion by an 

oxygen-to-carbon ratio of 3.47 by weight results in total net carbon production of 122 g C m-2

 y

-1. The 

excess of organic matter, derived from primary production, is redistributed within the system, and

eventually decomposed in the lower water column and in the sediments. The integrated annual oxygen

uptake rate in the lower water column at composite station C6* is 197 g O2 m-2

 y

-1, which converts to a 

value of 57 g C m-2

 y

-1 for the respiration process. Thus, on an annual basis, 47 percent of the surface net 

organic production at composite station C6* is decomposed in the lower water column and in the

sediments (Total Respiration:Net Production, TR:NP = 0.47) (see Figure 17). This model closely 

replicates field measurements (Justić et al. 2002). The oxygen and carbon dynamics at composite 

statation C6* are strongly correlated with nitrate flux from the Mississippi River (Justić et al. 2003). 

25 
TR

FOt 226

NP 423

(122)

197

(57)

D 197 O

1985 1986 1987 1988 1989 1990 1991 1992 1993 0

2

4

6

8

10 Bottom

Years

2

4

6

8

10

12 Surface

Observed

Model

Ox

ygen 

Concentrati

on (

mg O2 l-1)

TR

FOt 226

NP 423

(122)

197

(57)

D 197 O

TR

FOt 226

NP 423

(122)

197

(57)

D 197 O

1985 1986 1987 1988 1989 1990 1991 1992 1993 0

2

4

6

8

10 Bottom

Years

2

4

6

8

10

12 Surface

Observed

Model

Ox

ygen 

Concentrati

on (

mg O2 l-1)

1985 1986 1987 1988 1989 1990 1991 1992 1993 0

2

4

6

8

10 Bottom

Years

2

4

6

8

10

12 Surface

Observed

Model

Ox

ygen 

Concentrati

on (

mg O2 l-1)

Figure 17. Coupled physical-biological model of oxygen and carbon dynamics at

representative composite station C6* in the core of the hypoxic zone

(from Justić et al. 1996, 1997, 2002). TR is total respiration, NP is 

net production, FOt is flux of oxygen across the air-water interface,

DO is diffusion of oxygen across the pycnocline, values in black are

oxygen, and values in blue parentheses are oxygen converted to

carbon equivalents.

RELATIVE CONTRIBUTION OF INPUTS OF PRODUCED WATER VERSUS THOSE OF 

THE MISSISSIPPI RIVER (DIRECT AND INDIRECT) IN THE DYNAMICS OF HYPOXIA

Nitrogen 

One of the constituents of produced water, nitrogen, is an essential nutrient that supports primary 

production on the continental shelf. The Mississippi River delivers a large amount of nitrogen to the shelf 

that supports the high primary production there. The forms of nitrogen delivered by the Mississippi River 

are dissolved or particulate nitrogen and organic or inorganic nitrogen. The dissolved nitrate in the 

Mississippi River is the largest proportion of the total nitrogen (Table 6) (Goolsby et al. 1999). The 

average TKN in a produced water discharge is 150 mg l-1, of which ammonium is the predominant 

nitrogen form (OOC 2004).

26 
Table 6 

 

Mean Annual Flux of Nutrients from the Mississippi-Atchafalaya River Basin to the Gulf of Mexico, 

1980-1996 (adapted from Goolsby et al. 1999)

Nutrient Mean Flux 

(metric tons) 

Percent of Total 

Nitrogen (N), Total 1,567,900 100 

Nitrate 952,700 61

Ammonium 31,000 2

Dissolved organic N 376,000 24 

Particulate organic N 204,000 13 

A first order estimation of the amount of ammonium discharged onto the continental shelf area affected

by hypoxia (20,000 km2

) was compared to the amount of nitrogen discharged to the continental shelf

from the Mississippi and Atchafalaya rivers. This comparison assumes that the average produced water

volume within a 20-mi circumference of composite station C6* (813.6 km2

) is representative of the

average produced water volume over a 20,000 km2

 area. The produced water discharge and the

Mississippi River discharge were estimated on an annual basis. However, the produced water discharge 

through the year is fairly constant (Figure 18), but the load of nitrogen from the river peaks in April and

most is delivered between March and June. The conversion also assumes that the inputs of the numerous 

forms of nitrogen are all equivalent with regard to supporting primary production, which is not the case.

Calculations of components that would be relevant are difficult because of lack of data and the 

uncertainty of uptake of some forms of nitrogen by different taxa of phytoplankton and bacteria. Still, the 

comparison provides a relative statement of inputs. Conversion of the ammonium in produced water to

the area of hypoxia produces a value of 2.1 x 105

 kg y

-1. The amount of total nitrogen delivered to the 

shelf (assuming that it affects the same area) is 1.6 x 109

 kg y

-1. The produced water ammonium load is

therefore estimated to be 0.013 percent of the total nitrogen delivered by the Mississippi River system, 

0.008 percent of the total dissolved inorganic nitrogen and 0.002 percent of the total ammonium. 

27 
1984 1986 1 1988 990 1 1992 994 1 1996 998 2 2000

Year

002

0

1000000

2000000

3000000

4000000

5000000

Produced Water Discharge (bbl mo-1 )

Apr5mi

Oct5mi

Apr10mi

Oct10mi

Apr20mi

Oct20mi

Figure 18. Produced water discharge volume in April (during high Mississippi

River discharge) and October (during low Mississippi River 

discharge) by year for all discharging platforms in 5-mi, 10-mi, and

20-mi radius circles around composite station C6*. Data for 

produced water volumes provided by the Minerals Management

Service. 

Average concentration of dissolved inorganic nitrogen at composite station C6* over the period 1985-

1998 indicate that average surface values range from 8 to 10 μM in late winter and early spring when 

river discharge is high and below 1 μM after the peak of chlorophyll biomass in the spring and early

summer and when the river discharge is low in summer (Figure 4). Concentration of ammonium can be 

high in bottom waters when the oxygen level is close to anoxia, but otherwise falls mostly below 5 μM. 

Nitrate concentrations are mostly below 10 μM with the majority below 5 μM; highest values are in 

dissolved oxygen less than 2 mg l-1 (other than two outliers in Figure 19). A mean of 150 mg l-1

ammonium (equivalent to 8,333 μM NH4+

 ) in produced water would be discharged into ambient surface 

dissolved inorganic nitrogen (6 to 8 μM in spring and 1 μM in summer). If the produced water

ammonium is reduced to 0.1 percent of the discharge concentration within a short distance of the plume

and 0.01 percent of the discharge within 150 m [according to OOC (2004) dispersion model], the 

ammonium in the produced water discharge would be within the concentration of ambient dissolved 

inorganic nitrogen within 50 to 150 m from the discharge (see Figure 11). If a produced water were

discharged below the pycnocline into the minimal currents typical of summer bottom water, the

concentration of the ammonium in the discharge would be elevated in comparison to the ambient

conditions for both ammonium and nitrate. However, the diffusion or recycling rates of the bottom-water 

nitrogen compounds are not known. 

28 
0 1 2 3 4 5 6 7 8 9 0

5

10

15

20

25

30

Dissolved Oxygen (mg l

-1)

NH4 (μM)

0 1 2 3 4 5 6 7 8 9 0

5

10

15

20

25

30

35

40

NO3(μM)

Dissolved Oxygen (mg l-1)

0 1 2 3 4 5 6 7 8 9 0

5

10

15

20

25

30

Dissolved Oxygen (mg l

-1)

NH4 (μM)

0 1 2 3 4 5 6 7 8 9 0

5

10

15

20

25

30

35

40

NO3(μM)

Dissolved Oxygen (mg l-1)

Figure 19. Bottom water concentration of NH4

+

 and NO3

-

 (μM) at station C6* 

compared to bottom water dissolved oxygen (mg l-1) for period 1985-2002 

(from Rabalais et al. unpubl. data from hypoxia studies 1985-2005). 

Hydrocarbons 

Several assumptions and comparison of dissimilar data sets were necessary to develop first order 

estimates of the contribution of organic carbon from produced waters to the Mississippi River in 

supporting biochemical oxygen demand that leads to hypoxia. The assumptions lean to worst-case 

scenarios for the produced water carbon inputs, but the expected dilutions and dispersions correspond to 

available models. The oil and grease loadings do not distinguish among the various hydrocarbon 

compounds that might be in the effluent or the river. The compounds are of various categories—

aromatics, medium-molecular weight PAHs, acid extractable compounds, saturated hydrocarbons and

fatty acids. There is considerable variability among the many compounds with regard to volatility,

solubility, photo-oxidation, the likelihood of adhering to sedimenting particles, and the susceptibility to 

microbial degradation. All hydrocarbons included in the oil and grease content and loadings were 

included in calculations of organic carbon that would reach the lower water column and sediments and 

contribute to respiratory reduction of dissolved oxygen. This, of course, is not the case. 

The discharge of oil and grease from produced waters in U.S. Gulf of Mexico waters is 2,305 tonne y

-1

(primarily discharged into the northwestern Gulf) and from the Mississippi River is 525,638 tonne y

-1

(Tables 1 and 2). The riverine particulate organic carbon (POC) load is 3,209,536 tonne y-1, and the

dissolved organic carbon (DOC) load is 2,951,362 tonne y

-1 (Goolsby et al. 1999). The produced water 

oil and grease loads are for the entire U.S. Gulf of Mexico, but discharges west of the Mississippi River 

delta far exceed those to the east. The majority of the discharge from the Mississippi River moves

westward from the delta most of the year. If there were impacts from either source, they would most

likely affect the shelf to the west of the delta. The oil and grease load in the river discharge is expected to

dilute in the plume as it mixes with Gulf waters and follow the path of reduction outlined in Figure 9 for 

allochthonous carbon from the Mississippi River basin. If 4 percent of the oil and grease from the river

were to contribute to organic loading below the pycnocline, as is expected for the TOC load, then 21,025 

tonne y-1 of the Mississippi River oil and grease could potentially contribute to organic loading leading to 

hypoxia. The produced water oil and grease load for the Gulf of Mexico is 10.9 percent of the oil and

grease load from the Mississippi River that might contribute to hypoxia and 0.04 percent of the total 

Mississippi River oil and grease load. 

Without appropriate TOC, DOC and POC data for produced water discharges in the area of hypoxia, it

would be inappropriate to compare produced water oil and grease loads to riverine TOC data. However,

considering that the Gulf produced water oil and grease load is a relatively small percent of the

Mississippi River load that might contribute to hypoxia, it is reasonable to expect that this small

percentage would also be overwhelmed by the nutrient-enhanced surface in situ production offshore. The 

in situ production is estimated to be 96 percent of the organic matter loading that drives the oxygen and

carbon budgets of the continental shelf where hypoxia occurs, compared with the up to 4 percent riverine

organic loading that might contribute. The produced water contribution of organic carbon in the total 

29 
loading of organic carbon within the hypoxic area appears to be insignificant given the present body of

information. 

IMPLICATIONS 

The estimates used in this report to compare produced water inputs, either carbon or nitrogen, to similar

inputs from the Mississippi River are first order calculations with numerous assumptions. Estimates were 

made both across the large area where hypoxia occurs and in the area of South Timbalier Blocks 52 and

53 where more detailed data on oxygen and carbon dynamics are available for a suite of stations

(composite station C6*) in the core of the hypoxic zone (Figure 13). The inputs from the Mississippi

River are well characterized and modeled as to their effects on the continental shelf, including the

formation of hypoxia on fine temporal scale at composite station C6* and over a broad spatial scale from 

the Mississippi River to the upper Texas coast. 

More detailed calculations of produced water carbon and nitrogen inputs to the continental shelf may be 

needed to resolve which compounds are relevant to the stimulation of phytoplankton production and

respiration of fluxed organic matter, either directly or indirectly, and to what extent. The biochemical 

oxygen demand (BOD) of produced waters was not considered in this report, but may prove a useful line

of investigation to actual carbon loading that leads to oxygen consumption. There may also be a need to

distinguish between localized effects and regional effects. The suite of produced water discharges 

surrounding composite station C6* in OCS South Timbalier Blocks 52 and 53, however, appear to be an

average condition when compared with the distribution geographically and by volume of discharges that 

occur in the hypoxic zone (Figure 13). Inputs of produced water used to calculate loads of carbon and 

nitrogen in this report were assumed to be at or near the surface, but some discharges occur closer to the

bottom below the seasonal pycnocline. The fates of produced water effluents are likely to be different in

these two discharge scenarios (Figure 12). 

When considering whether or not to reduce produced water effluents that may contribute to respiratory

demand in waters of the hypoxic zone, data from this report and observations currently underway should

be placed within the context of what other mitigating measures might be taken within the Mississippi

River watershed (CENR 2000, Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001). 

The mitigating measures outlined in the Integrated Assessment and Action Plan are related primarily to 

the reduction of nutrients, both nitrogen and phosphorus, with a focus on nitrogen. The focus on nitrogen

derives from the strong empirical relationships of oxygen decrease through time with increasing nitrogen 

loads from the river and the close fit of models for oxygen dynamics on the shelf linked to nitrate flux.

Similar empirical relationships and modeling exercises with phosphorus are not robust. However,

management of both nitrogen and phosphorus is recommended as is the case elsewhere in the U.S. and 

globally.

The mitigation measures include a suite of agricultural best management practices, methods to facilitate 

the retention of nutrients and denitrification of nitrogen within the landscape, and tertiary treatment of

wastewater. There are differences among the many methods with regard to initial and long-term cost, and

immediate and long-term effectiveness of nutrient removal. Not all methods are conducive to the 

reduction of both nitrogen and phosphorus, but instead one or the other, and do not necessarily achieve

similar reductions for effort and cost expended. It is not known what potential reduction in nitrogen and 

organic carbon can be achieved for what cost through possible USEPA regulations on produced water 

discharge in the Gulf of Mexico OCS under a revised General Permit. 

The loads of nitrogen and organic carbon (in the form of oil and grease in the estimates) from produced 

water discharges in the hypoxic zone as estimated in this analysis are minimal compared to inputs from 

the Mississippi River and do not likely contribute in a significant way to the dynamics of hypoxia in the 

OCS. Current scientific knowledge is that nutrients from the Mississippi River are the primary cause of 

excess primary production that leads to hypoxia. The first order estimates of contribution of produced 

water carbon and nitrogen appear to support the current paradigm of causes of hypoxia. 

30 
SUMMARY 

Hypoxia is a seasonal, recurring feature of the Louisiana/Texas continental shelf adjacent to the

Mississippi and Atchafalaya river deltas. The coupled biological/physical system is driven by the 

freshwater discharge and nutrient loads from a river basin that encompasses 40 percent of the lower 48

United States. The strongest evidence in the current literature is for oxygen and carbon dynamics in the

hypoxic zone to be directly linked with seasonal stratification and nitrate flux from the Mississippi River. 

Increases in the overall productivity of the continental shelf ecosystem and the worsening of oxygen 

conditions below the pycnocline(s) is most closely correlated with changes in nitrate loads. The stratified

structure of the water column in which hypoxia can form and be maintained is required and physics often 

dictates the overall distribution of hypoxia. Changes in hypoxia over time are not correlated with annual 

changes in produced water discharges. There are no seasonal changes in produced water discharges as

there are for seasonal river discharge, nutrient flux and hypoxia. 

There is some evidence of hypersaline, oxygen-depleted plumes of produced water at the seabed during 

summer stratified conditions (e.g., selected production platforms in GOOMEX and Rabalais et al. unpubl. 

data from hypoxia studies 1985-2005). The plumes are not the source of the stratification but become 

more defined and differentiated in stratified water columns. The dissolved oxygen content of these 

hypersaline plumes is below that of the water 1 to 2 m higher in the water column and the bottom water 

surrounding the platform. The location of the discharge within the water column may affect the presence 

of a bottom saline plume. The produced water plume is seldom seen at the seabed during unstratified 

conditions. If discharged above a pycnocline typical of summertime hypoxic conditions and into water 

with a slow current speed, produced water discharge volumes up to 15,000 bbl d-1 would be diluted to less 

than 1 percent within 100 m of the discharge. 

There were many assumptions made and different data sets used for the first order estimates of produced 

water constituents that might contribute to the formation of hypoxia. These first order estimates of the 

contribution of carbon and nitrogen in produced water discharges compared to those of the Mississippi 

River indicated that the constituents from produced waters were minimal compared to those of the river. 

The produced water ammonium discharged within the area subject to hypoxia is estimated to be 0.013

percent of the total nitrogen delivered by the Mississippi River system, 0.008 percent of the total

dissolved inorganic nitrogen and 0.002 percent of the total ammonium. The ammonium in the produced

water discharge would reach the concentration of ambient dissolved inorganic nitrogen within 50 to 150 

m from the discharge. If discharged below the pycnocline in minimal currents typical of summer bottom 

water, the concentration of the ammonium in the discharge would exceed the ambient conditions for both 

ammonium and nitrate. 

Only up to 4 percent of the terrestrial carbon delivered to the Gulf of Mexico along with oil and grease 

from nonpoint source runoff are estimated to contribute to respiratory demand in water below the 

pycnocline in the area of hypoxia. The produced water oil and grease load for the Gulf of Mexico is 10.9

percent of the oil and grease load from the Mississippi River that might contribute to hypoxia. The 

remainder of the organic matter reaching the bottom and fueling respiratory consumption of dissolved 

oxygen comes from in situ phytoplankton production stimulated primarily by the nutrients from the 

Mississippi and Atchafalaya rivers. The produced water contribution of organic carbon in the total

loading of organic carbon within the hypoxic area appears to be insignificant given the present body of

information. 

31 
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37 
APPENDIX A COMPARATIVE CTD PROFILES 

Dissolved oxygen (mg l-1) and salinity (psu) profiles for stations C6 (triangles), C6B (circles) and C6C

(squares) for dates indicated; solid shapes are salinity and open shapes are dissolved oxygen. Data

source: Rabalais et al. unpubl data from hypoxia studies 1985-2005. 

A-1 
A-2 
A-3 
A-4 
A-5 
A-6 
A-7 
A-8 
A-9 
APPENDIX B AVERAGE PRODUCED WATER VOLUME IN A 5-, 10-, 

AND 20-MILE RADIUS OF COMPOSITE STATION C6* 

(RADIUS MEASURED FROM C6C LOCATION) (BBL 

MO-1) 

Year 5-Mile Radius 10-Mile Radius 20-Mile Radius

1984 269,408 405,651 1,520,797

1985 306,674 471,705 1,802,831

1986 325,443 557,275 1,806,944

1987 386,610 771,205 2,084,660

1988 503,472 945,987 2,374,878

1989 526,805 1,014,532 2,709,861

1990 547,367 968,061 2,524,706

1991 886,606 1,296,592 2,583,737

1992 887,682 1,246,444 2,384,940

1993 677,776 1,034,421 2,483,902

1994 784,031 1,185,625 2,619,488

1995 973,446 1,394,753 2,828,593

1996 1,241,297 1,796,231 3,109,922

1997 1,281,117 1,765,580 3,186,303

1998 1,057,323 1,861,618 3,428,311

1999 1,075,125 2,252,611 3,738,669

2000 1,196,975 2,603,766 3,875,918

2001 1,230,191 2,676,739 3,761,020

B-1 
The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

for most of our nationally owned public lands and natural resources. This includes fostering 

sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; 

preserving the environmental and cultural values of our national parks and historical places; 

and providing for the enjoyment of life through outdoor recreation. The Department assesses 

our energy and mineral resources and works to ensure that their development is in the best 

interests of all our people by encouraging stewardship and citizen participation in their care. 

The Department also has a major responsibility for American Indian reservation communities 

and for people who live in island territories under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) 

primary responsibilities are to manage the mineral resources located on the Nation's Outer 

Continental Shelf (OCS), collect revenue from the Federal OCS and onshore Federal and Indian 

lands, and distribute those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

administers the OCS competitive leasing program and oversees the safe and environmentally 

sound exploration and production of our Nation's offshore natural gas, oil and other mineral 

resources. The MMS Minerals Revenue Management meets its responsibilities by ensuring the 

efficient, timely and accurate collection and disbursement of revenue from mineral leasing and 

production due to Indian tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially 

affected parties and (2) carrying out its programs with an emphasis on working to enhance the 

quality of life for all Americans by lending MMS assistance and expertise to economic 

development and environmental protection.