ACCESS NUMBER: 36634 

STUDY TITLE: Contribution of Produced Water Discharge Oxygen Demand in the 

Development of Hypoxia 

REPORT TITLE: Relative Contribution of Produced Water Discharge Oxygen Demand 

in the Development of Hypoxia 

CONTRACT NUMBER(S): PO 0104-PO-36634 

SPONSORING OCS REGION: Gulf of Mexico Region 

APPLICABLE PLANNING AREA(S): Central Planning Area 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 2004 

COMPLETION DATE OF REPORT: April, 2005 

COST(S): FY 2005 $22,575; CUMULATIVE PROJECT COST: $22,575 

PROJECT MANAGER(S): Nancy Rabalais 

AFFILIATION (OF PROJECT MANAGER): LUMCOM 

ADDRESS: Louisiana Universities Marine Consortium, Cocodrie, LA 70344 

PRINCIPAL INVESTIGATOR: Nancy Rabalais 

KEY WORDS: hypoxia, produced water, Mississippi River, dissolved oxygen, nitrogen, 

biochemical oxygen demand, organic carbon 

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. 

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

a mixture of hypersaline water with varying volatile, water soluble, and more recalcitrant 

hydrocarbons, sulfur and nitrogen compounds, as well as residual chemicals added to 

assist in production or to treat the produced water prior to discharge. 

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 
ACCESS NUMBER: 36634 

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. A 

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

or NPDES regulation. The potential for produced water constituents to contribute to the 

biochemical oxygen demand (BOD) in offshore waters has not been studied. Relevant 

information exists in many studies that can be summarized and synthesized to address 

this question. 

OBJECTIVES: 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. 

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 the dynamics of hypoxia, the 

characterization of produced waters and their discharge into the northwestern Gulf of 

Mexico, and 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. 

DESCRIPTION: 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 a smaller area in the core of the hypoxic zone where 

more detailed data on oxygen and carbon dynamics are available. The inputs from the 

Mississippi River are well characterized and modeled as to their effects on the 

continental shelf over a broad spatial scale from the Mississippi River to the upper 

Texas coast. The comparisons of limited available produced water constituent loads 

with much better characterized Mississippi River data are made with numerous 

assumptions. 

How well multiple forms of data for organic carbon correspond and what nitrogen 

compound concentrations are in produced water are not known. The first order 

estimations lean to worst-case scenarios for the produced water carbon and nitrogen 
ACCESS NUMBER: 36634 

inputs, but the expected dilutions and dispersions correspond to available models. The 

oil and grease loadings do not track well to the total organic carbon content of produced 

water nor do they distinguish among the hydrocarbon compounds that vary with regard 

to volatility, solubility, photo-oxidation, 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 include 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. 

SIGNIFICANT CONCLUSIONS: First order estimations of the contribution of carbon 

and nitrogen in produced water discharges were compared to the Mississippi River 

contribution. Carbon and nitrogen in produced water contribute minimally in comparison 

to the river within the area subject to hypoxia. 

STUDY RESULTS: 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. Without 

appropriate total organic carbon, dissolved organic carbon and particulate organic 

carbon (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. 

STUDY PRODUCT(S): Rabalais, N. N. 2005. Relative Contribution of Produced Water 

Discharge Oxygen Demand in the Development of Hypoxia. OCS Study MMS 2005-

044. U.S. Dept. of the Interior Mineral s Management Service, Gulf of Mexico OCS 

Region, New Orleans, La. 47 pp. + Appendix 10pp.