Summary
Summer hypoxia (dissolved oxygen < 2 mg/L) in the bottom waters of the
northern Gulf of Mexico has received considerable scientific and policy
attention because of potential ecological and economic impacts. This hypoxic
zone forms off the Louisiana coast each summer and has increased from an
average of 8,300 km2 in 1985–1992 to over 16,000 km2 in 1993–2001,
reaching a record 22,000 km2 in 2002. The almost threefold increase
in nitrogen load from the Mississippi River Basin (MRB) to the Gulf since the
middle of the last century is the primary external driver for hypoxia.
A goal of the 2001 Federal Action Plan is to reduce the 5-year running
average size of the hypoxic zone to below 5,000 km2 by 2015. After
the Action Plan was developed, a new question arose as to whether sources other
than the MRB may also contribute significant quantities of oxygen-demanding
substances. One very visible potential source is the hundreds of offshore oil
and gas platforms located within or near the hypoxic zone, many of which
discharge varying volumes of produced water.
The objectives of this study were to assess the incremental impacts of
produced water discharges on dissolved oxygen in the northern Gulf of Mexico,
and to evaluate the significance of these discharges relative to loadings from
the MRB. Predictive simulations were conducted with three existing models of
Gulf hypoxia using produced water loads from an industry study. Scenarios were
designed that addressed loading uncertainties, settleability of suspended
constituents, and different assumptions on delivery locations for the produced
water loads. Model results correspond to the incremental impacts of produced
water loads, relative to the original model results, which included only loads
from the MRB.
The predicted incremental impacts of produced water loads on dissolved
oxygen in the northern Gulf of Mexico from all three models were small. Even
considering the predicted ranges between lower- and upper-bound results, these
impacts are likely to be within the errors of measurement for bottomwater
dissolved oxygen and hypoxic area at the spatial scale of the entire hypoxic
zone.
Introduction
Summer hypoxia in the bottom waters of the northern Gulf of Mexico has
received considerable scientific and policy attention because of potential
ecological and economic impacts from this very large zone of low oxygen, and
because of the implications for management within its massive watershed (CENR
2000; EPA 2001). These regions of oxygen concentrations below 2 mg/L that form
off the Louisiana coast each spring and summer increased from an average of
8,300 km2 in 1985–1992 to over 16,000 km2 in 1993–2001
(Rabalais et al. 2002), and reached a record 22,000 km2 in 2002.
There is significant interannual variability and no comprehensive records of
areal extent exist prior to 1985. Fig. 1 is a composite plot that shows the
frequency of occurrence of midsummer hypoxia in the Gulf of Mexico from 1985 to
1999.
An assessment of hypoxia causes and consequences (CENR 2000; Rabalais et al.
2002) concluded that the almost threefold increase in nitrogen load to the Gulf
(Goolsby et al. 2001) is the primary external driver that stimulated the
increase in hypoxia since the middle of the last century. This riverine
nitrogen input stimulates coastal algal production and the subsequent settling
of organic matter below the pycnocline. Because the pycnocline inhibits
vertical oxygen flux, decomposition of organic matter below the pycnocline
consumes oxygen faster than it is replenished, resulting in declining oxygen
concentrations during the period of stratification.
The Federal-State-Tribal Action Plan for reducing, mitigating, and
controlling hypoxia in the northern Gulf of Mexico (EPA 2001) included a goal
of reducing the 5-year running average size of the hypoxic zone to below 5,000
km2 by 2015. After the Action Plan was developed, a new question
arose as to whether sources other than the Mississippi River Basin (MRB) may
also contribute significant quantities of oxygen-demanding substances. One very
visible potential source is the hundreds of offshore oil and gas platforms
located within or near the hypoxic zone. Many of these platforms discharge
varying volumes of produced water. Produced water is trapped in underground
formations and is brought to the surface along with oil or gas.
Fig. 2 (J.P. Smith, ExxonMobil Upstream Research Company, personal
communication) shows the frequency of occurrence of midsummer hypoxia
superimposed on a lease block map of the Gulf of Mexico. The blue dots on the
map show platforms in the region. There are an estimated 287 platforms in the
hypoxia zone (Veil et al. 2005); however, not every platform is a produced
water discharge point. The red boxes indicate the lease block areas that are in
the hypoxic zone. Each box is marked with the number of lease blocks in the
area that have produced water discharges and that are in the hypoxic zone.
Until recently, only limited data characterizing oxygen demand, nutrient
concentrations, and loadings from offshore produced water discharges had been
collected. These discharges are authorized by a general permit issued by EPA
under the National Pollutant Discharge Elimination System (NPDES). As part of
the reissuance of this permit in 2004, EPA required that the industry provide
information on the amount of oxygen-demanding substances contained in the
produced water discharges.
Objectives
The objectives of this study were to assess the incremental impacts of
produced water discharges on dissolved oxygen conditions in the northern Gulf
of Mexico, and to evaluate the significance of these discharges relative to
loadings from the MRB. This study was conducted using the three existing models
of Gulf hypoxia described in Scavia et al. (2004). Results from this study will
provide EPA with an initial assessment of the appropriate forward path for how
to incorporate produced water discharges within the overall framework for
controlling nutrient loadings to the Gulf of Mexico as a management tool for
reducing the occurrence and extent of hypoxia.
This paper is based on a report submitted to EPA in July 2006 (Limno-Tech
2006) and it contains the principal study results and conclusions. The complete
study, including detailed descriptions of the models and all tabular and
graphical results, is documented in the report to EPA.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
9 January 2007
- Meeting paper published:
5 March 2007
- Revised manuscript received:
14 December 2007
- Manuscript approved:
18 December 2007
- Version of record:
15 June 2008
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