The three Louisiana coastal water areas that EPA is adding to the list of impaired State waters includes (out to the State three-mile limit): (1) Terrebonne Basin Coastal Bays and Gulf Waters; (2) Mississippi River Basin Coastal Bays and Gulf Waters; and (3) Barataria Basin Coastal Bays and Gulf Waters. The EPA letter states, in part, that "the three coastal waters that EPA is proposing to add to the State § 303( d) List are affected by low dissolved oxygen concentrations or hypoxia that may result, in part, from excessive nutrient loadings from the Mississippi and Atchafalaya Rivers." EPA noted in the letter that it "recognizes that nutrient load reductions may be needed from all states in the Mississippi River basin to reduce the size of the Gulf hypoxic zone and restore Louisiana coastal waters to full attainment of applicable water quality standards." And goes on to state that "continued State and Federal support in the planning and implementation of coastal restoration projects, including large-scale reintroductions of the Mississippi River and beneficial use of dredged material, provide excellent opportunities to both restore Louisiana's vanishing wetlands and reduce the nutrient load flowing directly into the Gulf of Mexico."
As described in a Record of Decision accompanying the letter, EPA found that "the State determined that three coastal segments . . . are exceeding applicable marine dissolved oxygen criterion but that those segments are not required to be listed on the State’s §303(d) list," instead placing them in "Category 4b . . . defined by the EPA’s 2008 Integrated Reporting Clarification Memorandum as waters for which 'other required control measures are expected to result in the attainment of an applicable water quality standard in a reasonable period of time.'" To support its finding, the State had cited ongoing efforts associated with the 2008 Gulf Hypoxia Action Plan (a plan describing a "national strategy to reduce, mitigate, and control hypoxia in the Northern Gulf of Mexico and improve water quality in the Mississippi River Basin"). Louisiana indicated that Action Plan efforts would ultimately result in water quality attainment for the three coastal sections. EPA, however, disagreed with the State's conclusions in that regard, noting that the "Gulf Hypoxia Action plan does not, by itself, meet the requirements" of applicable regulations and relevant EPA guidance in the matter.
With an upcoming publication of a formal Federal Register notice, EPA will open a 30-day public comment period on the proposed additions to the § 303( d) list and will consider any comments received before finalizing its decision.
The Louisiana coastal area involved in the EPA decision are part of what has been called the Gulf of Mexico "dead" or "hypoxic" zone, which the USGS describes as "an area along the Louisiana-Texas coast in which water near the bottom of the Gulf contains less than 2 parts per million of dissolved oxygen, causing a condition referred to as hypoxia. Hypoxia can cause fish to leave the area and can cause stress or death to bottom dwelling organisms that can’t move out of the hypoxic zone. Hypoxia is believed to be caused primarily by excess nutrients delivered from the Mississippi River in combination with seasonal stratification of Gulf waters. Excess nutrients promote algal and attendant zooplankton growth. The associated organic matter sinks to the bottom where it decomposes, consuming available oxygen. Stratification of fresh and saline waters prevents oxygen replenishment by mixing of oxygen-rich surface water with oxygen-depleted bottom water."
Most of the nutrients referred to by the USGS originate from agricultural runoff in the Mississippi River Basin (see, for example, this National Research Council 2009 report, "Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico").