Survival and growth of the dominant salt marsh grass Spartina alterniflora in an oil industry saline wastewater

Life Cycle of Oil and Gas Fields in the Mississippi River Delta: A Review

Oil and gas (O&G) activity has been pervasive in the Mississippi River Delta (MRD). Here we review the life cycle of O&G fields in the MRD focusing on the production history and resulting environmental impacts and show how cumulative impacts affect coastal ecosystems. Individual fields can last 40–60 years and most wells are in the final stages of production. Production increased rapidly reaching a peak around 1970 and then declined. Produced water lagged O&G and was generally higher during declining O&G production, making up about 70% of total liquids. Much of the wetland loss in the delta is associated with O&G activities. These have contributed in three major ways to wetland loss including alteration of surface hydrology, induced subsidence due to fluids removal and fault activation, and toxic stress due to spilled oil and produced water. Changes in surface hydrology are related to canal dredging and spoil placement. As canal density increases, the density of natural channels decreases. Interconnected canal networks often lead to saltwater intrusion. Spoil banks block natural overland flow affecting exchange of water, sediments, chemicals, and organisms. Lower wetland productivity and reduced sediment input leads to enhanced surficial subsidence. Spoil banks are not permanent but subside and compact over time and many spoil banks no longer have subaerial expression. Fluid withdrawal from O&G formations leads to induced subsidence and fault activation. Formation pore pressure decreases, which lowers the lateral confining stress acting in the formation due to poroelastic coupling between pore pressure and stress. This promotes normal faulting in an extensional geological environment like the MRD, which causes surface subsidence in the vicinity of the faults. Induced reservoir compaction results in a reduction of reservoir thickness. Induced subsidence occurs in two phases especially when production rate is high. The first phase is compaction of the reservoir itself while the second phase is caused by a slow drainage of pore pressure in bounding shales that induces time-delayed subsidence associated with shale compaction. This second phase can continue for decades, even after most O&G has been produced, resulting in subsidence over much of an oil field that can be greater than surface subsidence due to altered hydrology. Produced water is water brought to the surface during O&G extraction and an estimated 2 million barrels per day were discharged into Louisiana coastal wetlands and waters from nearly 700 sites. This water is a mixture of either liquid or gaseous hydrocarbons, high salinity (up to 300 ppt) water, dissolved and suspended solids such as sand or silt, and injected fluids and additives associated with exploration and production activities and it is toxic to many estuarine organisms including vegetation and fauna. Spilled oil has lethal and sub-lethal effects on a wide range of estuarine organisms. The cumulative effect of alterations in surface hydrology, induced subsidence, and toxins interact such that overall impacts are enhanced. Restoration of coastal wetlands degraded by O&G activities should be informed by these impacts.

Potential of Spartina maritima in restored salt marshes for phytoremediation of metals in a highly polluted estuary

Sedimentary abiotic environment, and concentration and stock of nine metals were analyzed in vegetation and sediments to evaluate the phytoremediation capacity of restored Spartina maritima prairies in the highly polluted Odiel Marshes (SW Iberian Peninsula). Samples were collected in two 10 -m long rows parallel to the tidal line at two sediments depths (0-2 cm and 2-20 cm). Metal concentrations were measured by inductively coupled plasma spectroscopy. Iron, aluminum, copper, and zinc were the most concentrated metals. Every metal, except nickel, showed higher concentration in the root zone than at the sediment surface, with values as high as ca. 70 g Fe kg(-1). The highest metal concentrations in S. maritima tissues were recorded in its roots (maximum for iron in Spartina roots: 4160.2 +/- 945.3 mg kg(-1)). Concentrations of aluminum and iron in leaves and roots were higher than in superficial sediments. Rhizosediments showed higher concentrations of every metal than plant tissues, except for nickel. Sediment metal stock in the first 20 cm deep was ca. 170.89 t ha(-1). Restored S. maritima prairies, with relative cover of 62 +/- 6%, accumulated ca. 22 kg metals ha(-1). Our results show S. maritima to be an useful biotool for phytoremediation projects in European salt marshes.