Shallow gas in near-surface sediments is another geologic hazard that can cause concern for oil and gas operators. Decomposition of trapped organic mater is the primary sources of biogenic gas. Thermogenic gas, originating in deeply buried source rocks, can migrate upward and also become trapped in shallow sediments. Drilling into gas can pose problems since a large amount of gas can lower the density of the mud and can contribute to seafloor instability and slope failure.

Studies developed under the Environmental Studies Program have been directed toward areas where more detailed geologic information was needed for management of the OCS mineral leasing program. These studies have provided assessments of operational constraints to oil and gas exploration and production. The data and mapped information are being utilized on a daily basis for tract evaluation, stipulation development and application, pipeline permit processing, protection of sensitive areas such as the Flower Garden Banks, preliminary platform emplacement, and pipeline routing.

(3) Nonenergy Marine Minerals

Several minerals in the north central Gulf of Mexico have the potential to become commercially exploitable. Deposits of quartz sand in Federal and State waters off the coasts of Mississippi and Alabama are of interest because of their potential use in the production of glass. Sulphur and salt deposits associated with the larger number of diapiric and domed structures in the OCS seafloor have mining potential. As of December 1994, there were two producing sulphur operations on the OCS. The Main Pass Sulphur Mine, located east of the modern Balize Delta lobe of the Mississippi River, has sulphur reserves that exceed all of the other remaining U.S. reserves combined. The other offshore sulphur operation is the Caminada Mine south of Grand Isle.

In addition to those discussed above, known mineral resources in adjacent coastal areas include phosphate, quartz, sand, sulphur, salt, oyster-shell, limestone, sand and gravel, and magnesia. As part of the national initiative to develop the marine mineral resources of the OCS, MMS and the Gulf coastal States completed preliminary economic reconnaissance studies of nonenergy marine minerals. Offshore sand deposits may provide material for coastal restoration, protection against accelerating rates of wetland loss, and beach replenishment. Offshore sand also has potential use in the production of glass. The Geological Survey of Alabama has completed two reports, in 1993 and 1995, entitled "Geologic, economic, and environmental characterization of selected near-term leasable offshore sand deposits and competing onshore sources for beach nourishment."

b. Physical Oceanography

The Gulf of Mexico is a semi-enclosed, subtropical sea with a surface area of approximately 1.6 million km2. The main physiographic regions of the Gulf basin are the continental shelf (including the Campeche, Mexican, and U.S. shelves), continental slopes and associated canyons, the Yucatan Channel and the Straits of Florida, and the abyssal plains. The continental shelf width along the U.S. coastline reaches a minimum off the Mississippi River, and evidence suggests that it effectively splits the shelf into the Texas-Louisiana western province and the Mississippi-Alabama-Florida eastern province. The Gulf is unique because it has two entrances: the Yucatan Channel and the Straits of Florida. The two entrances restrain communication from the deep Atlantic waters because of their limited sill depths to the central abyss which reach depths of 4,000 m. The Gulf's general circulation is dominated by the Loop Current and its associated eddies. Coastal and shelf circulation on the other hand is driven by several forcing mechanisms: wind stress, freshwater input, buoyancy and mass fluxes, and transfer of momentum and energy through the seaward boundary.

Sea surface temperatures in the Gulf range from nearly isothermal (29°C-30°C) in August to a sharp horizontal gradient in January, (25°C in the Loop core to 14°C-15°C) along the northern shelves. Surface salinities along the northern Gulf are seasonal. During months of low freshwater input, salinities near the coastline range between 29-32 parts per thousand (ppt). High, freshwater input conditions (spring-summer months) are characterized by strong horizontal salinity gradients and inner shelf values of less than 20 ppt (Wallace, 1980; Cochrane and Kelly, 1986). Vertical profiles of temperature, salinity, oxygen, and nutrients indicate the presence of five major water masses. The vertical distribution of temperature reveals that in January, the thermocline depth is about 30 to 61 m in the northeastern Gulf and 91 to 107 m in the northwestern Gulf. In May, the thermocline depth is about 46 m throughout the entire Gulf (Robinson, 1973).

Sharp discontinuities of temperature and/or salinity at the sea surface are known as fronts. Fronts in the open Gulf are associated with the Loop Current and eddies. A coastal front that lies about 30 to 50 km offshore is probably a permanent feature of the Gulf shelf. These fronts concentrate buoyant material such as spilled oil, detritus, or plankton.

The Loop Current is a highly variable current entering the Gulf through the Yucatan Channel and exiting through the Straits of Florida (as the Gulf Stream) after tracing an arc that may intrude as far north as the Mississippi-Alabama shelf. Velocities up to 300 cms1 have been measured, but a range of 100 to 200 cms1 is probably representative of this current. The volumetric flux of the Loop has been estimated at 30 million m's1.

The currents signature has been detected down to about 1,000 m. Below that level, there is evidence of a countercurrent. The "location" of the Loop Current is definable only in statistical terms, due to its great variability. Location probabilities during March, the month of greatest apparent intrusion, range from 100 percent in the core location at 25°N latitude, down to small probabilities (10%) near midshelf. A central tendency analysis indicates an average northern intrusion to 26.6°N latitude, within a wide envelope. Both analyses exclude the June-October period due to limitations on satellite data. When the Loop extends into or near shelf areas, instabilities may develop that can intrude warm water into the shelf or entrain cold water from the shelf.

Eddy-shedding is responsible for the first-order variability of the Loop and the principal mechanism coupling the circulation patterns of the eastern and western parts of the Gulf's basin. These eddies consist of warm water rotating in an anticyclonic or clockwise fashion. Major Loop Current eddies have diameters on the order of 300 to 400 km and may extend vertically to a depth of about 1,000 m. Once these eddies are free from the Loop, they travel into the western Gulf along various paths to a region between 25°N to 28°N latitude and 93°W to 96°W longitude. Travel speeds ranged from 2 to 5 kms1, and tangential velocities within the eddies have been reported from 50 to 200 cms1. As eddies travel westward, a decrease in size occurs due to mixing with resident waters and friction with the slope and shelf bottoms. The life of an individual eddy to its eventual assimilation by regional circulation in the western Gulf is about 1 year. The eddy-shedding period has been recently estimated between 6.5 and 9.5 months, with an average of 7.5 months (Hamilton et al., 1989). Because of their large size, once an eddy is shed, the Loop undergoes major dimensional adjustment and reorganizations. Eddies are frequently observed to affect local current patterns along the Louisiana/Texas slope, hydrographic properties, and possibly the biota of fixed platforms or hard bottoms. The heat and salt budgets of the Gulf are dependent on this importation, balanced by seasonal cooling and river input. Also, there is some evidence that these large reservoirs of warm water play some role in strengthening tropical cyclones when their paths coincide.

Smaller anticyclonic eddies have been observed to be generated by the Loop Current. They have diameters on the order of 100 km, and the few data available indicate a shallow vertical extent (ca. 200 m). Observations indicate a tendency to translate westward along the Louisiana/Texas slope. Also, cyclonic eddies associated with the eddy-shedding cycle have been observed in the eastern Gulf and the Louisiana/Texas slope. Their origin and role in the overall circulation are presently not well understood. A major eddy seems to be resident in the southwestern Gulf, however, recent evidence points toward a more complex, and less homogeneous structure.

Shelf circulation is complicated because of the large number of forces and seasonality of driving forces. A northward current driven by prevailing winds and the semipermanent anticyclonic eddy exist offshore of south Texas. A strong east-northeasterly current along the remaining Texas and Louisiana slope has been explained partly by the effects of the semipermanent anticyclonic eddy and a partner cyclonic eddy ("modon pair") and partly by the mass balance requirements of eddy movement. West of Cameron, Louisiana (93°W longitude), current measurements clearly show a strong response of coastal current to the winds, setting up a large-scale anticyclonic gyre. The inshore limb of the gyre is the westward or southwestward (downcoast) component that prevails along much of the coast, except in July-August.

Because the coast is concave, the shoreward prevailing wind results in a convergence of coastal currents at a location where the winds are normal to the shore or at the downcoast extent of the gyre. A prevailing countercurrent toward the northeast along the shelf edge constitutes the outer limb of the gyre. The convergence at the southwestern end of the gyre migrates seasonally with the direction of the prevailing wind, ranging from a point south of the Rio Grande in the fall to the Cameron area by July. The gyre is normally absent in July but reappears in August-September when a downcoast wind component develops (Cochrane and Kelly, 1986).

A circulation pattern on the Mississippi/Alabama (MA) shelf is emerging, but, improvements are still needed. The circulation on this area is driven mostly by Loop Current and associated eddies intrusions, tides, winds, and freshwater. Geometry and topography affects the region's circulation patterns. The inner extends to an area between 30 and 60 m isobaths, the outer shelf extends from these area to the shelf break at about 100 m (Kelly 1991). Even though tides are a significant component of the currents, wind forcing dominates the forcing of the mean inner shelf circulation (Vittor et al. 1985; Dinnel 1988; Kelly 1991). Even though drifter data suggest a westward flow near the delta and eastward flow farther offshore, Wiseman and Dinnel (1988) found weak currents during normal conditions. During Loop Current intrusions the current south of the delta exhibit large fluctuations. At water depths of 90 and 190 m in the MA shelf the flow is eastward on the average. The first circulation scheme described in the literature (see Vittor et al. 1985) suggest that winds with easterly components drive the surface and bottom water onshore. The onshore flow piles water near the coast and creates a pressure gradient that in time will stop onshore motion and drive bottom water alongshore. Under strong easterly winds, the pressure gradient could be strong enough to cause offshore flow. Northeasterly winds drives the bottom flow initially onshore, then topography deflects this flow westward. Under SE winds the flow is initially onshore and turns eastward by the topography. Using historical current and hydrographic data, Dinnel (1988) proposed another circulation scheme. His scheme calls for two counter rotating cells over the shelf. In Spring and Summer the inner shelf flow is westward and outer shelf is eastward. In midshelf, the flow results from an assumed convergence of the inner shelf cyclonic cell and the outer shelf anticyclonic cell. In Fall, the two counterrotating cells still prevail, but onshore flow from the delta in the west and offshore flow in the eastern side are indicated. In winter, the circulation consist of one cyclonic cell over the shelf with eastward flow over the slope. Loop Current interactions with the shelf which can occur in three possible ways (Kelly 1991) modulates the mean wind driven cyclonic circulation, and even reverse and dominate the shelf circulation.

The west Florida shelf circulation is dominated by tides, winds, eddy-like perturbations, and the Loop Current. The flow structure appears to consist of three regimes: the outer shelf, the mid-shelf, and the coastal boundary layer. Also, the Loop current and eddy-like perturbations are felt stronger in this region. During Loop intrusion events, upwelling of colder, nutrient-rich waters has been observed. In water depth less than 30 m the wind-driven flow is mostly alongshore and parallel to the isobaths. A weak mean flow is directed southward in the surface layer. In the coastal boundary layer, longshore currents driven by winds, tides, and density gradients predominate over the cross-shelf component (Science Application International Corporation (SAIC), 1986). The nonextreme wind-driven flow ranges from moderate to strong (25 to 50 cms1), and the tidal components are moderate (maximum speeds of about 15 cms1). Longshore currents due to winter northers, tropical storms, and hurricanes may range up to hundreds of cms1, depending on local topography, fetch, and duration.

Further information on the physical oceanography of the Gulf using surface drifters can be found in Parker et al. (1979) and Williams et al. (1977).

The chemical oceanography of the Gulf of Mexico is primarily influenced by its configuration and the large volumes of land runoff it receives. The Gulf of Mexico is a semi-enclosed waterbody with oceanic input through the Yucatan Channel via the Caribbean and with principal outflow through the Straits of Florida. The Mississippi River, as well as a host of other major drainage systems, result in freshwater inputs from approximately two-thirds of the area of the United States and more than one-half the area of Mexico. This large amount of runoff, with its nonoceanic composition, mixes into the surface water of the Gulf and makes the chemistry of parts of this system quite different from that of the open ocean. Sea surface salinities along the northern Gulf are seasonal. During months of low freshwater input, salinities near the coastline range between 29 to 32 ppt. High, freshwater input conditions during the spring and summer months result in strong horizontal salinity gradients and inner shelf values less than 20 ppt. The mixed layer, extending to a depth of approximately 100 to 150 m, is characterized by salinities between 36.0 and 36.5 ppt in the open Gulf (Barnard and Froelich, 1981). Dissolved oxygen values in the mixed layer average about 4.6 milliliters/Liter (ml/L), with certain seasonal variations, particularly a slight lowering during the summer months. Oxygen values generally decrease to about 3.5 ml/L with depth through the mixed layer (Barnard and Froelich, 1981). Vertical profiles of temperature, salinity, oxygen, and phosphate identify five major water masses down to 1,000 m. The principal nutrients, phosphate, nitrate, and silicate, generally are depleted in the surface mixed layer. Phosphates range from 0 to 0.25 parts per million (ppm), averaging 0.021 ppm; silicates predominantly from 0.048 to 1.9 ppm; and nitrates from 0.0031 to 0.14 ppm, averaging 0.014 ppm.

Two types of unusual watermasses can be found in the Gulf of Mexico: hypersaline basins and midshelf freshwater vents. Two basins containing hypersaline waters have been identified. Salinities as high as 196 ppt at a small pool on the East Flower Garden topographic high and 250 ppt in the Orca Basin have been measured (Barnard and Froelich, 1981; Addy and Behrens, 1980). The southwest Florida shelf contains a number of submarine freshwater springs found in association with extensive karst topography.

A common phenomenon in the Gulf, especially on the shelf, is the local presence of greatly elevated levels of suspended material with values above 1 ppm. Typically, sharp discontinuities of suspended particulate matter separate existent near-bottom layers of turbid water from overlying waters. These nephloid layers may be associated with resuspension of sediments by bottom currents, internal waves, intense at-depth biological

activity, or a complex combination of these factors. These features appear to occur naturally at nearly all locations on the shelf and upper slope environment, except the promontories of significant topographic highs (Brooks et al., 1981).

Degradation of the Gulf's marine waters is associated with coastal runoff and discharges, riverine inputs and, to a smaller extent, effluent discharges from offshore activities, primarily OCS oil and gas development and marine transportation. Not only do the river systems, particularly the Mississippi River, bring freshwaters to the Gulf, they carry large amounts of contaminants from the extensive agricultural activities, hundreds of cities, and thousands of industries, that drain their waters into the rivers that eventually reach Gulf waters. Offshore water quality problems that have been most apparent include floatable debris, hypoxic or oxygendepleted conditions, and toxic and pathogen contamination.

Offshore on the OCS, as of March 1996, approximately 32,500 wells have been drilled and about 3,800 platforms are producing. In 1993, approximately, 300 million barrels of crude oil and 4.6 tcf of gas was produced and shipped to shore by pipeline. Although such activity seems extensive, the maritime industry's use of Gulf waters is even greater. The volume of crude oil imported in 1993 through Gulf waters by tanker was approximately 1.5 billion barrels; about five times the volume piped from domestic production. Adding to this, about 236 million bbl of petroleum products were imported in Gulf waters, and 175 million bbl were exported. Although petroleum, both crude oil and petroleum products, is the commodity shipped the most through Gulf waters, vessel traffic associated with other commodities is extensive, and the Gulf has four of the top 10 busiest ports in the United States. All of these offshore activities discharge their effluent waters into the Gulf, and such extensive vessel traffic frequently results in large oil spills occurring. A description of nearshore and coastal inputs are discussed under coastal water quality, following this section.

Hypoxia has been found to be occurring in some areas of open Gulf bottom waters (Rabalais et. al., 1995). A zone of hypoxia affecting up to 16,500 square kilometers of bottom waters during mid-summer on the inner continental shelf from the Mississippi River delta to the upper Texas coast has been identified. Researchers have expressed concern that this zone may be increasing in frequency and intensity. Although the causes of this hypoxic zone have yet to be conclusively determined, high summer temperatures combined with freshwater runoff carrying large amounts of excess nutrients from the Mississippi River has been implicated. Benthic fauna studied within the area exhibited a reduction in species richness, abundance, and biomass that was much more severe than documented in other hypoxia-affected areas (Rabalais et al. 1994). Although the Mississippi and Alabama inner shelf has the potential for bottom water hypoxia, and low oxygen concentrations have been documented, such events are not considered frequent or widespread (Rabalais, 1992).

Red tides are a natural phenomenon in the Gulf, primarily off Florida, Texas, and Mexico, but may be influenced by anthropogenic inputs. The first documented case of red tide occurred in 1972. Red tides are caused by periodic blooms of a single-cell algae that produce potent toxins harmful to marine organisms and humans. They can result in severe economic and public health problems and are associated with fish kills and invertebrate mortalities. Some scientists are studying the question whether human activity, the increased nutrients loadings to Gulf waters from coastal runoff, contributes to the intensity of red tides. Between 1975 and 1990, red tides in the eastern Gulf of Mexico generally occurred in the fall and winter in the area between Tampa Bay and Charlotte Harbor, Florida (EPA, 1991).

(2) Coastal

Major activities affecting Gulf coastal water quality include those associated with the petrochemical industry, hazardous and oilfield wastes disposal sites, agricultural and livestock farming, power plants, pulp and paper