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USGS1979

Hydrocarbon geochemical analyses of Mid-Atlantic Outer Continental Shelf sediments: an environmental assessment

Concentration levels of hydrocarbon in 122 composite mid-Atlantic shelf surface sediment samples have been examined during the second year (1976-1977) of the Bureau of Land Management environmental assessment program. The objective of this study was to establish baseline concentrations and provide data for the "natural variability" curve for the mid-Atlantic region. Concentration levels ( μ g/g) have been determined for the total lipids recovered by liquid column chromatography, and hexane-soluble fraction (aliphatics) benzene fraction (aromatics) as resolved by gas chromatography for each composite sediment sample. Several geochemical parameter values were also determined among which were: 1) n-alkane odd-even ratio; 2) isoprenoid alkane/ n-alkane ratio; 3) pristine/phytane ratio; 4) pristane/n-C 17 ratio; and 5) phytane/n-C 18 ratio. Summary analyses of the concentrations and geochemical 18 parameters are included in this report. Surface sediments examined from the mid-Atlantic shelf contained very low concentrations of resolved aliphatic hydrocarbons, less than 5.0 μ g/gm sediment, and very low concentrations of resolved aromatic hydrocarbons, usually less than 1.0 μ g/gm sediment. Mass spectral analyses of selected hexane eluates confirmed the presence of a series of normal aliphatic alkanes ranging from to n-C 16 to n-C 31 as well as the isoprenoids pristane and phytane. A series of three hydrocarbon peaks in the n-C 20 to n-C 21 range were tentatively identified as n-C 25 unsaturated, branched homologs, The aromatic fractions contained a number of polynuclear aromatics including phenanthrenes, methylphenanthrenes, fluoranthene and pyrcne, benzofluoranthene, and alkyl benzenes. Data accumulated in the second year of the program, coupled with data from the first year's investigations (Miller et al., 1977), have provided the basis for establishing a preliminary natural. "variability curve" for the hydrocarbon concentration levels and distribution in these selected mid-Atlantic shelf samples. This curve shows the seasonal variability noted in both the aliphatic and aromatic hydrocarbons in these samples.

Mid-Atlantic Outer Continental Shelf↗

USGS1977

An environmental assessment of hydrocarbons in mid-Atlantic shelf sediments: 1975-1976 USGS-BLM program

During 1975-1976, the U.S. Geological Survey (U.S.G.S.), in conjunction with the Bureau of Land Management and the Virginia Institute of Marine Sciences, carried out an environmental assessment program to establish baseline concentration levels of hydrocarbons in the surface sediments of the Mid-Atlantic Shelf where leasing for petroleum exploration is anticipated. Reported here are the results of the individual hydrocarbon analyses for 150 composite surfacesediment samples.

Open-File Report↗

USGS1968

Ground-water hydrology of the Chad Basin in Bornu and Dikwa Emirates, northeastern Nigeria, with special emphasis on the flow life of the artesian system

Bornu and Dikwa Emirates lie in the Nigerian sector of the Chad Basin, a vast region of interior drainage encompassing about 600,000 square miles of north-central Africa. The report area includes about 25,000 square miles of the basin that lie in Nigeria. Most of the area is a featureless plain that slopes gently northeast and east from the uplands of central Nigeria towards Lake Chad. On its eastern side the lake has one surface outlet which overflows only during exceptionally high stages of the lake. This outlet spills into the channel of Bahr al Ghazal, which in turn drains into the Bod616 depression. Because the lake is shallow, the shoreline fluctuates markedly with high and low stages corresponding to the wet and dry seasons. The semiarid climate of Bornu and Dikwa Emirates is characterized by a long dry season and a short wet season that correspond to seasonal winds. Annual rainfall ranges from 15 inches in the northern part of the area to 32 inches in the southern. The Chad Basin in Dikwa and Bornu Emirates is underlain by interbedded sand and clay, collectively termed the Chad Formation. These alluvial and lactustrine sediments were deposited in or near Lake Chad whet it occupied a much greater area during Pliocene and Pleistocene time. The Chad Formation has a very slight primary dip in the direction of Lake Chad and conforms to the gentle slope of land surface. The known thickness of the formation ranges from a few feet where it overlies bedrock on the periphery of the basin to at least 1,800 feet at Maiduguri; however, its total thickness probably exceeds 2,000 feet in the central part of the basin. Three water-bearing units termed upper, middle, and lower zones occur within the Chad Formation. The upper zone yields water to numerous dug wells throughout the rural areas and also is .the major source of the Maiduguri municipal water .supply. The middle zone yields water from flowing artesian boreholes that have heads ranging from a few feet to 70 feet above land surface throughout a 13,000 square-mile area of the basin in Nigeria. The lower zone also yields water from flowing boreholes ; however, its areal extent has not been proved beyond the environs of Maiduguri. The present investigation is concerned primarily with the middle zone, which is the source of water for some 190 flowing boreholes used as little-watering points in the Nigerian sector of the Chad Basin. The thickness of loads of waterbearing sand in the middle zone ranges from less than 1 foot to 200 feet, and the artesian head ranges from land surface at Maiduguri to 70 feet above land surface at Lake Chad. The depth to the top of the middle zone in the area of flowing boreholes ranges from 500 to 1,250 feet below land surface. The waterbearing properties of the middle zone differ greatly from place to place. Also, the yields of individual flowing boreholes generally range from 50 to 20,000 imperial gallons per hour (gph). On the basis of water availability, the middle zone can be divided as follows : Areas of high-, moderate-, and low-yield artesian aquifer ; areas of low- and moderate-yield subartesian aquifer ; and an area where the yields from boreholes are insignificant or the aquifer is missing. Recommended maximum rates of long-term withdrawal from individual boreholes for the three artesian areas range from 100 to 5,000 gph with boreholes spaced 5 to 10 miles apart. By limiting flows to the recommended maximum rates, the boreholes should continue to flow for at least 30 years. The present average use per borehole (265 gph in 1965) is considerably less than the recommended maximum rates. Recharge to the upper zone occurs in significant but as yet unmeasured quantities, mostly in the vicinity of the major streams. Apparently, however, no significant amount of recharge reaches the middle zone from the Upper zone. Although the middle zone is, in effect, being 'mined' by existing flowing wells, the present (1965) rate of withdrawal i

Water Supply Paper↗