The development and application of statistical sampling regime to map out hydrocarbons in sediments.
Ahmed, Abdulwaheed Suleiman
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This thesis investigates and develops a stratified random sampling design for sediments in an offshore oil field environment. The sampling area was partitioned equally into 16 Zones, stratification were based on the near field and far field areas, and the number of samples in each Zone was chosen by proportional allocation, i. e. proportional to the available appropriate area (far field). Measurement techniques applied to the samples included laser granulometry, ultraviolet fluorescence, gas chromatography using mass selective detection or flame ionisation detection and elemental analysis. The total PAH concentrations (2- to 6-ring parent and alkylated PAHs, including the 16 US EPA PAHs) in sediments were relatively low (< 100 pg kg" dry weight). The PAH concentrations, Forties crude oil equivalent and diesel oil equivalent concentrations were generally higher in sediment of fine grain size and higher organic carbon loading. PAH distributions and concentration ratios Indicated a predominantly pyrolytic input, being dominated by the heavier, more persistent, 5- and 6-ring compounds, and with a high proportion of parent PAHs. The nalkane profiles of a number of the sediments contained small, high boiling, UCMs, indicative of weathered oil arising from a limited petrogenic input. Spatial structure analysis shows the existence of a trend in the variogram, and also the spatial pattern in the contour maps of the parameters measured, shows that the regionalized variable exhibited non-stationarity and were non-ergodic. The stratified random sampling scheme showed significant advantages over a classical grid sampling scheme when applied to the same area. Specifically, the stratified random sampling design gave much more reliable mean concentrations for all the parameters, achieving a much lower variance than the grid sampling. A further composite random sampling scheme was designed for sediments in the near-shore. The aim is to estimate a within-stratum mean value for each of the chosen measurement parameters with more thorough coverage (better representation), better precision and less variance at lower analytical cost. This scheme was trialed in two near-shore environments, the Clyde Estuary and the Firth of Forth. The results show no significant differences between the mean and distribution profile of the individual samples and the composite samples for all the parameters measured. This work utilised the best modern chemical analytical methods for the quantification of a range of hydrocarbon species, and utilised the results in a modem risk-based approach to environmental assessment. The new stratified random sampling design has been accepted for use in the national marine monitoring programme (NMMP) In the United Kingdom.