Geostatistical modelling of the spatial distribution of sewage pollution in coastal sediments

The spatial correlation of a series of sedimentary fecal stanols and sterols data obtained from 39 surface sediment sampling stations in the eastern waters of Hong Kong was analysed by geostatistical techniques. Coprostanol [5β] (5β(H)-holestan-3β-ol), cholestanol [5α] (5α(H)-colestan-3β-ol) and cholesterol abundantly found in human feces were used as molecular markers for sewage pollution monitoring in the coastal sediments. The [5β/(5β+5α)] ratio in the study area was found to range from 0.04 to 0.61 ( =0.27) and that of [5β/(5α+cholesterol)] ranged from 0.02 to 0.73 ( =0.19). Semi-variograms of the two ratios were found to fit isotropic spherical models: for [5β/(5β+5α)]: c0=4×10−3, c=6.5×10−3, a=12000, Dc=−0.024 and R=0.91; for 5β/(5α+cholesterol): c0=7.5×10−3, c=8.5×10−3, a=25000, Dc=−0.061 and R=0.67. Because of the more precise covariance model produced, the former stanol/sterol ratio was adopted as the sewage pollution marker for the kriging of contour map for sewage pollution. Three major sources of sewage pollution in the eastern waters of Hong Kong were highlighted, they were the densely populated Victoria Harbour, the sludge disposal site and the Inner Mirs Bay. A detailed dispersion pattern of sewage pollution around the sludge disposal site was studied. Our results demonstrate the usefulness of kriging in providing a holistic picture on the extent and spatial correlation of sewage pollution in coastal marine environment.