To investigate the potential sources of fatty alcohols arriving at a WWTP and entering the receiving waters, a study was conducted at Treborth North Wales using compound specific stable isotope mass spectrometry (C and H). Samples were collected from soils, marine sediments, detergents used in the catchment and in the WWTP. Total fatty alcohol concentrations decreased in the liquid phases through the treatment works with the majority of the compounds accumulating in the sludge (biosolids). Natural plant based detergents have dC values between -26 and -32‰ while petroleum-based detergents occupy a range between -25 and -30‰. The corresponding dH values are -250‰ for natural sourced materials and -50‰ for oil-based detergents which enable these two sources to be separated. The influent to the WWTP contained fatty alcohols which originated mainly from faecal sources and natural surfactants (~75%) with a smaller amount potentially derived from petroleum-based surfactants (~25%). The effluents from the WWTP contained mainly short chain compounds with a chain length less than C. Their dH stable isotope signature was different to the other potential sources examined and suggests bacterial synthesis during the treatment processes. The sludge had relatively high concentrations of fatty alcohols as would be expected from their low water solubility. The stable isotopic signatures were consistent with a mixture of faecal and detergent sources. The sludge in this area is routinely spread on agricultural land as a fertiliser and may find its way back into the sea via land runoff. On the basis of the mean discharge rates and the mean C concentration in the effluent, this WWTP would contribute ~300 g day to the receiving waters. The marine sediment samples had short chain fatty alcohols that are typical of marine production and with stable isotope values that indicate exclusive marine production for the C potentially mixed with terrestrial sources for the C and C compounds. Therefore, the fatty alcohols in the marine sediments are not the same as those that were discharged in the liquid effluent and these fatty alcohols were not the ones that entered the works through the influent but were synthesised or recycled within the works. © 2010 The Royal Society of Chemistry.