Partitioning of 232Th, 230Th and 228Th in the Surface Water of Northwest Peninsular Malaysia Coastal Waters as Tracer for SPM Sources

摘要

Natural thorium radioisotopes (232Th, 230Th, 228Th, 234Th) are well known to be particle reactive, thus they are often used to study the geochemical process of particle reactive matter. In this study, water samples were collected from Langkawi Island and Pangkor Island from six and ten sampling stations respectively, to determine the activity of 232Th, 230Th and 228Th in dissolved (DISS) and suspended particulate matter (SPM) phases, and estimate the sources and origin of SPM into the marine environment. The results in terms of volume activity concentration (mBq/L) shows that the activity level of 228ThDISS was higher than 228ThSPM at all sampling stations possibly due to the behaviour of the 228Th parent, 228Ra which is easily desorbed from SPM. At several stations far from the coast; stations L4 (4.58km), P7 (3.16 km), P6 (2.1 km), P10 (1.38 km), the activity of 232ThDISS was higher compared to 232ThSPM. These stations behave the same way as the open ocean in which 232ThDISS is contributed by atmospheric dust. Indeed, the partition of thorium between dissolved and SPM phase can be used to detect anthropogenic organic pollution as in station L6 in which thorium activity in dissolved phase is higher compared to in SPM phase. Moreover, the use of the (232Th/230Th)SPM ratio is useful to detect lithogenic input to the marine environment in which the ratio ranges from 0.13 to 0.5 and 0.7 to 3.16 for stations that are far and near to the coast respectively, except for station P1. However the data analysis shows that there is no clear pattern of kd values at both sampling sites, probably due to the adsorption and desorption processes of 232Th and 230Th that occurs in the water column. Nevertheless, as the SPM concentration (mg/L) increases, the kd value decreases except for the activity of 228Th which shows a strong negative correlation with SPM.