Investigation of porewater sampling methods for mercury and methylmercury

The feasibility of various techniques for the separation and quantification of sediment and sediment porewaters for total Hg and methylmercury (MMHg)- centrifugation, sediment filtration, whole core squeezing, and dialysis mem brane techniques (peepers)-were investigated in estuarine sediments. The co ncentration and distribution of iron, manganese, MMHg, and total Hg in pore waters were compared to ascertain which method provided the best technique for the collection of large volume porewater samples. Direct filtration tec hniques did not provide sufficient sample volume. Our studies confirmed the need to filter in an inert atmosphere. Processing in an inert atmosphere i s also required for centrifugation. Centrifugation was the most difficult m ethod, requiring extensive operator training and much attention to detail d uring sample processing. Core squeezing is a viable alternative but suffers from sample size problems and potential artifacts due to oxidation during processing. While peepers have advantages, the investigations here show tha t they need to be completely degassed before they provide reliable Hg speci ation measurements, especially as they were constructed from Teflon, which can store significant amounts of oxygen. The need to deoxygenate the peeper s compromises their use as a routine separation method for Hg. On the basis of the need for large volumes and the large number of samples to be collec ted, we concluded that centrifugation was the most reliable method for the determination of Hg and MMHg in estuarine porewaters.