Influence of filtration on concentrations of 62 elements analysed on crystalline bedrock groundwater samples by ICP-MS

Analyses of unfiltered and filtered (< 0.45 µm and < 0.10 µm) groundwater samples from 15 selected wells in crystalline bedrock aquifers of the Oslo area, Norway, have been studied for 62 chemical elements. While concentrations of almost all elements vary over several orders of magnitude between the individual wells, the discrepancy between filtered and unfiltered samples from the same well are rather small, not exceeding one order of magnitude. Many elements show no influence of filtration at all, while one element (Sn) suggests that filtration may actually introduce contamination to the samples. Correlation between unfiltered and filtered samples is high for most elements. The study shows that: (1) even unfiltered samples will satisfactorily reflect general water chemistry as long as drinking water (i.e. by definition rather 'clean' water, with low particulates) is collected; (2) filtered samples do not necessarily reflect 'true' solution chemistry (an elusive concept); and (3) the differences between samples filtered at < 0.45 µm and < 0.10 µm are so minimal for most elements, that the additional effort invested in ultra-filtration may not be justified for bedrock groundwater samples.