USE OF TAP WATER SAMPLES FOR MONITORING THE GEOGRAPHICAL VARIATION OFSTABLE ISOTOPES USED IN AUTHENTICITY STUDIES

Analysis of tap water samples is the best and most easily obtained inf ormation for estimating the pattern of stable isotope ratios in water across large areas. Usually such samples are not supplied continuously , but have to be collected and therefore stored. Storage of such water samples of about 20 mi volume in high-density polyethylene (HDPE) via ls for 15 years affected the O-18/O-16 ratio, depending on the tightne ss of closure, thus resulting in enrichments of up to about +2 parts p er thousand. Initially a set of 900 samples across the old federal sta tes (i.e. the former West Germany(1)) was collected and measured, firs t in 1979 and then a selected number were measured again in 1990 and i n 1995. The selected set of water samples was also used to determine t he D/H ratio (i.e. ratio of H-2 content to H-1 content) in 1995. The D /H ratio did not change in those stored samples, which maintained an o nly slightly altered O-18/O-16 ratio. Therefore, the D/H ratio is more suitable for characterizing the origin of stored water samples. Addit ional fresh samples from identical or comparable sites were supplied a fter 15 years, confirming the overall constancy of the isotopic patter n in the water supply of the old federal states. Since the geographica l pattern of the water intake has changed, about half of the samples w ere not of identical, but only of comparable, origin. Nevertheless the y showed a similar pattern to samples of identical origins collected 1 5 years previously. Thus, a set of limited tap water samples will be s ufficient to represent the isotopic variation of water across a large area if influences of the origin (e.g. river bank filtrate) and treatm ent (blending) can be checked. Since tap water is commonly used by the local food industry, its isotope composition can be used to trace wat er-containing material to its origin in order to check its authenticit y.