Environmental magnetic data are now routinely used to investigate compositional variability of rocks, sediments and soils. Data from samples taken in the Gwendraeth Estuary, South Wales, U.K., demonstrate levels of intra-site and intra-sample variability in magnetic properties could be significant in field settings that show relatively limited inter-site variability. Care must be taken in both sampling at individual field sites and also in sub-sampling field samples within the laboratory prior to analysis. In contrast, duplicate measurements on individual samples show relatively low levels of instrument variability with the equipment used in this study.