How is composite sampling applied spatially? How is it applied temporally?
Compositing can be performed on spatially distinct component samples if they have equivalent exposure potential. Thus, horizontal compositing should be limited to a particular medium or habitat type over which exposure is expected to be uniform. For example, sediment samples should not be composited across an entire bay if the receptor spends most of its time in shallow areas along the periphery. Likewise, vertical compositing is limited to layers in which exposure potential is uniform. For example, sediment dwelling receptors spend disproportionate amounts of time in the surface sediment layers. Thus, composite sampling should combine discrete samples from within, but not between 0-6, 6-12, and 12-24 inch sediment layers. Temporal composite sampling can be valuable in assessing average exposure in media if contaminant discharges are expected or known to be randomly distributed (i.e., surface water in a tidal river, air downwind from a discharge point). For example, when substantial ti
