Stormwater concentrations can vary widely during storms, whether a storm drain or river.  I do not know what you mean by MTE concentration, and 3x60 ml (are you compositing small samples).  Somewhat helpful may be to sample frequently during a flushing storm and plot concentration during flow, and you may need to separate rising limb of hydrograph, from falling limb.  You are probably not going to do well in estimating  range from a single sample, but if you take a sample during high flow after a long dry spell, that flush is often fairly high in contaminants.  Dont expect one grab sample to be representative of much of anything.   Sampling unpolluted well water in remote areas from a confined aquifer may be fairly representative in one sample.  Since I am unsure what MTE is, I hope this helps.

Thanks WIlliam and MET means Metal Trace Elements. Effectively smal samples can not represent the trend of contaminants and we are looking for a factor to estimate the range of contaminants concentrations. Such factor are used by company in charge of used and collected waters (some used a factor of 8) but we have not scientific evidence of suc factor