Why are there “filtered” and “unfiltered” samples?
Stormwater samples are processed prior to submittal to an analytical laboratory. Some of the water is filtered, removing the sediment suspended (particulates) in the water. This is done to measure the concentration of constituents actually dissolved in the water. The filtered samples represent clear water that a person might drink. The rest of the water is submitted to the analytical laboratory unfiltered, so the measured concentration includes both dissolved and suspended constituents. The unfiltered sample measures the total contaminants moving in the stream. It is unlikely that animals or humans would drink the water containing the high sediment loads present in the unfiltered samples. The concentration of the constituents that would be present in the suspended sediment alone can be calculated using the total suspended sediment (TSS) measurement. The concentration in suspended sediment is representative of part of what would be left in the channel when the sediments settle out after
