The answer to this is based on some guidance from about 15-20 years ago that identified that larger solids might be mobile in the formation under certain conditions. The use of a 5-micron, or even a 10-micron, filter size would allow us to see everything that was truly mobile in the colloidal transport. Unfortunately, filtration itself is a rather imperfect methodology, if I give you a filter and tell you that it's a 1-micron or a 5-micron filter, I am not guaranteeing you that every particle size smaller than that pore size will actually pass through the filter, but everything larger than that pore size will be tracked by the filter. In a study published in 1992 by Paul and Powell, it showed that particles between 2 to 2.5 microns passed through a 5-micron filter. So, in fact, a little bit less than half of the actual pore size would typically pass through the filter. It may give you a better indication of what's truly mobile with the ground water, but the best representation of what's going to be mobile is a low-flow sample that is not filtered at all. I, personally, would lean toward using a 10-micron filter if possible, but a 5-micron would be a better representation as opposed to 0.45.