Trying to write positive values

I'm working on subsurface vapor intrusion modeling. In solute transport module I need to define a coefficient for vapor sorption which is also a nonlinear function of concentration. The function includes ln(c) in which all concentration values should be positive. Since I have some very small values of negative concentration in some parts, so the error message shown up each time that can not evaluate the log of negative values. Now I'm trying to define "c" to be just positive. How can I do that?
I really appreciated your help


Simin