But MCLs aren't based on health. They're compromises that consider the cost of remediation and the availability of technology. And the goal , for TCE and radionuclides and many of the other Weldon Spring contaminants, is zero. In the real world of environmental remediation, though, zero's risible. Generally we believe there is some risk at every level of exposure.
Cancer starts with a single cell going bad.
So all you have to do is, excuse the expression, fuck up that first cell, and it doesn't take much chemical to do that. Draw a graph. In the upper right corner, put two dots, for high dose and high risk. In the lower left, draw dots for low dose and low risk. That's where you want to get to, so draw a line.