Is there an upper limit for purging and sampling as long as drawdown is not affected?

In terms of a time limit: there would be no time limitation on sampling it all depends on when the parameters are stabilized. In terms of an upper purging rate limit: I generally keep the flow rate to a liter per minute or less. The reason for that is not to exceed the 10th of a foot per second entrance velocity through the well screen zone that these screens were designed for. The velocity of a 10th of a foot per second gives us laminar flow, or non-turbulent flow, through the screen. If there is a higher velocity through the screen it produces a more turbulent flow and eventually you see more mixing and greater turbidity.