Let's look at the Uncertainty plot. Choose Uncertainty as the Data Component. Note that it ranges from 0.00 to 21.45. That location is a very appropriate next sampling site. Uncertainty does not have physical units, rather it is a dimensionless parameter for which higher values are bad and lower values are good.
High uncertainty means a high probability of poorly characterized contamination.
Low uncertainty means a low probability of uncharacterized contamination. The function used to compute uncertainty has been optimized to minimize the number of new sampling locations required to lower the maximum uncertainty anywhere in the model domain to a specific low value. The confidence levels and corresponding uncertainties are data dependent. If the measured data has regions of high concentration gradient, nearby areas away from sample locations will have lower confidence and higher uncertainty. Values above this level are clipped to this value.