Exceedance Threshold Unit

The thresholdUnit on an Exceedance records the unit used for the thresholdConcentration in the comparison — for example, ug/L or mg/kg. Together with resultUnit, it allows compliance staff to verify that the exceedance determination was made with matching units. This snapshot is stored on the Exceedance directly rather than retrieved from the ThresholdValue at query time, so the historical record of the exceedance is not affected by any future changes to the ThresholdValue's thresholdUnit configuration.