This is a pollution incident in a karst area. The pollutant enters a doline at the top of the system and arrives at the stream.

A stream monitoring point immediately downstream of the resurgence will detect the
pollution.

This indicates the difficulty of developing a monitoring system capable of detecting this type of incident. Water from the doline, with the pollutant, enters the well developed cave system.

Of the seven sampling boreholes, only one intersects the stream carrying the pollution. The probability of intersecting such a stream without prior knowledge of its position is extremely low.