A gamma-emitting tracer is added to sewage flowing into an estuary. A detector is used to record count rates at four locations, A to D, ordered from west to east. The background count rate is 20 counts per minute.
| Location | Count rate at 14:00 in counts per minute | Count rate at 14:10 in counts per minute |
|---|---|---|
| A | 23 | 22 |
| B | 240 | 29 |
| C | 25 | 210 |
| D | 22 | 24 |
Explain what the results show about the movement of the sewage and how the readings support your conclusion.