| Overall protective effectiveness | Total points Pt | Approximate residence time of percolating water in the soil and rock cover above the aquifer |
| very high | >4000 | >25 year |
| high | >2000 - 4000 | 10 - 25 years |
| moderate | >1000 - 2000 | 3 - 10 years |
| low | 500 - 1000 | several months to about 3 years |
| very low | 500 | a few days to about 1 year, in karstic rock often less |