| Year | Excellent quality or compliance with guide values | At least sufficient quality or compliance with mandatory values | Poor or non-complying | Closed | Quality classification not possible |
|---|---|---|---|---|---|
| 1990 | 68.26 | 79.81 | 9.16 | 0 | 11.03 |
| 1991 | 58.74 | 80.85 | 7.7 | 0.3 | 11.15 |
| 1992 | 71.13 | 82.28 | 7.87 | 0.38 | 9.47 |
| 1993 | 74.81 | 86.07 | 6.38 | 0.22 | 7.33 |
| 1994 | 76.75 | 88.89 | 6.14 | 0.16 | 4.82 |
| 1995 | 74.17 | 84.6 | 5.12 | 0.14 | 10.14 |
| 1996 | 82.14 | 91.64 | 4.62 | 0.11 | 3.64 |
| 1997 | 82.71 | 93.27 | 5.33 | 0.13 | 1.27 |
| 1998 | 83.8 | 94.6 | 4.51 | 0.14 | 0.75 |
| 1999 | 85.25 | 95.05 | 3.55 | 0.12 | 1.27 |
| 2000 | 85.76 | 95.08 | 2.73 | 0.12 | 2.07 |
| 2001 | 86.34 | 95.37 | 2.67 | 0.17 | 1.79 |
| 2002 | 86.98 | 95.75 | 1.75 | 1.03 | 1.48 |
| 2003 | 89.29 | 96.83 | 1.17 | 1.4 | 0.61 |
| 2004 | 88.46 | 96.69 | 1.48 | 1.5 | 0.32 |
| 2005 | 89.05 | 96.06 | 1.32 | 1.91 | 0.71 |
| 2006 | 88.41 | 96.14 | 1.51 | 1.95 | 0.4 |
| 2007 | 86.08 | 95.21 | 2.19 | 2.14 | 0.47 |
| 2008 | 84.98 | 96.27 | 1.43 | 2.11 | 0.19 |
| 2009 | 86.73 | 95.86 | 1.51 | 2.16 | 0.47 |
| 2010 | 80.6 | 92.52 | 1.12 | 0.3 | 6.06 |
| 2011 | 81.33 | 93.65 | 1.36 | 0.9 | 4.09 |
| 2012 | 82.03 | 95.53 | 1.64 | 0.05 | 2.78 |
| 2013 | 85.19 | 96.76 | 1.89 | 0.03 | 1.33 |