| Year | pH | Upper error | Lower error |
|---|---|---|---|
| 2001 | 8.084379 | 8.087379 | 8.081379 |
| 2002 | 8.084018 | 8.087018 | 8.081018 |
| 2003 | 8.082284 | 8.085284 | 8.079284 |
| 2004 | 8.080876 | 8.083876 | 8.077876 |
| 2005 | 8.078868 | 8.081868 | 8.075868 |
| 2006 | 8.077197 | 8.080197 | 8.074197 |
| 2007 | 8.075279 | 8.078279 | 8.072279 |
| 2008 | 8.073255 | 8.076255 | 8.070255 |
| 2009 | 8.072379 | 8.075379 | 8.069379 |
| 2010 | 8.070578 | 8.073578 | 8.067578 |
| 2011 | 8.068713 | 8.071713 | 8.065713 |
| 2012 | 8.0668 | 8.0698 | 8.0638 |
| 2013 | 8.064538 | 8.067538 | 8.061538 |
| 2014 | 8.063363 | 8.066363 | 8.060363 |
| 2015 | 8.061879 | 8.064879 | 8.058879 |
| 2016 | 8.059508 | 8.062508 | 8.056508 |