Maximum levels of a contaminant are important because of potential health or environmental effects of high concentrations. Scientists often find the "99th percentile" reading more helpful in tracking trends than a single, maximum reading. The annual 99th percentile concentration is the concentration that is higher than 99% of the concentrations recorded throughout the year. Another way of saying this is that only 1% of the ambient concentrations observed during the year fall above the 99th percentile concentration.
The annual 99th percentile concentrations of hydrogen sulphide have decreased over 1990 to 2007 in Edmonton, Fort McMurray and Fort Saskatchewan. Note that data for Fort McMurray only span 1990 – 1997. Null trends (change of 0% over 1990 to 2007) were found at the Calgary, Red Deer, Sherwood Park and Buffalo View Point stations. Statistically significant decreasing trends were found at Edmonton East (46%) and Fort Saskatchewan (41%) from 1990 to 2007.
Conversely, 99th percentile concentrations have increased in the oilsands area (North of Fort McMurray) by 62 to 323% since monitoring began in 1999 (not including the Buffalo Viewpoint station). In 2007, the highest peak concentrations of hydrogen sulphide were found at the Mildred Lake (16 ppb), Mannix (11 ppb) and Lower Camp (10 ppb) locations. A statistically significant increasing trend of 323% was found at the Mildred Lake station over 1999 to 2007. |
