and do not require that the data fo

and do not require that the data follow any particular distribution
(Helsel and Hirsch, 2002). The significance level (p) used for
hypothesis testing was compared to a threshold value (α) of 5%
(α=0.05) to evaluate whether the relation was statistically significant
(pbα). The Wilcoxon rank-sum test was used to test for
differences between the median values of a parameter in two sets of
samples. Contingency tables were used to test for differences between
detection frequencies in two sets of samples (Helsel and Hirsch,
2002).