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).