Our group took samples of contamination in sediments from around 20 non-related rivers, during 4 months.
I want to know if contaminant (micrograms per liter) is related to river and/or time, thus my first option would be 2-ways ANOVA. The factors are "river" (values 1 to 20) and "month" (values 1-4), and the response variable is contaminant content.
The problem is I have checked that the contaminant variable is non-normal and I didn't find a useful data transformation to avoid this.
So, which would be the best choice in this case? I have read that Kruskal-Wallis can be used with one factor and another one as blocking, but I'm not sure if it's my case (maybe because I don't fully understand factor blocking, my bad).
Any other non parametric 2-factos test I can use in this case? Thanks a lot