I am interested in statistics but I never had a course in college and I've had to learn

a few basic things myself. This time, I am working on something that is beyond my comprehension. I have a set of environmental data (20 soil samples) which I am playing around with. (See below.) I need to determine the outliers. It appears to me that most methods require normal or log-normal distribution. Using the free EPA proUCL, it tells me that my data is not normally distributed. (Looking at the graphs, I suspect that the highest 3 values are causing this?) If not normal, do I need to use non-parametric methods to determine outlier? What free software can I use for this purpose? Or, is there a method to justify normal or log-normal distribution? If I take 3 new samples in the field, at locations next to those with highest values, and prove consistency, will that help as a last resort? I would greatly appreciate any opinions and suggestions.

6,080

6,420

6,490

6,640

7,160

7,390

7,650

7,740

7,760

8,170

8,290

8,390

8,710

8,930

9,240

9,310

9,670

13,700

14,700

15,400