The use of multivariate statistics techniques (main components and factor analysis) it´s been becoming in habitual tool for hydrogeological and hydrochemical studies.

Thanks Roberto

I suggest to use fuzzy c-means clustering method.

If you are using the Hosking and Wallis approach to identify homogeneity of a region then the discordancy values that are calculated will give an indication of sites that are unusual in a statistical sense.  This should be compared to the physiographic characteristics of the sites to determine which sites should be removed to improve the homogeneity.  Further information can be found in: Burn, D.H. and N.K. Goel (2000). “The Formation of Groups for Regional Flood Frequency Analysis”, Hydrological Sciences Journal, 45(1), 97-112.

Thanks Donald. 

i have read the paper as well. was really helpful.