New perspectives on water chemistry and compositional data analysis

Water chemistry is commonly investigated to determine the suitability of water for various uses. With increased knowledge of aqueous chemistry, it has become possible to interpret the evolutionary processes that determine water composition and quality. This paper presents procedures for exploring and modeling the environment using compositional data from water analysis, utilizing statistical tools in an appropriate sample space. Our procedures build on a methodology based on log-ratios initiated by John Aitchison in the early 1980's. They are not only useful for interpreting the structure of the data, but also for characterizing and modeling the influence of geochemical processes acting on the environment. The geochemistry of water samples collected from wells on Vulcano Island (one of the Aeolian Islands of the Italian province of Sicily) will be used to illustrate the techniques, although an exhaustive overview would require many different examples. Vulcano island is a quiescent volcanic area where mobilization of chemical species by weathering of volcanic rocks and input of gaseous components from fumarolic activity has produced environmental changes expressed in the composition of phreatic waters at the surface and in the shallow subsurface. Changes in the chemical composition of waters in unconfined aquifers of the northwestern part of the island around the active crater appear to be useful in understanding the natural processes at work.