Trend analysis using nonparametric statistical techniques for detection and evaluation of spatial and temporal chemical changes at a hydrothermal exploitation (Felgueira Spa - Central Portugal)

Abstract

Like many thermomineral springs all over Europe, Felgueira Spa (Central Portugal) sulfide alkaline water has been used in therapeutic bathing for centuries. Two periods may now be depicted in the exploitation history of those low enthalpy resources: one characterized by the use of flowing water from springs and a dug well that lasted until the 1990´s when deep drilling wells were promoted to satisfy the increasing demand of medicinal water. After that, a second period of a more active resource extraction begins. Detection of statistical significant monotonic time trends in chemical composition for the period 1991-1998 was done using nonparametric statistical techniques, namely, Spearman rank correlation coefficient and Sen´s slope estimator.