Chemical composition of rainwater near two historical monuments: The ThannCollegiate (Alsace, France) and the Tours Cathedral (Indre et Loire, France)

In the present study, included within the framework of a Franco-German Rese arch Program for Conservation of Historical Monuments, the chemical composi tion of rainwater was investigated in Thann (Alsace, France) and in Tours ( Indre et Loire, France) between 1992 and 1993. A total of 78 and 24 samples , respectively, were collected, near the Thann collegiate and the Tours cat hedral on a weekly basis and analysed for Cl-, NO3-, SO42-, Na+, NH4+, K+, Mg2+ and Ca-2+. Results show pH ranged from 3.60 to 6.58 and from 5.49 to 7 .01 for Thann and Tours, respectively. In Thann, SO42- is the major anion a nd rainwater acidity is neutralised by compounds of terrigenic origin which come from the soil of the upper Rhine valley (Ca2+; 'loess') and the potas h mines of Alsace. In Tours, SO42- is also the major anion and the acidity is neutralised partially by atmospheric ammonia and partially by compounds of terrigenic origin and by dust from the erosion of the cathedral and the works of restoration, in the form of CaCO3.