Chemical and biological characteristics of dew and rain water in an urban coastal area (Bordeaux, France)

We report on a 1-year investigation (15 January 2002–14 January 2003) in Bordeaux, France, comparing the quality of dew water with respect to rain water. The following physico-chemical and bacteriological properties of dew and rain water were measured: pH, electric conductivity, cations (Na+, K+, Ca2+, Mg++, Zn++, Cu++), anions (Cl−, SO42−, NO3−, NO2−), hardness (TH, calcical, magnesial, permanent), complete alkalimetric title, dry residue and number of colony-forming unities (CFU) at 22 and 36 °C. The CO32−, HCO3−, HPO42− concentrations were found negligible. The ionic concentrations are in general lower in dew than in rain with NO2− as a noticeable exception. The mean rain pH (5.4) is lower than dew pH (6.3). The major ions are from the nearby Atlantic Ocean (within 50 km). Average ion concentrations are found below the World Health Organization (WHO) limit requirements for potable water; dew composition is close to low mineralized commercial spring waters for the analyzed ions. The biological analyses are concerned with CFU at 22 and 36 °C. Dew is seen to exceed on various occasions the WHO limits.