Abstract

In order to establish data to be used for comparison with future evolution related to climate change, physical, chemical and biological characteristics of dew and rainwater as collected during the 2005 dry season (plus a few data during the 2004 dry season) are reported. They have been collected in two characteristic tropical islands of French Polynesia, Tikehau (TKH), a low-lying coral atoll in the Tuamotu Archipelago, and the mountainous Tahiti island at the University of French Polynesia (TAH). Trade winds dominate the trajectory of air masses, ensuring constant temperature and humidity to the lower layers of the atmosphere where dew forms. In addition to the comparison of dew yields with a physical model using simple meteorological data (air and dew point temperatures, windspeed, cloud cover), the following parameters were studied: pH, electrical conductivity (EC), total dissolved solids (TDS), total hardness, suspended matter, ion concentration with major cations (Ca2+, K+, Na+, Mg2+, NH4+) and major anions (Cl−, SO42−, NO3−), reviviscible aerobe microorganisms and compared to two Polynesian spring waters (“Eau Royale” and “Vaimato”). Dew, with a chemical composition mainly consisting of Na+, Ca2+, Mg2+, Cl− and SO42−, exhibits much higher ion concentration than rain and compares well with the composition of local spring waters. The values of pH, EC and TDS are larger in dew than in rainwater. At TAH, the volume weighted mean (VWM) pH values of dew were 6.05 in 2004 and 5.23 in 2005, larger than the pH of rain (4.69). The VWM dew EC (at TAH, 203 μS·cm−1 in 2004 and 237 μS·cm−1 in 2005; 321 μS·cm−1 at TKH in 2005) and dew TDS (152 mg·L−1 at TAH and 225 mg·L−1 at TKH) were higher than the corresponding quantities in rain. Mean total hardness (TH) values of dew water are much higher than in rainwater (2.8 for dew versus 0.5 for rain at TAH and 5.7 for dew versus 0.5 for rain at TKH). Ions Na+, Mg2+ and Cl− are clearly of sea origin while the presence of Ca2+ is due to coral particles. From their chemical characteristics, dew and rainwater could be used as an alternative source of water in dry season but, due to the presence of reviviscible aerobe microorganisms at 22 °C and 36 °C (>300 CFU·mL−1), water must be disinfected to be potable.

Highlights

  • Due to its location in the middle of the Pacific Ocean, with a wind regime dominated by trade winds, chemical compositions of water from French Polynesia islands are greatly influenced by the presence of the sea

  • This work presents a comparison between rainwater and dew samples under humid tropical environments in French Polynesia at two sites, Tikehau and Tahiti islands, during the dry season of years 2004 and 2005

  • SO42− in dew is clearly from sea origin in Tikehau, it is in favor of mixed sea– anthropogenic origin in Tahiti

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Summary

Introduction

There is, to our knowledge, no report on chemical and biological studies of dew water and comparison with rainwater under tropical island conditions This is all the more regrettable since data are needed to establish future evolution due to climate change. The annual precipitations are sufficient for human use, with climate change, periodic droughts are more frequent during which the situation can become critical It is the objective of this paper to give information on dew characteristics and especially on its chemical and biological quality as compared to rain and spring water.

Measurements and Methods
Hourly and Nightly Dew Yield Potential with Energy Balance Model
Air Masses Trajectories
Chemical Analysis
Total Dissolved Solids
Total Hardness
Marine Influence
Biological Analysis
Findings
Concluding Remarks
Full Text
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