THE QUALITY OF FOG WATER COLLECTED FOR DOMESTIC AND AGRICULTURAL USE IN CHILE

Abstract

One exciting new application of meteorology is the prospect of using high-elevation fogs as an arid land's water resource. This has now become reality in northern Chile where a pilot project has used 50 fog collectors to generate an average of 7200 1 of water per day during three drought years. The chemical composition of the fog water is of primary importance and is examined in this paper.

A small, carefully cleaned fog-water collector was used at the site (elevation 780 m) to study the incoming fog (cloud). The ion and trace-element concentrations met Chilean and the World Health Organization's (WHO) drinking-water standards. The pH values, however, were at times extremely low. Samples from 1987 and 1988 were consistent with those from the larger dataset in 1989. The lowest observed pH was 3.46. The acidity was associated with high concentrations (89%) of excess sulfate in the 15 fog-water samples (based on Cl- as the seawater tracer element). The NO3-/SO4= equivalents ratio for the fog samples was 0.18, showing the dominance of SO4= in determining the acidity of the fog samples. The relative abundances of ions and trace elements in the dry deposition are very similar to those in the fog water, suggesting that the aerosols originate primarily from evaporated cloud droplets over the ocean. Based on enrichment-factor calculations (with Cl- as the indicator element for seawater and Al for the earth's crust), sea salts were the main source of Na+, Mg++, and Cl- in the fog water; soil dust was the main source of Fe, Al and Ti; and other sources provided Ca++, K+, NH4+, Br-, SO4=, NO3-, As, Cd, Pb, V, Mn, Ni, Cu, Sr, Sb, and Ba in the fog water. The use of enrichment factors based on the relative abundances in soil extracts suggests that As, V, Cu, and Sr may be available from wetted soil dust.

The output from the large (48 m2) fog collectors was also acceptable, except for several of the 24 trace elements, which exceeded the maximum allowable values in the first flush of water after a dry period of a few days. The pH values were again near 4 and would have to undergo a simple treatment to raise them to a value of 6 or more to meet the drinking-water standard. The output from a 2000-1 fog-water storage tank was completely acceptable and that from a 25000-1 storage tank completely acceptable, except for a low pH. In contrast, both the water presently being used in a nearby village and local spring water were unacceptable. It is concluded that fog water is an attractive alternative as a water supply even after collection on the large meshes at this site.