A stable isotopic study of fog: the Point Reyes Peninsula, California, U.S.A.

Abstract

Rain and fog were collected at four locations for almost two years for stable isotopic analysis, on the Point Reyes Peninsula on the north coast of California. The fog water ranged from −18 to −7%o in δD, and −3.1 to −1.7%o in δ 18O. The rain was isotopically more depleted, and ranged from −41 to −33%o in δD, and −7.0 − 5.8%o in δ 18O. Six groundwater samples were also collected on the Point Reyes Peninsula for stable isotopic analysis. These samples ranged from −32 to −29%o in δD and −5.8 to −5.2%o in δ 18O, and are ∼10%o more enriched in δD than the rain. A geographic variation in the δD of fog on the Point Reyes Peninsula is explained by the loss of ∼5% of the condensing atmospheric vapor in the form of fog drip. This explanation, coupled with the observed stable isotopic composition of groundwater, may indicate that a small portion of fog-drip water infiltrates and recharges the groundwater system. The stable isotopic ratios of California coastal fogs, collected on the Point Reyes Peninsula, were compared to those of mountain fogs collected in northern Kenya. Coastal fogs, collected in California, plot below the MWL and are consistently depleted in D and 18O with respect to SMOW. These coastal fogs are the result of an early-stage condensate representing a single-stage evaporation over the open ocean/condensation cycle at the coast. The mountain fogs, collected in northern Kenya, plot above the MWL, are consistently more enriched in deuterium than SMOW, and are thought to contain water that has been evapotranspired and recycled upwind.