Boron Deposition on Soil and Native Vegetation from Geothermal Emissions

Abstract

AbstractBoron deposition, soil B, and foliage B were sampled in a field study around two geothermal generating units with different operating histories; the study also determined relative sensitivities of several native tree species to B in soil and foliage. Boron emissions from the older cooling tower, operational for 11 yr, were nearly 5 times greater than emissions from the newer cooling tower, operational for 2.5 yr. This difference was due to substantially greater B concentrations in circulating water at the older unit. Boron deposition was as great as 129.6 kg ha−1 yr−1 at the older unit and 4.2 kg ha−1 yr−1 at the newer unit. Soil B (saturation extract) was as great as 44.0 mg L−1 at the older unit and 0.6 mg L−1 at the newer unit. At a given rate of B deposition, soil B was more than 4 times greater at the older unit due to its longer operating history. At a given soil B level, the deciduous bigleaf maple (Acer macrophyllum Pursh) contained double the foliage B of the broadleaf evergreen, interior live oak (Quercus wislizeni A. DC.), and about 5 times greater foliage B than the conifer, Digger pine (Pinus sabiniana Dougl.). Foliage B thresholds for detectable levels of visible injury due to B toxicity were about 500 mg kg−1 for both bigleaf maple and interior live oak. These relationships suggest that more rapid B accumulation is responsible for higher levels of visible injury due to B in bigleaf maples than in other native tree species.