On processes controlling the distribution and fluxes of boron in natural waters and air along transects across the South Island of New Zealand

The concentrations of boron were measured in rain, air and soil water along transects across the Southern Alps, New Zealand, during southern autumn and early winter 1999. Estimates of wet deposition, dry deposition, and flux of boron leaving the soil in soil water are presented. At the Main Divide of the Southern Alps, strong and constant orographic effects on rain-bearing fronts resulted in low concentrations and little variation of boron in rain. Areas with less orographic influence received varying boron concentrations in the rain depending on the origin and character of the fronts. Concentrations of boron in air were greater near the sea than in the intramontane basin. High mean concentrations in air go together with large variations caused by changes of wind direction and speed during frontal passages. Air reaching the intramontane basin was depleted of boron because of washout over the eastern and western ranges and by dry deposition. After a long dry summer spell, the first autumn flush of soil water contained high concentrations of boron at low soil water flow. Later, as the soil wetted up, the soil water concentration approached that of rain water at all sites. Wet deposition was least at sites away from the coast where there was little rain and less boron in it. High rainfall with low boron concentrations or mean rainfall with high boron concentrations occurred near the eastern and western divides. Dry deposition varied over shorter time intervals and distances than wet deposition. It was high near the coastlines during onshore winds and low in the intramontane basin, especially with weak convective turbulence. The concentrations of boron in springs resembled those in rain and soil water, increasing from the western coastal plain eastwards across the Southern Alps.