The behaviour of boron isotopes in natural waters and water-rock interactions.

The wide boron isotopic variations occurring in natural waters mainly are derived from the 20 fractionation between dissolved boric acid and borate anions, associated with the preferential removal from the system of 11B depleted borate ions by adsorption and/or minerals formation. Typical adsorbants of boron dissolved in groundwater are clay minerals of the aquifer matrix. Boron (and strontium) isotopes were used in investigating two alluvial aquifers in Tuscany, where boron concentration is often above 1 mg L-1 and may attain 8 mg L-1. The isotopic results indicate that, in the first case (Cecina River basin), the boron contamination is anthropogenic and derives from past discharge into streams of boron-rich industrial wastes. In the second case (Cornia Plain), the dissolved boron is released by boron-rich clayey sediments of the aquifer matrix and has, therefore, a natural origin.