Sodium Mobility in a Mini-Catchment Studied with Radioactive Tracers and Artificial Precipitation

Abstract

Seasalts in the precipitation may lead to a short-term acidification of the runoff from natural catchments due to ion-exchange reactions. It was decided to investigate this effect in the Storgama area which has acidic streamwater and lies in Telemark county, southern Norway. The mobility of 24Na as compared to that of tritiated water (HTO) was studied in a natural mini-catchment (98 m2, 61% barren granitic bedrock) using artificial precipitation with an intensity of about 5 mmh−1. The tracers were delivered as a pulse, and after 6 days (4 days with watering) an estimated 84% of the HTO reaching the plot was recovered in the outlets compared to 45% for 24Na. Sodium is therefore fairly mobile in the mini-catchment, but the results show that the positively charged sodium ions are delayed compared to the water movement as traced by HTO. From the observed sodium mobility and the [Na] values commonly occurring in natural precipitation in this area (average 15 μeql−1), it is estimated that the short-term acidification of the runoff due to seasalts is of minor importance in the mini-catcment which typically has [H+] ≈ 50 μeql−1 in runoff. This is probably also the case for the Storgama area as a whole.