Chemical composition and acidity of rainfall in the Alligator Rivers Region, Northern Territory, Australia

Abstract

Rainfall in the Alligator Rivers Region, Northern Territory, is essentially restricted to the period October to April. Rainwater samples from eight sites show that pH is consistently between 4.0 and 5.0, and confirms previous observations of the widespread occurrence of acidic rainwater across the northern parts of Australia. Measurements of anions (sulfate, nitrate, chloride, phosphate) and weak organic acids (formate and acetate) in rainfall, together with pH and conductivity, have shown the presence of local concentrations of ions and the presence of free acidity in excess of that attributed to dissolved carbon dioxide. Much of the acidity was associated with the presence of weak acids; for example, for all Magela samples 47% was weak acids compared with 25.5, 13 and 8% for hydrochloric, sulfuric and nitric acids, respectively. Acidity due to sulfate was therefore low, but sulfate in excess of that derived from sea-salt was of large magnitude; for example, 83% of sulfate for all Magela samples was excess. Possible sources of acidity are natural (burning of vegetation, soil dust, lightning, biogenic emissions from soils and oceans and volcanoes), and anthropogenic (sulfuric acid plant and electricity generating stations and minor emissions from calciners and sulfuric acid aerosols in open vats locally at Jabiru and nearby Nabarlek). The impact of sulfur emissions from the Ranger uranium mine/mill complex (sulfuric acid plant and electricity generating station) on rainwater acidity in the surrounding Jabiru area was examined, but the fate of the sulfur dioxide emissions was not known. Difficulties in establishing the significance of contributions from anthropogenic sources against this background of natural acidity are attributed to a lack of information on the proportions of chloride, sulfate and nitrate from these sources.