THE ORIGIN OF HIGH-NITRATE GROUND WATERS IN THE AUSTRALIAN ARID ZONE

Nitrate concentrations beyond the drinking-water limit of 10 mg l-1 NO3-N, are common in Australian arid-zone ground waters and are often associated with otherwise potable waters. In some aquifers nitrate-N concentrations of up to 80 mg l-1 have been found, and this is a severe constraint on water supply development for small settlements. Water-bore data indicate a correlation of high-nitrate ground waters with shallow unconfined aquifers. Aquifer hydrochemistry indicates that these ground waters were emplaced by episodic Holocene recharge events in an otherwise arid climate regime. Nitrate has been flushed through the unsaturated zone which apparently lacks denitrification activity. The nitrate originates by near-surface biological fixation and contributing organisms include cyanobacteria in soil crusts and bacteria in termite mounds with the highest soil nitrate concentrations found in the outer skin of termite mounds. Bacteria associated with the termites appear to fix nitrogen, which eventually appears in an inorganic form, principally as ammonia. Nitrate is produced by bacterial oxidation of the ammonia, and is leached to the outside of the termite mound by capillary action. Diffuse recharge from extreme rainfall events then flushes this nitrate to the water table.