CONTRIBUTION OF BEDROCK NITROGEN TO HIGH NITRATE CONCENTRATIONS IN STREAM WATER

Concentrations of nitrate in stream water throughout the world are rep orted to be elevated relative to natural background levels. This enric hment is commonly attributed to anthropogenic activities such as atmos pheric emissions(1), livestock feeding(2), agricultural runoff(3,4), t imber harvesting practices(5) and domestic/ industrial affluent discha rge(4,6). Here we show that bedrock containing appreciable concentrati ons of fixed nitrogen contribute a surprisingly large amount of nitrat e to surface waters in certain California watersheds, to an extent tha t even small areas of these rocks have a profound influence on water q uality. As 75% of the rocks now exposed at the Earth's surface are sed imentary in origin(7), and as these rocks contain about 20% of the glo bal nitrogen inventory(8), 'geological' nitrogen may be a large and hi therto unappreciated source of nitrate to surface waters. Such a natur al nitrate source may be especially significant given that nitrate con tamination at very low levels can contribute to surface water eutrophi cation(9), may cause infant methaemoglobinaemia ('blue baby' syndrome) (6) and has been implicated in certain cancers(6). In addition, geolog ical nitrogen may be a source of the 'missing' nitrogen noted in sever al biogeochemical studies of ecosystem nitrogen budgets(1).