Concentrations of nitrate in stream water throughout the world are reported to be elevated relative to natural background levels.This enrichment is commonly attributed to anthropogenic activities such as atmospheric emissions , livestock feeding , agricultural runoff [3,4], timber harvesting practices  and domestic/industrial effluent discharge [4,6]. Here we show that bedrock containing appreciable concentrations of fixed nitrogen contribute a surprisingly large amount of nitrate to surface waters in certain California watersheds, to an extent that even small areas of these rocks have a profound influence on water quality. As 75% of the rocks now exposed at the Earth's surface are sedimentary in origin , and as these rocks contain about 20% of the global nitrogen inventory , 'geological' nitrogen may be a large and hitherto unappreciated source of nitrate to surface waters. Such a natural nitrate source may be especially significant given that nitrate contamination at very low levels can contribute to surface water eutrophication , may cause infant methaemoglobinaemia ('blue baby' syndrome)  and has been implicated in certain cancers . In addition, geological nitrogen may be a source of the 'missing' nitrogen noted in several biogeochemical studies of ecosystem nitrogen budgets .