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(1), livestock feeding(2), agricultural runoff(3,4), timber harvesting practices(5) and domestic/ industrial affluent 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(7), and as these rocks contain about 20% of the global nitrogen inventory(8), ＇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(9), may cause infant methaemoglobinaemia (＇blue baby＇ syndrome)(6) and has been implicated in certain cancers(6). In addition, geological nitrogen may be a source of the ＇missing＇ nitrogen noted in several biogeochemical studies of ecosystem nitrogen budgets(1).