Contribution of bedrock nitrogen to high nitrate concentrations in stream water

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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 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 [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].

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