Identifying the sources of elevated uranium in surface waters within watersheds of tributaries to the San Juan River, Four Corners Region, USA

Weathered uranium- (U-) rich sediment and mining and production activity could contribute to elevated concentrations of U in surface waters. In the San Juan River watershed in the Four Corners region of the United States, U-rich deposits are common in many geological formations and, historically, U mining in the region was prevalent. The goal of this work was to identify potential sources of U to the San Juan River from ephemeral and perennial tributaries during monsoonal storms. Data show that most of the tributaries in the watershed have elevated U concentrations in surface-water samples, greater than the US Environmental Protection Agency maximum contaminant level of 30 micrograms per litre (µg l^-1) for U. Although parts of the watershed have been mined for U, not all elevated concentrations were found in regions that drained from mined areas. The two highest total U concentrations in surface water in the region were from the Chinle Creek near Bluff watershed (362 µg l^-1) and Gallegos Canyon watershed (206 µg l^-1), the former having many U mines and the latter with none. The lack of an obvious effect from mining activity and U in the local geological formations to the concentrations of U in the surface water suggests that another source could be the primary contributor of U to the San Juan River watershed.