Widespread Presence of Naturally Occurring Perchlorate in High Plains of Texas and New Mexico

摘要

Perchlorate (ClO_4~-) occurrence in groundwater has previously been linked to industrial releases and the historic use of Chilean nitrate fertilizers. However, recently a number of occurrences have been identified for which there is no obvious anthropogenic source. Groundwaterfrom an area of 155 000 km~2 in 56 counties in northwest Texas and eastern New Mexico is impacted by the presence of ClO_4~-. Concentrations were generally low (<4 ppb), although some areas are impacted by concentrations up to 200 ppb. ClO_4~- distribution is not related to well type (public water system, domestic, agricultural, or water-table monitoring) or aquifer (Ogallala, Edward Trinity High Plains, Edwards Trinity Plateau, Seymour, or Cenozoic). Results from vertically nested wells strongly indicate a surface source. The source of ClO_4~- appears to most likely be atmospheric deposition. Evidence supporting this hypothesis primarily relates to the presence of ClO_4~- in tritium-free older water, the lack of relation between land use and concentration distribution, the inability of potential anthropogenic sources to account for the estimated mass of ClO_4~-, and the positive relationship between conserved anions (e.g., IO_3~-, Cl~-, SO_4~(-2)) and ClO_4~-. The ClO_4~- distribution appears to be mainly related to evaporative concentration and unsaturated transport. This process has led to higher ClO_4~- and other ion concentrations in groundwater where the water table is relatively shallow, and in areas with lower saturated thickness. Irrigation may have accelerated this process in some areas by increasing the transport of accumulated salts and by increasing the number of evaporative cycles. Results from this study highlight the potential for ClO_4~- to impact groundwater in arid and semi-arid areas through long-term atmospheric deposition.