文摘
Perchlorate (ClO4-) occurrence in groundwater haspreviously been linked to industrial releases and thehistoric use of Chilean nitrate fertilizers. However, recentlya number of occurrences have been identified for whichthere is no obvious anthropogenic source. Groundwater froman area of 155 000 km2 in 56 counties in northwestTexas and eastern New Mexico is impacted by the presenceof ClO4-. Concentrations were generally low (<4 ppb),although some areas are impacted by concentrations upto 200 ppb. ClO4- distribution is not related to well type (publicwater system, domestic, agricultural, or water-tablemonitoring) or aquifer (Ogallala, Edward Trinity High Plains,Edwards Trinity Plateau, Seymour, or Cenozoic). Resultsfrom vertically nested wells strongly indicate a surface source.The source of ClO4- appears to most likely be atmosphericdeposition. Evidence supporting this hypothesis primarilyrelates to the presence of ClO4- in tritium-free older water,the lack of relation between land use and concentrationdistribution, the inability of potential anthropogenic sourcesto account for the estimated mass of ClO4-, and thepositive relationship between conserved anions (e.g., IO3-,Cl-, SO4-2) and ClO4-. The ClO4- distribution appears tobe mainly related to evaporative concentration andunsaturated transport. This process has led to higherClO4- and other ion concentrations in groundwater wherethe water table is relatively shallow, and in areas withlower saturated thickness. Irrigation may have acceleratedthis process in some areas by increasing the transportof accumulated salts and by increasing the numberof evaporative cycles. Results from this study highlightthe potential for ClO4- to impact groundwater in arid and semi-arid areas through long-term atmospheric deposition.