Perchlorate Deposition in Wet Precipitation across North America
W. Andrew Jackson
Department of Civil and Environmental Engineering,
Texas Tech University,
Lubbock, Texas USA
Perchlorate (ClO4-) contamination continues to be an important environmental issue due to its potential health impacts, the number of areas with affected groundwater, and the possibility of new legislation. Perchlorate is primarily used as an oxidant in solid rocket propellants and to a lesser extent on a mass basis in flares, fireworks, and some explosives. Natural perchlorate has been known to exist for over a century but until recently was thought to only occur in the Atacama Desert. Natural perchlorate is believed to be of atmospheric origin, yet no systematic study has been conducted to evaluate perchlorate deposition rate and possible seasonal or spatial variations. Perchlorate wet deposition was evaluated weekly at 16 NADP sites across the continental US as well as Puerto Rico and Alaska for a three year period (October 2004-October 2007). In addition, 8 additional sites were evaluated in the third year of the study. Perchlorate (ClO4-) analysis used sequential ion chromatography-mass spectroscopy-mass spectroscopy (IC-MS/MS) with a lower quantitation limit of 5 ng/L. Perchlorate concentrations varied from <5 ng/L to a high of 102 ng/L with a mean of 14.1 ± 13.5 ng/L for the 1578 total samples. Perchlorate annual deposition by site ranged from a low of 12.5 (TX) to 157 mg/ha-year (NE) and averaged 65 ± 30 mg/ha-year for all sites. Perchlorate concentrations and deposition were generally highest in May-August declining to lows in December-February. Perchlorate concentration is significantly (p < 0.001) related to all other monitored ions (Ca+2, Na+, K+, NH4+, NO3-, Cl,-, SO4-2, and pH) except Mg+2, although the regression coefficients are generally quite low (< 0.3), except for Ca+2, NH4+, and NO3- (r = 0.40, 0.41, and 0.38 respectively). Average annual perchlorate deposition was correlated (r > 0.5; p < 0.001) with Ca+2, K+, NH4+, NO3-, Cl-, and SO4-2 . Deposition of ClO4- by precipitation in the continental United States (excluding AK, HI, and Puerto Rico) while diffuse, represents a potential annual net mass flux of 51,000 kg, a value comparable to the estimated annual environmental releases from other known ClO4- sources.