title: Mobilization of naturally occurring perchlorate related to land-use change in the southern High Plains, Texas.
abstract: Perchlorate (Cl4-) reservoirs that accumulated in semiarid unsaturated zones, similar to chloride (Cl-), can contaminate underlying aquifers if they are mobilized. The purpose of this study was to evaluate Cl)4- mobilization related to land-use change from natural to agricultural ecosystems in the southern High Plains (SHP, USA), where large ClO4- concentrations (< or =60 microg/L) are found in the underlying Ogallala aquifer. Boreholes were drilled and sampled beneath natural ecosystems (3 boreholes) and beneath nonirrigated (rainfed, 7 boreholes) agricultural ecosystems. Large ClO4- reservoirs (361-934 g ClO4-/ha; peaks 47-111 gamma ClO4-/L pore water), that accumulated for up to approximately 30,000 yr under natural ecosystems, are being displaced to depths of 2.2 to >9.2 m in sampled boreholes under rainfed agriculture by increased drainage/recharge. High correlations between ClO4- and Cl- under natural areas (r = 0.81) and rainfed agricultural areas (r = 0.88) indicate that accumulation and mobilization of ClO4- can be predicted from Cl- data. Minimal analysis of ClO4- (e.g., two points, minimum and maximum Cl- concentrations in each profile) can be used to predict ClO4- inventories to within 9% of estimates based on detailed sampling. A pooled linear regression model based on all data in this study (99 points) predicts ClO4- inventories to within 19% of measured inventories. Continued mobilization of pre-existing unsaturated zone ClO4- reservoirs (361-934 g/ha) could increase the current groundwater ClO4- values by a further 8-21 microg/L in the SHP.
authors.affiliation: Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924, USA.
authors.forename: Bridget R
authors.initials: BR
authors.lastname: Scanlon
authors.forename: Robert C
authors.initials: RC
authors.lastname: Reedy
authors.forename: W Andrew
authors.initials: WA
authors.lastname: Jackson
authors.forename: Balaji
authors.initials: B
authors.lastname: Rao
mesh_terms: D000383:Agriculture; D002712:Chlorides / Q000032:analysis; D017753:Ecosystem; D005843:Geography; D009038:Motion; D010472:Perchlorates / Q000737:chemistry*; D011891:Rain; D012044:Regression Analysis; D012987:Soil; D013781:Texas; D014881:Water Supply
chemical_list: D002712:Chlorides; D010472:Perchlorates; D012987:Soil; C494474:perchlorate