es070281b_si_001.pdf (462.51 kB)
Unsaturated Zone Arsenic Distribution and Implications for Groundwater Contamination
journal contribution
posted on 2007-10-15, 00:00 authored by Robert C. Reedy, Bridget R. Scanlon, Jean-Philippe Nicot, J. Andrew TachovskyArsenic compounds have been applied at the land
surface as pesticides in agricultural areas globally. The
purpose of this study was to evaluate the fate of anthropogenic
arsenic applications related to agriculture, using arsenic
applications on cotton in the southern High Plains (SHP),
Texas, as a case study and examining possible linkages with
contamination of the underlying Ogallala aquifer in this
region, where 36% of wells exceed the new EPA 10 μg/L
standard. Unsaturated zone soil samples were collected from
boreholes beneath natural ecosystems (grassland/shrubland) to provide a control (no arsenic application) (5
profiles) and cotton cropland (20 profiles) for analyses
of water-extractable arsenic, vanadium, phosphate, chloride,
and nitrate. Natural ecosystem profiles have high arsenic
concentrations at depth (maximum of 7.2−69.6 μg As/kg dry soil at 5.9−21.4 m depth) that are attributed to a
geologic source. Most profiles beneath cotton cropland have
high arsenic concentrations within the upper meter
(profile means 1.7 to 31.6 μg/kg) that correlate with
phosphate (r = 0.70, p < 0.01) and are attributed to
anthropogenic arsenic application associated with phosphate
fertilizer application. High arsenic concentrations at >1
m depth (profile means ≤36.3 μg/kg) found in cropland profiles
are attributed to a geologic source because of similarity
with profiles beneath natural ecosystems, lack of correlation
with phosphate, and pore-water ages that predate
anthropogenic arsenic application in many profiles. GIS
analyses showed poor correlations between groundwater
arsenic and percent cultivated land (r = −0.15, p <
0.01), groundwater nitrate (r = 0.30, p < 0.01), and water
table depth (r = −0.31, p < 0.01), further supporting the idea
that anthropogenic-derived arsenic in the shallow
subsurface is not linked to groundwater arsenic contamination
in this region.