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ARSENIC UPTAKE BY POPLARS AND IMPLICATIONS FOR USE IN HYDRAULIC CONTROL OF GROUNDWATERLewis-Russ, A., R. Henning, D. Fenske, E. Hicks, J. Haramut, M. Goan, J. Perkins, and C. Bury. Proceedings of the Sixth International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2008). Battelle Press, Columbus, OH. ISBN 1-57477-163-9, Abstract B-080, 2008
A pilot test in the upper Midwest was implemented to evaluate the
applicability of hybrid poplars to ground-water withdrawal and to assess the
degree of metals transport. The test site is located adjacent to a river that
experiences diurnal fluctuations of up to one foot in elevation. The
ground-water table is shallow (4 to 6 feet below ground surface) and contains
elevated arsenic concentrations. The remedial method being considered for the
site is ground-water containment using a cut-off wall that would surround the
site. To avoid potential mounding of ground water within the containment wall,
water must be removed, but previous experience indicates ex situ treatment
would be difficult due to the site's chemistry. The design calls for
ground-water removal via extraction by tree roots, with an engineered pumping
system in reserve, if needed. For the pilot test, nearly one acre of poplars
containing over 1,000 trees was planted in June 2006 using the trenching
method. Ten piezometers were then installed and instrumented with transducers
and dataloggers. Water level fluctuations in the river and precipitation data
also were recorded. Initial data indicate a ground-water depression formed in
the center of the tree plot. Also, diurnal fluctuations decreased abruptly in
magnitude following the first hard freeze. These results provide evidence that
the trees are affecting ground-water levels. The plot has been extended and
monitoring continues. To evaluate translocation of arsenic, samples of leaves
and other tree tissue were obtained from newly planted trees, existing trees
on site, and associated soil and ground water. Information from the literature
indicates that arsenic content of tissue is affected by soil concentrations,
plant species, and time of year, as well as other factors. Tree tissue samples
were obtained in the fall before leaf drop, as well as in July and August
2006. Analytical results indicate that arsenic concentrations are less than
the site-specific soil background concentration of 6.3 m/kg; hence, movement
of arsenic from ground water to the soil surface should not prevent use of
trees for hydraulic control at this site.
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