Biogeochemistry of Arsenic in Contaminated Soils of Superfund SitesEPA Grant Number: R830842
Title: Biogeochemistry of Arsenic in Contaminated Soils of Superfund Sites
Investigators: Sarkar, Dibyendu , Datta, Rupali
Institution: The University of Texas at San Antonio
EPA Project Officer: Carleton, James N
Project Period: August 1, 2003 through July 31, 2005 (Extended to July 31, 2007)
Project Amount: $391,473
RFA: Superfund Minority Institutions Program: Hazardous Substance Research (2002) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Safer Chemicals , Hazardous Waste/Remediation
The Agency for Toxic Substances and Disease Registry has ranked arsenic as number 1 in its 2001 Priority List of Hazardous substances at Superfund sites. There are many Superfund sites where soils are severely contaminated with arsenic as a result of over-application of arsenical compounds. Since arsenic is classified as a Group-A carcinogen, an elevated health risk is associated with long-term human exposure to arsenic in such contaminated soils. Studies suggest that human bioavailability of arsenic is much greater in water than in soils, indicating that human health risk assessment using the water model potentially overestimates the risk associated with ingestion of arsenic from soils. It also sets higher limits on soil-cleanup goals, essentially translating to significant over-expenditure during remediation. The overall goal of the proposed project is to enable more accurate risk assessment for human exposure to arsenic in soils by determining how biogeochemical properties of soils influence human bioavailability of arsenic. The specific objectives of the proposed study are: (1) to examine the relationship between geochemical speciation of arsenic and its bioavailability as a function of soil properties in a realistic soil-plant-water system; (2) to evaluate the use of low-cost chemical amendments in decreasing soil arsenic bioavailability; and (3) to identify the physiological and genetic mechanisms behind arsenic uptake and detoxification in plants.
The relationship between arsenic speciation and bioavailability as a function of soil chemistry will be evaluated in a greenhouse setting. It is hypothesized that soil speciation of arsenic will dictate the portion of total arsenic actually available for absorption in the human gastrointestinal system. Five different soil types have been chosen based on their potential differences with respect to arsenic reactivity. These soils will be treated with three arsenic compounds (two inorganic and one organic) at a very high rate to mimic contaminated conditions. The leachates and the harvested tissues will be analyzed for arsenic and uptake will be calculated. The "operationally defined" geochemical forms of arsenic will be correlated with "in-vitro" bioavailable arsenic to determine which soil-arsenic forms are more likely to be bioavailable in a given condition. Bioavailable arsenic will be correlated with soil properties. Bioavailability studies will also be conducted after amending the soils with low-cost chemical compounds with high arsenic retention potentials. If these compounds are able to increase arsenic retention capability of the soils, the bioavailable fraction of soil-arsenic will decrease. The influence of such compounds on arsenic uptake by plants will be studied. In order to identify the molecular biological mechanisms behind arsenic detoxification in plant systems, brake fern (Pteris vittata), the only known arsenic hyperaccumulator will be used along with rice (Oryza sativa), a high biomass, fast-growing crop plant. Since extensive data on the rice genome is available in an existing database, the approach will be to first clone the genes of interest in rice. Similar studies will then be performed on brake fern, with little known genomic information. The mode of arsenic-mediated phytochelatin induction will be compared in rice and brake fern, and the effects of arsenic compounds on gene-expression and activities of the enzymes involved in the phytochelatin-mediated detoxification pathway will be investigated.
The proposed research is expected to yield the following outcomes: first, development of proper understanding of geochemical speciation of arsenic in soils of variable physico-chemical properties; second, demonstration that low-cost chemical amendments have the potential to irreversibly transform soluble arsenic forms to insoluble, adsorbed phases, thereby reducing arsenic bioavailability; and third, demonstration that the interaction between arsenic and phytochelatins can be genetically characterized, since phytochelatin-assisted detoxification holds the key to arsenic phytoremediation. Collectively, this new knowledge is expected to have a major positive impact on understanding how soil biogeochemical properties influence human bioavailability of arsenic. Consequently, its application is expected to vastly improve the current guidelines on human health risk assessment associated with direct exposure to high doses of arsenic in Superfund soils, and in the process, lower cost of remediation by setting more realistic soil clean-up goals.