Fate, transport, and bioavailability of arsenic in manured and contaminated soils of Delaware

Gardner, Sheila
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University of Delaware
Over the past several years, trace element contamination, specifically arsenic (As), in soils and water has become an alarming environmental issue. All metal/metalloids are potentially hazardous at some concentration (Langdon et al., 2003), and As is known to be carcinogenic, phytotoxic, and biotoxic at extremely low concentrations. The most toxic forms of As are arsine gas, followed by inorganic trivalent compounds, organic trivalent compounds, inorganic pentavalent compounds, and finally elemental As (Cullen and Reimer, 1989). Inorganic As has been listed by the United States Environmental Protection Agency (USEPA) as a Class A human carcinogen and has been linked to bladder, kidney, liver, lung, and skin cancers, as well as impaired nerve function (Research Triangle Institute, 1998). In 2001 the USEPA reduced the allowable levels of As for oral intake in drinking water from 50 µg L-1 to 10 µg L-1, and states have been required to comply with this regulations since 2006. Delaware has complied with this standard since 2001. In 2004, the Delaware Department of Natural Resources and Environmental Control (DNREC) established an Interim Arsenic Soil Cleanup Standard for Residential Properties at 23 mg kg-1 for surface soils from zero to two feet below ground surface, and 101 mg kg-1 for subsurface soils from two to six feet below ground surface, which was a change from the previous standard of 11 mg kg-1. DNREC established the 10-5cancer risk (the potential risk for one additional cancer death caused by exposure to a carcinogen in a human population of 100,000 in a lifetime) for As at 4 mg kg-1, the hazard index value (the numerical value obtained by dividing a person’s expected daily intake of a non-carcinogen by a level which is not expected to produce toxic effects) at 23 mg kg-1, and the background level at 11 mg kg-1. Delaware Senate Bill 68, introduced in April 2005, established an As cleanup level of 6 mg kg-1; however, the bill was not passed into law based on previous research showing that the average background As concentration in Delaware soils is 11 mg kg-1 (DNREC, 2008). The primary sources of As introduction into the environment in Delaware are pesticides, poultry litter, and historical tanneries. An organic As compound known as Roxarsone is incorporated into poultry feeds to control Coccidiosis, increase growth rate, improve feed utilization, enhance pigmentation, and may be effective in suppressing Salmonella (Alpharma, 1999). Most of the As is excreted by the chicken and is incorporated into poultry litter (PL – a mixture of bedding material and manure). The use of PL as an agricultural soil amendment has been an ongoing process for many years on the Delmarva Peninsula, and it is apparent that the fate of As in soils, especially those that are sandy and prone to leaching, needs to be studied.