Mapping Background Levels of Arsenic In Soil and Stream Sediments to Aid In the Determination of Regulations for Land Application of by-Products

Many agricultural and industrial by-products can be used as aggregate, soil fertilizers, and soil conditioners. However, the concentration of potentially toxic metals and metalloids present in the by-products may limit their use. Government agencies are determining acceptable concentrations and application rates for a wide variety of by-products (e.g., foundry sands, animal manure). While assessing biological risks is essential, application guidelines should also take into account natural background levels. Soil and stream sediment samples from the USGS National Geochemical Survey database were used to assess natural arsenic (As) background levels in the State of Ohio. Relatively undisturbed sampling sites were selected to minimize the influence of anthropogenic inputs. Concentration was measured by atomic absorption in topsoil, subsoil, and stream sediment samples collected statewide. The median concentration was 10 mg kg^-1. A map of As concentration patterns was made using indicator kriging. Five areas of elevated concentrations (relative to the median concentration) were identified and interpreted as to their geologic sources. Many states use a guideline of 20 mg kg^-1 for soils in residential areas. A map showing the probability of exceeding this threshold was made. A 1,660 km² area in north-central Ohio contains natural levels of As with a greater than 50% probability of exceeding the guideline. Land application regulations could be made more efficient through consideration of the spatial variation of natural background levels.