Patterns of Soil Arsenic in the Conterminous United States

Arsenic (As) is one of the most dangerous contaminants in the periodic table as it is highly toxic at low concentrations and can often go undetected. Exposure to As occurs via consumption, most commonly through drinking water but also in foods, especially crops grown in As contaminated soils. Soil As is a naturally occurring element in the earth’s crust. The observed patterns can result from both natural processes and anthropogenic forces. While substantial studies have been conducted on As in water, much less has been done to either broadly characterize As distributions in soil or identify the processes driving these distributions. A major limitation to further study has been the lack of thorough soil analysis available across wide spatial area. In 2013 the United States Geological Survey (USGS) published geochemical and mineralogical data for soils that they had collected from all over the conterminous United States. The data is extensive but lacks explanation as to how the minerals came to be located in different geologic locations in the U.S. In this thesis I leverage the raw data provided by the USGS to describe for the first time As distribution in the soils for the whole of the conterminous U.S. and to provide deeper interpretation into the processes responsible for the patterns observed here. I examine depth profiles in soil As and use this data to help identify areas where anthropogenic forces are responsible for high levels of As observed in the top most soil layer. In many parts of the U.S. soil As is derived from underlying shale deposits. Furthermore, I identify a strong relationship between the extent of the last glaciaciation and high concentrations of soil As in the northern U.S., indicating that glacial processes have influenced the high levels of As in the soil. Other hotspots in the U.S., specifically in Texas and the Mississippi River can in part be explained by agricultural practices such as historical use of arsenic-based pesticides on crops such as tobacco, cotton, and rice. Exposure through rice consumption is of particular concern, as it is a crop that absorbs As, and is being grown in the southern Mississippi River floodplain where there are high concentrations of soil As. This thesis provides a comprehensive assessment of soil As levels throughout the U.S. and identifies the processes responsible for those patterns.