Residential Properties and Arsenic Concentrations
In Washington State, residential properties must be located on lands with minimal arsenic concentrations. By regulation, this limit is 20 parts per million, or 20 milligrams per kilogram.
Residential properties fell under the categories of mobile homes, multi-family residences, single-family residences, and residential outbuildings.
A total of 5,875 residential properties were found to sit on land that has over the limit of 20ppm arsenic concentration. Although this is only 2.5% of the 237,052 residential properties in Pierce County, it is not an insignificant number and would certainly matter to the people who reside in those areas.
Of the 5,875 afftected residential properties, 0.5% are mobile homes, 1.0% are residential outbuildings, 13.0% are multi-family residences, and 85.5% are single-family residences.
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Note: This data is from 2010, thus the information is not up to date. Also, military land is not included in this analysis due to the lack of land use data. This is why there are bare patches on the map.
Initial Steps
The process began with a Pierce County base map, 2010 census blocks, existing land use, and the dissolved arsenic sample layer from the Soil Toxins and Child Density project.
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First, residential properties were separated from the land use layer using "select by attribute." These selections were then exported into a new feature class layer of residential properties.
Selected residential properties.
Pierce County base map and residential properties.
Selected arsenic samples >20mg/kg
and resulting layer (below).
Selected census blocks containing arsenic samples >20mg/kg
and resulting layer (below).
Selecting Attributes and Locations
Next, concentrations of arsenic over 20mg/kg were selected by attribute in the dissolved arsenic sample table. This was followed by a selection by location of 2010 census blocks containing the previously selected arsenic samples.
Selected residential properties with >20mg/kg arsenic
and resulting layer (below).
Final Output
Finally, the residential properties underwent a selection by location via "intersect" using those census blocks containing >20mg/kg arsenic points. These newly selected properties were exported as a new feature class layer, showing which residences are near dangerous levels of arsenic in the soils.