Recently, arsenic speciation has been done on Vashon-Maury Island to help assess the geologic and geochemical conditions associated with higher arsenic concentrations.  Initially, a bedrock source was suggested by a correlation of higher arsenic concentrations versus deep for wells in other parts of the county.  However, some of the wells with high arsenic concentrations were noted to be shallow or in areas with deep unconsolidated sediments, where an intact bedrock source would be far away.

 The ambient sampling included analysis of total phosphorus and a correlation was noted between the arsenic and phosphorus concentrations.  This raised the possibility that the source of the arsenic to the ground water is a geochemical process similar to that occurring in Bangladesh, where buried Pleistocene peat deposits appear to be releasing arsenic through the anoxic reduction of iron oxyhydroxide. 

 Based on the result of the various monitoring and peat sampling, it appears that degrading periglacial peat deposits are another likely source of naturally-occurring arsenic in ground water supplies in King County.