Overbank sediments from central Mexico: an evaluation of their use in regional geochemical mapping and in studies of contamination from modern and historical mining

Overbank sediment sequences in central Mexico display vertical changes in chemistry which can be related to both anthropogenic contamination and natural geological sources and processes. They also show significant lateral chemical variation, at both local and regional levels, which makes the design of a sampling strategy for regional geochemical mapping or contamination studies difficult. This variation is particularly pronounced in drainage basins which have been contaminated by mining activity and limits the use of overbank sediment as a systematic regional geochemical mapping medium. To be used with confidence, overbank sediments require detailed studies of fluvial geomorphology and history, accompanied by accurate age dating. Active drainage sediments are a more viable alternative in areas of historical mining activity, but must be examined carefully in the light of the regional background where levels of contamination are low or the contaminants are buried within the river floodplain.