Arsenic Geochemistry

Arsenic concentrations in the solid geological materials composing MN aquifers are not exceptionally high, yet 50% of the 869 domestic wells tested in west-central MN had arsenic concentrations greater than 10 ug/L – this seeming paradox is the hallmark of geologically sourced arsenic contamination. Our research will address the geochemical form of arsenic (speciation) and the biogeochemical processes that lead to arsenic build up in groundwater.

May 14-15, 2009

We traveled to Hibbing, MN to collect geologic materials from archived well cores stored by the MN DNR. Our goal was to sample all of the major stratigraphic interfaces to approximately 200 ft. depth in 10-15 cores - for example, where sand-and-gravel layers meet glacial tills. These geologic interfaces might be critical geochemical zones where arsenic is leached from glacial tills and concentrated in groundwater.

Lindsey Briscoe, Sarah Nicholas, and Ryan Lesniewski

Core storage warehouse - Ryan moving palets of core boxes.

The archived rotary-sonic cores are stored in long narrow wooden boxes. Jordan (DNR) removed each core, box-by-box, from the warehouse shelves and placed on palets. Then we moved the palets (usually 3/core with up to 20 boxes/palet) to an adjacent sampling room. The boxes are heavy (~40 lbs.) and filled with splinters.

Sarah and Brandy

Ryan and Lindsey

Iron nodules in sand layer

Here are examples of the materials we sampled. Sarah Nicholas (Ph.D. student) and Sara Baldvins (Chem. Eng. UROP undergraduate) will conduct sequential extractions on these materials to measure operationally defined arsenic species. In July, Sarah and Brandy will measure arsenic speciation directly using X-ray absorption spectroscopy (XAS) at the Advanced Photon Source, Argonne National Laboratory.