TRACKING ARSENIC UP-ICE FROM MINNESOTA

In Minnesota, arsenic is widespread in soil, its glacial parent material, but only some associated aquifers. The high arsenic wells are linked to the New Ulm Formation of the Des Moines lobe and other similar units. In Blue Earth County high arsenic wells are in aquifers in contact with 2 of the 9 members of the New Ulm Formation.

The paradox is that no correlation has been found between absolute concentration of As in the solids and As in groundwater. Des Moines lobe till members range from 0.1 to 10 mgkg^-1 As with concentrations up to 17 mgkg^-1 in glaciolacustrine deposits. The crustal average is ~2.5 mg kg^-1.

The solid phase As speciation is important in determining the potential for release of As into water. For this reason, identifying the parent material and the acting diagenetic processes may be useful in predicting As release to groundwater.

We investigate the evolution of tributaries to the Des Moines lobe in the Canadian Prairies and track ice flow lines using textures and grain count data to determine if certain till members can be linked to a high arsenic member of the Pierre Shale. The shale subcrops over much of the till source area in Manitoba and is easily recognized in the 1-2mm fraction of tills. The high arsenic Gammon Ferruginous Member ranges in thickness from 56 m to 0.10 m but is thin near its eastern margin along its only known outcrops along the northern flank of the Manitoba Escarpment and the eastern flank of the Pembina Hills. It is at these locations that it would have been available for incorporation at specific times as certain tributary ice streams contributed to the Des Moines lobe.

The Gammon Ferruginous is the lowest Member of the Pierre Shale and lies above the calcareous speckled shale at the top of the Boyne Member of the Carlile Formation and beneath the interbedded bentonite and black shale of the Pembina Member of the Pierre Shale. The Boyne and Gammon Ferruginous members have high total organic C and are enriched in pyrite and siderite compared to the members above and below these horizons in the Carlile and the Pierre. The Gammon Ferruginous Member has anomalously high concentrations of arsenic, rare earths and other metals or transition metals such as Pt, Pd, Cu, Ni, Yt, V and Zn. It is also enriched in Th and U compared to the other Upper Cretaceous members.