PHASE PARTITIONING AND MOBILITY OF MANGANESE ACROSS SIX CRITICAL ZONE OBSERVATORIES: COMPARING PLANT AND BEDROCK CONTROLS

Manganese contamination of surface soils and groundwater may negatively impact wildlife, agriculture and human across the US. Surface soils can have elevated Mn concentrations (>1000 mg kg^-1) due to human pollution but also from biological uplift and surface retention by plants. Groundwaters that serve as drinking water sources can be contaminated with Mn exceeding US EPA secondary drinking water standards from dissolution of minerals bearing Mn and downward transport of Mn from the vadose zone. To evaluate Mn mobility and bioavailability, a sequential extraction of Mn (organic matter + exchangeable phase, secondary oxide phase, and residual mineral phase) was performed on soil profiles and filtered Mn in surface waters at each CZO. The residual Mn was the largest fraction, ranging 416 – 1608 μg g^-1 and 53 – 96% of total Mn. Mn in the organic and secondary oxide fraction were comparable in concentrations (9 – 1077 μg g^-1) and in % of the total Mn (2 – 28%) among CZOs. Calhoun CZO had the lowest Mn concentrations likely due to the degree of weathering while Shale Hills had the highest concentrations from regional pollution and soil parent material. Eel River CZO also had high Mn concentrations due to parent material. Southern Sierra, Boulder Creek and Luquillo CZOs had similar combined concentrations with the residual mineral phase as the dominant fraction. Mn in streamwater ranged from 20 – 940 μmol L^-1 and were similar except Luquillo CZO had Mn concentrations an order of magnitude greater than the other five CZOs.