Hubbard Brook Experimental Forest: Watershed 3 Manganese Transects

An area of Watershed 3 in Hubbard Brook Experimental Forest containing
a seep, a spring and a swale was found to contain hot spots for soil
manganese. The source of the Mn was thought to be groundwater
discharge, and this area was studied intensively due to its
implications for disproportionately affecting headwater stream
chemistry. Soils were sampled along a grid pattern; O and B horizon
samples were collected and analyzed for dithionite-extractable (total
secondary) Mn & Fe, pH, total organic C and N, moisture content,
and Cr oxidation potential (indicative of Mn oxide reactivity). The
data were spatially interpolated to identify areas of Mn enrichment
and other chemical variables related to topography and groundwater
discharge. Two hot spots for soil Mn were found: one in a poorly
drained, flowing spring and one in a moderately well-drained swale,
which had similarly high Mn (6000-9000 mg
kg-1 soil). However, the spring soils had
high Cr oxidation potential while the nearby swale soils did not,
indicating a difference in Mn reactivity. A third location in the
study area was a poorly drained seep with water that was not visibly
flowing, and these soils had lower quantities of Mn and low Cr
oxidation potential.