Boron water quality for the Plynlimon catchments C. Neal Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, OXON, OX10 8BB, UK
Abstract. Boron concentrations in rainfall, throughfall and stemflow
for Spruce stands, mist, streamwater and groundwater are compared with
chloride to assess atmospheric sources and catchment input-output balances
for the Plynlimon catchments. In rainfall, boron concentration averages
about 4.5 μg-B l-1 and approximately two thirds of this comes from anthropogenic
sources. In through-fall and stemflow, boron concentrations are approximately
a factor of ten times higher than in rainfall. This increase is associated
with enhanced scavenging of mist and dry deposition by the trees. As the sampling sites were close to a forest edge, this degree
of scavenging is probably far higher than in the centre of the forest.
The throughfall and stemflow concentrations of boron show some evidence
of periodic variations with time with peak concentrations occurring during
the summer months indicating some vegetational cycling. In mist, boron
concentrations are almost twenty times higher than in rainfall and anthropogenic
sources account for about 86% of this. Within the Plynlimon streams, boron concentrations are about
1.4 to 1.7 times higher than in rainfall. However, after allowance for
mist and dry deposition contributions to atmospheric deposition, it seems
that, on average, about 30% of the boron input is retained within the catchment.
For the forested catchments, felling results in a disruption of the biological
cycle and a small increase in boron leaching from the catchment results
in the net retention by the catchment being slightly reduced. Despite the
net uptake by the catchment, there is clear evidence of a boron component
of weathering from the bedrock. This is shown by an increased boron concentration
in a stream influenced by a nearby borehole which increased groundwater
inputs. The weathering component for boron is also observed in Plynlimon
groundwaters as boron concentrations and boron to chloride ratios are higher
than for the streams. For these Goundwaters, increases in boron concentrations
are matched linearly by increases in the concentration of the principal
ase cation weathering component in the bedrock, calcium. However, the bedrock
weathering term is not uniform as the ratio of boron to calcium concentration
varies for the different boreholes sampled.
Citation: Neal, C.: Boron water quality for the Plynlimon catchments, Hydrol. Earth Syst. Sci., 1, 619-626, doi:10.5194/hess-1-619-1997, 1997.