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Sodium and chloride levels in rainfall, mist, streamwater and groundwater at the Plynlimon catchments, mid-Wales: inferences on hydrological and chemical controls
1Centre for Ecology and Hydrology, Crowmarsh Gifford, Wallingford, Oxon OX10 8BB,UK.
2Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767, USA.
e-mail for corresponding author: cn@ceh.ac.uk
Abstract. Variations in sodium and chloride in atmospheric inputs (rainfall and mist), stream runoff and groundwater stores are documented for the upper Severn River (Afon Hafren and Afon Hore catchments), Plynlimon, mid-Wales. The results show five salient features.
- Sodium and chloride concentrations are highly variable and highly
correlated in rainfall and mist. The sodium-chloride relationship in
rainfall has a slope close to the sodium/chloride ratio in sea-water,
and an intercept that is not significantly different from zero. This
indicates that sea-salt is the dominant source of both sodium and
chloride in rainfall, which would be expected given the maritime nature
of the metrology. For mist, there is also a straight line with
near-zero intercept, but with a slightly higher gradient than the
sea-salt ratio, presumably due to small additional sodium inputs from
other sources.
- There is an approximate input-output balance for both sodium and
chloride, with the exception of one groundwater well, in which high
chemical weathering results in an anomalous high
Na/Cl ratio. Thus, atmospheric deposition is the dominant source of
both sodium and chloride in groundwater and
streamflow.
- The fluctuations in sodium and chloride concentrations in the
streams and groundwaters are strongly damped compared to those in the
rain and the mist, reflecting the storage and mixing of waters in the
subsurface.
- On all timescales, from weeks to years, sodium fluctuations are
more strongly damped than chloride fluctuations in
streamflow. The additional damping of sodium is consistent with ion
exchange buffering of sodium in the catchment soils.
- Sodium and chloride concentrations are linearly correlated in the streams and groundwaters, but the slope is almost universally less than the sea-salt ratio and there is a non-zero intercept. The Na/Cl ratio in streamflow and groundwater is higher than the sea-salt ratio when salinity is low and lower than the sea-salt ratio when salinity is high. This pattern of behaviour is again consistent with ion exchange buffering of sodium in the catchment soils.
Keywords: Sodium; chloride; rainfall; mist; stream water; groundwater; Plynlimon; Hafren; Hore; Tanllwyth; moorland; Sitka spruce; forest; cation exchange; fractal
Final Revised Paper (PDF, 2197 KB)
Citation: Neal, C. and Kirchner, J. W.: Sodium and chloride levels in rainfall, mist, streamwater and groundwater at the Plynlimon catchments, mid-Wales: inferences on hydrological and chemical controls, Hydrol. Earth Syst. Sci., 4, 295-310, 2000. Bibtex EndNote Reference Manager
