The hydrochemistry of the headwaters of the River Severn, Plynlimon C. Neal, J. Wilkinson, M. Neal, M. Harrow, H. Wickham, L. Hill, and C. Morfitt Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, OXON, OX10 8BB, UK.
Abstract. Water quality data spanning 13 years and covering an
extensive range of major, minor and trace elements in rain and stream waters at Plynlimon in mid Wales, are presented.
Rainfall water chemistry is highly variable due to varying proportions of marine and
pollutant derived constituents associated with patterns of atmospheric circulation. Stream
waters, being composed of different proportions of waters from three chemically distinct sources
at any one time (atmospheric deposition, the soil system and deeper groundwaters),
are also chemically highly variable. For example, components predominantly derived from deposition
such as chloride change only in response to sea-salt deposition episodes. Solutes associated
with bedrock weathering such as calcium, magnesium, and alkalinity decrease with
increasing flow, those associated with the upper soil layers such as aluminium, many transition
metals, dissolved organic carbon and hydrogen ions increase with increasing flow. The
nutrients (e.g. nitrate, boron, bromide and iodine) exhibit strong seasonal cycles associated
with cycles of vegetation growth and decay.
The changes in stream water chemistry resulting
from tree harvesting in the Afon Hore catchment are shown to have run their course within a period of eight
years. Nutrient increases in the first few years following the commencement of felling have returned
to or fallen below pre-felling values. Aluminium changes are shown to be complicated by
changes in nitrate and calcium. Aluminium concentrations initially increased and have fallen
below their pre-felling value. Data for chloride suggest a reduction in capture of dry and
mist deposition; this indicates the importance of understanding reduced deposition as a result of felling.
Felling has also affected the soil micro-climate which experiences greater fluctuations
in temperature and an increase in the concentration of constituents associated with organic
Input-output mass balance estimates show that atmospheric
inputs of many constituents are retained strongly by the catchment (e.g. ammonium, phosphate, barium,
boron, lead and iodine). In contrast, many of the transition elements as well as divalent base
cations, aluminium and alkalinity show a net release from the catchment. Conservative constituents
such as chloride and sodium show a net input-output balance.
Citation: Neal, C., Wilkinson, J., Neal, M., Harrow, M., Wickham, H., Hill, L., and Morfitt, C.: The hydrochemistry of the headwaters of the River Severn, Plynlimon, Hydrol. Earth Syst. Sci., 1, 583-617, doi:10.5194/hess-1-583-1997, 1997.