Acid neutralization capacity measurements in surface and ground waters in the Upper River Severn, Plynlimon: from hydrograph splitting to water flow pathways C. Neal1, T. Hill1, S. Hill1, and B. Reynolds2 1Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, OXON, OX10 8BB, UK. 2Institute of Terrestrial Ecology, Bangor Research Unit, University of Wales Bangor, Deniol Road, Gwynedd, LL57 2UP, UK.
Abstract. Acid Neutralization Capacity (ANC) data for ephemeral
stream and shallow groundwater for the catchments of the upper River Severn show a highly heterogeneous system
of within-catchment water flow pathways and chemical weathering on scales of less than
100m. Ephemeral streams draining permeable soils seem to be supplied mainly from shallow groundwater
sources. For these streams, large systematic differences in pH and alkalinity occur
due to the variability of the groundwater sources and variability in water residence times. However,
the variability cannot be gauged on the basis of broad based physical information collected in
the field as geology, catchment gradients and forest structure are very similar. In contrast,
ephemeral streams draining impermeable soils are of more uniform chemistry as surface
runoff is mainly supplied from the soil zone. Groundwater ANC varies considerably over space
and time. In general, the groundwaters have higher ANCs than the ephemeral streams. This is due
to increased chemical weathering from the inorganic materials in the lower soils and groundwater
areas and possibly longer residence times. However, during the winter months the groundwater
ANCs tend to be at their lowest due to additional event driven acidic soil water contributions
and intermediate groundwater residence times.
The results indicate the inappropriateness of a
blanket approach to classifying stream vulnerability to acidification simply on the basis of soil sensitivity.
However, the results may well indicate good news for the environmental management of acidic and acid
sensitive systems. For example, they clearly indicate a large potential supply of weathering
components within the groundwater zone to reduce or mitigate the acidifying effects of land
use change and acidic deposition without the environmental needs for Aiming. Furthermore,
the high variability of ephemeral stream runoff means that certain areas of catchments where
there are specific problems associated with acidification can be identified for focused
remediation work for the situation where liming is required.
The case for focused field campaigns and caution
against over reliance on blanket modelling approaches is suggested. The results negate the conventional generalizations
within hydrology of how water moves through catchments to generate streamflow events
(from Hortonian overland flow to catchment contributing areas).
Citation: Neal, C., Hill, T., Hill, S., and Reynolds, B.: Acid neutralization capacity measurements in surface and ground waters in the Upper River Severn, Plynlimon: from hydrograph splitting to water flow pathways, Hydrol. Earth Syst. Sci., 1, 687-696, doi:10.5194/hess-1-687-1997, 1997.