| The water quality of the Nant Tanllwyth stream in the Plynlimon region of
mid-Wales is related to the key hydrobiogeological controls and the effects of conifer
harvesting based on an analysis of rain, cloud, stream and groundwater measurements. The
results show the normal patterns of stream water quality response to hydrology. Thus, there
is a high damping of atmospheric inputs due to storage in a highly heterogeneous soil and
groundwater system. Correspondingly, there is a highly dynamic response for components such
as calcium, bicarbonate and aluminium. This response links to the relative inputs of acidic
and aluminium-bearing soil waters under high flow conditions and base enriched bicarbonate
bearing waters from the groundwater areas under baseflow conditions. The introduction of a
deep borehole near the main stem of the river opened up a groundwater flow route to the
stream and other parts of the catchment. There were two aspects to this. Firstly, it caused
a change to the stream water quality, particularly under baseflow conditions, by increasing
the concentrations of calcium and magnesium and by reducing the acidity. The monitoring
shows that this change has persisted for over eight years and that there is no sign of
reversion to pre-borehole times. Secondly, it caused a change in the groundwater level
and chemistry at a borehole on the other side of the river. This feature shows that the
fracture system is of hydrogeochemical and hydrogeological complexity. The effects of
conifer harvesting are remarkable. At the local scale, felling leads to the expected short
term increase in nitrate, ammonium and phosphate from the disturbance of the soil and the
reduction in uptake into the vegetation. Correspondingly, there is a reduction in sodium
and chloride linked to reduced scavenging of atmospheric inputs from cloud water by the
vegetation and also due to increased dilution potential due to reductions in transpiration
by the trees. However, within the main stream, virtually no change is observed in stream
water quality with felling, except for a decrease rather than an expected increase in
nitrate concentration. It seems that the increase in phosphate, for a system that is
essentially phosphate limiting, has stimulated biological activity in the stream leading
to increased uptake of nitrate and ammonium. However, there is little change in the
sodium and chloride in the stream and there are important issues of the representative
nature of studying small scale drainage areas. Boreholes, introduced to assess the impacts
of the felling programme, show unexpected variations in groundwater chemistry. These
variations are associated with the complexity of both flow routing and the chemical
reactivity within the groundwater and lower soil zones, rather than changes that can be
linked specifically to felling. The implications of the study are discussed in relation
to both process understanding and forestry management practices.
Keywords: water quality, acidification, conifer, harvesting, pH, nitrate, ammonium,
phosphate, nutrients, alkalinity, Plynlimon, Wales, Tanllwyth, streams, groundwater |