Modelling the long-term changes in stream, soil and ground water chemistry for an acid moorland in the Welsh uplands: The influence of variations in chemical weathering

A long-term model of stream acidification, MAGIC, has been applied to an acidic moorland catchment, the Gwy, in mid-Wales. This application has been used to examine the effects of variability of weathering on soil, groundwater and stream water chemistry from 1844 to the present day and onwards to 2080 with a predicted acidic oxide reduction of 60% in the future, The results show that weathering initially affects soil, groundwater and stream water chemistry profoundly, Despite this, the results indicate that a simple two-layer (one soil and one groundwater end-member) model can still provide a good prediction of long-term stream water quality even when each soil and groundwater area is heterogeneous in composition. The work provides support for the use of the lumped MAGIC model. However, for the predictions to be applicable it is shown that it is critical that the relative contribution of waters and soil exchange materials from the hydrochemically distinct regions within the catchment are adequately represented within the model. This probably means that further field work is required to examine source area contributions.