Prediction of future short-term stream chemistry — a modelling approach

The problem of predicting future short-term chemical behaviour in acidic and acid-sensitive streams is addressed. A relatively simple method is presented which combines a chemical technique for splitting the hydrograph into ground water and soil water components, based on the conservative component acid neutralization capacity, with the long-term hydrochemical model (MAGIC) in a ‘two-box’ mode. This method, coupled with a chemical speciation program (ALCHEMI), is used to assess short-term variations in stream chemistry with change in atmospheric deposition chemistry. The method is applied for a semi-natural moorland catchment in mid-Wales (Afon Gwy). For both hydrogen ion and inorganic aluminium, the modelled stream mixing relationships are non-linear, particularly at low hydrogen ion concentrations. The results also show that the relationship between hydrogen ion and inorganic aluminium concentrations varies with time in the stream. This result has important implications as it implies that aluminium concentrations will not recover, with reduced anthropogenic sulphur deposition, as rapidly as has previously been thought. Two methods, for use with critical load evaluation of ecological stress, are presented for describing the changes occurring: the hydrogen ion and inorganic aluminium concentration duration curve; the hydrogen ion and inorganic aluminium incident frequenty diagram.