SPATIAL VARIABILITY OF CANOPY THROUGHFALL AND GROUNDWATER SULFATE CONCENTRATIONS UNDER A PINE STAND

Calculation of solute flux into groundwater by recharge must account f or the spatial variability of water Bur and recharge solute concentrat ion. Solute concentration of the recharge can be characterized by samp ling the uppermost groundwater. In an earlier study under pine (Pinus silvestris L.) on sandy soil, solute concentrations at the water table along a transect fluctuated in a recurrent pattern, probably caused b y heterogenous solute input via canopy throughfall. To obtain more ins ight into the spatial variability of deposition processes and solute c oncentrations in recharge, transect sampling of the uppermost groundwa ter was repeated in 1989, and again in 1993. Solute deposition by thro ughfall was also measured in 1993. The data were analyzed by spectral and time series methods. Sulfate (SO4) concentrations in canopy throug hfall and uppermost groundwater shelved recurrent fluctuations corresp onding to canopy coverage of the ground. Sulfate maxima coincided with canopy edges. The dominant fluctuation frequency of SO4 concentration s in 1993 was lower than in 1989 because of canopy enlargement by pine tree growth. Detailed analysis of the 1993 SO4 concentrations reveale d superimposed fluctuations with a frequency valid for the 1989 canopy edge distribution. This can be attributed to an actual patterned SO4 retrieval from solid aluminum-hydroxo-sulfate (Al-OH-SO4) phases, stor ed in the subsoil under previous higher SO4 deposition rates.