I have chemical analysis of one catchment which can be considered more or less homogeneous and also stream water analysis, I would like to know how elemental ratios (to Na for example) are related both in the geology and in the water. The rocks are mostly andesites. Can I expect the ratios to be similar, and based on your expertise can you recommend a methodology to relate both variables?
There's no simple answer to this. The elemental ratios are a function of proximity to the ocean (Na, Cl, Br, I, etc), relative abundance and mode of weathering of feldspar, rate of weathering, rock Mg-Fe ratio and redox state (especially in relation to iron), the degree of rock mineralization (SO4, Cl, F), rock heterogeneity, and how close the rock-water system is to thermodynamic equilibrium. There are many good books on the subject, of which my long-standing favorite is John Hem's classic US Geological Survey Water Supply Paper 2254, Study and Interpretation of the Chemical Characteristics of Natural Water, 3rd Edition, available online as a PDF.