Elemental ratios in rocks and stream water
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.