Marcos,

If you are concerned with hydrographic separations, there are two papers
that I suggest you read written by my colleague Dave Genereux:

Genereux, D.P. and Hooper, R.P.  1998  Oxygen and Hydrogen Isotopes in
Rainfall-Runoff Studies  Chp 10. in Isotope Tracers in Catchment
Hydrology  Kendall, C. and McDonnell, J.J.  Elsevier  319-346.

Genereux, D.P.  1998  Quantifying uncertainty in tracer bsed hydrograph
separations.  Water Resources Research  v34(4) 915-919.

Basically you have to have good separation between groundwater and surface
water to isotopically separate storm hydrographs, and this may be true only
during certain times of the year.  To remedy this situation you may have to
go to a three component tracer based hydrographic separation using, for
example, isotopes and major ion chemistry or alkanlinity.  The second paper
presents a mathematical treatment of uncertainty in two and three component
systems.

Good luck & hope this helps,

Bill