Just twenty per cent

A recent paper on GRL, based on isotope studies claims that the contribution of fresh water to the oceans, could be just 20% (Wow!), the rest coming from what is termed the 'subterranean estuary,' which some researchers think supply the lion's share of terrestrial nutrients to the oceans.

The paper, by Kwon et al., is the result of many years of work, and is open access clicking on the link below. 

Reference

Kwon, E. Y., G. Kim, F. Primeau, W. S. Moore, H.-M. Cho, T. DeVries, J. L. Sarmiento, M. A. Charette, and Y.-K. Cho (2014), Global estimate ofsubmarine groundwater dischargebased on an observationally constrainedradium isotope model, Geophys. Res. Lett., 41, 8438–8444, doi:10.1002/ 2014GL061574.