  We present a general, model-independent, quantum statistical treatment of the
connection between the quantum and hydrodynamical pictures of reservoir driven
macroscopic systems. This treatment is centred on the large scale properties of
locally conserved hydrodynamical observables and is designed to form a bridge
between quantum microdynamics and classical macroscopic continuum mechanics,
rather than a derivation of the latter from the former. The key assumptions on
which the treatment is based are hypotheses of chaoticity and local equilibrium
for the hydrodynamical fluctuations around nonequilibrium steady states,
together with an extension of Onsager's regrssion hypothesis to those states.
On this basis, we establish canonical generalisations of both the Onsager
reciprocity relations and the Onsager-Machlup fluctuation theory to
nonequilibrium steady states, and we show that the spatial correlations of the
hydrodynamical fluctuations are generically of long range in these states.
