As per my knowledge in TOC analyzer(shimadzu model) IC will be purged out and in water only two forms of carbon remains that is NPOC(non purgable organic carbon) and POC(purgable organic carbon).
TOC=NPOC-POC, POC is present only in low quantity so it was neglected,
Hence TOC=NPOC
TOC increases due to microbial proliferation in water due to storage.
So increase in TOC means microbial contamination also high,