I’m pulling together go-to resources for estimating mass discharge and dealing with back-diffusion at chlorinated solvent sites in layered sands and silts. I still lean on REMChlor‑MD and MT3DMS, plus a simple flux spreadsheet using wells at 20 m spacing, but I’d love recs you trust for deciding when to pivot from pump‑and‑treat to ISCR or bio. My standing advice: quantify source strength and the transmissive vs. low‑k storage fractions first — the transport and remediation timelines get a lot more honest.