Effective, considerate shale play water management support operations and protect the environment. A parameter often overlooked is total dissolved solids (TDS). Knowledge of TDS is important to meet these dual goals. Subsurface TDS typically increases with depth. However, produced-water samples from the Eagle Ford Shale show a strong TDS decrease by a factor of ~10 with increasing well depth (~200,000 ppm at ~2.5 km to 18,000 ppm at ~3.6 km). Water stable isotopes strongly suggest that the low TDS is not due to dilution by meteoric water. Rather, it is attributed to smectite-to-illite conversion, in which the smectite interlayer water is released into the pore space. Depth, temperature, and other related indicators (source for K, excess silica) support such a mechanism. In addition, water-isotope patterns and 87Sr/86Sr ratios suggest a conversion operating in a closed system. Order-of-magnitude calculations show that the 8% of mixed-layer clay present on average in the Lower Eagle Ford Shale is sufficient to dilute brines to observed levels. Stakeholders could then have a more optimistic outlook on water recycling and on using produced water for other uses (irrigation, municipal) because the low salinity is an intrinsic property of the formation rather than due to short-term mixing.