Mixed. Most modern systems handle softener brine fine. Older systems with low-volume drain fields (under 750 sq ft) may suffer salt accumulation. Best practice: route softener discharge to a separate dry well.
More detail
Water-softener regeneration brine contains sodium chloride at concentrations 5-20x higher than typical wastewater. The two concerns. (1) Sodium accumulation in the soil at the drain-field absorption surface can affect soil structure (sodium displaces calcium and magnesium in soil chemistry), reducing percolation over years to decades of regular discharge. (2) Modern septic bacterial populations tolerate the periodic salt slug in regeneration cycles fine; older systems with marginal drain-field capacity show more sensitivity. Cincinnati-area homes with high-iron well water (some Indian Hill, Goshen, Morrow) commonly have softeners; routing the softener discharge to a separate dry well or daylight is straightforward and inexpensive ($300-$700 retrofit) and eliminates the long-term concern. New construction in Greater Cincinnati typically includes the separate softener discharge by default. Cincinnati high-iron-water context: Indian Hill, Goshen, Morrow, and outer Mason all have well-water hardness or iron content that triggers softener installation. For those homes, routing the softener discharge to a separate dry well or daylight (not the septic) is the standard recommendation. Retrofit cost $300-$700 if the original installation tied into the septic. New softener installations should plumb separately by default.