Brackish
Etymology
From Dutch or Low German "brak" ("salty, briny") plus the English "-ish." It entered English through the sea-trading contact between English and Dutch sailors.
adjective · /BRAK-ish/
Of water, slightly salty — a mix of fresh and sea water — and by extension, unpleasant to taste.
What it names
The in-between saltiness of estuaries and marshes, where river meets sea.
Example
The well near the coast had turned brackish.
Related idea
Brackish water is what defines an estuary, the zone where a river’s fresh water blends into the salt of the sea.