Sodium is a naturally occurring constituent in drinking water,
and typically originates as water dissolves rocks and soils as
it flows in streams and rivers or percolates underground. A sodium
content of 20 mg/L or less in drinking water meets EPA's guideline
for 'sodium-free' diets.
The short answer is "Consult
with your physician."
What we can tell you is that food tends to be the most important
source of sodium in a person's diet.
The recommended Daily Value for sodium is 2400 mg. 'Low
sodium' diets are typically between 1000 and 3000 mg per day.
If you drink a gallon of water (16 8-oz glasses) in a day you'll
get less than 160 mg. That's if your water contains 40mg/liter
(154mg/gallon) of sodium.
In contrast you'll get to a whopping 2400 mg of sodium
by eating just two 1/4 pound hamburgers. In the U.S., adults average
about 4000 - 6000 mg per day.