[CliffordK]

I suppose one of the risks of road salt usage is that if there is semi-arid land downstream of the area that applies the roadsalt, that uses the river water for irrigation, then it puts those farmers at risk.

Not all states use Road Salt.  Oregon does not.
http://www.subaruforester.org/vbulletin/f76/map-states-use-road-salt-27855/
I believe that Washington does use it in the Puget Sound area, but at least that salt only travels a short distance to reach the Pacific Ocean.  Other areas in the country have a much longer downstream impact.

I consider the salt to be extremely destructive, and would hope highway officials would pursue alternatives.  A non corrosive liquid de-icer is often used here in Oregon.

I also have refused to use salt de-icers for sidewalks.

I was surprised to see on the map that California is listed as using road salt.  If the salt is a problem for the agriculture there, I would think they would prohibit the use of road salt, as well as working with all the upstream states, Nevada, Utah, and New Mexico to find alternatives. (most of Idaho's drainage is northward and their application is listed as minimal).

Apparently Australia has been developing salt tolerant wheat varieties.  Perhaps the salt will become a natural herbicide.
http://www.sciencedaily.com/releases/2010/04/100423094622.htm
http://www.sciencedaily.com/releases/2012/03/120311150717.htm

Anyway, compared to the approximately 10 million tons of salt used on the roads in the USA each year, a couple of pounds of salt used along a walkway would be pretty insignificant.  However, I wouldn't consider it an optimal solution.

[Bored chemist]

The question has been around for a while.
http://en.wikipedia.org/wiki/Paracelsus#Contributions_to_toxicology
and I don't think they have solved it yet.
This
http://echa.europa.eu/documents/10162/13632/information_requirements_r8_en.pdf
talks about threshold and non threshold effects.
The issue is even more complicated with things like salt, copper iron or even oxygen where too much is bad for you but not enough is also bad news.

[CliffordK]

There are a number of dose related values that you may see.

LD50 is the Lethal Dose in which 50% of the sample group dies.

http://en.wikipedia.org/wiki/Median_lethal_dose

But, as BC mentioned, it can be complicated.  For example, some toxins build up in the body, or otherwise are more represented by a cumulative dose.

There is also a concept of Hormesis, in which low doses of a normally toxic substance may in fact be beneficial. 

http://en.wikipedia.org/wiki/Hormesis

Radiation exposure, for example, is something that some people consider any dose as being a risk, but in fact there is an amount of hormesis effect, so low dose radiation may in fact reduce the cancer risk.

http://en.wikipedia.org/wiki/Radiation_hormesis

[Bored chemist]

It's a long way from clear that hormesis is real.
As that wiki page says "Quoting results from a literature database research, the Académie des Sciences — Académie nationale de Médecine (French Academy of Sciences — National Academy of Medicine) stated in their 2005 report concerning the effects of low-level radiation that many laboratory studies have observed radiation hormesis.[6][7] However, they cautioned that it is not yet known if radiation hormesis occurs outside the laboratory, or in humans. 8 "

[Lmnre]

If it's a small area, cover it with sheets of black plastic, which should get rid of every kind of plant. Non-toxic, reusable, etc.

[CliffordK]

I have heard that biodiesel will kill grass and weeds for about a year or so after a spill, then they will come back quite well.

You can mulch your flowerbeds with bark.  Perhaps using a landscape fabric below the bark.  The landscape fabric is supposed to be more porous than black plastic, and doesn't float up as badly.

I dump my grass clippings around my flowerbeds which will also help control the weeds.  Then the clippings go away over the winter.