This article may be the answer to your questions. I wish to be useful.

Best wishes,

Dear A N Singh,

Ayşe Yazlik provided a very useful article. Few more things are added and hope they would be of any use.

First, regarding the permissible limit in agricultural soil, referring to the article by Walters J and other such literatures, the half life of 2,4-D is very short (less than 15 days). In soil, 2,4-D is reported to affect seed germination (link below)

http://www.jstor.org/stable/pdf/4040499.pdf?seq=1#page_scan_tab_contents

However, in most cases the effect is in terms of delay in seed germination, and decrease in seed germination to less than 80% is at a concentration above 0.07% (700 ppm).

http://science.sciencemag.org/content/105/2724/283

In some works, the dose of applied 2,4-D affecting seed germination (less than 80%) had been reported highr (1000 ppm for Hibiscus). We observed no detectable level (0.1 ppm) of 2,4-D residue in soil (sandy clay loam) after application for 10 crop cycles of transplanted rice-rice sequence (applied 0.75 kg a.i./ha twice in a year).

Regarding the second point of query, i.e. permissible limit in riparian soil the following information may be observed:

Canada’s Guidelines for Canadian Drinking Water Quality has established a maximum acceptable concentration (MAC) of 0.1 mg/L.

http://healthycanadians.gc.ca/publications/healthy-living-vie-saine/water-dichlorophenoxyacetic-eau/index-eng.php

This had been fixed assuming intake of 1.5 litre water for a person having average bodyweight of 70 kg. So, a soil can take higher load of 2,4-D with a short half life.

Similar to soil, the half life of 2,4-D in water is also short (about 9 days). We did not observe any detectable level (0.05 ppm) of 2,4-D residue in water at 15 days after application (1 kg a.i./ha from an initial applied concentration of 0.13 ppm).

The riparian vegetation is usually dominated by grassy species, while the target species for 2,4-D is broadleaved plants. Thus, for riparian soils too the 2,4-D concentration affecting biodiversity may be set similar to agricultural soils.

I am so thankful to borah sir and Ayse for providing a good information and suggestion definitely it help me. Dear Borah Sir, Please send me full article at my mail [email protected] because I'm unable to download the linked articles (http://science.sciencemag.org/content/105/2724/283, and http://healthycanadians.gc.ca/publications/healthy-living-vie-saine/water-dichlorophenoxyacetic-eau/index-eng.php)