If you wish to pursue M.tech in Environmental Engineering then here are some of the colleges given below you can prefer:

IIT Bombay

NIT Agartala

NIT Tiruchirappalli etc.

The admission is based on GATE Entrance Exam. To check the cutoff you can go through with the given below links.

IIT Bombay-

http://www.iitb.ac.in/newacadhome/Cutofffiles/cutoff2018-19/Ceseengg.html

NIT Trichy

https://engineering.careers360.com/articles/gate-cutoff-for-nit-trichy

NIT Agartala

https://engineering.careers360.com/articles/gate-cutoff-for-nit-agartala

If you wish to know about any particular college then let me know.

Good Luck!