ground water level sensor

May be you can try measure ground resistance.
Try to search www for humidity sensors and irrigation .

Is the measurement possible in a well or through soil?

its through soil..........some radar may be used.......some computer analysis thereafter of the recieved signal.........i have been searching the net

What you need for this application is not just a radar but special Ground Penetration Radar. It doesn't look to be the cheapest option and on top of that the analyzing software will have a lot of troubles interpreting quite umbigious results .. problems very similar to acoustic devices ..

http://www.cpeo.org/techtree/ttdescript/gpr.htm
http://www.geoinformatics.com/freedownloads/gprs.pdf

how about ultrasonics...!!