It's not an easy afternoons job, but it can be done. Again, it depends on several factors; from soil type, to risk of adjacent contamination from fertilizers/additives, to equipment availability. Not sure of expedient methods other than solar stills or digging into surface springs. One would be wise to have bleach in your BOB.
Anyway, here's a link to a blog that offered some thoughts to consider.
http://massiehouse.blogspot.com/2008...ging-well.html
(Can't certify the reliability of this info. YMMV.)