Another option to a well is a storage tank. Since you already have a spring.While a cistern is normally associated with a rain capture system it can hold water from any source. I essence it is a below ground concrete water tank.
Such a tank could easily store enough water to carry you over most expected drought or other disaster situation.
The Rural Art of Capping A Spring
http://www.motherearthnews.com/Modern-Homesteading/1985-05-01/The-Rural-Art-of-Capping-a-Spring.aspxCapping and developing a spring
http://www.sungravity.com/bulletin__3.htmlSpring Development
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/wqe11302/$FILE/716a15.pdf
Small Farm Spring Development
http://www.in.nrcs.usda.gov/smallscalefarmers/Small%20Farms%20Spring%20Development%20(IN)-web.pdf
Developing Springs as a Water Source
http://www.prairiewaternews.ca/water/vol13no1/story4a.html