A Zelkova tree's spread is 30-40 feet, according to the first link below. Since it has an extensive root system, it will cause damage to a septic system that is 8-10 feet from it, when it reaches maturity. The largest roots will be under the dripline (the soil under the branches), however the entire root system can spread up to 1.5 times the height of the tree. (see second link, first page) For a Zelkova tree that reaches a height of 60-80 feet, (see first link) that could mean a root system that extends 120 feet. A Zelkova is considered to be a fast-growing shade tree, so it should be moved, while it is still immature. (see third link)
http://pubs.caes.uga.edu/caespubs/pubcd/L350.htm#chart -second on the list
http://www.cahe.nmsu.edu:16080/pubs/_h/H-328.pdf -gives parameters of growth of roots on the first page
http://pubs.caes.uga.edu/caespubs/pubcd/L350.htm -the chart on the first link comes from this article
http://groups.ucanr.org/slosson/documents/1999-20002075.pdf - an additional source, in which an experiment was conducted diagramming root systems of zelkova and two other tree types. Bear in mind, these diagrams were done when the trees were young saplings. The diagram shows that the root system is shallow, but has the most horizontal spread of the three.