A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Online public documents
Parenthetical citations–reference list style: citing specific types of sources
Part II. Source Citation

To cite online public documents, follow the relevant examples presented elsewhere in 19.9. In addition, include the URL and the date you accessed the material (see 15.4.1). Note that a URL alone is not sufficient; you must provide the full facts of publication, as far as they can be determined, so that a reader can search for the source if the URL changes. If page numbers are not available, you may identify the location of a cited passage in a parenthetical citation by adding a descriptive locator (such as a preceding subheading) following the word under.

R: U.S. Bureau of the Census. 2004. Income, Poverty, and Health Insurance Coverage in the United States, 2004. http://www.census.gov/prod/2005pubs/p60—229.pdf (accessed December 19, 2005).

P: (U.S. Bureau of the Census 2004, under “Ratio of Income to Poverty Level”)

or

(U.S. Bureau of the Census 2004)