Using non-mathematical terminology, to start the discussion.
Define induced drag:
The drag which is produced from the tip vortices produced by a finite size of wing, and is directly proportional to useful lift.
define aspect ratio:
The wing span to average chord, with >10:1 being required for an efficient wing.
explain how aspect ratio affects induced drag:
By reducing the strength of the tip vortices, the induced drag goes down, so the wing becomes more efficient.
explain how proximity to the ground affects aspect ratio:
Although there is no physical alteration of aspect ratio, the ground restricts the downwash and hence the strength of the tip vortices. The effect on the flow field is as if the wing aspect ratio had been increased.
Hmmm, good links.