Rain and groundwater levels

Early tuesday afternoon, I was in my basement, mucking about with my sump pumps, and I noticed that the water level was about 10" down from the top of the sump, which is basically just a hole in the floor. Later that afternoon, there was a sudden flash rainstorm. I checked the sump again, and saw that the water level had risen to nearly the top of the sump, a rise of about 8" from earlier. I know water seeks its own level, so I assume that was the level of the groundwater, but what I’m not sure about is: does that mean we actually got 8" of rain in a half-hour long storm? That seems excessive. Or is there some other factor in play, such as the relative impermiability of large chucks of the ground that are covered with asphalt, houses, and the like, that can somehow “concentrate” the ground water?

Well, that’s part of it. All the water that falls on the roof of your house has to go somewhere. Depending on whether you have gutters or not, it’ll all go right to your foundation. Same with the driveway. And your yard probably isn’t flat, so all the water gets concentrated in the lowest spot which could very well be your sump.

Just a 1/4" of rain over a large area could account for 8" of water in your sump. All depends on the relative areas of rainfall to basement area/sump, etc.
If your house is situated in a flat area all rainfall will seek the lowest area to accumulate in … your basement.
If you have enough elevation to provide for gravity drainage some ditching/drain lines might be in order.