Foundation stability

I've got a 52 year old house that's built on a crawl with block walls. The house is nearly at the low spot of the street. When there is a hard rain, 3-4 times a year, the water table rises enough to be into the crawl space. Last spring it got to within 3-4 inches of my furnace. Usually within about 12 hours the water table drops below the crawl space floor elevation. The foundation is approximately 35'x25'. There is a lot of clay in the soil.
I have put a sump pump in on the lower end of the crawl space to pump out the water so my furnace stays out of danger. The hole is about 2' x 2' x 1.5' and I've used a sturdy plastic planter box as a liner. This setup worked pretty well during the last hard rain we had. I'm pumping the water to a nearby drop box inlet.
My question is: Is there any risk to my foundation by doing this? After the pump cycles the water would flow back into the sump and seep up through the surrounding area faster than usual (as if it gradually rose with the water table). Correct?
I'm assuming this has been happening since the house was built in 1951 and the foundation doesn't look any different than any other 50 year old one. It doesn't seem to have caused any problems.
Thanks for any advice.
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