How to eliminate odor from crawl space

I'm trying to eliminate damp odor originating from a crawl space.

Here's the situation:

• Half of the house has a basement. The other half is a crawl space accessible from the basement through an access opening. The access opening and another "vent" opening share the crawl space air with the basement.
• The crawl space is surrounded by foundation walls with no windows. The floor is stones covered with overlapping heavy-duty plastic sheets (nothing is sealed).
• There are two HVAC zones. The basement has one air handler for that half of the house, including air vets and a return in the basement. The crawl space has an air handler for its half of the house. It, too, has vents and a return in the crawl space to ventilate the crawl space.
• The house is adjacent to a community water catchment area and the water table gets high during rainy periods. There is a sump pump in the basement that is fed from the gravel layer under the basement floor, and it runs continuously in periods of wet weather. There is a foundation wall and footer between the basement and crawl space, so I suspect the crawl space doesn't drain to the sump pump.
• The house in in good shape in terms of exterior water (the rain gutters are good and discharge far from the house, and the grading is OK). There are no signs of water in the basement.

Here's the problem. During wet weather, the crawl space gets a "wet ground" smell that wafts out of the stone floor. The HVAC return picks it up and adds it to the household air on that side of the house. So half of the house gets the aroma.

I stuck a dehumidifier in the crawl space but it doesn't affect the smell. The air handler has an electrostatic air cleaner, but that doesn't filter smells. So I'm looking for a solution to keep the aroma out of the household area.

What I've considered:

• I could seal the crawl space return, but that would starve the air handler of 1/3 of its air volume. Or I could duct it to the vent hole in the basement wall and pull better air. However, I would need to close the vents in the crawl space or it would just circulate the crawl space air by a longer route to the return. So again, there's an air volume problem.
• If I isolate the HVAC from the crawl space air, the crawl space will no longer be ventilated. I assume the odor would become worse in the crawl space, possibly even a mold problem.
• I'm not sure how practical it would be to seal the floor. There is no way to get concrete or any heavy material into the crawl space. Maybe adding another layer of plastic sheeting would help if there's a way to seal it and seal it to the walls (duct tape?). The floor area is on the order of 1,400 sq. ft., so it isn't a trivial area to seal.
• I thought about putting a sump pump in the crawl space. However, There is probably 3 1/2' to 4' of gravel to get below the basement floor level to where the water actually is, and the sump pit would go below that. So it would be more of a well than a sump pit. I can pump the water out, but the ground will still be wet, so I'm not sure that would affect the odor.
• I've seen various deodorizer products you stick in the ductwork. They get very mixed reviews.

Any recommendations on how to solve the problem?