I don’t know what I’d do without Joe Ponessa, the Rutgers professor emeritus who, time after time, for as long as I have been writing this column, has stepped in to bail me out of my ignorance.

This time, it’s about cat urine, an issue that a reader asked about a few weeks back.

Cat urine is an especially difficult contaminant to deal with, especially if it’s a long-term problem, he says.

While Ponessa is not sure anything would fully eliminate odors from long-term staining, there are a couple of easy things he suggests trying before resorting to some kind of coating.

A commercial pet stain and odor-removal product would be a first choice. Another worthwhile alternative would be to cover the stained areas with activated charcoal, available at pet stores and perhaps pharmacies.

This is a treated charcoal with legendary ability to absorb chemicals and odors, functioning like a chemical magnet. This would be spread on the affected areas and renewed every couple of days. He would try this for a week or two.

Activated charcoal is used in fishtank filtration systems, as well as in air purifiers, and is prescribed for and fed to some poisoning patients because of its ability to take up certain types of poison from the stomach.

"I’m not sure how effective this would be, but it’s cheap and easy enough to do," he says.

By the way, "the ultimate resource for products to deal with severe stains and odors is a mortuary supply company," Ponessa adds.

As always, thanks.

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Q: I have a black-slate-top end table that I have had for more than 25 years. Last year, my granddaughter-in-law placed a large pumpkin on the table. We did not realize that it was leaking until it was too late.

I have tried to remove the spots from the slate but have not had much luck. I have tried white vinegar, toothpaste, and furniture wax.

Do you have any other suggestions on how to get rid of these unsightly spots?

A: What I saw online, at eHow, is this:

Combine half a cup of vinegar, half a cup of lemon juice, and half a cup of baking soda in a bowl. This should form a paste. If necessary, add a little water or more baking soda to make a thick paste. Apply paste to the stains, lay a damp cloth over it, and leave it for up to 20 minutes. Scrub.

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Q: We dug out dirt next to our foundation and spread tar on the wall. We covered with tar paper and filled back the dirt.

We still get seepage after a big rain. We read that sodium bentonite can be used, but don’t know where to obtain it. Do you have any suggestions?

A: Sodium bentonite, actually western sodium bentonite clay, is used as a pond sealer. They say it is environmentally safe, but I don’t know anything else about it.

It is applied with hand tools, the manufacturer says. It comes in 50-pound bags. Check online for a supplier, typing in "pond sealant."

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Q: In my bathroom I have a sink, toilet, and bathtub. When I flush the toilet or after running the water from the sink, it makes this plunk, plunk noise about five to eight times.

The bathroom tub does not make this noise, neither does the kitchen sink nor the sink in the basement. This has been going on for about two months now.

I tried plunging the sink, but that did not help. What are your thoughts on this matter? Is it a big problem, and is there a solution?

A: Proper drainage requires constant and unimpeded air flow. When the drain is partially blocked — hair and soap I’ve noticed — that plunking sound occurs.

If all the drains are connected to the sink, and the sink is partially clogged, it may clunk.

You may want to start with that.