Answer: You can try Oxy-Clean, the oxygenated bleach that we use to clean mildew off 18th-century headstones in our churchyard.
It could be oil, according to some sources, leaching from mulch in the flower gardens, or black algae, say others.
Regular cleaning may be necessary, whatever the cause.
Q: The water in my house in Avalon has developed a rotten-eggs odor. I first noticed it after Sandy hit, when a friend took me to the Shore to check the house.
I thought at the time the odor might have been caused by the storm affecting the water quality, but when I was there this weekend, the smell was still apparent in all the water faucets.
It is stronger in the hot water, but that is probably because hot water releases gas more easily than cold.
I have read that this gas is not necessarily a health threat, and the only step I have taken so far is to buy lots of bottled water for drinking and cooking.
I really have no idea where to begin to find the solution to the problem. I asked my neighbors, and none of them has noticed any odor.
A: The best explanation for the rotten-egg smell - hydrogen sulfide gas, which occurs naturally in groundwater - comes from a D.C.-area company called Magnolia Plumbing, Heating & Cooling: www.magnoliaplumbing.com.
The odor also can be produced by sulfur bacteria or chemical reactions inside water heaters, or by pollution, which is probably the case, at least initially, with Sandy.
The rotten-egg smell does not mean your water is unsanitary, except in the rare case that the gas is caused by sewage pollution.
Hydrogen sulfide gas in the air can be harmful in high concentrations, Magnolia's plumbers say.
When removing the gas from the water, which is possible, it's important to vent the gas outside so it doesn't collect in low-lying places like well pits, basements, or in enclosed areas like well houses.
How can you find the problem, and correct it? This is where calling your plumber becomes an important step.
If the rotten-egg smell only comes out of hot-water faucets, there's probably a problem with the water heater.
If the smell is coming from both faucets, but only comes from water that has been treated by a water softener, the problem is most likely sulfur bacteria in the water softener.
If the smell comes on strong when either the hot or cold faucets are first turned on, but diminishes after a little while, you probably have sulfur bacteria in the well or distribution system.
If the smell is strong when you first turn the faucets on and doesn't go away, there's probably hydrogen sulfide gas in the groundwater.
While I'm a fan of do-it-yourself, I limit what I do to things that are relatively routine or provide a great deal of satisfaction, such as building a coffee table or a bookcase or painting the dining room or the exterior of the house.
I leave the important stuff to professionals, and you should, too.
It's worth the money to get it done properly and quickly, rather than try to do it yourself and end up having to spend even more to extricate yourself from a situation you have made much worse.
The rotten-eggs smell has an obvious source. Your plumber, experienced with Shore houses, will find it.
Questions? E-mail Alan J. Heavens at firstname.lastname@example.org or write him at The Inquirer, Box 8263, Philadelphia 19101. Volume prohibits individual replies.