Strange smelling gravel?

[Alchemilla]

Hello grapes,

I am trying to turn a concrete/gravel/decking desert into a back garden.

A large part of the area had been covered in slate chippings over a weed membrane with about two inches of a grey gravel/sand mix underneath. (I assume as some sort of token sub-base / drainage. ) Whilst digging over the site, I would sometimes notice an odd smell, but didn't really think anything of it.

Today, I dug a hole and planted some perennials in another part of the garden where the slate covering had still been intact. Whilst kneeling down, I noticed that the odd smell seemed to come from the grey gravel/sand stuff underneath the weed membrane. It seemed so far-fetched, I almost thought I was imagining things, so I got my husband to verify and he confirmed it. (Does anyone else do this? )

The smell is quite unpleasant, sort of heavy and chemical-like. Reminds me a bit of the smell when a road is being freshly surfaced. Does anyone have any idea what that could be? Or maybe found something similar in their garden?

I am wondering now if there is anything toxic in that gravel stuff that my family will end up eating if I grow fruit and vegetables in the garden...

[Bigmallly]

Pets urinating on gravel can leave an offensive smell..............Pour some water down & see if it helps.

[Alchemilla]

Apologies BM, that did make me laugh. (How whiffy is Gorilla urine? )

I obviously didn't explain that very well. The sand/gravel layer covered an area of about 10x10 metres and the smell appeared whenever the stuff is exposed to air. I think it would take one hell of a pet to cover that space.

Also, there is rain aplenty here in Wales, so for all intents and purposes, the gravel has had hundreds of litres of water run through it for at least five years and still smells like that.

Based on my internet searchings so far, my suspicion is that this sand/gravel was created by crushing up old road surfaces and then selling it on as "aggregate". In which case, the resulting "aggregate" likely contains a number of carcinogenic organic chemicals as well as heavy metals such a lead.

Surely, that can't be legal in the UK?

[veggiechicken]

Does the "gravel" beneath the membrane have good drainage? The smell may disperse if you leave it exposed to air for a while.
if you're worried about the contents of the aggregate, can you have some samples tested?

As for the rain in South Wales, I'm so glad it rained overnight as its the first for weeks!

[Snoop Puss]

My first thought was: cracked pipes underground or a body...

(Sorry.)

[Kirk]

I would guess it is just a build up of bacteria and such like. If you think about it they use layers of sand and gravel to "home" bacteria in to help clean water through sewage plants. Those big rotating sprayers are releasing semi-polluted water onto a sand/gravel bed that houses bactreria in it to remove pollutants from the water. A bit more "friendly" is similar in a large aquarium filter, but the same idea. And yes an aquarium filter has an "odd" smell when you eventually decide to clean it, sort of neither horrendous or pleasant.

I would guess that that is where the smell is generated from. Your sand and gravel layer would be a nice medium for stuff that size to grow in.

Not sure what you can do other then remove it all. I assume it is soil/clay layer, sand/gravel, membrane then chippings so you will be maintaining a moist/damp environment at the samd/gravel layer.

[Snadger]

Road 'planings' are used regularily for farm roads and paths etc. These are comapatively large sized lumps of tarmac and gravel mixed. If this is what it is it would need stripping from land completely to enable anything to grow on it. Crushed slag was also used but this would have a rotten eggs (sulphurus) smell rather than a smell of tar.

[Alchemilla]

Thanks all for the helpful suggestions!

VC, the smelly grit is on compacted clay - so poorly drained. The point Kirk made about bacteria may be relevant. I guess bacteria as such would be no problem, as I could just mix it in with the soil as originally planned and the bacteria would die as the conditions no longer suit them?

I could have the soil tested, but I'd rather not spend 100 pounds on lab tests if not strictly necessary... (and it would also need to at least have an inkling what contaminants to test for) Although I might end up doing that as it looks like I'll have to test for PCBs anyway due to the site apparently being near an old power station...

Snadger, it doesn't smell like sulphurus, so I don't think it is slag. Still suspect it might be "road planings" as it has a grey-blueish colour, but doesn't look like slate... Although there are no pieces bigger than 1cm in it. The smell lessens when the grit has been exposed to the air for a while.