[jalacu]

The River Dunn, Hungerford, West Berkshire. Chalk stream.
I could wait of course but the pavement is a bit of a mess and I was trying to be neighbourly.

[Borek]

If I used 500ml of HCl at 10%

The River Dunn, Hungerford, West Berkshire. Chalk stream.

Looks perfectly safe to me. Chalk waters are rather alkaline and have high buffering capabilities, judging from wikipedia pictures River Dunn is large enough to quickly dilute the acid, and 50 g is quite small amount.

Also, amount of heat will be quite low, especially taking into account fact you are using the acid in the open, where the heat can quickly dissipate. I wouldn't be surprised if the heat effects were below "manual detection limits".

Doesn't mean you will be not violating some local regulations, plus, there is always a possibility that the acid will leech something nasty from the asphalt. Unlikely IMHO, but can't be completely ruled out.