Which side of the engine block as you stand at the front of the vehicle? Gearbox side or cambelt side?
It's obviously difficult to advise on the limited information but a good place to start fault finding is:
Go to any reputable local garage and ask if they can do a "sniff test" This will test the coolant system via the plastic header tank for any chemicals and gasses related to the engine oil and exhaust side of the block mixing with coolant which is a tell tale sign of head gasket failure... No matter how small. It will confirm or eliminate the possiblility. Most garages can perform yhis test free or for a nominal labour charge... £10 max is money well spent IMO.
It could be the plastic expansion tank itself cracked, its a common issue on all Alfa's during one point in their life, in particular if they have had overheating issues in the past.
A quick test is to remove the tank, bung the pipe work from it in order to prevent further coolant loss.
Now with the tank removed, bung the exit points for the coolant pipes etc with the fingers of rubber gloves and elastic bands... Now fill the tank with boiling water and screw the filler cap back on... Now turn upside down, dry off any spilt water and watch for any steam rising from the plastic tank itself. I tend to do this on a cold day ouside so its easier to spot... If all is ok thats another potential fault eliminated so simply empty the tank of the boiling water and refit it back onto the car.
If you are getting tell tale coolant marks running down the side of the block and drying with heat of the engine on the gearbox side, the chances are it could be something simple like a thermostat gasket or even a housing gasket failure... Remove the induction pipe work for a much better look. Use a small vanity mirror and a torch.
If the tell tale signs are there, then it will involve draining the system, removing the pipes and thermostat housing etc. from the block. Use a sharp stanly blade flat and carefully to remove the old paper gasket gunk... Give it a clean dry surface for the new gaskets and refit. If you cant get hold of a gasket readily, blue hylomar sealant is just as good.
If all is good from the housings etc and you have eliminated yet an other issue, move onto the rubber pipe work, again it's best to semi or completely drain the system and take off the jubilee clips one at a time... Inspect the pipe end for splits and pay attention to where the jubilee clips have dug into the rubber coolant pipes, replace as needed. Move onto the next suspect pipes one at a time until you have eliminated all the possibilities.
Now if it is coming from the cambelt side, the chances are it could be the waterpump gasket that isn't sitting right and if that is the case, you have recourse with the garage who fitted the new pump for you. They will have to recify it FOC.
Now before we blame it all on that, lets do a "belts and braces test".... The radiator.... A well known bugbear of all Alfa's.... They can look in very, very good condition from above but can be rotten at the bottom. It involves removing the bottom engine stone gaurd and getting under the car to inspect the bottom of the radiator thoroughly with a torch... You are looking for missing or damaged fins that look like WWWWWWWWW between the main water channels.... Coolant loss should be apparent but another test to confirm any failure is to start the engine and warm the coolant up to normal temperature until the radiator fans kick in. With the engine guard removed it will be easy to see major leaks on a piece of cardbord placed underneath the car directly under the radiator....
If no coolant drops it could still be minor and "steaming" out so it involves getting under again with a torch and inspecting for any suspect damage areas that steam is slowly drifting out of the radiator... Bear in mind that on wet days this can be normal as the radiator heats up and drys off any rain around the area... Best to check the aradiator on a dry cold day... It is more obvious then... Any of the above faults, then its time for a new radiator....
May I suggest that a new thermostat at the same time will pay dividends and more than likely pay for itself in MPG within a matter of weeks anyway... 2-3 years of a good, well sorted coolant system is worth every penny and every scraped knuckle you encounter
Hope this helps