Where I work (UK) incidents on stand such as fuel leak/traffic collision/vehicle fire etc usually get noticed/advised to Airfield Ops first and they are first on scene.
They will declare a "Ground Incident" via RT to GMC (Ground Movement Controller) in tower.
That gets the same response as a Mayday. The RT call will identify location/nature of incident.
Further information can be provided either via GMC or direct to Fire Chief via RT
An aircraft aware of own or nearby incident just needs to advise GMC and they will declare the Ground Incident.