In the Cutaadels present state it appears that alchemica deposits are completely trafficable (i.e. some parcels are completely surrounded by alchemica). @coderdan could you comment on this?
Rather than a spillover mechanic perhaps alchemica deposits “bubble up” collectible alchemica to the surface. This would ensure these bonus alchemica remain public on accessible alchemica road ways (unless of course nearby owners wall off an entire estate, however this would likely be extremely cost prohibitive initially).
The added advantage of alchemica “bubble up” is potential to give non boost deposits (various ponds, lakes etc not currently attached to parcels) the ability to generate alchemica and draw attention to what will be interesting features throughout the citaadel.