The outcome is case by case and subjective. Opinions about an aquished being influenced by a single vulnerable “plugged” well vary by available anecdotes and other types of proxies.

Short answer: anything is better than zero, yet also consider that plugging methods and outcomes are not consistently the same. Therefore, the question isn’t “setback versus plug well” it’s “setback versus plug type versus hydrogeology”.

Scenarios synonymous with “plugged well”:

• Casing top capped (clam-type or welded), but casing left unfilled with <<k material;
• screens purposely obstructed, yet casing left unfilled with <<k material;
• full casing obstructed, casing filled with <<k material, yet sanitary seal and/or grouting in annular space is/becomes dilapidated/fractured/weathered)

Although this has been researched amongst hydrogeologists and others, the conclusions about what is an appropriate plugging method comes down to intuition and/or good faith judgement.

Going “full gold standard” RE plugging (pull casing and tremie pipe fill entire bore hole), is expensive and frowned upon by both laymen well owners and projects focused upon optimizing resources.

Continuously consider that well head protection isn’t exclusively about a single well being a source of water, it’s also about any well being a pathway to a source of water.

Therefore, in the absence of standardized plugging (engineering controls), take the route of observing the setbacks recommended for active wells (managerial controls; i.e., wells that are not filled with <<k material) and adjust and record conclusions accordingly.