Quality–Quantity Coupled Evaluation of Groundwater in a Typical Industrial City of the Guangdong–Hong Kong–Macao Greater Bay Area

Groundwater in the coastal industrial cities of the Guangdong–Hong Kong–Macao Greater Bay Area faces rising pressure from saline–tidal intrusion, multi-source contamination, and intensive abstraction. Effective management therefore requires an integrated view of water quality and resource availability. A total of 369 groundwater samples were collected from Quaternary porous and fractured bedrock aquifers during the wet and dry seasons. Major ions and key pollutants were analyzed, and overall quality was assessed using the improved Nemerow pollution index. A 3D transient FEFLOW model calibrated for 2022–2024 was combined with Nemerow quality classes to quantify season-specific exploitable resources by grade. The results indicate that NO3−, Mn, and NO3–N are the dominant pollutants (0–202.05 mg/L, 0.001–8.91 mg/L, and 0–108 mg/L, respectively). Nemerow grading shows Class IV prevailing (47.4–54.5%), with higher Class V proportions in fractured groundwater (27.3–34.5%) than in porous groundwater (14.0–15.5%); overall quality deteriorates in the dry season. Annual mean sustainable exploitable resources are 2.72 × 108 m3/a (porous aquifers) and 1.25 × 108 m3/a (fractured aquifers). These results provide a quantitative basis for season- and quality-informed groundwater development and protection in coastal industrial cities.