The quality of groundwater resources in Kerman Province has been seriously threatened by the arid and semi-arid climate, urbanization, industrial development, and excessive abstraction. This study aims to assess the long-term quality of groundwater resources in the central, southern, and western areas of the province by analyzing water quality data from 1197 observation wells over a 22-year period. To this purpose, the exploratory principal axis factoring (PAF) technique was used to determine the weights of the water quality parameters and calculate the Water Quality Index (WQI). The exploratory analysis of weights indicated that TDS, EC, Cl⁻, and Na⁺, with weights greater than 0.15, were the most influential variables affecting the WQI. Spatial analysis of the water quality index revealed that in the southern, central, and eastern parts of Kerman Province, water quality remains excellent to good during both the dry and wet seasons. Conversely, in the northern and western regions, water quality drops to poor or non-potable levels. The Mann–Kendall test and Sen’s slope analysis showed that the annual average WQI had a significant increasing trend at the 99% confidence level, with a Sen’s slope of 1.49. This suggests a consistent decline in groundwater quality, moving from the poor category to very poor and eventually reaching non-potable conditions. The findings highlight the urgency of implementing management strategies to prevent further decline in groundwater quality and to protect the long-term sustainability of regional water resources.