Spatiotemporal Variations and Driving Factors of Water Hardness in Drinking-Water Sources in Taihu Lake (2011–2023)

Water hardness, an important factor influencing both human health and aquatic ecosystems, is controlled by natural processes and human activities. This study examined spatiotemporal variations in water hardness in Jinshu Port (JP) and Yuyang Mountain (YM) water sources in Suzhou from 2011 to 2023. The JP source exhibited a higher total hardness (92–182 mg/L) than the YM source (87–179 mg/L), and both sites showed clear seasonal patterns. Long-term trends diverged: the JP source remained stable, while the YM source declined significantly. Carbonate hardness increased, whereas non-carbonate hardness decreased in both sites. These changes were associated with the acid rain frequency, which correlated positively with non-carbonate hardness but negatively with carbonate hardness. Land use also strongly affected hardness: farmland-dominated rivers in Huxi (90–210 mg/L) had higher levels than forest-dominated rivers in Zhexi (76–164 mg/L). Water-soluble calcium and magnesium in farmland soils were about 4.5 times higher than those in forest soils and roughly doubled with fertilization. Overall, human activities—including land use, fertilizer application, and acid rain—strongly influenced hardness patterns. Over the past decade, the hardness in both regions has generally remained stable with a slight decrease, suggesting that the strict environmental protection in the Taihu Lake Basin effectively mitigated anthropogenic impacts on water sources.