Water quality and sediment geochemistry in lakes of Yunnan Province, Southern China.

Yungui Plateau lakes in southwestern China are economically important, although few have been studied previously. Water and sediments of 24 lakes throughout Yunnan Province were sampled in October 1994. We describe the chemical and physical characteristics of Yunnan lakes, and address effects of regional geology and human influences on water quality and sediment type. Water quality differs between deep Yunnan lakes of tectonic origin and shallow solution basins. Shallow lakes generally have higher nutrient concentrations and appear to be more susceptible to riparian disturbance than deeper lakes. Shallow lakes with high macrophyte standing crops, nevertheless, exhibit nutrient-poor waters. Principal ions Ca ²⁺ , Mg ²⁺ , and HCO ₃ ^– reflect regional carbonate geology, except in Cheng Hai, which is a sodium bicarbonate lake. Specific conductance and δ ¹⁸ O are positively correlated, indicating that evaporation concentrates both solutes and ¹⁸ O. Large, shallow lakes in southeastern Yunnan exhibit ¹⁸ O-enriched waters because of substantial evaporation, whereas small, deep lakes are ¹⁸ O-depleted. Lake waters are ¹⁸ O-depleted in small, shallow basins that receive substantial rainwater input relative to their small volumes. ¹⁸ O enrichment in Cheng Hai suggests that a recent 5-m water-level decline in this lake was caused by increased evaporation or diversion of freshwater inflow. Yunnan watersheds have undergone substantial deforestation, agricultural cultivation, soil erosion, and industrialization. Limnetic nutrient concentrations indicate that human activities have affected water quality. Organic matter content is low in sediments because of increased non-carbonate, clastic sediment yield from watersheds. Environmental policies are needed to balance ecological constraints with economic activities that impact water quality.