Hardness-dependent water quality criteria for cadmium and an ecological risk assessment of the Shaying River Basin, China.

Hardness is one important water quality parameter that influences the toxicity of cadmium. Several studies have derived water quality criteria (WQC) for cadmium, but most of these studies did not consider environmental factors. Moreover, few studies considered environmental factors when carrying out ecological risk assessments (ERA) based on environmental factors. In this research, six native aquatic organisms in the Shaying River were adopted to conduct toxicity tests for cadmium. By combining published toxicity data for cadmium with hardness values and toxicity data from this study, hardness-dependent WQC were established. When normalized to a hardness of 100 mg/L CaCO, the criterion maximum concentration (CMC) of 6.46 μg/L and criterion continuous concentration (CCC) of 1.49 μg/L in the Shaying River Basin were derived according to the USEPA guidelines. The acute predicted no effect concentrations (PNECs) derived by species sensitivity distribution (SSD) methods based on log-logistic, log-normal and Burr Type III models were 1.03, 2.41 and 1.66 μg/L, respectively. Recommended WQC values finally expressed as a function of hardness: (1) CMC=(1.136672-0.041838 × lnH) × e; and (2) CCC=(1.101672-0.041838 × lnH) × e. In addition, three tiers of ERA of cadmium in surface waters were conducted based on hardness obtained during different seasons in the Shaying River using the hazard quotient (HQ), the margin of safety (MOS), and the joint probability (JPC) methods. In tiered 1, 2, and 3 ERA, cadmium exposure concentrations were standardized to a hardness of 100 mg/L. The three levels of the ERA method in the tiered framework gave consistent results: the ecological risks of cadmium in the Shaying River Basin were at acceptable levels. The present study provides a reference for the derivation of WQC and risk assessment of pollution affected by differences in aquatic species and water quality factors such as hardness.