Laboratory and field study on changes in water quality and increase in dissolved iron during riverbank filtration

Abstract

Changes in the water quality by the riverbank filtration (RBF) process were investigated in the field-scale demonstration sites. The overall water quality was improved by RBF, but Fe²⁺ concentration significantly increased in the riverbank-filtered water more than in the river water. This result would be caused by the interaction between the iron minerals and the river water in the aquifer and the influx of the hinterland groundwater into RBF wells. Dissolution properties of iron from the aquifer soils cored at the sites were evaluated through incubation experiment considering various values of redox potential (Eh), dissolved oxygen (DO), and hydrogen-ion concentration exponent (pH). These results presented that at the incubator with the final Eh of 470 mV, DO of 3.4, and pH of 4.53, the iron from the aquifer soil was most dissolved, and the pyrite and siderite contents in the aquifer soil decreased significantly from 11.5 to 6.22% and from 50.8 to 24.5%, respectively. Based on changes of ion concentrations (such as Fe²⁺, Fe³⁺, SO₄^2- and NO₃^-) and iron species in the incubators, it was believed that pyrite and siderite minerals in the aquifer soils cause an increase in the Fe²⁺ concentration with the absence of DO and an increase in the Fe²⁺ and Fe³⁺ concentrations with the presence of DO. The dissolution rates of iron minerals into Fe²⁺ and Fe³⁺ were dependent on Eh, pH, and DO and were more sensitive to Eh and pH than DO. The results of this study can provide information on RBF site selection and its operation.

Keywords: Dissolved iron; Iron species; Natural attenuation; Redox environment; Riverbank filtration; Water quality.