Pumping iron and manganese: Geochemistry and engineering of groundwater well systems with challenging redox conditions

Abstract

Wells facilitate extraction and injection of groundwater, both for heating/cooling and drinking water purposes. Their ability to supply groundwater of sufficient quantity and quality may be impeded with time. To a large extent this is due to redox processes associated with iron and manganese, which are dissolved in anoxic groundwater and precipitate to form oxide minerals under oxic conditions. The oxide minerals may clog filters, pumps, etc., and give the water an unpleasant color. Iron and manganese problems have been studied in open-loop ground source heat pump systems in Melhus and drinking water wells at Ringerike waterworks, from collection and analysis of hydrochemistry data, incrustations samples, video surveys of wells, hydraulic and thermal system monitoring data, and laboratory experiments.

The conducted research shows that oxidation of iron in open-loop ground source heat pump systems in Melhus is governed by iron oxidizing bacteria, and their preferred hydrochemical growth habitat. On the other hand, oxidation of manganese is slower, and therefore represents a bigger issue in water supply systems where the residence time is longer and oxygen is more easily available. At Ringerike groundwater works, manganese concentrations started to increase after 19 years of operations, probably due to sorption front slowly moving from the bank filtration to the well field area. This underlines that iron and manganese should be treated as two independent elements, with separate problems, monitoring strategies and remediation actions associated with groundwater extraction and injection schemes.