The occurrence and geological sources of naturally high iron in the Middle Devonian aquifer system, Estonia

Groundwater pumped from the terrigenous Middle Devonian (D2) aquifer system is naturally rich in iron (Fe), making it a challenge to fulfil the requirements for drinking water quality. The total iron (Fetot) concentrations are above the limit value set for drinking water (0.2 mg/L) in 81% of the analysed water samples. The highest Fetot values reach up to 26 mg/L in some locations of southern Estonia. Due to the reducing conditions in the aquifer system, most of the Fetotconcentrations are caused by a high Fe2+ content. Infiltrated aerobic water becomes anaerobic and Fe3+ reducing along a deep flow path, leading to the downgradient increase in dissolved Fe concentrations. In order to study the natural sources of Fe in the Middle Devonian aquifer system, rock samples from the Narva, Aruküla, Burtnieki and Gauja stages were used for chemical analyses and leaching experiments. The whole-rock chemical analyses showed large variation in the Fe2O3 content (1.20–9.91%), whereas the values were higher in aquifer-forming siltstones than in sandstones. The amount of the leached Fe in groundwater is partly controlled by the granulometric composition of terrigenous rocks. The highest leached Fetot (up to 1.7 mg/L) concentrations were detected in the rocks where the share of the sand fraction is over 70%. As a rule, water is abstracted from  sandstones having large pores and good groundwater yield, therefore water quality problems could only be solved by installing Fe removal facilities in southern Estonia.