Groundwater abstracted from the Middle Devonian aquifer system is the main source of drinking water in South Estonia. High iron and manganese concentrations in groundwater are the greatest problems in this region. The total iron concentrations up to 16 mg L ' are mainly caused by a high Fe"* content in water, pointing to the dominance of reducing conditions in the aquifer system. A pilot study was carried out to estimate the effectiveness of 20 groundwater purification plants with eight different water treatment systems (aeration combined with Manganese Greensand, Birm, Nevtraco, Hydrolit-Mn, Magno-Dol and quartz sand filters) in Vöru County. The results demonstrate that in most cases the systems with pre-aeration effectively purify groundwater from iron, but only 13 out of 20 water treatment plants achieved a reduction of iron concentration to the level fixed in drinking water requirements (0.2 mg L"'). Manganese content decreased helow the maximum allowed concentration in only 25% of systems and in cases where the filter media was Birm or quartz sand and pre-oxidation was applied. The study showed that the high level of iron purification does not guarantee effective removal of manganese.
Groundwater pumped from the terrigenous Middle Devonian (D 2 ) aquifer system is naturally rich in iron (Fe), making it a challenge to fulfil the requirements for drinking water quality. The total iron (Fe tot ) concentrations are above the limit value set for drinking water (0.2 mg/L) in 81% of the analysed water samples. The highest Fe tot 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 Fe tot concentrations are caused by a high Fe 2+ content. Infiltrated aerobic water becomes anaerobic and Fe 3+ 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 wholerock chemical analyses showed large variation in the Fe 2 O 3 content (1.20-9.91%), whereas the values were higher in aquiferforming 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 Fe tot (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.
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