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The role of biodegradation in the attenuation of natural organic matter (NOM) was investigated in long-term experiments that simulate artificial ground water recharge (AGR) for drinking water production. Lake water containing 5.8 mg L(-1) total organic carbon (TOC) was continuously fed into an 18.5-m-long sand column. During the 941 d of operation, on average 76 and 81% of TOC was removed within the first 0.6 m and the entire column length, respectively. Large molecular size fractions (approximately 1800-2200 Da) of NOM were removed more efficiently than smaller ones (approximately 250-1400 Da). The biodegradation of dissolved organic carbon (DOC) within the first 0.6 m, measured by the stable inorganic carbon isotope (delta13C) method, depended on temperature and hydraulic load: The extent of mineralization was 32% at 6 degrees C (Day 442) and 38% at 23 degrees C (Day 708) with a 0.3 m3 (m2d)(-1) hydraulic load and 52% at 5.5 degrees C (Day 883) with a 3.1 m3 (m2d) (-1) hydraulic load. The rest of the DOC removal was likely due to entrapment or sorption onto the sand particles. Decreases in DOC and the total cell counts in the water along the column were positively correlated (r = 0.99; P = 0.001). The accumulation of biomass was minor, with the highest concentration amounting to 7.2 mg g(-1) dw of sand. In summary, this study demonstrated that biodegradation has a key role in NOM removal in AGR and is dependent on temperature.

The glacigenic Virttaankangas aquifer in SW Finland is characterized by groundwaters with high pH values of up to 9.5. High pH values exceeding 9 are uncommon for shallow groundwaters from unconsolidated glacigenic formations on crystalline bedrock. To evaluate the geochemical evolution of groundwater, water samples were collected from 17 sites. The samples were analysed for the isotopic composition of oxygen, hydrogen, dissolved inorganic carbon (DIC) and strontium, the activities of radiocarbon and tritium, and for the basic chemical composition. Groundwaters in the Virttaankangas aquifer system show a considerable range in water pH from 6.4 to 9.5. The isotopic and chemical characteristics of the groundwater in the aquifer units were observed to vary depending on the aquifer mineralogy, groundwater residence time and the openness of the system to CO2 exchange. The pH values were noted to increase in parallel with the contents of Ca and DIC and δ13CDIC. In higher pH values, the groundwaters had attained saturation with respect to calcite. These chemical changes were interpreted to result from dissolution of calcite, present in trace amounts in the Virttaankangas sediments. Specifically, the highest pH values were attributed to calcite dissolution under closed system conditions at a low partial pressure of CO2.

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