The geochemistry of selenium in groundwaters in Finland

The background level, seasonal variation, speciation, and geographical distribution of selenium (Se) were studied in dug and drilled wells and in natural groundwaters in Finland. The total Se concentration of well water samples (n = 256) ranged from 4.1 to 2720 ng/l, with a median of 68.4 ng/l. The median Se concentration of groundwater from locations in natural surroundings was 51.5 ng/l (n = 41) and 50.5 ng/l (n = 39) in 1989 and 1990, respectively. The season had only a minor effect on both well and groundwater Se concentrations. Selenate was the most abundant species in groundwater. Locally, high concentrations of groundwater Se may originate from bedrock. The proportions of both selenite and Se associated with humic substances were less than 8 and 15%, respectively. The median Se concentration of 37 infiltration water samples was 28.6 ng/l (range 6.1–137 ng/l). The annual net deposition of Se from precipitation into soils was estimated to be 0.49 g/ha in the southern and 0.33 g/ha in the northern parts of Finland. Se from precipitation had no impact on groundwater Se. The effect of agricultural activities including Se supplemented fertilization on the Se concentration of groundwater in cultivated areas could not be demonstrated.