Iron and artificial recharge of groundwater

High iron concentrations may be a problem when groundwater is used for drinking water supply. The objective of this field study has been to clarify the factors that control the presence of iron in the water phase at artificial groundwater recharge with pond infiltration. The iron concentration in water was monitored over a 7-month period at a site with pond infiltration. Water was sampled once a month from 10 to 115 cm depth in the filtersand of the pond, and analyzed for iron. The filtersand was sampled before and after the water sampling period so that accumulation of iron could be quantified. It was found that at the 20 cm level in the filtersand the iron concentration was reduced to a level fully acceptable in drinking water. The analysis of the soil showed that 86% of the accumulated iron was retained in the top 5 cm of the filtersand. According to a budget considering the amount of iron coming to the filtersand and the amount leaving it, 138 g m– 2 should have been retained during 15 months while 145 g m– 2 was actually found by analysis. The findings show that the iron retention process was fast, both in time and travel distance. The iron retention efficiency would thus not be improved by a longer residence time. This suggests that even smaller geologic formations may be used for pond infiltration. The results also indicate that the iron retention was a largely abiotic process. The problem with gradually increasing iron concentrations at induced recharge is discussed in view of studies of iron-reducing microorganisms.