Groundwater contamination by arsenic in Vietnam poses a serious health threat to millions of people. In the larger Hanoi area, elevated arsenic levels are present in both, the Holocene and Pleistocene aquifers. Family-based tubewells predominantly tap the Holocene aquifer, while the Hanoi water works extract more than 600,000 m 3 /day of groundwater from the Pleistocene aquifer. Detailed groundwater and sediment investigations were conducted at three locations exhibiting distinct geochemical conditions, i.e., i) high levels of dissolved arsenic (av. 121 µg/L) at the river bank, ii) low levels of dissolved arsenic (av. 21 µg/L) at the river bank and, iii) medium levels of dissolved arsenic (60 µg/L) in an area of buried peat and excessive groundwater abstraction. Seasonal fluctuations in water chemistry were studied over a time span of 14 months. Sediment-bound arsenic (1.3–22 µg/g) is in a natural range. Arsenic correlates with iron ( r 2 > 0.8) with variation related to grain size. Sediment leaching experiments showed that arsenic can readily be mobilized at each of the three locations. Low levels of arsenic in groundwater (< 10 µg/L) generally exhibit manganese reducing conditions, whereas elevated levels are caused by reductive dissolution under iron- and sulphate reducing conditions. Average arsenic concentrations in groundwater are twofold higher at the river bank than in the peat area. The lower levels of arsenic contamination in the peat area are likely controlled by the high abundance of iron present in both the aqueous and sediment phases. With median molar Fe/As ratios of 350 in water and 8700 in the sediments of the peat area, reduced iron possibly forms new mineral phases that resorb (or sequester) previously released arsenic to the sediment. Despite similar redox conditions, resorption is much less significant at the river bank (Fe/As(aq) = 68, Fe/As(s) = 4700), and hence, arsenic concentrations in groundwater reach considerably higher levels. Drawdown of Holocene water to the Pleistocene aquifer in the peat area, caused by the pumping for the Hanoi water works, clearly promotes reducing conditions in Pleistocene groundwater. This demonstrates that excessive abstraction of water from deep wells, i.e., wells tapping water below the arsenic burdened depth, can cause a downward shift of iron-reducing conditions and concurrently mobilize arsenic along the way. Vertical migration of reduced groundwater may also impact aquifers under natural hydrological conditions. Seepage of DOC-enriched groundwater derived from degradation of organic matter in the clayey sediments at the river bank was observed to enhance (and maintain) iron-reducing conditions in the aquifer where organic matter is scarce. Once the aquifer becomes reduced, arsenic is released from the aquifer solid-hosts but additionally derives from the arsenic-enriched groundwater seeping from the clay into the aquifer. This behaviour is an important mechanism for arsenic contamination in aquifers that might not necessarily contain enough organic matter in their sediments to induce reducing conditions independently. Keywords Hydrology Water isotopes iron-arsenic ratio sediment extraction seasonal fluctuations irrigation 1 Introduction Arsenic is a persistent contaminant in groundwater and drinking water in the Red River Delta of Vietnam ( Berg et al., 2001; Trang et al., 2005; Berg et al., 2006, 2007 ). In the last 10–12 years, people have moved away from using surface water and water from shallow dug wells as sources for drinking water, in favour of groundwater pumped from individual private (family-based) tubewells. Long-term exposure to arsenic can affect human health and is a significant cause of skin pigmentation, hyperkeratosis, cancer, cardiovascular disease and may affect the mental development of children, among other possible adverse effects ( Smith et al., 2000; Wasserman et al., 2004; Kapaj et al., 2006 ). The World Health Organization (WHO) recommends 10 μg arsenic per litre as drinking water guideline. Vulnerable areas for arsenic contamination are typically young Quaternary deltaic and alluvial sediments containing highly reducing aquifers. Arsenic concentrations can have a very heterogeneous distribution, for which the reasons are not yet fully understood. The general geochemical mechanisms of arsenic mobilization under reducing conditions is widely attributed to microbial and/or chemical reductive dissolution of arsenic-bearing iron minerals in the aquifer sediments ( Nickson et al., 2000; Dowling et al., 2002; Harvey et al., 2002; Stuben et al., 2003; Horneman et al., 2004; Islam et al., 2004; Zheng et al., 2004; Charlet and Polya, 2006; Postma et al., 2007 ). Others suggest that arsenic may be released from soil minerals at oxic–anoxic boundaries and could subsequently be drawn down from the near-surface through the aquifer to well-depths ( Polizzotto et al., 2006 ). However, both theories do not explain why neighbouring wells can differ in arsenic concentration by 1–2 orders of magnitude, or why some highly reducing areas have lower arsenic levels than others. This problem is partly due to the fact that geological and geochemical conditions are typically very patchy in unconsolidated sediments of young alluvial environments. Some studies have linked the mobility of arsenic in groundwater to the abundance of solid-phase hosts such as carbonates ( Akai et al., 2004 ), silicates, sulphates and iron(hydr)oxides ( McArthur et al., 2001; Roman-Ross et al., 2006 ). Recent publications show that arsenic is not only reductively released from the sediments, but dissolved arsenic can again be reincorporated to solid-phase hosts, i.e., arsenic may be retained by evolving new phases that incorporate arsenic and iron ( Herbel and Fendorf, 2006 ), the sorption density of As(III) on iron oxide phases increases with increasing Fe(II)(aq) ( Dixit and Hering,...