many large asian cities 
 rely on groundwater for their water supply , 
 and the abstraction of groundwater has commonly caused the development 
 of large depression cones .
in addition , some of these cities , exemplified by kolkata and hanoi , are situated on top of aquifers 
 that are seriously contaminated by geogenic arsenic .
generally , the 
 uppermost holocene aquifers are the most - contaminated by arsenic , 
 while underlying pleistocene aquifers , in most cases , are low in arsenic . because groundwater abstraction for municipal water supply occurs 
 from the deeper pleistocene aquifers , with groundwater low in arsenic , 
 the change in groundwater flow patterns may cause the migration of 
 arsenic - rich groundwater from upper aquifers into the underlying low - arsenic 
 aquifers .
the effect of municipal groundwater abstraction pumping 
 also influences the water quality on a regional scale because depression 
 cones extend far beyond the city limits .
both 
 hanoi and dhaka are located along major rivers , and hydrogeological 
 studies have shown groundwater abstraction to cause a very 
 significant infiltration of river water into the underlying aquifers . 

the recharge of aquifers by losing rivers , often called bank infiltration , 
 enhances the available water resource , and managed and monitored bank 
 infiltration is a strategy that has been used extensively , particularly 
 in europe . apart from increasing the quantity of the groundwater resource , 
 bank infiltration also may improve the quality of the infiltrating 
 surface water . during bank infiltration , processes taking place at 
 the surface water 
groundwater interface can remove nutrients , 
 organic carbon ( including organic contaminants ) , and pathogens such 
 as microbes and viruses .
however , the effect 
 of red river bank infiltration on the water quality of the aquifers 
 below hanoi is completely unknown , even though it is considered the 
 main source of groundwater recharge below the city .
the city of hanoi has a leaky sewerage system as 
 well as open channels 
 carrying wastewaters and untreated sewage water through the city , 
 both of which are leaking into the underlying aquifers .
in addition , the aquifers beneath 
 hanoi are contaminated by arsenic with concentrations exceeding 300 
 g / l .
in contrast , the water of the red river 
 running along the city of hanoi is very low in dissolved arsenic .
therefore , one could hope that 
 the large scale infiltration of river water can improve the water quality in the aquifers and also flush out 
 the arsenic .
the time scale needed for flushing arsenic from the aquifer 
 is currently unknown but must depend on infiltration rates , groundwater 
 flow patterns , and residence times as well as the geochemical processes 
 controlling the groundwater arsenic content .
the latter includes the 
 mobilization of arsenic by reductive dissolution of fe - oxides and 
 the adsorption and desorption of the mobilized arsenite .
the objective of the present 
 study is to investigate the rate of infiltration of red river water 
 into the underlying aquifer , clarify the geochemical processes occurring 
 between the infiltrating water and the aquifer sediment , and predict 
 the effect of river water infiltration on the groundwater arsenic 
 content .
our field 
 locality at nam du 
 ( 205825.43 n , 1055409.73 
 e ) is located on the eastern boundary of the city of hanoi , vietnam , 
 directly on the shore of the red river .
the geology of the area has 
 been investigated extensively in relation with the establishment of 
 the nam du well field in 2004 . generally , 
 the geological sequence in the area consist of a top clay layer underlain 
 by a up to 40 m thick holocene aquifer consisting of fine- to medium - grained 
 sand , comprising more fine- or coarse - grained layers .
the sandy aquifer 
 is underlaid by an aquitard of variable thickness ( 110 m ) 
 followed by a coarse - grained sand to gravel aquifer of pleistocene 
 age .
remote sensing shows the nam du 
 area to belong to the meander belts of the red river .
a detailed geological profile of our study area , based 
 on natural  logging of boreholes and sediment description , 
 is shown in figure 1 .
uppermost there is an up to 11 m thick layer of clay and silty 
 clay , which is absent close to the river .
below follows an up to 40 
 m layer of fine- to medium - sized sand .
underneath , there is an up 
 to 20 m thick layer of coarse sand and gravel . between the two sand 
 formations ,
finally , at the bottom , there is a layer of claystone  siltstone . 
 in accordance with the geological description of the area , the upper aquifer with fine - grained sand
is 
 considered to be of holocene age , while the underlying coarse sand  gravel 
 layer is interpreted as being of pleistocene age .
the intermediate 
 clay lenses are probably remnants of the aquitard separating the holocene 
 and the pleistocene aquifers , which elsewhere is found as a more - continuous 
 layer .
the upper layers 
 of clay and fine  medium - grained sand belong to the holocene , 
 the coarse sand  gravel to the pleistocene , and the silt 
the low measured electrical conductivity 
 ( ec ) values ( 200270 s / cm ) indicate the presence of 
 river water in the whole system .
arsenic is low to absent in the river 
 water and upper 17 m of the groundwater but is mobilized further down .
the depth of 
 the river channel at nam du varies between 13.5 and 17.0 m below the 
 ground level at the nest of boreholes on the shore ( figure 1 ) .
the mid channel depth is 
 15.3 m and is situated within the holocene aquifer .
the groundwater 
 below hanoi has been exploited since 1894 , and groundwater abstraction 
 presently amounts to > 900  000 m / day .
groundwater is being pumped mainly from six 
 major well fields distributed throughout the city . as a result ,
large coalescing depression cones have developed 
 below the city of hanoi in both the holocene and pleistocene aquifers , 
 which are partially connected . the well field at nam 
 du
the pumping station included in our field site 
 ( figure 1 ) is part 
 of this gallery of pumping stations , but the flow toward the pumping 
 station is the result of the drawdown caused by the entire pumping 
 activity below hanoi .
the hydrostatic 
 pressure in our wells screened in the pleistocene aquifer is about 
 6 m lower than those in the nest screened in the holocene aquifer 
 ( figure 1 ) , indicating 
 a strong downward gradient induced by the pumping .
the pumping combined 
 with the less - permeable holocene deposit on top of the highly permeable 
 pleistocene causes predominantly vertically downward groundwater flow , 
 providing the shortest passage through the holocene .
the pressure 
 difference between the holocene and pleistocene aquifers is also found 
 elsewhere in the city , but its magnitude varies from locality to locality . to investigate the 
 hydrogeological and geochemical processes , we have collected three 
 sets of data ( figure 1 ) ; ( a ) detailed water sampling of the sediment porewater performed 
 on a sandbank in the river , ( b ) water sampling in a nest of boreholes 
 located on the river shore with screens installed at different depths 
 in the holocene aquifer , and ( c ) water sampling in boreholes and the 
 nearest pumping station screened in the pleistocene aquifer . to sample the interface between river and groundwater
, we used a 
 1 cm diameter drive - point piezometer and a 50 ml syringe for extracting 
 the water .
sampling was carried out in shallow water , approximately 
 25 cm deep , on a sandbank in the river located about 50 m from the 
 shore , with a depth resolution of 0.20.5 m until 3 m below 
 the surface ( figure 1 ) . at each depth
the internal volume of the piezometer was emptied 
 at least three times before taking the sample .
on the shore , a nest 
 of 11 wells ( figure 1 ) was installed in the depth range 4.940 m. additionally , 
 four wells ranging in depth from 40 to 60 m were installed in the 
 pleistocene aquifer between the pumping station and the nest at the 
 shore ( figure 1 ) .
the 
 wells were equipped with 60 mm pvc casings , a 1 m long screen , and 
 1 m sand trap .
a quartz sand filter pack was installed , and the well 
 was sealed using bentonite .
the top end of the pvc casing was sealed 
 to prevent the entrance of surface water during flooding .
to further 
 avoid leakage along the casing , the top of the wells were embedded 
 in a concrete platform , each well within a protective steel casing 
 and steel screw cap . directly after completion , the wells were pumped 
 to remove the water affected by the drilling operation .
sampling of 
 the wells was done more than three months after their installation 
 in 20102011 .
o2 , ph , and electrical conductivity 
 ( ec ) were measured with probes in a flow cell mounted directly on 
 the sampling tube .
samples for other parameters were collected in 
 50 ml syringes and filtered through 0.2 m sartorius minisart 
 cellulose acetate filters .
aqueous as(v ) and as(iii ) were separated 
 by passing the water through a disposable anion exchange cartridge .
the cartridge was , together with the filter , 
 carefully flushed with n2 before use .
arsenite was determined 
 as the as concentration in the water filtered through a cartridge , 
 and as(v ) was calculated as the difference between the total as and 
 as(iii ) concentrations . in the field ,
samples for cations 
 were preserved with 2% of a 7 m hno3 solution and refrigerated 
 until analysis in the laboratory .
samples for nh4 , cl and so4 were 
 collected in 20 ml polypropylene vials and frozen immediately after 
 sampling . in the laboratory cations
arsenic was determined on the same instrument 
 using a hvg hydride generator and a graphite furnace .
anions were 
 analyzed by ion chromatography using a shimadzu lc20ad / hic-20a super . 

detection limits were as , 0.013 m ; mn , 0.91 m ; ca , 0.50 
 m ; nh4 , 5.6 m ; po4 , 1.1 m ; 
 no3 , 3.2 m ; and so4 2.1 , m .
samples for tritium  helium dating 
 were taken in duplicate 
 in clamped copper tubes in 2015 and analyzed by the institut fr 
 umweltphysik , universitt bremen .
the accuracy 
 of the dates is given by the standard deviation as 0.8 years 
 ( jrgen sltenfu , personal communication )
tritium  helium 
 dating 
 of water sampled from the boreholes in the nest screened in the holocene 
 aquifer ( figure 1 ) 
 is shown in figure 2 .
the groundwater is extremely young in screens at all depths , indicating 
 the fast infiltration of river water into the underlying aquifer . 

this 
 piezometer is located in one corner of the well cluster , and here , 
 a clay layer ( not shown in figure 1 ) was encountered during drilling from 26 to 28 m depth , 
 which was not present in the other drillings just a few meters away . 
 below this clay layer
, the groundwater apparently has a larger residence 
 time , yielding a higher groundwater age .
the 30 m depth screen also 
 shows a maximum in the dissolved as(iii ) concentration ( figure 3 ) and in the nh4 concentration ( not shown ) , reflecting that the longer groundwater 
 residence time allows more as(iii ) and nh4 to build up . 

the remaining screens yield groundwater ages ranging from less than 
 one year to 1.3 years in the screen at 40 m depth .
duplicate measurements 
 are , except for the 30 m screen ( figure 2 ) , within 0.05 years , indicating a high precision . 
 however , the estimated accuracy of 0.8 years is high compared 
 to the measured ages . in addition there is uncertainty as to whether 
 infiltration occurs from the deeper or the more shallow parts of the 
 channel . using the vertical distance between the mean channel depth 
 of 15.3 m and the groundwater age of 1.3  0.8 years at 40 m 
 yields a vertical transport rate of 19 m / year with an uncertainty 
 range of 1249 m / year .
helium 
 groundwater ages , plotted in duplicate at all depths , show the presence 
 of young water down to 40 m depth .
the electrical conductivity ( ec ) 
 in the groundwater is close to the value of 205 s / cm in the 
 river water .
the partial pressure of co2 is 10 in river water and 10 to 10 in the groundwater .
effect of redox processes on the groundwater chemistry in the holocene 
 aquifer at nam du .
symbols designate field data , and lines indicate 
 reactive transport model predictions with adsorption of as(iii ) disabled . 
 in the model , the upper cell consumes 0.27 mm doc , which reduces o2 and no3 in the infiltrating water .
further down , 
 the oxidation of organic carbon and reduction of as - containing fe - oxides 
 are controlled by rate equations in the model of postma et al .
model lines are given for 60 years but are nearly 
 identical for other times < 100 year .
a further indicator for river water intruding into the aquifer 
 is the electrical conductivity of the groundwater ( figure 2 )
. shallow groundwater on the 
 shores of the red river 30 km nw of hanoi , unaffected by pumping , 
 has an ec in the range 7501000 s / cm .
river water , however , has an ec of around 200 s / cm , 
 and the difference is due to mineral weathering mainly occurring in 
 the soil zone .
the strong contrast in ec between river water and normal 
 groundwater makes the ec a good indicator for intrusion of river water 
 into aquifers . as shown in figures 1 and 2 , river water and groundwater in the holocene and 
 pleistocene aquifers , as well as water of the pumping station , all 
 have an ec in the range of 200320 s / cm , indicating 
 rapid recharge from the river to be the dominant source of water in 
 the aquifer .
an additional useful tracer parameter is the partial 
 pressure of 
 co2 ( pco2 ) .
river 
 water has a low pco2 value 
 of 10 , close to that in the atmosphere ( pco2 = 10 ) . 

however , water infiltrating through soils , or through organic rich 
 sediment , is exposed to root respiration and organic decay that generate 
 co2 . at our field site 30 km upstream along the red river , 
 the groundwater , therefore , has a pco2 in the range of 10 to 10.figure 2 shows how all the water in the aquifer below the river 
 has a pco2 in the range of 
 10 to 10 .
these low partial 
 pressures of co2 also indicate the recharge of river water 
 into the aquifer without being exposed to extensive organic matter 
 decay or respiration .
this is consistent with fast recharge of water 
 through the bottom of the river .
the river water has an o2 concentration of 0.24 mmol / l , 
 only slightly below the concentration 
 of 0.27
mol / l of nitrate . 
 at a depth of a few meters below the surface ,
both oxygen and nitrate 
 have disappeared from the groundwater ( figure 3 ) . at the same time , the river water and 
 one sample at 1 m depth contain about 0.6
mmol / l dissolved organic 
 carbon ( doc ) , while the remaining groundwater has a doc concentration 
 near 0.1 mmol / l .
as the river water infiltrates , the doc appears to 
 be filtered by the uppermost sediment .
stoichiometric calculation 
 suggest that the oxidation of 0.27 mmol / l doc would be sufficient 
 for the complete removal of dissolved oxygen and nitrate from the 
 infiltrating river water .
there is no evidence for fe - oxides functioning 
 as electron acceptor for doc oxidation in the top sediment layers 
 because fe(ii ) appears in the groundwater at greater depth .
this pattern 
 of doc filtration by the uppermost sediment layer and concomitant 
 removal of dissolved o2 and nitrate was also observed at 
 other bank infiltration sites .
below 10 m depth , the fe(ii ) 
 concentration starts to increase in the infiltrating water ( figure 3 ) , probably because 
 the degradation of sedimentary organic carbon causes reductive dissolution 
 of fe - oxides .
similar fe(ii ) increases attributed to reductive dissolution 
 of fe - oxides have been observed at other sites with ongoing bank infiltration .
the distribution of arsenic in the river and groundwater is 
 displayed in figure 1 and in detail for the holocene aquifer in figure 3 .
the river water and the shallow groundwater 
 are free of arsenic , while it is present in the deeper groundwater 
 of the holocene , the pleistocene aquifers , and the water of the pumping 
 station .
this distribution indicates that the infiltrating river water 
 is mobilizing arsenic from the aquifer sediment .
found the rate of arsenic mobilization from the sediment into the 
 groundwater to depend on the burial age of the aquifer sediment with 
 the youngest sediments being most reactive .
the geochemical processes 
 controlling the groundwater arsenic content were quantified in a 1d 
 reactive transport model , up to a burial age of 6000 years , using 
 the code phreeqc-3 . in the model
, the 
 oxidation of organic matter causes the reduction of arsenic - containing 
 fe - oxides , resulting in the release of fe(ii ) and as(iii ) to the groundwater . 

both organic carbon and fe - oxides become consumed over time and less - reactive 
 as well .
the oxidation of organic matter and the reduction of fe - oxides 
 are controlled by separate rate equations that quantify their decreasing 
 rates over time ( eqs 1 and 2 in the supporting information ) .
the rate equations were calibrated using the 
 groundwater chemistry of four sites on the red river floodplain 30 
 km northwest of hanoi , where both the 
 burial age of the holocene aquifer sediment and the vertical flow 
 rate of the groundwater are known .
the application of the model 
 developed for the field area 30 km 
 north of hanoi to the nam du field site assumes that floodplain sediment 
 is similar at the two sites .
this appears a reasonable assumption 
 given that remote sensing shows that 
 both sites to belong to the red river meander belts , and they also 
 have comparable holocene sediments .
accordingly , we have applied the 
 model to the nam du data ( see the supporting information for the phreeqc input file ) .
the main difference between the two 
 field sites is the much - faster vertical flow rate at nam du of 19 
 m / year compared to 0.5 m / year in the undisturbed aquifers north of 
 hanoi .
the burial age of aquifer sediments 
 adjacent to the river at nam du is not known , but as an end - member 
 case , we assume the aquifer to be very young ( < 100 years ) . with 
 that assumption ,
the organic carbon and iron oxides have the highest 
 possible reactivity , and the calculated release rates of fe(ii ) and 
 as(iii ) will accordingly be maximum rates .
any older sediment would 
 consequently show lower fe(ii ) and as(iii ) release rates .
the composition 
 of the solution entering into the 1d model is that of river water . 

the results calculated by the model for 60 years of infiltration are 
 shown as the solid lines in figure 3 but would be nearly identical for other times < 100 
 years . to simulate , in addition , the consumption of doc in the uppermost 
 sediment , we have added a constant rate of organic carbon degradation 
 of 5.1 mm c / year in the upper layer of the model , matching the removal 
 of o2 and no3 in the incoming water by doc oxidation 
 ( figure 3 ) . below the 
 top layer
, the oxidation rate of sedimentary organic carbon is calculated 
 from the rate equation , and the main electron accepting process is 
 the reductive dissolution of fe - oxides .
figure 3 shows the modeled release of fe(ii ) to the 
 groundwater , which corresponds to a rate of 35 m fe / year . in 
 the bottom 4 m of the profile , a small amount of fe(ii ) becomes precipitated 
 as siderite , but higher up in the profile , all fe(ii ) stays in solution . 

a comparison with field data shows the modeled values to be slightly 
 higher than the field data , but the assumption that the holocene aquifer 
 at nam du is young appears reasonable .
otherwise , the modeled fe(ii ) 
 concentration would be much higher than the measured fe(ii ) values 
 because the fe(ii ) release of an old low - reaction sediment would be 
 significantly lower .
fe - oxides being reduced in the model contain 
 1.2 mmol as(v ) per 
 mol of fe , a value based on analysis of fe - oxides by reductive dissolution 
 in sediment samples from four sites with a different burial age of 
 the aquifer sediment on the red river floodplain . during the reduction of fe - oxides in the aquifer , as(v ) 

becomes reduced to as(iii ) and is released to the groundwater . using 
 the model , we can evaluate how much of the as measured in the groundwater 
 could possibly be derived from reductive dissolution of as - containing 
 fe - oxides from sediment that is considered to be < 100 years old . 

to 
 keep all as(iii ) released from reduced fe - oxides in solution , we have 
 disabled the adsorption of as(iii ) in the model .
figure 3 compares the measured as(iii ) 
 in the groundwater with the model prediction assuming reductive dissolution 
 of as - containing fe - oxide being the only source of as(iii ) in the 
 water .
the modeled as profile in figure 3 corresponds to an as(iii ) release rate of 
 0.052 m as(iii)/year .
for the 19 m / year vertical flow rate , 
 the modeled as(iii ) concentration at 40 m depth is 0.11 m , 
 while the uncertainty range of 1249 m / year in the vertical 
 flow rate corresponds to a as(iii ) concentration range of 0.040.17 
 m .
figure 3 clearly 
 shows the modeled as concentration , released by reductive dissolution 
 of fe - oxides in 70 year old sediment , to be much lower than the as 
 concentration measured in the aquifer . any older sediment would have 
 a lower reactivity and , therefore , a lower as(iii ) release rate
therefore , 
 most of the as(iii ) present in the groundwater can not be released 
 by reductive dissolution of as - containing fe - oxides .
note that the 
 same release rate of 0.052 m as(iii)/year yields a modeled 
 groundwater arsenic concentration of up to 4 mol as(iii)/l 
 at the dan phuong locality upstream along the red river .
because the reduction of fe - oxides , and thereby 
 the release of as , is kinetically controlled , the resulting groundwater 
 concentration will depend on the groundwater flow rate . at the dan phuong site ,
the downward groundwater 
 velocity is 0.5 m / year , while at nam 
 du , it is 19 m / year .
the same as(iii ) release rate therefore produces 
 groundwater with a as(iii ) concentration that is almost 40 times higher 
 at dan phuong than at nam du .
the importance of reaction kinetics 
 on arsenic mobilization is also evident from the much - higher arsenic 
 concentration found in porewater in sediment along the shores of the 
 red river , 6 km further downstream at van phuc .
vertical flow rates here are 0.13.1 m / year , which is an order of magnitude lower than the 
 vertical flow rate we derive for the bottom of the river channel , 
 and therefore result in a higher arsenic concentration .
in fact , the 
 main role of redox processes for arsenic mobility in the nam du aquifer 
 is to maintain anoxic conditions in the aquifer , thereby keeping arsenic 
 in its as(iii ) form .
if the oxidation of arsenic to as(v ) should occur , 
 it would cause a much - stronger arsenic retardation because as(v ) adsorbs 
 much more strongly to the aquifer sediment .
groundwater must have been high 
 in as(iii ) before water supply pumping by the city of hanoi induced 
 river water infiltration into the aquifer because high as(iii ) concentrations 
 are seen everywhere else on the red river floodplain in aquifers contained 
 in young sediments . dissolved as(iii ) in the groundwater 
 is in equilibrium with as(iii ) adsorbed onto the sediment , and the 
 results of a forced gradient experiment and push  pull tests indicate 
 that arsenic sorption rapidly responds to changes in the water chemistry . 
 the infiltration of low - arsenic river water is therefore expected 
 to cause desorption of arsenic from the aquifer sediment .
the distribution 
 of as(iii ) in the groundwater over depth , with low as(iii ) in the 
 upper 15 m and higher as(iii ) further down ( figure 3 ) , is consequently interpreted as a leaching 
 front , and its position will be determined by the number of pore volumes 
 of groundwater flushed through the aquifer and the adsorption isotherm . in a previous study , we experimentally 
 investigated the adsorption of as(iii ) onto the holocene aquifer sediment 
 at the nam du field site using a sediment sample from 5.5 to 6 m in 
 depth .
the results showed a linear adsorption isotherm up to an aqueous 
 concentration of at least 10 m as(iii ) corresponding to a 
 distribution coefficient , kd of 
 12.6 l / kg .
the adsorption of as(iii ) was also found to be completely 
 reversible . 
 for the measured aqueous concentration of 1 m as(iii )
( figure 3 ) and a porosity 
 of 0.36 , the isotherm predicts an adsorbed 
 concentration of 59 mol as(iii ) per l of contacting groundwater , 
 corresponding to a retardation of 60 .
accordingly , it will take the 
 flushing of 60 pore volumes of groundwater through the holocene aquifer 
 to desorb all adsorbed as(iii ) . starting with an initial concentration 
 of 1 m as(iii ) in the groundwater , close to what is measured 
 in the groundwater , and the corresponding adsorbed as(iii ) concentration 
 of 59 mol / l
, we have calculated how the as(iii ) versus depth 
 profile would look like at different times and compared it to the 
 measured as(iii ) concentration ( figure 4 ) . as the water moves downward , the as(iii ) concentration 
 increases along the isotherm until it reaches the concentration that 
 was present in the aquifer before river water infiltration began . 

because the adsorption isotherm is linear , the shape of the leaching 
 front will be independent of the as(iii ) concentration . in the model , 
 the vertical flow rate is 19 m / year , and the leaching front moves 
 downward with 0.32 m / year , and after 60 years of modeled river water 
 infiltration , there is a reasonable correspondence with the field 
 data ( figure 4 ) . using 
 the uncertainty in the infiltration rate from 12 to 49 m / year
, the 
 range in front progression is 0.20 to 0.82 m / year , and the time for 
 the leaching front to reach the same depth is from 96 to 24 years . 

although groundwater abstraction in hanoi started 100 years ago , we do not know when river water started to infiltrate 
 into the aquifer at nam du .
van geen et al . constructed a flow model for van phuc , on the next bend of the river 
 6 km further south . here
, the pumping of the hanoi water supply has 
 caused a flow reversal from a gaining to a losing river .
the timing 
 of the flow reversal was estimated to be between 1951 and 1971 .
our 
 estimate of 60  36 years of river water infiltration is within 
 the time space of groundwater abstraction by the hanoi water supply 
 but clearly associated with considerable uncertainty .
lines are leaching profiles calculated 
 using a kd of 12.6 l / kg and a 
 flow velocity of 19 m / year , derived from tritium  helium dating . 

the calculations were carried out for an initial groundwater concentration 
 of 1 m as(iii ) .
groundwater flow modeling suggest that infiltrating red river water is 
 the main source of groundwater recharge into the aquifers beneath 
 hanoi , which are heavily pumped by the municipal water supply .
infiltrating 
 river water may be beneficial for the groundwater quality in at least 
 two ways .
first , the river water contains no arsenic , and one may 
 hope that its infiltration will lower the groundwater arsenic content 
 over time .
it is of major importance 
 to understand the hydrogeological and geochemical processes occurring 
 at the river 
our data from nam du suggest 
 that a very fast recharge is taking place from the river channel . 

however , another recent study at the van phuc site located 6 km downstream from our site at nam du suggests 
 much - slower infiltration to occur along the shores of the river . in 
 future work
, it would be important to delineate the relative importance 
 of the two modes of recharge , particularly because they seem to result 
 in a different groundwater arsenic content . while we find at nam du 
 that , due to the high infiltration , the kinetically controlled reductive 
 dissolution of as - containing fe - oxides only contributes insignificantly 
 to the groundwater arsenic content , the study at van phuc suggest 
 that the slow infiltration rates allow much - higher arsenic concentrations 
 to build up in the groundwater .
an additional 
 reason for the much - higher arsenic concentrations found in porewater 
 of the shoreface sediment could also be that van phuc is located just 
 downstream of the channel at yen my , which drains most of the untreated 
 urban wastewater from the city of hanoi into the red river , providing an ample source of reactive carbon . while the infiltrating river water may readily displace the original 
 groundwater as the result of pumping ,
our results indicate that a 
 decrease in the arsenic concentration in the pumped water will take 
 much longer .
our data indicates that it probably will take another 
 65  39 years to flush the adsorbed as(iii ) from the holocene 
 aquifer . to reach the screens of the groundwater abstraction wells
van 
 geen et al . determined the retardation 
 of as(iii ) in pleistocene sands to be 1620 , which is a factor 
 of 34 lower than what we find in the holocene .
in addition , 
 many pleistocene sediment layers consist of highly 
 permeable gravels that probably have an even - lower retardation . flushing 
 of arsenic from the pleistocene aquifers is therefore expected to 
 proceed much faster than from the holocene aquifers .
thus , bank infiltration 
 of red river water into the holocene and pleistocene aquifers will 
 likely cause the groundwater arsenic concentration to decrease , but 
 it will take several decades .
 recharge 
 of red river water into arsenic - contaminated aquifers 
 below hanoi was investigated . 
 the groundwater age at 40 m depth in 
 the aquifer underlying the river was 1.3  0.8 years , determined 
 by tritium  
 helium dating . 
 this corresponds to a vertical flow 
 rate into the aquifer of 19 m / year . 
 electrical conductivity and partial 
 pressure of co2 ( pco2 ) indicate that water recharged from the river is present in both 
 the sandy holocene and gravelly pleistocene aquifers and is also abstracted 
 by the pumping station . 
 infiltrating river water becomes anoxic in 
 the uppermost aquifer due to the oxidation of dissolved organic carbon . 
 
 further downward , sedimentary carbon oxidation causes the reduction 
 of as - containing fe - oxides . 
 because the release of arsenic by reduction 
 of fe - oxides is controlled by the reaction rate , arsenic entering 
 the solution becomes highly diluted in the high water flux and contributes 
 little to the groundwater arsenic concentration . 
 instead , 
 the as concentration 
 in the groundwater of up to 1 m is due to equilibrium - controlled 
 desorption of arsenic , adsorbed to the sediment before river water 
 started to infiltrate due to municipal pumping . 
 calculations indicate 
 that it will take several decades of river water infiltration to leach 
 arsenic from the holocene aquifer to below the world health organization 
 limit of 10 g / l . 
Introduction
Materials and Methods
Results
and Discussion