... andcoarsegrainsize[4,5].Finesedimentscompos edmainlyofsand in the lowerpartandweatheredclay in the upper part of VinhPhuc Formation(Q13vp). The thicknessof this ... percentage. In general, the higher the intervalofconcentration, the lessquantity of samples is. So that, the freq uencydistributionisskewedto the left(Fig.7). In thiscase, the averagevalueissignificantlydifferentfrom the median.Indeed, the averagevalueequals140.88mg/l,while the medianequals26.37mg/lwith ... isdividedintointervalsof100mg/lexcept the lastintervalthathas the valuefrom1000upto9000mg/l.Itisobviousthat the frequencydistributionof Na+ skews to the left. The ...