Mitigation and Remedy of Groundwater Arsenic Menace in India

More documents

Recommendations

Info

Mitigation and Remedy of Groundwater Arsenic Menace in India : A Vision Documentsome scientists believe that the reductive desorption and dissolution of arsenic adsorbed ontoiron oxyhydroxides in recent sediments is the most probable mechanism of arsenic mobilizationin groundwater.The oxidation hypotheses lack the merit as no arsenopyrite or pyrite rich in arsenic couldbe detected in widespread areas. Very low concentration of sulphate in groundwater is alsocontrary to pyrite oxidation hypothesis. On the other hand, the reduction hypothesis is gettingconsiderable support from various studies. The soils in deeply submerged areas promote growthof aquatic weeds and increased agricultural wastes, most of which are mixed up with soil andburied underground and finally decay anaerobically. The anaerobic process provides a reducingenvironment of low redox condition for dissolution of iron and arsenic from solid phase to theliquid phase in groundwater.Microbial arsenic reduction is also emerging as a potentially important factor in aquaticarsenic mobility. Bacterial action e.g. Thiobacillus Ferroxidant can assist the oxidation ofFe 2+ (aq) in the presence of dissolved oxygen. Investigations have revealed microbialtransformations of As (V), a form that absorbs strongly sediment solids to As (III), a form that isoften much more mobile. As this process generates energy for the microbes involved, it has thepotential to modify arsenic speciation of a system both rapidly and extensively.Some scientists argued that arsenic contamination has been created by imprudenthuman interventions on nature. The modern system of agriculture has introduced anenvironment in which huge amount of fertilizers, pesticides, agrochemicals and groundwater forirrigation caused physical interventions changing the natural flow of irrigation setting of thecountry.From the available information and results of various research works, it is consideredthat a combination of oxidation and reduction phenomena coupled with changes introduced byorganic matter present in the sediments might have contributed largely for such high mobilizationof arsenic. Large scale groundwater withdrawal during summer months causes lowering ofWater table and enhances oxidized zone. During monsoon months the sediments are subjected toreducing environments with large quantities of rainfall recharge. As redox conditions becomeincreasingly reducing, the ferric iron is reduced to, resulting in mobilization of some of the adsorbedarsenic. The Wetland cultivation with increased microbial activity further aggravates theproblem.Ground water in Bengal Deltaic Plain is anoxic in nature and mostly Calcium bicarbonatetype in which sedimentary iron [both Fe (II) and Fe (III)] deposited by the meandering rivercarries arsenic. Sediment mineralogy and texture along with organic matter play crucial role inrelease of As in groundwater. High redox sensitive species (As, Fe and Mn), high alkalinity andabsence of dissolved oxygen and nitrates suggest the microbial mediated and thermodynamicallyNIH & CGWB 53
Sources and Causes of Groundwater Arsenic Contamination in Ganga-Brahmaputra Plainsfavoured redox processes (denitrification→ iron reduction). The presence of amorphousFe-oxide together with surface bound phosphate in hotspot areas, and Fe-oxides as well ascarbonate and phosphate minerals are also playing significant role in arsenic mobilization. It hasbeen observed that As (III) is more dominant in near-surface aquifers rich in organic matter.Release of arsenic to groundwater is mainly caused by reductive dissolution ofhydrated iron oxide (HFO) and corresponding oxidation of organic matter. Groundwater flow,particularly during recharge of aquifers brings dissolved organic matter, in contact with HFO,promoting its bio-mediated reductive dissolution and consequent release of iron and arsenic togroundwater. High concentrations of arsenic are common in alluvial aquifers of the BengalBasin and Mid Ganga Plain and arise because erosion of Himalayan region supplies immaturesediments with low sulphate loadings of FeOOH on mineral grains, to a depositional environmentthat is rich in organic matter so that complete reduction of FeOOH is common. The role ofhuman interventions, in increasing recharge of groundwater consequent to groundwaterwithdrawal during pre-monsoon period and enhancing the process of reduction, may also bequite significant.Although level of information in Lower Ganga Plain is quite substantial, research workon mobilization of arsenic in Mid Ganga Plain and Brahmaputra Plain has to be geared up toarrive at a more precise understanding on the processes involved.3.8 SummaryFindings of many research studies are yet to explain precisely actual causes, sourcesand mechanisms in a boarder perspective. The studies have mostly remained in search of causes,mechanisms and in identification of 'pros and cons' at micro-level. Groundwater flow andcontaminant transport modeling studies could settle down suggesting different options of aquifermanagement to provide arsenic free groundwater from a contaminated aquifer withoutinfluencing much mobilization of arsenic contaminated water. Efforts for in-situ remediation ofarsenic contaminated aquifer have already been initiated in West Bengal. It is strongly believedthat there is a need for integrated research to identify genesis of arsenic and its mechanism ofrelease and mobilization in the soil-water system. There is also a need for developing methodsfor in-situ remediation and groundwater management strategies in arsenic contaminated areas.The primary source or the parental outcrops of arsenic in the Ganga-Brahmaputra basin is yet tobe established. However, the occurrence of arsenic in groundwater in the BDP and Gangeticplains has been recognized as of geological origin with spread out resulting from the mobilizationunder natural hydro-geologic conditions. Two hypotheses recommending mechanism of arsenicmobilization in the groundwater, one - oxidation of arsenopyrite or arsenic rich pyrite in soilstrata, and the other, reductive dissolution of arsenic from soils, the later case 'the reductivedissolution' is recognized as the most potential reason than the former as the dominant54NIH & CGWB
Page 5:
ACKNOWLEDGEMENTThe Government of In
Page 8 and 9:
nearly 33 villages in 4 districts i
Page 10:
(iii) Whether ex-situ removal techn
Page 14 and 15:
6.4 Experiences during Arsenic Remo
Page 16 and 17:
List of FiguresFigure no. Figure ca
Page 18 and 19:
List of TablesTable no. Table Capti
Page 20 and 21:
Mitigation and Remedy of Groundwate
Page 22 and 23: Mitigation and Remedy of Groundwate
Page 102 and 103: 5.2.2 Co-precipitationMitigation an
Page 122 and 123:
Page 124 and 125:
5.8 SummaryMitigation and Remedy of
Page 126 and 127:
Page 128 and 129:
Page 130 and 131:
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
Page 142 and 143:
Page 144 and 145:
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
Page 156 and 157:
Page 158 and 159:
Page 160 and 161:
Page 162 and 163:
Page 164 and 165:
Page 166 and 167:
Page 168 and 169:
Page 170 and 171:
Page 172 and 173:
Page 174 and 175:
Page 176 and 177:
Page 178 and 179:
Page 180 and 181:
Page 182 and 183:
Page 184 and 185:
Page 186 and 187:
Page 188 and 189:
Page 190 and 191:
Page 192 and 193:
Page 194 and 195:
Page 196 and 197:
Page 198 and 199:
Page 200 and 201:
Page 202 and 203:
show all

Mitigation and Remedy of Groundwater Arsenic Menace in India

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?