Ground Water Chemistry

Unlike surface waters, the quality of sub-surface water varies widely depending on local aquifer conditions. For example, in the coastal regions of Tamilnadu, salinity of groundwater due to the intrusion of seawater in to the sub-surface aquifer is a major problem. Due to excess withdrawal of groundwater, the water table has fallen too far below thereby allowing the seawater to percolate.

Similarly in Gujarat due to uncontrolled withdrawal of groundwater, water is becoming highly saline apart from the fact that depth of the water table reaches at reaching at places, even beyond 200 metres.

 

Chemistry (mg/l) of Groundwater from different parts of India

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pH      EC   F       Cl      NO3           HCO3                                      PO4             SO4              SiO2          Ca     Mg    Na  K         States 

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7.4      2.3    2.3    249    24      426    --       79     12      59      49      187    96   A.P.

7.1      0.2    0.4    3        0        68      --       7       --      17      4        4        3     Assam

7.4      1.6    0.3    138    7        860    --       38     --      55      18      307    4     Bihar   

7.6      5.0    0.4    571    1        418    0.25   149   34      126    180    446    58   Delhi

7.1      2.0    2.4    305    460    730    --       21     --       28      40      400    4     Gujarat           

7.1    1.2     0.8    280    1        312    --       27     --       88      50      122    3     Karnataka                

7.1    0.3     0.1    23      4        116    --       11     --       36      7        15      1     Kerala

7.1    1.1     0.1    145    1        220    --       70     --       65      27      58      47   Orissa                         

--      --       2.9    104    17      383    0.007 114   --       62      14      161    --    Punjab                        

7.7    5.4     5.6    750    262    1039  --       474   --       68      76      1089  24   Rajasthan                 

7.3    1.3     2.3    338    1        281    --       31     --       56      56      178    1     T.N.                        

7.3    0.7     0.5    28      10      297    --       39     --       62      18      44      12   U.P.

7.2    0.6     1.2    8        48      383    --       2       --       92      17      10      4     W.B.

 

But for the higher salinity, the groundwater also has the same types of dissolved solids as surface waters but all in enhanced quantity. Table summarizes example of groundwater quality in many parts of India. There are two specific examples where the groundwater quality has been receiving wide attention. First relates to the fluoride problem and the other relates to the arsenic problem.

Dissolved fluoride in water is derived by mineral-water interaction; surface waters generally have lower fluoride and within limits (generally below 1mg/l) for health requirements. But groundwater tends to have higher fluoride in many places in India leading to fluorosis, a major health problem. Ananthapur district in Andhra Pradesh and Ajmer district in Rajasthan are known to have acute fluorosis in many villages where the dissolved fluoride could be as high as 4 to 7mg/l. There are in addition, many other localities such as parts of Gurgaon district in Haryana, Salem district in Tamil Nadu and some villages in Agra district in UP that are known to have higher fluoride levels in groundwater. Such waters hence cannot be used without some treatment to remove the excess fluoride.

Similarly more than 7000 wells in several districts in west Bengal and also Bangladesh have high dissolved arsenic, usually more than 50 µg/l (0.05mg/l). There are also other regions such as the Khetri and Zawar mining areas in Rajasthan that has at places elevated levels of dissolved arsenic. Groundwater in these areas cannot be used without some treatment to remove the excess arsenic. In addition, bacteriological population, though not reported for many groundwater regions, may be locally important particularly in urban areas where due to soil and urban wastewater seepage organisms may grow in the water below the water table.