this is only because iron is most dissolved metal in ground water bodies (in natural condition) than any other metal. These water bodies are in reducing condition which is favorable for iron in dissolved Fe(+2) state

Any metal with Eo< +0.82 V can be oxidized by H2O in the presence of O2.

[FOR Mehjabeen Javed :

Since your self happens to be a student of Zoology, I am omitting the reason behind this value (0.82V), but to set record state, I, humbly, attach a file for my RG colleagues who are from Chemistry/Physics]

For

Mn,Zn, Cr, Fe , Co, Ni, Cu metals

the

SOP values of the metals are:

1,18 ,0.76, 0.74,0.44,0.28,0.26 , -0.34V.

[a)Now Mn with value 1.18V(more than 0.82 V) and Cu with value -0.34 ( less than 0.82)

are not expected to react with oxygen on the presence of water.

(b) Theoretically, all other metals(Zn, Cr, Fe , Co, Ni) should undergo corrosion . AND DO REACT WITH H2O in presence of O2 .But excepting Fe, all the above named metals form protective lMETAL OXIDE(Zn, , Co, Ni) or the METAL HYDROXIDE(Cr) layers while the WHOLE OF iron reacts to give Fe2O3/ [(Fe^2)4(Fe^3)2(OH) 12][CO3.2H2O]*. and thus the polluted water contains maximum concentration of Fe(II) and Fe(III) ions while In case of other metals we observe comparatively small concentration of Zn(II), Cr(III), Co(II) and Ni(II)

*This rust is called GRI [Green Rust I].

Two typographical mistakes are regretted:

State---- straight

Cu----- has negative SOP.