Methods for analysis of groundwater quality—Part 71:Determination of α-hexachlorocyclohexane,β-hexachlorocyclohexane,γ-hexachlorocyclohexane,δ-hexachlorocyclohexane,hexachlorobenzene,1,1-dichloro-2,2-bis(4-chorophenyl)ethylene(p,p’-DDE)1,1-dichloro-2,2-bis(4-chlorophenyl)ethane(p,p’-DDD),1,1,-trichloro-2-(2-chlorophenyl)-2-(4-chlorophenyle)ethane (o,p’-DDT)and 1,1,1-thrichloro-2,2-bis(4-chloropheyle)ethane(p,p’-DDT)-Gas chromatography