The 1,2-Dichloropropane in the return water of the cooling tower

Dear all,

The return water of the cooling tower we analytics contain 1,2-Dichloropropane, the concentration is  0.00093/ 0.00090 mg/L, while the 1,2-Dichloropropane of the supply water is not detector (N.D.), the concentration of the method detection limit (MDL) is 0.00041mg/L. Could someone tell us kindly : the formation mechanism of 1,2-Dichloropropane , and how to prevent the trace value detected ?