A CONSTRUCTION PROJECT TO REPLACE A WATER MAIN ON WEST SOUTH STREET IN MARSHALLTOWN BETWEEN SOUTH CENTER AND SOUTH 6TH STREETS BEGINS MONDAY.  FOR A GOOD PORTION OF THE PROJECT, THERE WILL ONLY BE ONE LANE TRAFFIC ON WEST SOUTH STREET, SO MOTORISTS ARE ADVSIED TO SEEK ALTERNATE ROUTES.  THE PROJECT IS EXPECTED TO LAST ABOUT FIVE MONTHS.