MANAGEMENT OF CONTAMINATED GROUNDWATER DURING CONSTRUCTION DEWATERING – A CONTINGENCY APPROACH TO CRISIS RESOLUTION

A Miami Beach construction project required three months of continuous high-flow dewatering. The local environmental agency ordered dewatering to cease due to discovery of elevated ammonia concentrations in the discharge. It was decided to both design a treatment system and concurrently conduct a statistical evaluation to determine if ammonia concentrations were within natural background concentrations. A breakpoint chlorination system was designed to include chemical feed, mixing, and reaction systems for sodium hypochlorite and sodium bisulfite; flocculation and clarification for settleable solids removal; and, granular activated carbon filters to remove trihalomethanes. Design was based on laboratory analyses of raw water, jar testing to identify chemical dosages and trihalomethane formation rates, and rapid small-scale column testing to estimate carbon breakthrough times. As the design was being finalized, the statistical evaluation concluded that natural background concentrations of ammonia substantially exceeded the regulatory standard, and the agency allowed dewatering to resume without treatment.