The practicability of ships arriving to the Great Lakes to conduct ballast water exchange plus treatment-Analysis of shipping patterns

Ballast water exchange (BWE) has historically been the most oft-used approach to manage ballast water with the goal of reducing biological invasions. Henceforth, most ships will exclusively perform ballast water treatment (BWT) to comply with limits on concentrations of living organisms in discharge water, although the combination of BWE and BWT may reduce the risk of invasion more than BWT alone, particularly for ships arriving to freshwater ports, such as those in the North American Great Lakes. Whether the BWE and BWT is practicable rests upon several factors, particularly the time available to perform both operations in the appropriate area and safely during a voyage. We investigated records of ships arriving to U.S. ports from 2004 through 2014 to characterize the frequency and location of BWE. The National Ballast Information Clearinghouse dataset contained information on 872 × 103 arrivals in the 11-year span, which is summarized herein. In U.S. ports within the Great Lakes, about 1% of vessels arriving-or 921 arrivals over 11 years-conducted BWE. Most vessels arriving to Great Lakes ports did not discharge ballast water, but of those that did, > 98% of the volume discharged was from coastwise voyages. In ships arriving from overseas, ballast water was held for at least 4 days after BWE. Ships performing BWE while offshore would still need to meet the limits for living organisms in discharged water, which would require BWT while underway doubling the use rate of the shipboard treatment system, but also making some BWE procedures (e.g., flow-through exchange) impracticable.