FAQ: Invasive species in ballast water

What is ballast water and why do tankers use it?

Tankers without cargo take on sea water, known as ballast water, to ensure vessel stability during ocean voyages.

What is the difference between segregated and unsegregated ballast water?

Crude oil tankers that serve the Valdez Marine Terminal either load ballast water into “empty” oil cargo tanks or dedicated ballast water tanks for the return trip to Valdez.

When ballast water is stored in an empty oil cargo tank it is typically referred to as “unsegregated” or “dirty.” Although every effort is made at the refinery to completely unload oil from the cargo tanks prior to loading the tanks with ballast water, some residual oil inevitably remains on the tank walls and floor and mixes with the ballast water, creating an oily water mixture that requires treatment prior to discharge into the ocean.

Most new tankers are designed with segregated ballast water tanks, but tankers sometimes need to take on additional ballast water in oil tanks during severe weather .

Studies completed on unsegregated oily ballast water at the Valdez Marine Terminal have shown that the amount of hydrocarbons in the oily ballast water are lethal to aquatic species in the tanks. Based on this research, it is unlikely that non-indigenous species in unsegregated ballast water would survive the trip. As a result, unsegregated ballast has been eliminated from the the Councils’ NIS project scope of concern, and the project’s emphasis has concentrated on the control and treatment of segregated ballast water.

What happens to segregated ballast water when it reaches Valdez?

Ballast water is pumped out of the ballast water tanks and discharged directly into Prince William Sound and Port Valdez. If left untreated, non-indigenous species may potentially survive the trip in segregated ballast tanks and pose an invasion risk to the waters of Prince William Sound. In recent years, new technologies allow on-board treatment of ballast water to reduce the transfer of living organisms into new environments. See also: How do we manage ballast water to prevent invasive species?

What happens to unsegregated oily ballast water when it reaches Valdez?

Prior to loading crude oil, the tankers with unsegregated ballast water in the cargo tanks must pump the water ashore for treatment. The unsegregated ballast water typically contains 0.5-1.5% crude oil and is pumped to the Ballast Water Treatment Facility before being discharged into Port Valdez. Treatment of contaminated ballast water is regulated by the Environmental Protection Agency under a National Pollutant Discharge Elimination System Permit and a mixing zone permit issued by the Alaska Department of Environmental Conservation. The oil concentration is reduced to a few parts per million prior to being discharged into Port Valdez.

Where is the ballast water from?

Approximately 86% of the ballast water discharged into Port Valdez originates from the ports of Puget Sound, San Francisco, and Long Beach. The residence time of segregated ballast water shipped from those ports is typically 5-10 days. This short residence time favors the survival of transported non-indigenous aquatic organisms in ballast water. Repeat inoculation of competent organisms on a high volume basis poses a serious NIS risk for the waters of Prince William Sound.

How do we manage ballast water to prevent invasive species?

The primary method of managing ballast water has traditionally been ballast water exchange. In this process, ships exchange coastal ballast water (and organisms) with open ocean water, away from shore. The organisms in this mid-ocean water are less likely to survive in a coastal environment.

Ballast water is exchanged by either flushing tanks three times the tanks volume (flow-through exchange) or emptying and refilling tanks (empty-refill exchange). While ballast water exchange removes many non-indigenous species, it is not 100% effective and does not treat tank sediments or non-pumpable ballast water. Ballast water exchange can also be operationally unsafe, for example during rough weather. A vessel captain may use his or her discretion, and if it is unsafe, can claim an exemption to this rule.

On-board ballast water management systems are increasingly becoming required to remove or kill non-indigenous species. The technology in these systems varies. Types of treatments include filtration, cyclonic separation, ozone, ultraviolet irradiation, in-transit heat, ultrasound, chlorine dioxide, or a combination of these technologies. Systems have been approved internationally and in the United States for installation and use. You can find an explanation of types of ballast water treatment systems on Mr. Marine’s website: Discover the Types of Ballast Water Treatment

Who regulates ballast water?

The International Maritime Organization has created guidelines and a Ballast Water Management Convention for global-scale management of ballast water. In the United States, the U.S. Coast Guard and the U.S. Environmental Protection Agency have various applicable regulations, including:

  • National Aquatic Nuisance Prevention and Control Act of 1990 (USCG)
  • National Invasive Species Act of 1996 (USCG)
  • 2008 Vessel General Permit (EPA)
  • Vessel Incidental Discharge Act of 2020 (EPA)

Several states also operate ballast water management programs, including California, Oregon, and Washington. Alaska does not have a state-specific program.


The most common trade routes for oil tankers leaving Port Valdez are to refineries in:

• Puget Sound, Washington: Anacortes, Cherry Point, Ferndale, Port Angeles, Tacoma
• San Francisco Bay, California: Benicia, Martinez, Richmond
• Southern California: El Segundo, LA/Long Beach

A limited number of shipments have also been made to refineries in Cook Inlet, Alaska (Nikiski) and Hawaii (Barber’s Point).


In May 1996, a previous ban on export of crude oil from the Valdez Marine Terminal to foreign ports was lifted by Presidential order. Shipment of oil to foreign ports increased the risk of non-indigenous aquatic species being transported back to Prince William Sound in tanker ballast water.

During 1996-2002 the majority of the crude oil (about 95%) continued to be shipped to U.S. refineries; however, some crude oil (5% or less) was shipped to Japan, China, Korea, and Taiwan.

The Presidential order requires exporting tankers to adopt a mandatory program of deep water ballast exchange in at least 2,000 meters water depth. Exceptions to this requirement can be made by the tanker captain to ensure the safety of the vessel and crew. Record keeping requirements are in place to document that deep water ballast exchange is being conducted. (15 CFR 754.2). Tankers sailing to domestic ports are not required to practice deep water ballast exchange


The Valdez Marine Terminal and the Trans-Alaska Pipeline System began operations in 1977. Between 1977 and 1996, 11 billion barrels of oil were shipped from Port Valdez to refineries in U.S. ports. (Prior 1996, U.S. law prohibited the export of crude oil from the Valdez Marine Terminal to foreign ports, with one exception to the Caribbean.)

No ballast water treatment or management plan (including ballast exchange) was required during this time period.


Skip to content