Mustard gas would last for up to a week on land. It was said to have caused only 5% fatalities despite its ghastly wounds, so it was mainly an area denial weapon. If used on beaches, then its effects in water also have to be considered:
Mustard Gas (Sulphur Mustard)
Contact with sea water turns mustard gas from its normal liquid state to viscous or even solid one. Hydrolyses with water only occurs after thorough mixing. Since it is heavier than water it sinks and it is believed that it can provide a continuing local source of poison for some time. Dangerous oily film of sulphur mustard remains on the water surface. HT is considered practically insoluble; that sulfur mustard freezes at 13-15°C and might become a semisolid at temperatures near the freezing point (such as those found at the bottom of water pools); and that hydrolysis occurs slowly, forming a thin “monolayer,” after which reaction rates for the entire volume of agent droplet or mass are negligible. In soil and water, some sulfur mustard evaporates into the air and the rest breaks down in minutes to days depending on environmental conditions.
At room temperature, mustard agent is a liquid with low volatility and is very stable during storage. The melting-point for pure mustard agent is 14.4 oC. In order to be able to effectively use mustard agent at lower temperatures, it has been mixed with lewisite in some types of ammunition in a ratio of 2:3. This mixture has a freezing-point of -26 oC. During the Second World War, a form of mustard agent with high viscosity was manufactured by means of the addition of a polymer. This is the first known example of a thickened CW agent.
At the beginning of September, the water temperature in the English Channel is 17°C but temperatures of 15 degrees are common, and 13 degrees are possible. Note of Sea Temperatures in the English Channel, 1929 to 1949, and Plymouth Sunshine and Light by WRG Atkins, and PG Jenkins, Journal of the Marine Biological Association of the United Kingdome Vol 31 No2 1952 pp. 328-329
In aqueous solutions, mustard agent decomposes into non-poisonous products by means of hydrolysis. This reaction is catalyzed by alkali. However, only dissolved mustard agent reacts, which means that the decomposition proceeds very slowly. If sulfur mustard is put in water, it dissolves within minutes if the water is stirred, and slowly if is not. When it does dissolve, it reacts with water and changes to other compounds. The time necessary for a quantity of sulfur mustard that is dissolved in water to decrease by half is about 2 minutes at 40 °C (104 °F). If large amounts of sulfur mustard are spilled into water, most of the sulfur mustard will change to other compounds very slowly or not at all.
Incidents are still occurring annually in the neighbourhood of Sweden where people risk injury from mustard agent. This largely involves fishermen who are exposed to mustard agent brought to the surface by fishing nets. The background is found in the dumping of chemical weapons after the Second World War in waters off the Danish and Swedish coasts. Many fishing ports in south Sweden and Denmark have resources to care for injured people and to decontaminate equipment contaminated by mustard agent. Certain resources are also available on the fishing vessels.