A remediation plan to decontaminate soil and ground water below the Methuen Public Works yard is in place, and work should be underway by November. The new city budget includes $190,000 for the project, which will be carried out by Common Sense Environmental Inc. of New Bedford. The contamination was first detected in 2007, and is believed to have come from underground gasoline tanks that some city officials estimate were at least 50 years old. Several underground oil and gas storage tanks were in the area at one point; some were removed 20 years ago and others were emptied and anchored with concrete. The project is expected to take about two years to complete.