Monthly monitoring for potentially harmful levels of gases is to be carried out in an area of Fort William.
Highland Council said more than 100 homes and 10 businesses in the An Aird area of the town would be kept up to date on the investigation.
Households were first notified of possible contaminated land nearby in January.
The probe concerns landfilling materials including dredging, commercial and municipal wastes.
The local authority said as the waste decomposed the potentially harmful gases methane and carbon dioxide were produced by microbes in soil.
A total of 58 boreholes have been dug to gather samples of soil and groundwater.
Landfill gases
These were being analysed and the results will help determine what other action needs to be taken.
The council said if significant contamination was found, local residents and businesses would be contacted directly at the earliest opportunity.
In March, a community woodland on a former landfill site was closed after heavy metals, arsenic and asbestos were detected in the soil and surface water.
Highland Council said it took the action at Newtonhill, about a mile from Wick town centre, as a precaution while further tests were carried out.
Separate studies of landfill gases found no risk to 25 nearby homes.
Last year, properties were demolished and contaminated soil removed from the site of a former gas works in Easter Ross.
Residents of flats from 25 to 71 Joss Street were relocated.
Chemicals benzene and benzo(a)pyrene were traced in the gardens of the properties in 2004.
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