The Baltimore Sun ran an article with some further information including:
the levels have been rising since spring
The letters, sent Friday, come nearly two months after the state received the results of groundwater testing at Meller's Food Mart, a convenience store and former Sunoco station. The state's delay in notifying the county Health Department could be a violation of Maryland law.
the county Health Department did not receive word of detection until Aug. 31 - nearly seven weeks after the findings
One of the wells also showed benzene, another gas additive, at 171 parts per billion.