Maryland Department of Natural Resources

St. Mary's County Observation-Well Network

Project Details

The St. Mary's County water-level network is one of five observation-well networks maintained by MGS and funded through cooperative agreements with Anne Arundel, Calvert, Charles, Queen Anne’s, and St. Mary’s counties.

The primary objective of these networks is to monitor the effects of water-supply withdrawals on groundwater levels at both a local (well field) and regional scale. More specifically the water-level data are used to help assess (1) long-term sustainability of the water supply; (2) well interference (drawdown at each well in a multiple-well system added to drawdowns at the other wells); (3) potential for increased development of the aquifers; (4) potential for salt-water intrusion, where applicable; and (5) the role of ground-water extraction in land subsidence.

The St. Mary's County observation-well network, initiated in 1997, is currently funded through a cooperative agreement between Maryland Geological Survey and St. Mary's County Metropolitan Commission. The network consists of 17 wells located mainly in the central part of the County. Aquifers monitored include the Upper Patapsco (11 wells), and Aquia (6 wells). Frequency of measurement is semiannual (spring and fall).



Current Water Level Trends


hydrograph for observation well SM Fe 31 in the Aquia aquifer

Aquia aquifer


After many years of steady water-level decline, the Aquia aquifer has shown a more stable trend in recent years. This is mainly due to a shift of usage from the Aquia to the Upper Patapsco over the last few years. The reason for this change is that levels of arsenic in the Aquia throughout much of St. Mary's county exceed the recently revised U.S. Environmental Protection Agency Maximum Contaminant Level. For more information, see Report of Investigations 78 or this map

hydrograph for observation well SM De 59 in the Upper Patapsco aquifer

Upper Patapsco aquifer


Water levels in the Upper Patapsco aquifer show a gradually declining trend since 2007 when they were first measured in the St. Mary's county observation well network. Wells in the Upper Patapsco have assumed a more important role in water supply as Aquia wells are abandoned due to the presence of arsenic in exceedence of the EPA drinking water standard. In general, the water levels show an annual high in the spring, and an annual low in the fall.