St Marys County, Maryland: drinking water report. St. Marys County occupies the southern tip of Maryland's Western Shore, bordered by the Chesapeake Bay
St. Marys County occupies the southern tip of Maryland's Western Shore, bordered by the Chesapeake Bay and the Potomac and Patuxent Rivers. The county includes Lexington Park, California, and Leonardtown, with residents drawing water from the Patuxent River, deep aquifers in the Aquia and Patapsco formations, and several smaller community systems. Much of the population relies on private wells tapping the Nanjemoy-Marlboro aquifer, while the Metropolitan Commission serves larger population centers through surface water and groundwater sources.
St. Marys County faces water quality challenges typical of coastal Maryland communities, particularly concerning per- and polyfluoroalkyl substances (PFAS) and agricultural contamination. The region's military presence, including Naval Air Station Patuxent River, has historically introduced firefighting foam containing PFAS into groundwater systems. Maryland's statewide UCMR5 sampling has detected PFAS compounds in multiple water systems across southern Maryland counties, with patterns suggesting both point-source contamination from military and industrial sites and diffuse agricultural impacts. The Patuxent River watershed has documented elevated nutrient loads from agricultural runoff, which can affect treatment plant performance and create conditions favoring disinfection byproduct formation.
Lead and copper remain concerns in older housing stock, particularly in Leonardtown and established neighborhoods near the naval base where homes built before 1986 may have lead service lines or copper pipes with lead solder. Maryland's lead and copper rule monitoring shows that aging infrastructure in communities established during mid-century military expansion creates localized risk areas. The county's combination of groundwater and surface water sources means residents face different contamination profiles depending on their provider. Those on private wells, which serve approximately 40 percent of county residents, have no mandatory testing requirements and may face elevated nitrates from septic systems and agricultural fertilizers, alongside naturally occurring arsenic and radium from the Coastal Plain aquifer geology.
The Chesapeake Bay's influence creates additional complexity. Saltwater intrusion affects some coastal wells, particularly during drought conditions, while tidal fluctuations can concentrate pollutants in nearshore areas. The county's wastewater treatment capacity struggles during peak summer population influxes, when the permanent population of roughly 113,000 swells with tourists and seasonal residents. This seasonal stress can affect downstream water quality and create conditions where source water protection becomes critical for public systems drawing from the Patuxent River.
Test your water if you rely on a private well, focusing on nitrates, bacteria, arsenic, and PFAS if you live near current or former military facilities. Municipal system customers should request their utility's Consumer Confidence Report and ask specifically about PFAS testing results and lead service line inventories. Check your water to see current contamination data for your specific address, review our water filter guide to identify treatment options appropriate for your contamination profile, request your detailed report for comprehensive testing history, and visit the Maryland state page for context on statewide regulatory efforts and testing requirements.