waterbycounty
EPA SDWIS

St. Mary's County Water Quality

St. Mary's County, Maryland

Water Grade

C

Water Score

55.6

Violations

9

State Rank

#11

of 24 (1 = best)

EPA SDWIS Compliance

Drinking Water Quality

Water Quality Grade

C

Based on EPA compliance history and violation data

Water Score

55.6/100

Higher = better quality

Health Violations

9

Health-based violations

Violation Rate

9.9%

Systems with violations

Water Advisory: St. Mary's County

Water Verdict

St. Mary's County receives a fair water quality assessment with a grade of C and a score of 55.6 out of 100. The water supply meets baseline federal standards, but there may be periods of elevated contaminant levels or infrastructure concerns worth monitoring.

Violation Context

St. Mary's County has recorded 9 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 9.9 violations per 1,000 residents, this rate is high and signals significant water quality management issues.

Consumer Guidance

Tap water in St. Mary's County meets baseline standards, but residents who are immunocompromised or have young children may want to use an NSF-certified water filter as a precaution. With 9 recorded health violations, staying informed about utility communications and boil-water notices is especially important. For long-term peace of mind, request your utility's latest Consumer Confidence Report and consider independent water testing if you have specific health concerns.

Regional Context

St. Mary's County has better water quality than the average county in Maryland. Its water score is 5.1 points higher than the state average, indicating stronger water system performance relative to neighboring counties.

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

42.2%

35 of 83 assessed

Moderate concern

Top Impairment Causes

  • 1

    FECAL COLIFORM

  • 2

    TOTAL SUSPENDED SOLIDS (TSS)

  • 3

    CAUSE UNKNOWN

Source: EPA ATTAINS · Reporting cycle 2022

Impairment is determined under the Clean Water Act §303(d): a water body is impaired when it fails to meet state-defined quality standards for designated uses (drinking, recreation, aquatic life). Assessment coverage varies by state — counties without assessed water bodies are not shown.

Past 5 years

Water Quality Monitoring

Monitoring Sites

174

Active in the past 5 years

Measurements Recorded

75K

74,738 total readings

Most Measured

  • Physical
  • Nutrient
  • Microbiological

Categories measured most frequently

Data from the EPA Water Quality Portal (WQP), aggregating monitoring records from federal, state, and tribal sources. Each measurement represents a single sample analyzed for a specific characteristic (e.g., E. coli, pH, dissolved oxygen, nitrogen).

Live USGS Streamgage

River & Stream Conditions

Current Discharge

3.45cfs

May 14, 6:15 PM UTC

vs Long-Term Average

13%

Well below typical

Primary Streamgage

St Marys River at Great Mills, MD

USGS site
01661500
Drainage area
24 sq mi
Long-term mean
26.3 cfs

One representative streamgage (the one with the largest drainage area in the county). Many counties have multiple gauges — this view summarizes the primary one. The long-term mean is the full-record annual average; "% of typical" compares the latest reading against that average.

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Frequently Asked Questions

What is the water quality in St. Mary's County, Maryland?
St. Mary's County, Maryland has a drinking-water quality grade of C with a score of 55.6/100, based on EPA SDWIS compliance data. The county has 9 health-based drinking water violations over the past 5 years. Watershed health, monitoring records, and live streamflow are reported separately on this page.
Are there any water violations in St. Mary's County?
St. Mary's County has 9 health-based drinking water violations recorded by the EPA over the past 5 years. Health-based violations indicate instances where contaminant levels exceeded EPA Maximum Contaminant Levels (MCLs). Violations may have been resolved — check with your local water utility for current status.
How healthy are the watersheds in St. Mary's County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 42.2% of St. Mary's County's 83 assessed water bodies are classified as impaired (35 impaired). The top reported causes are FECAL COLIFORM, TOTAL SUSPENDED SOLIDS (TSS), CAUSE UNKNOWN. Impairment means the water body fails to meet state quality standards for at least one designated use — drinking water source, recreation, aquatic life, or fish consumption. Note: watershed impairment doesn't always translate to tap-water issues; treatment plants can remove most regulated contaminants.
How much water-quality monitoring happens in St. Mary's County?
EPA's Water Quality Portal records 74,738 measurements from 174 monitoring sites in St. Mary's County over the past five years. The most frequently measured characteristic groups are Physical, Nutrient, Microbiological. Each measurement is a single sample analyzed for one characteristic (E. coli, pH, dissolved oxygen, etc.). High monitoring density means more scientific evidence behind any reported signal — it does not by itself indicate water quality.
What's happening with rivers in St. Mary's County right now?
St. Mary's County's primary USGS streamgage on the St Marys River is currently reading 3.45 cubic feet per second — 13% of the long-term mean of 26.33 cfs. This is well below typical — often a signal of drought stress on source water. For genuine real-time data, visit waterdata.usgs.gov.
How does St. Mary's County water compare to the Maryland average?
St. Mary's County's SDWIS water quality score of 55.6/100 is higher than the Maryland state average of 50.5. The average water quality grade across Maryland is D, based on data from 24 counties with available SDWIS data.
Is tap water safe to drink in St. Mary's County?
Based on EPA SDWIS data, St. Mary's County has a water quality grade of C (55.6/100). This indicates moderate compliance. Some violations have been recorded but overall standards are maintained. The grade speaks to the public water system, not the watershed — for watershed-level concerns, see the Watershed Health zone. For the most up-to-date information, contact your local water utility or review your Consumer Confidence Report (CCR).
Why does St. Mary's County have so many water violations?
St. Mary's County has 9 health-based drinking water violations on record from the EPA SDWIS database. A higher violation count can result from aging infrastructure, underfunded water utilities, agricultural runoff contamination, or industrial pollution. Counties with more water systems may also see more violations simply due to scale. Residents concerned about water quality should consider independent water testing and home filtration systems.
How does St. Mary's County rank for water quality in Maryland?
St. Mary's County ranks #11 out of 24 counties in Maryland by SDWIS water quality score (1 = best). With a score of 55.6/100, it falls in the middle third of counties statewide. The ranking reflects EPA SDWIS compliance only — not watershed impairment, monitoring density, or streamflow, which are tracked separately on this page.

Counties with Similar Water Quality

Carroll County

Maryland

C

57.8

Water score

Prince George's County

Maryland

C

58.5

Water score

Harford County

Maryland

C

60.8

Water score

Data Sources

Drinking-water compliance data from the EPA Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database. Scores reflect compliance history and health-based violation counts.

Watershed health and impaired-waterway data from the EPA ATTAINS Clean Water Act §303(d) assessments — state-reported, EPA-finalized.

Water-quality monitoring counts from the EPA Water Quality Portal (WQP) — federated USGS, EPA, and state agency sampling records over a rolling 5-year window.

Live streamflow from the USGS National Water Information System (NWIS) — continuous discharge measurements from the largest-drainage gauge in each county, compared against the full-record long-term annual mean.

Disclaimer: This data is informational only. It is not health, legal, or professional advice. For concerns about your specific water supply, contact your local water utility.

By Logan Johnson, Founder & Data EditorUpdated Reviewed by Logan Johnson, Founder & Data Editor

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