Reports
The influence of ground water on nitrate loads of streams in the upper Rock Creek Basin, Montgomery County, Maryland
2000, Duigon, M.T., Bolton, D.W., and Cooper, B.F.
Open File Report 2000-02-13
Key Findings
The upper Rock Creek basin lies within the Piedmont physiographic province in Montgomery County, Maryland. It comprises the drainage areas of Rock Creek and North Branch Rock Creek above a point just downstream of their confluence, an area of almost 30 square miles. The upper Rock Creek basin is tributary to the Potomac River and, subsequently, to Chesapeake Bay. Efforts to reduce nutrient loads in streams and reservoirs throughout the Bay watershed require an assessment of the ground-water contribution of nutrients to the streams in order to facilitate planning and development actions consistent with minimizing nutrient production. Just over half of the upper Rock Creek basin is urbanized, a little more than one quarter is forested, and the rest is in agricultural use. Community sewerage is available along the eastern and western parts of the upper Rock Creek basin. This brief study determined:
- Dissolved nitrate+nitrite concentrations in samples collected from eight wells ranged from 0.66 to 5.0 mg/L as N
- Dissolved nitrate+nitrite concentrations in samples collected from 19 stream sites during base-flow conditions ranged from less than 0.050 to 3.15 mg/L as N
- All concentrations were within the range of nitrate concentrations measured in ground water sampled by the Geological Survey from schists and gneisses elsewhere in the Maryland Piedmont (less than 0.050 to 25 mg/L as N)
- None of the concentrations exceeded the U.S. Environmental Protection Agency's maximum contaminant level of 10 mg/L as N
- Statistical analyses of 240 Maryland Piedmont wells selected from the U.S. Geological Survey data base show associations of higher nitrate concentrations with agricultural land use, lower nitrate concentrations with undeveloped (forested) land, and no association of nitrate concentrations with residential use
- Nitrate concentrations in base-flow samples and areal annual base-flow nitrate loads from the upper Rock Creek basin tend to be greater in basins having more agricultural land use, although the relationships contain much scatter
- The larger subbasins within the upper Rock Creek basin are more urbanized, rendering landuse-base-flow nitrate relations less clear
- Areas in the upper Rock Creek basin that are unsuited for septic systems generally lie along the streams and are found in areas underlain by serpentinite where soil cover is thin
- Areas that are most suited for septic systems tend to lie in uplands between streams areas of marginal or highly variable suitability for septic systems are located in urbanized areas, in areas where mapped soils have marginally-suited properties, and where slopes range from 8 to 25 percent
- Some additional septic systems could be installed in many of the basins without significant impact on water quality, but the maximum number cannot be estimated from current information
