Hydrogeology & Hydrology Program
Nitrate Loads in the Rock Creek Basin	contact: David Bolton (dbolton@mgs.md.gov)

Summary and Conclusions:

The upper Rock Creek basin in central Montgomery County, Maryland, has a drainage area of about 29.6 mi2. Nineteen subbasins ranging in size from 0.011 to 2.663 mi2 were delineated for an investigation into relations between the nitrate content of ground water and land-use factors. Dissolved nitrate plus nitrite concentrations were measured in samples taken from eight wells in the upper Rock Creek basin and from 19 stream sites under base-flow conditions to assess ground-water contributions to the nitrate load of the upper portion of Rock Creek in Montgomery County. Concentrations in samples from wells ranged from 0.66 to 5.0 mg/L as N, and concentrations from base-flow samples ranged from less than 0.050 to 3.15 mg/L as N. None of the concentrations exceeded the maximum contaminant level established by the USEPA (10 mg/L as N), but several had concentrations greater than background levels (variable, but generally considered 1 to 3 mg/L as N). The nitrate + nitrite concentrations in the sampled wells are within the range of values seen elsewhere in the Piedmont physiographic province of Maryland (which range from less than 0.05 to 25 mg/L as N).

Statistical analyses of land use surrounding 240 wells in the Piedmont disclose significant associations between higher nitrate concentrations and higher proportions of agricultural use, higher nitrate concentrations and lower proportions of undeveloped (mostly forested) land, and no relationship between nitrate concentrations and residential land use. Base-flow nitrate concentrations or annual loads per square mile in streams are significantly (although not very noticably) correlated with percentage agricultural land use (r = 0.58, p = 0.009) or percent cropland (r = 0.55, p = 0.014). Percent urban land use, septic-system density, percent of basin served by community sewerage, and percent cropland are all correlated with drainage area, consequently, the effects of these land-use factors on base-flow nitrate loads may be somewhat confounded by the effect of larger basin size. Larger basins may produce more nitrate in base flow each year, but agricultural land use is associated with higher loads per square mile, and septic-system density appears to vary independently of base-flow nitrate.

Community sanitary sewers are available along the eastern and western parts of the upper Rock Creek basin, but a number of septic systems are still in use in some of the subbasins that are almost entirely sewered. Areas unsuited for septic systems generally lie along the streams and are also found in areas underlain by serpentinite which tend to have thin soil. Areas that are the most suited tend to lie in uplands between streams. Areas of marginal or highly variable suitability are located in urbanized areas (because of very localized ground disturbance); in areas where mapped soils have marginally-suited properties (such as depth to the water table or to bedrock close to the minimum required); and where land-surface slopes range from about 8 to 25 percent. Hydrogeologic conditions affecting suitability for septic systems in the upper Rock Creek basin are similar to conditions elsewhere in Montgomery County, except the basin lacks those geologic units, found elsewhere in the county, in which obtaining individual domestic water supplies is a significant problem.

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updated 3/3/05