METEORIC 10Be CONCENTRATIONS IN THE POTOMAC RIVER BASIN

We measured meteoric ¹⁰Be in sand collected from 3 sites on the main-stem Potomac River (basin areas of 18616, 15528, and 1411 km²) and from 12 tributaries (16 km² to 2642 km²). Ten samples came from USGS gaging station sites where suspended sediment data are available. Meteoric ¹⁰Be concentration are uncorrelated with both basin area (R² = 0.07) and modern sediment yields (R² =0.08).

Our meteoric ¹⁰Be concentrations range from 0.6 to 5.5*10⁸ atoms/g with an average of 2.5+/-1.3*10⁸ atoms/g (1s). Small basins (16 to 34 km²), one in the Coastal Plain and 3 in the Piedmont, near Washington, DC have the lowest meteoric ¹⁰Be concentrations (0.6 to 1.5*10⁸ atoms/g). The highest concentration of ¹⁰Be (5.5*10⁸ atoms/g) is found on Conococheague Creek, (basin area 796 km²), the northernmost tributary of the Potomac. There is a decreasing downstream trend in ¹⁰Be concentrations on the Potomac (4.3*10⁸, 2.6*10⁸ and 2.0*10⁸ atoms/g).

Brown et al. (1988) computed a ¹⁰Be Erosion Index (EI), a basin-scale ratio between atmospheric ¹⁰Be loading and ¹⁰Be leaving the basin on sediment. We sampled a location (station 01638500, main branch Potomac) where Brown et al. 1988 calculated an EI of 0.77; today the site has an EI of 1.3. The change results from differences in ¹⁰Be concentration (4.2*10⁸ atoms/g (Brown), 2.6*10⁸ atoms/g (this study)) and sediment yield (2.6 mg/(cm²*yr) (Brown et al.), 4.1 mg/(cm²*yr) (Gellis et al., 2004)). Potomac sediment samples, collected by Brown et al. (n=2), have an average ¹⁰Be concentration of 6.1+/- 2.4*10⁸ atoms/g, about twice the average of our measurements (n=15). Brown's EIs ranged from 0.77-3.77 (average 2.26); ours (n=10) range from 0.25 to 3.33 (average 1.75); the differences in ¹⁰Be concentration and EI are not significant (p-values of 0.74 and 0.88, respectively).

Assuming steady state and a ¹⁰Be delivery rate of 1.3*10⁶ atoms/(cm²*yr), we can interpret measured meteoric ¹⁰Be concentrations as erosion rates. Calculated erosion rates range from 9 m/My to 77 m/My with an average of 25 m/My, similar to that reported along the Appalachian Mountains and adjacent Piedmont. However, some erosion rates are inconsistent with other estimates a difference likely reflecting violation of the steady state assumption caused by soil stripping from colonial and post colonial agriculture and development.