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A review of seven studies addressing soil flux related to manual hoeing and weeding in Asia and Africa indicates that tillage erosion from hand-held implements on steep hillslopes can alter topography and influence the distribution of soil and nutrients within fields. Soil flux resulting from manual tillage increases with slope in a near-linear fashion until the slope angle approaches the angle of repose for displaced aggregates. On steeper slope gradients, the increase in soil flux is nonlinear because displaced clods roll, bounce, or slide relatively long distances downslope before coming to rest or breaking down. Because of this ravel-like transport process, soil flux is modeled most appropriately across a wide range of gradients as a nonlinear process. A modified nonlinear ravel model predicted soil fluxes in the reviewed studies better than a linear model, but it was not superior to a general exponential model. Additional field studies are needed to increase the basic understanding and predictability of soil erosion by manual tillage on steep slopes, as this activity is still practiced in many areas of the world.

The study of fluvial bed sediments is essential for deciphering the impact of anthropogenic activities on water quality and drainage basin integrity. In this study, a systematic sampling design was employed to characterize the spatial variation of lead (Pb) concentrations in bed sediment of urban streams in the Palolo drainage basin, southeastern Oahu, Hawaii. Potentially bioavailable Pb was assessed with a dilute 0.5 N HCl extraction of the <63 μm grain-size fraction from the upper bed sediment layer of 169 samples from Palolo, Pukele, and Waiomao streams. Contamination of bed sediments was associated with the direct transport of legacy Pb from the leaded gasoline era to stream channels via a dense network of storm drains linked to road surfaces throughout the basin. The Palolo Stream had the highest median Pb concentration (134 mg/kg), and the greatest road and storm drain densities, the greatest population, and the most vehicle numbers. Lower median Pb concentrations were associated with the less impacted Pukele Stream (24 mg/kg), and Waiomao Stream (7 mg/kg). The median Pb enrichment ratio values followed the sequence of Palolo (68) > Pukele (19) > Waiomao (8). Comparisons to sediment quality guidelines and potential toxicity estimates using a logistic regression model (LRM) indicated a significant potential risk of Palolo Stream bed sediments to bottom-dwelling organisms. [ABSTRACT FROM AUTHOR]