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[1] Surface water chemistry in the main stem and source points of the Rio Icacos basin (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors regulating spatial variability in major solutes in a rapidly weathering landscape. We sampled along the main stem as well as at small source points at high elevation where fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base cations were lower at the source points than in the main stem, and were lowest in lowelevation source points. Calcium and sodium were the dominant cations at all sampling points after sea-salt correction, reflecting the weathering of plagioclase feldspar throughout the basin. The partial pressure of carbon dioxide (pCO 2 ) tended to be higher, and HCO 3 À concentrations were lower, in the low-elevation source points than at other positions in the landscape. When coupled with the relatively low concentrations of Si and base cations, this suggests that the availability of primary reactive minerals, rather than carbonic acid concentrations, limits weathering in these low-elevation sources. Mechanical denudation appears to enhance chemical weathering rates not only by refreshing reactive mineral surfaces but also by contributing carbon dioxide from the decomposition of organic-rich material in landslides, which occur frequently. The spatial variability of major solutes appears to depend primarily on the availability of fresh primary reactive minerals, carbon dioxide concentrations, and hydrolysis conditions.

Water quality in less-developed countries is often subject to substantial degradation, but is rarely studied in a systematic way. The concentration and flux of major ions, carbon, nitrogen, silicon, and trace metals in the heavily urbanized Bagmati River within Kathmandu Valley, Nepal, are reported. The concentrations of all chemical species increased with distance downstream with the exceptions of protons and nitrate, and showed strong relationships with population density adjacent to the river. Total dissolved nitrogen (TDN), dominated by NH4, was found in high concentrations along the Bagmati drainage system. The export of dissolved organic carbon (DOC) and TDN were 23 and 33 tons km(-2) year(-1), respectively, at the outlet point of the Kathmandu Valley, much higher than in relatively undeveloped watersheds. The cationic and silica fluxes were 106 and 18 tons km(-2) year(-1) at the outlet of the Bagmati within Kathmandu Valley, and 36 and 32 tons km(-2) year(-1) from the relatively pristine headwater area. The difference between headwaters and the urban site suggests that the apparent weathering flux is three times higher than the actual weathering rate in the heavily urbanized Bagmati basin. Fluxes of cations and silica are above the world average, as well as fluxes from densely populated North American and European watersheds. End-member composition of anthropogenic sources like sewage or agricultural runoff is needed to understand the drivers of this high rate of apparent weathering