Storm flow in forested basins on the Canadian Shield is largely supplied by subsurface water; however, mechanisms by which this water reaches the stream remain unclear. Side slope contributions to storm flow were studied using throughflow trenches on slopes in a headwater basin near Dorset, Ontario. Discharge, soil water content, and chemical and isotopic signatures of subsurface water were monitored at each site. Four hypotheses were tested: (1) most flow occurs at the soil-bedrock interface on shield slopes with thin soil; (2) a significant fraction of event water moves vertically to bedrock via preferential flow pathways and laterally over the bedrock surface; (3) relative preevent water contribution to subsurface flow on shield slopes is a function of soil thickness; and (4) a significant portion of event water flux in storm flow from forested basins with shallow soil cover is supplied from side slopes via subsurface flow along the soil-bedrock interface. Hypothesis 1 was confirmed from hydrometric observations during spring and fall rainstorms. Hypotheses 2 and 3 were supported by temporal trends in dissolved organic carbon and •80 in flow at the soil-bedrock interface and by isotopic hydrograph separations (IHSs) of hillslope runoff. Comparison with the streamflow IHS indicated that event water flux from the basin in excess of that attributable to direct precipitation onto near-channel saturated areas could be supplied by flow along the bedrock surface (hypothesis 4). Flow at the soil-bedrock interface on side slopes also contributed ---25% of preevent water flux from the basin. Much of the event water component of basin storm flow may travel considerable distances via subsurface routes and is not necessarily contributed by surface runoff processes (Horton flow or saturation overland flow). Therefore the assumption that event water undergoes little interaction with the soil during its passage downslope may be unwarranted here. One concept which may account for rapid subsurface transfer of preevent water to stream channels is the groundwater ridging mechanism [Sklash and Farvolden, 1979]. This process is limited to the near-channel area and results from rapid conversion of the tension-saturated zone (capillary fringe) tophreatic water by infiltrating event water. The subsequent rise in the near-stream water table increases the hydraulic gradient to the stream and/or the size of seepage faces, resulting in pronounced groundwater discharge to the channel. This rapid Copyright 1995 by the American Geophysical Union. Paper number 94WR03286. 0043-1397/95/94WR-03286505.00 rise in the water table has been noted in laboratory [Abdul and Gillham, 1984] and field investigations [Novakowski and Gillham, 1988; Abdul and Gillham, 1989] and is supported by numerical simulation [SMash and Farvolden, 1979]. However, studies on the Canadian Shield have questioned the ability of groundwater ridging to serve as the dominant means of supplying storm flow to the stream during snowmelt. Thus Buttle and Sami [1992] found that gro...

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Phosphorus and nitrogen mass balances of five wetlands (two beaver ponds, two conifer-Sphagnum swamps and one sedge fen) situated in three catchments in central Ontario, Canada, were measured. Monthly and annual input-output budgets of total phosphorus (TP), total nitrogen (TN), total organic nitrogen (TON), total inorganic nitrogen (TIN), ammonium ion (NH: -N), nitrate (NO; -N) and dissolved organic carbon (DOC) were estimated for the five wetlands during the 1982-83 and 1983-84 water years. Except for the deepest beaver pond (3.2m) which had annual TP retention of -44% (-0.030 0.015 g m-2 yr-I), the wetlands retained < 0.001 to 0.015 gm-2 yr-I; however, this was less than 20% of the inputs and the estimated budget uncertainties were equal to or greater than the retention rates. Annual TN retentions ranged from -0.44 to 0.56gm-'yr-' (-12 to 4%) but were not significantly different from zero. The wetlands transformed nitrogen by retaining TIN (16 to 80% RT) and exporting an equivalent amount as TON (-7 to 102% RT). The beaver ponds, however, retained NO, while NH: was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH: and NO,. DOC fluxes into and out of the beaver ponds were equal (-18 and 4% RT) but output from the conifer swamps exceeded input by r 90%. Marked seasonal trends in nutrient retention were observed. Nutrient retention coincided with low stream flow, increased evapotranspiration and biotic uptake during the summer. Net nutrient export occurred during the winter and spring when stream flows were highest and biotic uptake was low.

Separating the different forms of aqueous aluminum is essential for determining their relative toxicity in acidified streams and lakes. A short extraction with oxine at pH 8.3 separates inorganic monomeric and organic aluminum from the polymeric and amorphous forms. The organic fraction can be eliminated by equilibrium dialysis prior to extraction. In many cases, the inorganic fraction can also be estimated from free and total fluoride measurements. The two methods agree well in artificial solutions (R2 = 0.997) and reasonably well in natural waters (R2 = 0.82), although the computed estimates tend to be slightly lower than the directly measured values. During spring snowmelt in the Muskoka–Haliburton region of central Ontario, very little polymeric and amorphous aluminum was found, and most of the inorganic monomeric fraction was in the fluoride complexes. I present an empirical model that predicts inorganic monomeric aluminum in these waters (R2 = 0.90) using total organic carbon and the aluminum measured by a short extraction without dialysis.

Abstract. A seven year chemical budget of a minerotrophic conifer swamp located on the PreCambrian Shield, Ontario, Canada and subjected to anthropogenically acidified deposition is presented. Contrary to other published studies, this swamp retains sulphate during wet years and exports it during dry years. Alkalinity is always retained (acidity is exported) and base cations are almost always exported. It is predicted that if this pattern continues, the cation exchange sites of the swamp will become increasingly saturated with protons and aluminum ions, and the waters passing through the swamp will be increasingly acidified.