Bruce D. LaZerte

Runoff Production in a Forested, Shallow Soil, Canadian Shield Basin

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 18O 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.

Hydrograph separation : a comparison of geochemical and isotopic tracers

Abstract A chemical hydrograph separation can be used to study runoff processes on a watershed scale provided the soil kinetics of the geochemical tracer are well documented. To separate streamflow into surface (SF) and subsurface flow (SSF), the supply of the tracer from the soil should be uniform throughout the soil profile and independent of the residence time of the soil/groundwater. This paper examines the use of two weathering products, dissolved silica and magnesium, for separating spring runoff into SF and SSF in small headwater streams on the Canadian Shield. Field observations and leaching experiments in the laboratory suggested that both tracers were released very rapidly from the podzolic soils. Silica allowed a well-defined separation of streamflow into SF and SSF since it is absent in melt/rain water and its concentration varies little throughout the soil profile. Magnesium was used in those streams with wetlands when silica was not behaving conservatively. Both geochemical tracers suggested higher contributions of SSF to total spring runoff (> 90%) than estimates (≈ 72%) based on the stable isotope deuterium. This difference may result from laterally flowing soil-water that did not mix with the phreatic/groundwater reservoir.

Phosphorus and nitrogen retention in five Precambrian shield wetlands

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 (NH4+ -N), nitrate (NO3− -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 (3.2 m) which had annual TP retention of −44% (−0.030 ± 0.015 g m−2 yr−1), the wetlands retained < 0.001 to 0.015 g M−2 yr−1 ; however, this wasless 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 0.56 g m−2 yr−1 (−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 NO3− while NH4+ was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH4+ and NO3−. DOC fluxes into and out of the beaver ponds were equal (−18 and 4% RT) but output from the conifer swamps exceeded input by > 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.

The impact of drought and acidification on the chemical exports from a minerotrophic conifer swamp

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.

Metals and acidification: An overview

Both upland soils and lake sediments appear to retain atmospherically deposited trace metals (e.g. Pb) even under acid conditions. The abundant, local, mineralogically derived metals (e.g. Al and Mn) are exported from upland soils under acidic conditions, but are usually retained by lake sediments. Acidification, however, reduces the extent of retention in lake sediments and soils, potentially inducing elevated metal levels in lake water. Only under conditions of extreme acidification and with the more mobile metals do lakes become net exporters of metals. Surface depletion of metals in sediment cores may not be the result of recent acidification.

Manganese speciation in dilute waters of the Precambrian shield, Canada

Abstract In contrast to its physical state in some geographical areas, manganese in waters draining the Precambrian shield in Canada is mostly non-particulate. Using a differential pulse anodic stripping voltammetry speciation technique, we demonstrate a trend from ionic Mn(II) dominance at low pH to an increasing proportion of colloidal manganese oxyhydroxides at pH ⩾ 5.5 in non-dystrophic waters. The relationship of Mn(II) with pH in the oxic, non-dystrophic samples is consistent with a Hausmannite or Feitknechtite controlling phase.

Predictability of geochemical buffering and runoff acidification in spatially heterogeneous catchments

Catchments are more spatially variable than catchment models assume them to be. Does spatial heterogeneity undermine the validity of spatially aggregated catchment geochemical models? Here we test whether catchment spatial heterogeneity invalidates a recently proposed technique that uses the bulk chemistry of runoff to predict how key water quality variables (e.g., titratable alkalinity and concentrations of base cations, hydrogen ions, and inorganic aluminum) will respond to changes in stream water in acid anion concentrations. We show that these predictions are valid for hypothetical heterogeneous catchments, created by mathematically “mixing” measured runoff chemistry data from chemically diverse streams. The chemical information in the combined runoff yields reliable acidification predictions, without any direct information concerning either the runoff chemistries of the individual source streams or the geochemical properties of their catchments. These results show that catchment acid buffering can be predicted directly from runoff chemistry in spatially heterogeneous catchments, even if the individual source regions cannot be identified and their geochemical characteristics are unknown.

Trophic state decrease after lanthanum-modified bentonite (Phoslock) application to a hyper-eutrophic polymictic urban lake frequented by Canada geese (Branta canadensis)

Abstract Urban lakes are important assets to highly populated regions; however, extensive usage and other influences degrade their water quality, which then requires rehabilitation and maintenance. Hyper-eutrophic Swan Lake, Greater Toronto, Canada (5.5 ha, 4.4 m maximum depth) was a gravel pit that became degraded by elevated total phosphorus (TP) concentrations, mostly from internal P sources. Because Swan Lake is a terminal lake with limited flushing and small external load, a phosphate adsorbing and sediment capping agent, lanthanum-modified bentonite (Phoslock), was applied in spring 2013 to intercept the internal load. Average TP concentration decreased from 0.247 to 0.099 mg/L in the first and 0.060 mg/L in the second post-treatment year. A TP mass balance model adequately predicted post-treatment annual average TP concentration by not including the pre-treatment internal load estimate of 650 to 1100 mg/m2/yr. Phytoplankton biomass decreased only in the second post-treatment year, when Secchi transparency (highly correlated with chlorophyll concentration) increased to a growing season average of 1.4 m (range 0.7–2.7) compared to 0.5 m (0.37–0.63) before treatment. We explain the lack of response in the first treatment year with a relatively late application (29 Apr–1 May 2013), when P released from the winter bottom sediments had already been taken up by phytoplankton. Recently, a growing population of waterfowl (mostly Canada goose, Branta canadensis) were the highest contributors of nutrients (75%), as indicated by a mass balance based on literature-derived goose P export and biweekly bird census. We recommend waterfowl management or repeated treatment to further improve water quality.