Alan R. Hill

Subsurface denitrification in a forest riparian zone: Interactions between hydrology and supplies of nitrate and organic carbon

The influence of hydrology andpatterns of supply of electron donors and acceptors onsubsurface denitrification was studied in a forestriparian zone along the Boyne River in southernOntario that received high nitrogen inputs from a sandaquifer. Two hypotheses were tested: (1) subsurfacedenitrification is restricted to localized zones ofhigh activity; (2) denitrification zones occur atsites where groundwater flow paths transportNO3− to supplies of available organiccarbon. A plume of nitrate-rich groundwater withconcentrations of 10–30 mg N L−1 flowed laterallyat depths of 1.5–5 m in sands beneath peat for ahorizontal distance of 100–140 m across the riparianzone to within 30–50 m of the river. In situ acetyleneinjections to piezometers revealed that significantdenitrification was restricted to a narrow zone ofsteep NO3− and N2O decline at theplume margins. The location of these denitrificationsites in areas with steep gradients of groundwater DOCincrease supported hypothesis 2. Many of thesedenitrification “hotspots” occurred near interfacesbetween sands and either peats or buried river channeldeposits. Field experiments involving in situadditions of either glucose or NO3− topiezometers indicated that denitrification wasC-limited in a large subsurface area of the riparianzone, and became N-limited beyond the narrow zone ofNO3− consumption. These data suggest thatdenitrification may not effectively removeNO3− from groundwater transported at depththrough permeable riparian sediments unlessinteraction occurs with localized supplies of organicmatter.

Groundwater-surface water interactions in headwater forested wetlands of the Canadian Shield

Abstract Groundwater and surface water interaction in two conifer swamps located in headwater catchments with contrasting till depth, typical of the southern Canadian Shield, were studied from June 1990 to August 1992. Both swamps had little influence on the regulation or attenuation of seasonal runoff response in the catchment. The two valley bottom swamps were connected to local aquifers but the upland-wetland connection was continuous in the catchment with deeper till and ephemeral in the catchment with thin till-rock ridges. Groundwater movement through the wetlands was restricted mainly to the surface peat layer in both wetlands, because a large portion of inputs from shallow soil layers and stream inflows enter near the peat surface. However, differences in upland-wetland connections resulted in contrasting hydrologic regimes in the two swamps. During seasons with larger inputs, both swamps were hydrologically connected to uplands and had a similar hydrology characterized by a high water table, rapid storm response, and predominance of saturated overland flow. In summer, upland inputs were absent in the catchment with thin till-rock ridges, resulting in cessation of baseflow and a lower water table that varied in response to variations in rainfall. Continuous upland inputs throughout the summer in the catchment with deeper tills (1–3 m) sustained baseflow and kept the water table near the peat surface. This study demonstrates the control of morphology and shallow subsurface geology on the hydrology of valley bottom swamps influenced by local aquifers.

Hyporheic zone hydrology and nitrogen dynamics in relation to the streambed topography of a N-rich stream

The influence of riffle-pool units on hyporheic zone hydrology and nitrogen dynamics was investigated in Brougham Creek, a N-rich agricultural stream in Ontario, Canada. Subsurface hydraulic gradients, differences in background stream and groundwater concentrations of conservative ions, and the movement of a bromide tracer indicated the downwelling of stream water at the head of riffles and upwelling in riffle-pool transitions under base flow conditions. Channel water also flowed laterally into the floodplain at the upstream end of riffles and followed a subsurface concentric flow path for distances of up to 20 m before returning to the stream at the transition from riffles to pools. Differences in observed vs predicted concentrations based on background chloride patterns indicated that the hyporheic zone was a sink for nitrate and a source for ammonium. The removal of nitrate in the streambed was confirmed by the loss of nitrate in relation to co-injected bromide in areas of downwelling stream water in two riffles. Average stream water nitrate-N concentrations of 1.0 mg/L were often depleted to <0.005 mg/L near the sediment-water interface. Consequently, an extensive volume of the hyporheic zone in the streambed and floodplain had a large unused potential for nitrate removal. Conceptual models based mainly on studies of streams with low nutrient concentrations have emphasized the extent of surface-subsurface exchanges and water residence times in the hyporheic zone as important controls on stream nutrient retention. In contrast, we suggest that nitrate retention in N-rich streams is influenced more by the size of surface water storage zones which increase the residence time of channel water in contact with the major sites of rapid nitrate depletion adjacent to the sediment-water interface.

Ground water flow paths in relation to nitrogen chemistry in the near-stream zone.

Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L−1, 1.2–1.8 mg L−1 and 4.6–9.1 mg L−1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L−1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L−1 and 0.4–2.2 mg L−1 respectively. Ammonium — N concentrations (20–60 µg L−1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.

Factors affecting the export of nitrate-nitrogen from drainage basins in southern Ontario

Abstract The export of nitrate-N from 21 watersheds near Toronto, Ontario was measured over a 25-month period. The annual average loss ranges from 1.41 to 7.31 kg ha −1 . Analysis of these data indicated a very significant correlation between nitrate exports and percentage of each watershed in crops and abandoned agricultural land. A number of soil and topographic variables also had significant correlations with stream nitrate levels. The causal relationships underlying these simple correlations are difficult to evaluate because of considerable multicollinearity between land use and watershed physiography. The separate analysis of stream discharge indicated that agricultural watersheds have lower mean annual discharges (1 s −1 km −2 ) than those watersheds containing extensive areas of forest and abandoned agricultural land. Variations in annual discharge therefore tended to reduce the contrast in nitrate exports between agricultural and non-agricultural watersheds. The influence of watershed characteristics on nitrate levels exhibited considerable seasonal contrasts. During summer low flow conditions nitrate concentration and loss were not correlated with watershed land use, and instead were affected primarily by localised pollution sources and by biological processes within the streams.

Runoff mechanisms in a forested groundwater discharge wetland

Abstract Hydrometric measurements and geochemical hydrograph separation techniques were used in experimental plots to determine the processes that govern stormflow production in a forested swamp situated in a regional groundwater discharge zone. Hydrometric measurements suggested that saturated overland flow from permanently saturated areas created by discharging groundwater was the major storm runoff mechanism. Chemical separation analysis, using a portable irrigation system, indicated that over 70% of the runoff was pre-event water. Groundwater ridging did not contribute significantly to wetland runoff, whereas preferential flow through pipes accounted for a minor component of the pre-event water at peak streamflow and the majority of the pre-event flow in the recession. Rapid mixing of event water with a significantly larger pool of pre-event water standing in the saturated areas is responsible for the rapid appearance of the pre-event water in surface generated stormflow. The rapid groundwater-surface water mixing in the saturated areas has implications for the interpretation of runoff mechanisms based solely on chemical separation.

Sulphate dynamics in relation to groundwater-surface water interactions in headwater wetlands of the southern Canadian Shield

The spatial and temporal distribution of sulphate (SO4) concentrations in peat pore water and the outlet streams of two forested swamps was related to variations in the magnitude of upland runoff, wetland water levels and flow path. The swamps were located in headwater catchments with contrasting till depths typical of the southern Canadian Shield. Inputs of SO4 from shallow hillslope tills and streams showed little seasonal variation in either source or concentration in both swamps. Sulphate dynamics at the outlet stream reflected hydrological and biogeochemical processes within the valley wetlands, which in turn were partly controlled by catchment hydrogeology. During high runoff, maximum water table elevations and peak surface flow in the swamps resulted in upland inputs largely bypassing anoxic peat. Consequently, SO4 concentrations of 8–10 mg/l at the swamp outlets were similar to stream and groundwater inputs. During periods of low flow, concentrations of SO4 at the swamp outlets declined to less than 3 mg/l. At this time lower water table elevations resulted in increased interaction of input water with anoxic peats, and therefore, SO4 reduction. Contrasts in till depth and the nature of groundwater flow between catchments resulted in differences in SO4 dynamics between years and swamps. In dry summers the absence of groundwater inputs to the swamp in the catchment with thin till resulted in a large water table drawdown and re-oxidation of accumulated S, which contributed to maximum SO4 concentrations (up to 35 mg/l) during storm runoff. Continuous groundwater input to the swamp in the catchment with deeper till was critical to maintaining saturated surfaces and efficient SO4 retention during both dry and wet summers. A conceptual model of wetland SO4 retention and export, based on catchment hydrogeology, is developed to generalize the SO4 dynamics of valley bottom wetlands at the landscape scale.

Episodic sulphate export from wetlands in acidified headwater catchments: Prediction at the landscape scale

Sulphate (SO4−2) concentrations in 34 intensively measured Canadian Shield streams near the Dorset Research Centre, central Ontario, were used to test a hydrogeologic model that uses simple measures of wetland area and till depth to identify catchments that produce SO4−2 pulses. Mean annual measured maximum SO4−2 concentrations were significantly greater in shallow till (<1 m depth) catchments containing wetlands than catchments covered with deeper tills (>1 m depth) containing wetlands or catchments with no wetlands. Average maximum SO4−2 concentrations in wetland catchments during years with dry summers were >20 mg/L in 19 of 20 catchments with average till depths of <1 m, whereas concentrations were <20 mg/L in 5 of 6 watersheds with average till depths of >1 m. Peaks in mean annual maximum SO4−2 concentrations from wetland catchments with shallow till occurred during summers with rain fall 150–200 mm less than potential evaporation estimates. There were no significant differences in mean average annual SO4−2 concentration among the different catchments during wet summers, with SO4−2 concentrations ranging from 6 to 13 mg/L. These observations suggest that a large portion of the temporal and spatial variation in SO4−2 chemistry and export can be predicted in headwater catchments of the Canadian Shield and perhaps in other landscapes where till depth influences upland-wetland hydrologic connections.

Soil N mineralization and nitrification in relation to nitrogen solution chemistry in a small forested watershed

Spatial variations in soil processes regulating mineral N losses to streams were studied in a small watershed near Toronto, Ontario. Annual net N mineralization in the 0–8 cm soil was measured in adjacent upland and riparian forest stands using in situ soil incubations from April 1985 to 1987. Mean annual rates of soil N mineralization and nitrification were higher in a maple soil (93.8 and 87.0 kg.ha−1) than in a pine soil (23.3 and 8.2 kg.ha−1 ). Very low mean rates of mineralization (3.3 kg.ha−1) and nitrification (3.4 kg.ha−1) were found in a riparian hemlock stand. Average NO3-N concentrations in soil solutions were 0.3–1.0 mg.L−1 in the maple stand and >0.06mg.L−1 in the pine stand. Concentrations of NO3−N in shallow ground water and stream water were 3–4× greater in a maple subwatershed than in a pine subwatershed. Rapid N uptake by vegetation was an important mechanism reducing solution losses of NO3−N in the maple stand. Low rates of nitrification were mainly responsible for negligible NO3−N solution losses in the pine stand.

Sulphate Mobilization and Pore Water Chemistry in Relation to Groundwater Hydrology and Summer Drought in two Conifer Swamps on the Canadian Shield

Variations in sulphate (SO42-) concentration of porewater and net SO42- mobilization were related to differences in water level fluctuations during wet and dry summers in two conifer swamps located in catchments which differed in till depth and seasonality of groundwater flow. Sulphate depletion at the surface and in 20 cm porewater coincided with anoxia and occurred mainly during the summer when water levels were near the peat surface and water flow rates were low in both catchments. There was an inverse relationship between net SO42- mobilization and water level elevation relative to the peat surface, explaining variation in SO42- dynamics between the swamps during summer drought periods. Aeration of peat to 40 cm and a large net SO42- mobilization (10–70 mg SO42- m-2 d-1) occurred during a dry summer in which the water level dropped to 60 cm below the surface in the swamp receiving ephemeral groundwater inputs from shallow tills within the catchment. This resulted in high SO42- concentrations in the surface water and porewater (30–50 mg L-1), and elevated SO42- concentrations remained through the fall and winter. In contrast, within the swamp located in the catchment with greater till depth (> 1 m), continuous groundwater inputs maintained surface saturation during the dry summer, and SO42- mobilization and concentrations of SO42- in the pore water during the following fall did not increase. Susceptibility to large water table drawdown and mobilization of accumulated SO42- is influenced by the occurrence of ephemeral vs. continuous groundwater inputs to valley swamps during dry summer periods in the Canadian Shield landscape. This study reveals that extrapolation of results of SO42- cycling from one wetland to another requires knowledge of the hydrogeology of the catchment in which the wetlands are located.