Mike Stone

Fractal dimensions of individual flocs and floc populations in streams

The fractal dimension of an individual floc is a measure of the complexity of its external shape. Fractal dimensions can also be used to characterize floc populations, in which case the fractal dimension indicates how the shape of the smaller flocs relates to that of the larger flocs. The objective of this study is to compare the fractal dimensions of floc populations with those of individual flocs, and to evaluate how well both indicate contributions of sediment sources and reflect the nature and extent of flocculation in streams. Suspended solids were collected prior to and during snowmelt at upstream and downstream sites in two southern Ontario streams with contrasting riparian zones. An image analysis system was used to determine area, longest axis and perimeter of flocs. The area–perimeter relationship was used to calculate the fractal dimension, D, that characterizes the floc population. For each sample, the fractal dimension, Di , of the 28 to 30 largest individual flocs was determined from the perimeter–step-length relationship. Prior to snowmelt, the mean value of Di ranged from 1·19 (Cedar Creek, downstream) to 1·22 (Strawberry Creek, upstream and downstream). A comparison of the means using t-tests indicates that most samples on this day had comparable mean values of Di . During snowmelt, there was no significant change in the mean value of Di at the Cedar Creek sites. In contrast, for Strawberry Creek the mean value of Di at both sites increased significantly, from 1·22 prior to snowmelt to 1·34 during snowmelt. This increase reflects the contribution of sediment-laden overland flow to the sediment load. At three of the sampling sites, the increase in fractal dimensions was accompanied by a decreases in effective particle size, which can be explained by an increase in bed shear stress. A comparison of fractal dimensions of individual flocs in a sample with the fractal dimensions of the floc populations indicates that both fractal dimensions provide similar information about the temporal changes in sediment source contributions, about the contrasting effectiveness of the riparian buffer zones in the two basins, and about the hydraulic conditions in the streams. Nevertheless, determining the individual fractal dimensions of a set of large flocs in a sample is very time consuming. Using fractal dimensions of floc populations is therefore the preferred method to characterize suspended matter. Copyright

Fractal dimensions of suspended solids in streams: comparison of sampling and analysis techniques

Fluvial suspended sediment typically consists of a variety of complex, composite particles referred to as flocs. Floc characteristics are determined by factors such as the source, size and geochemical properties of the primary particles, chemical and biological coagulation processes in the water column and shear stress and turbulence levels in the stream. Studies of floc morphology have used two contrasting methods of sampling and analysis. In the first method, particles settle on a microscope slide and are observed from below using an inverted microscope. The second method uses filtration at no or low vacuum and particles deposited on the filter are observed with a microscope. Floc morphology can be quantified using fractal dimensions. The aims of the present study were to examine the effect of the two sampling methods on the fractal dimensions of particle populations, and to evaluate for each method how well the fractal dimensions at the various sampling sites reflect basin conditions. Suspended solids were collected in triplicate on inverted microscope slides and on 0·45 μm Millipore HA filters in two southern Ontario streams with contrasting riparian zones during a minor runoff event resulting from the melt of a freshly fallen snowpack. An image analysis system was used to determine area, longest axis and perimeter of particles. The morphology of the particle population of each sample was characterized using four fractal dimensions (D, D1, D2 and DK). Systematic differences in fractal dimensions obtained with the two methods were observed. For the settling method, outlines of larger particles were frequently blurred because of the distance between the focal plane (the top of the inverted microscope slides) and the plane of the particle outline. In this method, the blurring of large particles can cause an increase in the projected area and length of the particle. The effect on the particle perimeter is unpredictable because it depends on the amount of detail lost through blurring and its effect on the apparent increase in particle size. Because of blurring, D and D1 tend to be systematically lower for the settling method, whereas the net effect on D2 is unpredictable. Particle size distributions derived from settling are typically coarser because small, low density particles may remain in the water column and all particles may not deposit on the slides. This loss of fines results in systematically lower DK values for the settling method compared with the filtration method. Fractal dimensions and particle size distributions obtained with the filtration method were sensitive to and clearly indicated differences between drainage basins and between sites within each basin. These differences were explained by basin characteristics and conditions. Fractal dimensions and particle size distributions obtained with the settling method were less sensitive to drainage basin characteristics and conditions, which limits their usefulness as process indicators. Copyright

HYDROGEOLOGICAL EVALUATION OF A SOUTHERN ONTARIO KETTLE‐HOLE PEATLAND AND ITS LINKAGE TO A REGIONAL AQUIFER

This paper examines a southern Ontario kettle-hole peatland (Spongy Lake) to determine its hydrogeological linkage with local and regional water tables. The water table in the peat deposit and lake are perched 6 m above the regional aquifer, and there are strong lateral and downward hydraulic gradients. The horizontal hydraulic gradient (Δh/Δz) measured at the edge of the peatland ranged between 0.15 and 0.23 and the vertical gradient reached −1.24 (i.e., downward flow). At depths less than 1.0 m, saturated hydraulic conductivity (Ks) ranged from 10−7 to 10−5 m s−1 and increased in magnitude with proximity to the peat surface. In an intermediate zone (1.0–4.0 m depth), Ks values ranged from 10−8 to 10−7 m s−1, while deeper clay materials had Ks values ranging from 10−9 to 10−8 m s−1. A clay layer directly below the deep peat limits downward seepage of water (one to two orders of magnitude less than evaporation). During periods of relatively high water, most seepage loss occurs laterally at the interface between mineral sediment and the peat. Spongy Lake is an important recharge zone for the regional aquifer, and the hydrologic and ecological integrity of the system should be protected.

Nitrate transport in shallow groundwater at the stream-riparian interface in an urbanizing catchment

Drive point peizometers were installed at the stream–riparian interface in a small urbanizing southern Ontario catchment to measure the effect of buffers (presence/ absence) and land use (urban/agricultural) on the movement of NO− 3-N in shallow groundwater from the riparian area to the stream. Mean NO− 3-N concentrations ranged from 1.0 to 1.3 mg L−1 with maximum values of 9.4 mg L−1. Holding land use constant, there was no significant difference (p>0.05) in NO− 33-N concentration between buffered and unbuffered sites. Nitrate-N levels were not significantly different (p>0.05) as a function of land use. The lack of difference between sites as a function of buffer absence/presence and land use is probably due to the placement of some peizometers in low conductivity materials that limited groundwater flow from the riparian zone to the stream. Subsurface factors controlling the hydraulic gradient are important in defining buffer effectiveness and buffer zones should not be used indiscrim inately as a management tool in urban and agricultural landscapes to control nitrate-N loading in shallow groundwater to streams without detailed knowledge of the hydrogeo logic environment.

Distribution and Movement of Nitrate in Soils from Snowpack in a Stream Riparian Zone, Waterloo, Ontario

Stream riparian zones are landscape features that retain nutrients and enhance water quality. However, little is known about winter controls on nutrient transport in riparian zones. In this study we examine the distribution and movement of nitrate (NO3−) between snowpack, underlying soils and groundwater in a riparian zone to quantify processes which control NO3− transport through seasonally-frozen surface soils. Soils and vegetation in buffer zones attenuate nitrate during the growing season. However, this ability is uncertain with vegetation senescence, and when soils freeze and seasonal snowcover can contain an important nitrate load and later release it to surface waters during melt. At our study site snowcover reached a maximum depth of 32 cm following the major snowfall event from Julian Day (JD) 41 to 47, 2000. During this event, the snow water equivalent (SWE) increased twofold to 4.7 cm. A melt event starting on JD 53 resulted in a SWE loss of 2 cm. Snowpack NO3− concentrations reached a maximum value of 1.4 mg L−1 and a maximum loading of 51.7 mg m−2. During the main melt event, snowpack loading reduced to 36.9 mg m−2 and concentrations of NO3− in the snow decreased to 0.55 mg L−1. Over the study period, groundwater NO3− concentrations were relatively constant near 0.25 mg L−1. However, evidence of mixing of groundwater with stream water is strongly suggested by higher NO3− concentrations in near-stream groundwater (0.75 mg L−1), which was under hydrostatic pressure caused by the stream ice-cover. No identifiable NO3− pulse was observed during the main snowmelt period because persistent soil frost promoted depression storage, overland flow and shallow throughflow. Our results indicate that overland flow and shallow throughflow were the likely pathways for NO3− export from this system. Clearly these processes cannot be ignored when quantifying snowpack NO3− through riparian buffers with frozen soils.