Water Qual. Res. J. Canada, 2006
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Volume 41, No. 4, 383–397
Copyright © 2006, CAWQ
Chloride as an Indicator of Non-point Source Contaminant Migration in a Shallow Alluvial Aquifer Bryer R. Manwell1 and M. Cathryn Ryan2* Department of Civil Engineering,1 and Department of Geology and Geophysics,2 University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4
Non-point source (NPS) contamination in the context of surface and ground water interaction was investigated in the Elbow River, Alberta, Canada. Groundwater flow direction, water table elevation, baseflow recession, chloride, nitrate and microbiological measurements were made to determine the interaction of several small tributaries and a small hamlet with Elbow River-associated groundwater. Groundwater flow in the alluvial aquifer adjacent to the Elbow River is predominantly subparallel to the river, but can vary seasonally by as much as 30°. Bedrock constricts the alluvial aquifer downgradient of the tributaries and the hamlet, causing groundwater to emerge into the Elbow River at this point. Chloride mass flux estimates suggest that septic effluent from the hamlet enters the groundwater immediately downgradient of the hamlet and discharges to the river from 5 to 12 km downgradient. A tributary creek which joins the Elbow River just above the bedrock constriction also contributes significant chloride to the river, and had significantly higher bacterial loads than the Elbow River, suggesting that land uses on this creek are also a significant source of contamination. Geologic sources, cattle grazing on the alluvial aquifer, road salting, golf course fertilizer use and wildlife are also potential contaminant sources. Key words: hyporheic zone, chloride, alluvial aquifer, water quality
Introduction
2000). On the reach scale, ground- and river-water exchange also occurs around riffles and pools (Danielopol 1989; Harvey and Bencala 1993; Stanford and Ward 1993; Woessner 2000). On the channel scale, morphological effects such as bedrock constraints can have significant influences on ground- and surface-water exchange (Stanford and Ward 1993; Wroblicky et al. 1998; Poole 2000; Kasahara and Wondzell 2003; Conant 2004). When contaminated groundwater migrates long distances before interacting with stream water, river water quality may be impacted far from the source of the contamination, with significant lag times (Modica et al. 1998; Pringle and Triska 2000). Using the current conceptual model; which holds that contaminants enter rivers in the same reach as their sources are located, these contaminant sources would not be readily recognized. The zone of ground- and river-water mixing below and adjacent to the stream channel is termed the hyporheic zone (Duff and Triska 2000; Woessner 2000; Fellows et al. 2001). Describing stream reaches as gaining, losing, flow-through or parallel flow can be useful in unraveling the complexity of hyporheic interaction (Woessner 2000). The classification is based on the difference in stream stage and head distribution of the groundwater. Groundwater in the vicinity of surface water can be discharging, recharging or flowing parallel to the stream. Seasonal fluctuations in the water table can cause stream reaches to change flow regimes spatially and temporally (Modica et al. 1998; Eberts and George 2000; Soulsby et al. 2001; Alexander 2003; Storey 2003).
Three rivers supply the major Alberta population centres with drinking water: the North Saskatchewan, which supplies the greater Edmonton area, and the Bow and Elbow, which supply the Calgary region (Corbett and Lalonde 2004). Together, these rivers supply drinking water to more than 58% of Alberta’s population (Alberta Environment 2005; Alberta Municipal Affairs 2005). Provincial and municipal authorities maintain water quality sampling programs on these rivers. Implicit in these river water quality-sampling programs is a conceptual model, which holds that contaminants enter rivers in the same reach that the sources are found (U.S. Geological Survey 1999). Thus, river sampling stations are typically located immediately up- and down-gradient of potential contamination sources such as wastewater treatment plants. This method is useful for point-source discharges to rivers, but it is a poor method for detecting non-point groundwater discharges to rivers. Groundwater flow in permeable, river-connected aquifers tends to be sub-parallel to the river valley (i.e., in the direction of maximum topographic gradient). If rivers parallel the river valley, then groundwater can flow for significant distances without ground- and river-water exchange (Pringle and Triska 2000). Conversely, when rivers meander across the river valley, groundwater flow often intersects the river, allowing exchange (Woessner * Corresponding author;
[email protected] 383
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During seasonal mountain runoff, stream discharge can increase by as much as several orders of magnitude. This seasonal variation in river discharge can cause rapid increases in the adjacent groundwater table with associated groundwater recharge by river water in a process known as ‘bank storage’ (Todd 1955; Winter et al. 1998). As the river returns to base flow, bank storage water re-enters the river, with potential river water quality impacts from indirect sources (Fryar et al. 2000). Baseflow is the water that sustains river discharge during the winter months. Baseflow recession analysis is a technique used to estimate the baseflow component of the stream hydrograph. This estimate of baseflow has also been used to estimate groundwater storage for any given year (Butler 1957; Meyboom 1961a; Farvolden 1963; Freeze and Cherry 1979; Domenico and Schwartz 1990; Eberts and George 2000). Baseflow recession analysis separates one component of stream discharge, baseflow, from the other two main components, direct runoff and interflow. Direct runoff and interflow water from mountain melt have short or no residence time as alluvial water. Early baseflow is predominantly bank storage water that has recently become part of the shallow groundwater flow system (Meyboom 1961b). Groundwater that enters the river in the latter part of the low flow season generally has been under the surface longer. As this water has had more time in the subsurface, it has had more time to be affected by contaminants migrating from the surface. Therefore, groundwater entering the river in the latter part of the low flow season may be of lower quality than groundwater entering the river in the early part of the low flow season (Runge et al. 1989). An historic assessment of the Elbow River comparing water quality indicators between 1970 and 1999 levels showed significant increases in coliform bacteria, turbidity and total dissolved phosphorus during this time (Sosiak 1999). A more recent study confirmed the increase in degradation of water quality (Sosiak and Dixon 2004). Potential non-point sources of water quality degradation were reported to be septic systems, cattle grazing and wildlife. Chloride is a tracer of sewage and manure contamination (Lockwood et al. 1995). The main sources of chloride in rivers are sea salt (in precipitation), weathering of halite and anthropogenic inputs (Berner and Berner 1987). Elevated chloride concentrations occur in manure-impacted groundwater (Rodvang et al. 2004). Chloride is also present in significant concentrations in domestic sewage (Fair et al. 1971; Runge et al. 1989) and in sewage effluent plumes in groundwater (LeBlanc 1985; DeSimone and Howes 1998), but not typically present at concentrations above 3 mg L–1 in the Elbow River (Beers and Sosiak 1993). Although there is relatively little data on chloride concentrations in the alluvial aquifer, they are thought to be low. In general there are low (i.e.,
