Exploring Source Water Influences Using Isoscapes

Figure 2-9 shows surface water δ¹⁸O isoscapes from biweekly baseflow sampling corresponding to Figure 2-4. Under wetter spring and early summer conditions (early period), surface water δ¹⁸O remained relatively uniform over the entire watershed with concentrations in many instances overlapping the range of independently sampled groundwater. Headwater tributaries draining small wetlands, like A1 and A5, quickly exhibited evaporative enrichment (by the second survey). Starting on July 4^th, a ~20 day period occurred with little to no precipitation. Evaporative enrichment in ¹⁸O was more pronounced at sites A1 and A5 by July 17^th (red areas). A general evaporative enrichment of surface waters can be seen throughout the observation record, which also allows us to identify locations where groundwater inflows continue to strongly influence surface water δ¹⁸O. Headwater locations (G1, W2, WL3, C1, C3 (dark blue areas)) within all three subwatersheds remain similar to groundwater δ¹⁸O as spring drainage and rains diminished. The inferred influence of groundwater inflows on surface water coincide with areas of coarse-grained materials (Figure 2-2), promoting connections that maintain streamflow at low-flow times of year.

Sampling was focused on baseflow conditions but spring and summer rainstorms were also monitoring for influence on surface water δ¹⁸O signatures. On July 23^rd a large precipitation

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event occurred with recorded amounts ranging from ~18 to 71 mm and with δ¹⁸O and δ²H values of -5.5 to -7.08‰ and -32.99 to -41.79‰, respectively (Figure 2-4a and Appendix A). As seen in Figure 2-9 and Figure 2-4b, three days later on July 26^th baseflow sampling showed no significant changes in the isoscape (as noted earlier, A1was dry). As shown in Figure 2-4, flow between July 23^rd and 26^th became relatively constant at 0.3 m³/s. Leading up to sampling on August 13^th, there were several additional precipitation events (~30 to 68 mm) followed by a large rainstorm (38.1 to 45.2 mm) on Aug 13^th (night/early morning before sampling). Volume-weighted signatures of total rainfall for this period are δ¹⁸O and δ²H of -8.68 to -7.76‰ and -57.17 to -50.01‰, respectively, for P1 and P3 sites, respectively (data for P2 unavailable) (Figure 2-1). In this case, the August 13^th isoscape shows a dampening of the previous pattern, with some sites becoming more negative in signature (e.g. A1, A4, W6 and A5) and some sites becoming more positive (e.g. C1, W2, and WL3) (Figure 2-9 and Table 2-3) due to rainfall or surface water contributions. With these additional rain contributions, we also see a corresponding increase in W10 discharge, with a peak flow increasing to 0.4 m³/s. However, following the August 13^th rain event there was a two week span with no rainfall leading up to August 28^th, and δ¹⁸O returned to patterns previously seen in July 17^th and 26^th isoscapes (Figure 2-9). This suggests that antecedent conditions are playing a role. For example, Devito et al. (2005b) reported a forested catchment in glaciated Boreal Plains of Alberta (Canada), with thin clay-rich glacial till, had increased runoff with increased saturation of soil water. With successive rainfall events occurring within a few days of each other and having >30 mm per event resulted in saturation of the underlying soil layer and produced runoff. In the Wasi watershed, it is likely that rain events leading up to August 13^th saturated the thin overburden and produced runoff leading to a dampening effect on surface water δ¹⁸O concentrations.

Previously, studies have been conducted using water isotopes and isoscapes to examine source water contributions (Bowen et al., 2012; Brooks et al., 2012). In this study, water isotope sampling was overlaid onto an existing water quality program (Kendall et al., 2010) in an effort to better understand source contributions to streamflow over a spring-summer period. Due to the large number of sampling sites used, it was not feasible to generate the streamflow records necessary for quantitative baseflow separation. Some supporting discharge data at select sites is available through the NBMCA. However, the large spatial coverage provided by the sampling sites allows the use of an isoscapes approach to provide insights into spatial variance in source

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water. As such, the key findings of this research are the discovery of regions of inferred groundwater influence corresponding with coarse-grained sediments particularly identifiable during low-flow, late summer conditions. This finding is in agreement with published literature in various hydrogeologic settings. Two case studies were compared with extreme differences in geologic substrate with regard to groundwater and surface water interaction; the Dismal River in Nebraska, and the Park River in North Dakota. Winter (2007) argued that streams originating in highly permeable terrain have a constant supply of groundwater and relatively stable flow. In contrast, streams passing over low-permeable terrain (such as glacial till) will typically have little exchange with groundwater and highly variable flows. In the Wasi watershed, isotopic signatures of headwater samples closely reflected groundwater sourced from coarse-grained Quaternary deposits; however, samples collected downstream of Wasi Lake and Graham Lake resembled lake water isotopic signatures. Wassenaar et al. (2011) found similar results in the Okanagan River system, downstream of Okanagan Lake. Due to the short residence time, few evaporative effects on isotopic signatures in stream samples were noted.

This case study maps the dynamics of source water contributions to streamflow in the Wasi watershed. At the watershed scale, isoscapes depict patterns of surface water δ¹⁸O as influenced by groundwater, meteoric and evaporative influences in a complex headwater system.

These patterns were only discernable in late summer, when flows are low and surface waters show strong evaporative enrichment. Under these conditions, “hot spots” (i.e. surface water δ¹⁸O dominated either by evaporative processes or groundwater influences) and “hot moments” (i.e.

early and late period or biweekly timescales) are readily identifiable at the watershed scale, and can provide an entry point for pattern analysis (Kendell et al., 2010). In piggybacking upon NBMCA’s established phosphorous monitoring program, stable isotope results from this study provide information about spatial and temporal source water contributions to streamflow. This information will be used in understanding nutrient transport, and ultimately, ecosystem functioning and health. The stable water isotope data results from this study has established foundational data about key hydrological components for use in future water monitoring and modeling programs.

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Figure 2-9: Isoscape of biweekly surface water δ¹⁸O (‰) from May 7 through August 28, contoured at 0.5. See Figure 2-1 for sampling site labels. Catchment characteristics provided by the NBMCA (2011). Provincial data layer of Ontario for streams, waterbodies, and wetlands were provided by Digital Moving Target Indicator (DMTI) CanMap® datasets (2005). Surface water δ¹⁸O interpolations had a regression root-mean-square error <3.33. See Appendix E for corresponding biweekly isoscapes of surface water δ²H (‰).

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