Explaining groundwater isotopic variation

Across well types (dug and drilled) and well depths (164.4 to 6.1 m) (Appendix B), the maximum temporal variability at any one location for δ¹⁸O and δ²H was ≤ -0.32 and ≤ -1.78‰, respectively, throughout the sampling season (Table 2-1 and Table 2-2, respectively). The constant isotopic compositions in groundwater indicates a well-mixed system throughout the sampling period (Quiroz Londoño et al., 2008). The temporal stability found in groundwater’s isotopic composition suggests the majority of observed variation in groundwater signatures (2.1 and 15.4‰ for δ¹⁸O and δ²H, respectively) were from differences from site to site (i.e. in space) influenced more by hydrological features than by summer precipitation (Mandal et al., 2011).

Although groundwater δ¹⁸O and δ²H signatures varied little temporally, they did vary spatially.

Generally, groundwater δ¹⁸O concentrations became more negative in a linear trend from outlet to headwaters by ~2‰ (Figure 2-8). To illustrate this, groundwater sites (Figure 2-1) are grouped into three clusters (A, B & C) (Figure 2-8). Groups were created by visually comparing δ¹⁸O concentrations and their locations with respect to Quaternary geology, creating clusters that seemed reasonable. Moving inland from the outlet, δ¹⁸O and δ²H in groundwater was most enriched in heavy isotopes (most positive value) in cluster A and most depleted of heavy isotopes in cluster C (Figure 2-8). Published values for the altitude effect of ~0.3‰/100 m can be assumed for elevations less than 5000 m (Poage & Chamberlain, 2001). With an elevation change of ~275 m from outlet to headwaters in the Wasi watershed, we estimate the altitude effect could account for 0.83‰ variation in recharge to groundwater. A doubling of the lapse rate (~-0.6‰/100 m) would be needed to fully account for the observed spatial variation in groundwater δ¹⁸O, suggesting that an additional mechanism may be contributing to spatial variation in groundwater signatures.

Different pathways of recharge through various porous networks and degree of saturation above the water table can also produce spatial variations in groundwater isotopic concentrations (Clark & Fritz, 1997). Recharge through direct infiltration can be a mixture of pre-existing soil moisture and rainfall. With this process repeated, groundwater can have a range of isotopic compositions that plot below the WAP, yet next to the LMWL (Clark & Fritz, 1997). In the Wasi

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watershed, there are spatial differences in Quaternary geology (Figure 2-2) roughly coinciding with each of the three clusters (Figure 2-8) that could affect recharge. Sample sites in cluster A are mainly located within bedrock/bedrock drift complexes, sample sites in cluster B are mainly located in fine-textured lacustrine deposits, and sample sites in cluster C are located predominately in coarse-textured glaciolacustrine deposits. Coarse-textured materials with higher permeability, which can promote increased recharge during spring freshets, likely accounting in part for the most negative isotopic concentrations shown by cluster C. Although there were 3 drilled wells (not shown) that did not follow this trend, it is likely that, generally, differing recharge rates of glacial deposits in conjunction with the altitude effect contributes to the spatial variation in the majority of groundwater’s isotopic concentrations.

Groundwater isotopic signatures in the Wasi watershed varied spatially, with very limited temporal variation. This finding is in agreement with isotopic signatures cited in previous literature. In the Oldman River watershed, a considerably larger basin of 28,200 km² in southern Alberta (Canada), groundwater δ¹⁸O from sampled wells had spatial variability between ~2 and

~3‰ (Rock & Mayer, 2007). While the Wasi watershed is comparatively smaller (235 km²), groundwater recharge in headwater systems can be complex, particularly in glaciated till (Clark

& Fritz, 1997; Winter, 2007). Although a trend towards an altitude effect was not observed in isotopic signatures of precipitation over the study period, this was likely due to the short period of record. However, a lapse rate of 0.3‰/100 m could explain nearly half the spatial variation in groundwater δ¹⁸O by ~2‰ in the Wasi watershed, some evidence suggests that Quaternary deposits may influence preferential groundwater recharge in areas of coarse-grained materials.

Generally, over the course of the sampling period, groundwater δ¹⁸O signatures ranged between -13.57 to -11.68‰ with the most temporal difference of 0.32‰ (Table 2-1) occurring at site 1459 in the middle of the watershed in fine-textured deposits (Figure 2-2). With groundwater isotopic signatures from both dug and drilled wells plotting close to the LMWL and below the WAP for this region, this suggests groundwater is well mixed hydrological unit with recharge likely occurring during spring melt and previous years fall precipitation.

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Figure 2-5: Isotopic composition of rain (diamond), surface water (triangle), and groundwater (circle) samples in the Wasi watershed. The open square denotes Ottawa’s weighted annual precipitation (WAP) value for δ¹⁸O and δ²H of -11.21 and -77.46‰, respectively (IAEA, 2001b).

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Figure 2-6: Early and Late period average isoscapes of surface water δ¹⁸O (‰). Catchment characteristics provided by the NBMCA (2011). See Appendix E for corresponding isoscapes of δ²H. Provincial data layer of Ontario for streams, waterbodies, and wetlands were provided by Digital Moving Target Indicator (DMTI) CanMap® datasets (2005).

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Table 2-1: δ¹⁸O of groundwater samples. See Figure 2-1 for site locations and Appendix C for full data.

Site ‰ N 29-May-12 13-Jun-12 29-Jun-12 10-Jul-12 24-Jul 9-Aug-12 24-Aug-12 Min (‰) Max (‰) Range (‰)

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Table 2-2: δ²H of groundwater samples. See Figure 2-1 for site locations and Appendix C for full data.

Site ‰ N 29-May-12 13-Jun-12 29-Jun-12 10-Jul-12 24-Jul 9-Aug-12 24-Aug-12 Min (‰) Max (‰) Range (‰)

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Table 2-3: δ¹⁸O of surface water samples. Blanks indicate no data. See Figure 2-1 for site locations. Not all sites were sampled on the same day, see Appendix C for full data.

Site ‰ N 7-May 24-May 4-Jun 18-Jun 4-Jul 17-Jul 26-Jul 13-Aug 28-Aug Min ‰ Max ‰ Range ‰

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Table 2-4: δ²H of surface water samples. Blanks indicate no data. See Figure 2-1 for site locations. Not all sites were sampled on the same day, see Appendix C for full data.

Site ‰ N 7-May 24-May 4-Jun 18-Jun 4-Jul 17-Jul 26-Jul 13-Aug 28-Aug Min ‰ Max ‰ Range ‰

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Figure 2-7: Surface water δ¹⁸O (‰) in each subwatershed with sample locations in order of most downstream (left hand side) to most upstream (right hand side). Top panel shows early period; lower panel shows late period. Asterisks denote headwater tributaries and arrows mark the confluence of Graham and Chiswick Creek (from left to right) with the Wasi River. No data was collected from W1A in the late period due to inaccessibility.

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Figure 2-8: Spatial variability of dug (grey circle) and drilled (open circle) groundwater δ¹⁸O and δ²H (‰) samples. The open square denotes Ottawa’s weighted annual precipitation (WAP) value for δ¹⁸O and δ²H of -11.21 and -77.46‰, respectively (IAEA, 2001a). Watershed boundary 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).

38 early and late averages in δ¹⁸O and δ²H were 0.08 and 0.77‰, respectively. This consistency in isotopic concentrations between early and late period is likely due to the artesian well (Arti) (Figure 2-1), which discharges into a roadside ditch a few hundred meters upstream of C1.

However, average surface water at site C3 became more negative in δ¹⁸O and δ²H than the early period by 0.74‰ and 3.95‰, respectively, under low-flows and drier conditions in the late period. A likely explanation for this could be the occurrence of saturation overland flow in the early period where a raised water table near stream areas can result in mixing of groundwater

One kilometer downstream of sample site G1, Graham Lake (GrLk) differs in δ¹⁸O and δ²H between early and late periods by ~1 and ~6‰, respectively. Downstream of site GrLk, sites along the mainstem of Graham Creek (G2, A2, G4, & G6) had isotopic signatures similar to Graham Lake (GrLk) during both early and late periods (Figure 2-1 & Figure 2-7). A small tributary (A5) drains ~5 km² area of wetland. This site presents increasingly more positive isotopic concentrations from early to late period, likely due to evaporative processes often seen in attenuated waters discharged by wetlands (Clark & Fritz, 1997). In contrast, a second tributary (A2) (Figure 2-1) continues to show isotopic concentrations similar to GrLk (Figure 2-7) during both early and late periods. No trend of enrichment or depletion in ¹⁸O at surface water sites downstream of GrLk in the late period suggests that groundwater discharge and in stream