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Spatial variations in tap water isotopes across Canada: Tracing water from precipitation to distribution and assess regional water resources [1]

['Shelina A. Bhuiyan', 'Department Of Earth', 'Environmental Science', 'University Of Ottawa', 'Ottawa', 'On', 'Yusuf Jameel', 'Department Of Civil', 'Environmental Engineering', 'Massachusetts Institute Of Technology']

Date: 2023-01

As demonstrated in other studies [ 19 , 20 , 23 , 37 ], the spatially coherent regional patterns of tap water δ 2 H ( Fig 2 ) and their strong correlation with local precipitation (annual/summer) ( Fig 5 and Fig C in S1 Text and Table 2 ) indicate that precipitation is the primary control of tap water δ 2 H composition in Canada. The annual and summer water balance models improve the predictability of δ 2 H values of Tap River and Tap Lake , but not Tap Groundwater ( Fig 6 and Fig D in S1 Text and Table 3 ), providing insights into post precipitation processes. The water balance modeling approach described above does not account for isotopic fractionation due to evaporation, or for infiltration. As infiltration rates can vary seasonally, this might influence the predicted δ 2 H values. In this study, we interpreted residual δ 2 H values between our predicted local surface water and observed tap water ( Fig 7 ) as reflecting either evaporative losses (for negative residuals) or other processes not accounted for in the water balance modeling [ 20 ].

4.2 Regional patterns in observed δ2H values of tap water

4.2.1 East Coast regions (New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland). In the East Coast regions, more positive δ2H values and d-excess values in tap water (Figs 2 and 3) coincide with warm and humid summers and a year round rainy climate [38, 39]. This pattern is irrespective of the source of the tap water samples, and indicates modern precipitation is the primary source of tap water in these regions. We found some small positive R precipitation and R surface values for Tap Groundwater (~38%) and Tap River (~33%) compared to Tap Lake (~17%) (in red, Fig E in S1 Text and Fig 7). Similarly, Gibson et al. [28] observed positive δ2H residuals between the predicted δ2H values in precipitation and observed δ2H values in eastern Canadian streams, suggesting evaporation into humid oceanic air masses can lead to isotopic enrichment of surface waters along high slope evaporation lines. Many of the Tap Groundwater samples in the East Coast regions (~36%) have low (more negative) d-excess values (< 8.5 ‰) (Fig 3) indicating significant evaporative losses. Also ~62% of the total Tap Groundwater samples showed negative R precipitation and R surface values (in blue, Fig E in S1 Text and Fig 7), which also supports evaporative losses in these waters. Comparatively, a recent study suggested that the Maritime regions exhibit some of the lowest evaporation related losses in Canada [40]. The lower d-excess values and large negative R precipitation and R surface values likely reflect a combination of misclassification of municipal water sources and water management processes occurring during the storage and distribution of water to residents. For example, some Tap Groundwater samples in Nova Scotia originate from Middle Lake Road Wells where groundwater is stored on surface reservoirs (Data A in S1 Text, available at https://doi.org/10.6084/m9.figshare.19243518) [41]. Another example is, the City of Saint John in New Brunswick which uses the South Bay Treatment Facility and Loch Lomond Drinking Water Treatment Facility for water treatment and storage [42]. It is likely our Tap Groundwater samples from St John is indicative of evaporation losses during treatment and storage at those facilities, as water loss is common through this process [43]. We also explored satellite images by using the latitude and longitude of each of the anomalous Tap Groundwater samples with high evaporation signals and found out some of them directly fall close to lakes, pounds and surface reservoirs (Data A in S1 Text, available at https://doi.org/10.6084/m9.figshare.19243518). So it is likely some of the anomalous Tap Groundwater samples may have been misclassified by the municipalities or that these municipalities store groundwater on surface reservoirs. All these factors could have contributed to the evaporation signals found in a number of Tap Groundwater samples from the East Coast regions. In the future, as temperatures warm, such isotope signals would be practical to assess water management strategy and quantify losses of exploited groundwater. In dry regions, evaporative losses from reservoirs can run in excess of several million dollars for large cities [18, 25]. Approximately 58% and 83% of the total Tap Lake samples display low (more negative) d-excess (<8.5 ‰) and negative R precipitation and R surface values respectively. These negative d-excess and negative residuals are found mainly in Newfoundland and Nova Scotia suggesting significant evaporative losses from these coastal lakes. Most of these samples originate from small lakes or artificial pounds such as Lake George, Little Lake, Sand Lake, Landrie Lake, Lake Major and Rodney Lake, for which higher evaporative losses is expected (Data A in S1 Text, available at https://doi.org/10.6084/m9.figshare.19243518). Many of these lakes are used to supply water to small towns or communities. These regions could benefit from isotopic monitoring to assess the long-term losses of water due to natural and local water management strategies and to improve the sustainability of their water management practises.

4.2.2 The Great Lakes regions (Ontario and Quebec). In the Great Lakes regions, more positive δ2H values dominate for tap water, similar to what is observed in precipitation for this region [44]. However, these tap waters show an interesting combination of positive and negative d-excess values for Tap Groundwater and Tap Lake respectively. Tap Groundwater samples have d-excess similar to those found in precipitation in these regions, suggesting limited evaporative losses [28]. The more positive d-excess of the Tap Groundwater reflects the amount of recycled water fluxes (‘lake-effect’ precipitation events) in the Great Lakes regions, as suggested by earlier studies [45, 46]. Aquifers that recharge near the lakes have more positive d-excess values than areas that are further away from these lakes [47]. Conversely, Tap Lake have more negative d-excess values, and negative R precipitation and R surface values, suggesting they have undergone more evaporative losses with its associated fractionation [12]. Bowen et al. [19] showed similar patterns of “low d-excess regions” around the Great Lakes in the United States, however in Bowen et al. [19], the sources for those tap water samples were not known. Here, we show that Tap Lake can undergo significant evaporation in these regions [48]. Such high evaporative losses could be partially due to tap water management related issues as recent study suggests this region to have very limited evaporative losses [40]. Except for a few small lakes such as Aspey Lake, Lauzon Lake, Lake Sassagianga and Lake Wawa, most of the Tap Lake samples in these regions are sourced from the Great Lakes (Data A in S1 Text, available at https://doi.org/10.6084/m9.figshare.19243518). The risks and issues associated with these water resources with respect to climate change occurs over longer timescales and requires a good understanding of the long-term water balance of the Great Lakes [48–50]. Long-term seasonal and multi-annual isotopic monitoring of tap waters could be used to identify the long-term effect of climate or water management practices on tap water supplied by lake waters in the Great Lakes region.

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[1] Url: https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000068

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