Surface waters are widely used as sources of drinking water in Canada. The identification of the main sources of pollutants to surface waters, often associated with increasing urbanization, is needed to improve management strategies. We used stable water isotopes (δ¹⁸O and δD), and nutrient and fecal coliform mass fluxes to estimate the seasonal contributions of water and pollutants of different inflows to Lake St. Charles (LSC). Our results indicate that out of six identified LSC water sources, groundwater (GW), Hurons River (HR) and small stream tributaries (SSTs) represent the major contributors (in terms of water volume). The three other water sources, including two wastewater treatment plants (WWTPs), Delage Lake (DL) and precipitation (rain and snow), represent smaller seasonal contributions. During the high-flow seasons (spring and fall), LSC receives water mainly from SSTs and HR, while during the low-flow season (summer), the lake relies mainly on GW inflows. Estimates of point source (WWTPs) versus non-point source (primarily septic installations) concentrations of nutrients and fecal coliforms show that the latter represents a seasonally variable and greater source of pollutants (5% WWTPs vs. 95% septic installations). HR and SSTs, draining densely occupied areas of single housing units with individual septic installations, are categorized as non-point sources, and as such are considered major nutrient and fecal coliform contributors. The higher precipitation in spring and fall caused increased fluxes of nutrients and fecal coliforms through higher stream discharge. Due to the seasonally variable and difficult to predict discharge dynamics of SSTs and HR, these two sources represent a potentially larger threat to LSC water quality. We advocate that improved management requires reducing the number of individual septic installations in the LSC watershed.

在加拿大，地表水被广泛用作饮用水水源。需要确定通常与城市化进程相关的地表水污染物的主要来源，以改善管理策略。我们使用稳定同位素水（δ ¹⁸O和δD）以及营养和粪便大肠菌群质量通量，以估算水和不同流入圣查尔斯湖（LSC）的污染物的季节贡献。我们的结果表明，在确定的六个LSC水源中，地下水（GW），休伦斯河（HR）和小溪支流（SST）是主要的贡献者（就水量而言）。其他三个水源，包括两个废水处理厂（WWTP），德拉奇湖（DL）和降水（雨和雪），代表了较小的季节性贡献。在高流量季节（春季和秋季），LSC主要从SST和HR接收水，而在低流量季节（夏季），湖泊主要依靠GW的流入。估算点源（WWTPs）与非点源（主要是化粪池设施）的养分和粪便大肠菌的浓度，表明后者代表了季节性变化和更大的污染物源（5％WWTP与95％化粪池）。HR和SST排泄了带有单个化粪池设施的单个住房单元的密集居住区，被归类为非点源，因此被认为是主要的营养和粪便大肠菌群贡献者。春季和秋季较高的降水量通过较高的流量排放导致营养物质和粪大肠菌的通量增加。由于季节性变化并且难以预测SST和HR的排放动态，这两个来源对LSC水质构成了潜在的更大威胁。