Data Repository: Spring and stream water evolution in a rapidly urbanizing watershed, Austin, TX

Quantifying urban development impacts on fresh water quality and quantity is critical, especially as growing populations concentrate in urban centers and with climate change projections of increased hydrologic extremes. We investigate geochemical processes through which municipal supply and waste water, carbonate bedrock, and soils impact stream and spring water compositions within the Bull Creek watershed (Austin, Texas). This watershed exhibits a sharp geographic divide between urban and rural land. Urban and rural waters were assessed to quantify relative influences of municipal water on stream and spring water elemental compositions and 87Sr/86Sr values. Higher 87Sr/86Sr for samples from urban sites relative to rural sites can be accounted for by two processes: (1) water leakage from municipal infrastructure and/or irrigation, or (2) ion exchange as precipitation infiltrates through soils with varying 87Sr/86Sr. Irrigated soils have higher 87Sr/86Sr than unirrigated soils, indicating that irrigated municipal water resets soil compositions, and that process (1) is a dominant driver of urban stream and spring water evolution. Geochemical modeling results indicate that urban waters consist of 50% to 95% municipal water. Geochemical modeling further demonstrates the evolution of municipal water as it infiltrates as groundwater and undergoes water-rock interaction. These results are compared with groundwater compositions on a regional scale to infer local flow paths and relative groundwater residences times of municipal water. This study provides a geochemical modeling framework that quantifies both the significance of municipal water on urban stream water and soil compositions, and the role of municipal water within urbanized watersheds and aquifers.量化城市发展对淡水质量与水量的影响至关重要，尤其是在人口持续向城市中心集聚，且气候变化预测显示水文极端事件愈发频发的背景下。本研究以德克萨斯州奥斯汀市布尔溪流域为研究对象，探究市政供水与废水、碳酸盐基岩及土壤通过何种地球化学过程影响溪流与泉水的水质组成。该流域存在显著的城乡土地利用地理分界，研究通过对比城乡水体样本，量化市政供水对溪流与泉水的元素组成及87Sr/86Sr（87Strontium/86Strontium）比值的相对影响。城区点位样本的87Sr/86Sr比值高于农村点位，这一现象可通过两种过程解释：（1）市政基础设施的输水泄漏或灌溉用水渗漏；（2）降水穿过锶同位素比值各异的土壤时发生的离子交换作用。灌溉土壤的87Sr/86Sr比值高于未灌溉土壤，这表明市政灌溉用水会重塑土壤的化学组成，且过程（1）是城区溪流与泉水水质演化的主要驱动因素。地球化学模拟结果显示，城区水体中市政供水占比可达50%至95%。该模拟还进一步揭示了市政供水作为地下水入渗并经历水-岩相互作用时的演化过程。本研究将上述结果与区域尺度的地下水水质组成进行对比，以此推断市政供水的局部径流路径及相对地下水停留时长。本研究构建了一套地球化学模拟框架，可同时量化市政供水对城区溪流及土壤化学组成的影响程度，以及市政供水在城市化流域与含水层中的作用。