ABSTRACT

An environmental isotope study was conducted to assess the cause for the freshening of seawater observed in the Mahim Bay, Mumbai, India after a storm event during the southwest monsoon period. Water samples were collected from the various locations of the coastal water and the suspected inland water sources such as rain, river and groundwater and analysed for major ion species and stable isotopes (δ¹⁸O and δ²H). Dissolved radon (²²²Rn) in the coastal water was monitored in-situ. Field monitoring survey in the coastal water indicated lower electrical conductivity (1,730 μS/cm) near Mahim Mosque compared to the surrounding shelf waters. Relatively high excess ²²²Rn activities (up to 55 Bq/m³) were observed in the Mahim Bay, even after 13 days of seawater freshening event. Based on the hydrochemical and isotope results, various prevailing hypotheses on the occurrence of low salinity water in the Mahim Bay were tested. It is inferred that the low salinity coastal water was associated with groundwater discharge occurring in the Mahim Bay and in the Mithi River and were unlikely due to the overflow of Vihar and Powai Lakes in the catchment of Mithi River and surface runoff because of the rain/storm events. Temporal variations of electrical conductivity and stable isotopic composition of coastal water in the Mahim Bay showed that the groundwater inputs were decreasing after the storm event. ²²²Rn was found to be a useful tracer for distinguishing the subsurface flow of water to the coastal system.

摘要

进行了一项环境同位素研究，以评估在西南季风期间发生风暴事件后在印度孟买马欣湾观察到的海水变淡的原因。从沿海水域和疑似内陆水源（如雨水、河流和地下水）的各个位置收集水样，并分析主要离子种类和稳定同位素（δ ¹⁸ O 和 δ ² H）。对沿海水中的溶解氡 ( ²²² Rn) 进行了原位监测。沿海水域的现场监测调查表明，与周围大陆架水域相比，马赫姆清真寺附近的电导率较低 (1,730 μS/cm)。相对较高的过量²²² Rn 活性（高达 55 Bq/m ³) 在 Mahim Bay 被观察到，即使在 13 天的海水淡化事件之后。根据水化学和同位素结果，对马欣湾低盐度水存在的各种流行假设进行了检验。推断低盐度沿海水与发生在 Mahim 湾和 Mithi 河中的地下水排放有关，并且不太可能是由于 Mithi 河集水区的 Vihar 和 Powai 湖溢流以及降雨造成的地表径流/风暴事件。Mahim 湾沿岸水电导率和稳定同位素组成的时间变化表明，风暴事件后地下水输入量正在减少。²²² Rn 被发现是一种有用的示踪剂，可用于区分流向沿海系统的地下水流。