**1. Introduction**

Environmental, economic, and climate changes in many parts of the world put serious pressure on water resources, making a reliable alternative water supply a critical global concern [1]. Rainwater harvesting is a sustainable water managemen<sup>t</sup> practice that involves collecting and storing rainwater for later use. The collected rainwater can then be used for a variety of purposes, including irrigation, landscaping, and even household uses [1]. Since rainwater harvesting is an off-grid water supply [2], it contributes to reducing the pressure on the central systems, decreasing the reliance on freshwater abstraction [3], particularly in areas where water is in short supply or where the demand for water exceeds the available supply.

When determining the suitability of the water resource, water quality becomes a prominent issue. The harvested rainwater is generally of sufficient quality for non-drinking purposes [4]. However, due to various reasons (e.g., nutrient inputs, temperature increase, draught, etc.), cyanobacteria proliferation can occur in rainwater harvesting ponds and damage the water quality. Furthermore, some species can excrete cyanotoxins that may enter the agricultural fields by irrigation and cause environmental and public health problems. Thus, it is important to evaluate the potential contaminants of harvested rainwater.

The goal of this research was to evaluate the water quality of rainwater harvesting ponds in Istanbul used for irrigation and assess their potential risks for non-potable usage.

### **2. Materials and Methods**

The samples were collected from 17 rainwater harvesting ponds (S1–S17) located in Istanbul (Türkiye) during the summer of 2022 (Figure 1). These ponds were built between 1965–1989 in Istanbul and used for irrigation purposes in the neighboring agricultural fields. The active water volumes of the ponds varied between 11,000–7,000,000 m<sup>3</sup> in which S11 had the highest volume followed by S16 (1,406,405 m3) and S15 (1,103,386 m3).

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**Citation:** Ozbayram, E.G.; Köker, L.; O ˘guz Çam, A.; Akçaalan, R.; Albay, M. Water Quality and Risk Assessment in Rainwater Harvesting Ponds. *Environ. Sci. Proc.* **2023**, *25*, 28. https://doi.org/10.3390/ ECWS-7-14245

Academic Editor: Athanasios Loukas

Published: 16 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Sampling locations (S): Green dots indicate the rainwater harvesting ponds with cyanobacterial blooms.

The water samples collected from the surface were kept under dark and cold conditions and brought to the laboratory immediately. Water temperature, pH, dissolved oxygen (DO), and conductivity were measured in situ via a portable multiparameter (650 MDS, YSI, Yellow Springs, OH, USA) at each sampling site. Total nitrogen (TN) and total phosphorus (TP) were analyzed according to the methods outlined by the American Public Health Association [5]. The chlorophyll-*a* concentration was determined using the method described in ISO 10260 [6]. Phytoplankton samples were fixed by Lugol's iodine solution, and the phytoplankton enumeration was performed according to Utermöhl (1958) [7]. The microcystin concentrations were measured using liquid chromatography–high-resolution mass spectrometry (LC-HRMS) [8].
