## 2 Research Methodology
### Study Area  
Delhi, a vibrant cosmopolitan city with a rich and diverse culture, is the capital of the world's largest democracy, India. Historically known as Indraprastha, Delhi spans an area of 1483 square kilometers along the banks of the River Yamuna. Geographically, Delhi is located at 28.38\({}^{\circ}\) N latitude and 77.13\({}^{\circ}\) E longitude in the northern part of India. The city features three major geographical regions: the Yamuna floodplain, the ridge, and the Gangetic Plains. The Yamuna floodplain, with its fertile soil, is ideal for agriculture but is susceptible to frequent flooding. The ridge, with an average elevation of 293 m above sea level, is the most prominent feature of the area. Delhi consists of 200 densely populated villages. It shares its borders with Haryana to the west and Uttar Pradesh to the east.  
The area of Delhi is about 1484 sq. km. The rural area of Delhi is 783 sq. km. (302 sq. miles), and the urban area is 700 sq. km (270 sq. miles). Delhi's length is approximately 51.9 km (32 miles), while its width is about 48.48 km (30 miles). Delhi lies between 28\({}^{\circ}\)24\({}^{\prime}\)15\({}^{\prime\prime}\) and 28\({}^{\circ}\)53\({}^{\prime}\)00\({}^{\prime\prime}\) north latitude and 76\({}^{\circ}\)50\({}^{\prime}\) east longitude [World Energy Council 2010].  
The study area in Delhi focuses on the Okhla Industrial Area, Nangloi, and Karol Bagh to assess water quality during the pre-monsoon, monsoon, and post-monsoon seasons. The base map of the study area was created using Survey of India topographic sheets and digitized with ArcGIS software (Figure 1). GPS technology was employed to pinpoint the location of each sampling site, and the coordinates were subsequently imported into the GIS platform for further analysis.  
Figure 1: Location map showing study sites of Delhi region.  
The Okhla Industrial Area--Okhla is situated near the Okhla barrage in South Delhi. It is also known as the Okhla Industrial Area or the Okhla Industrial Estate. It is an industrial suburb divided into three phases, namely Okhla Industrial Area Phase 1, Phase 2, and Phase 3. The coordinates of the Okhla Industrial Area are 28.5317\({}^{\circ}\) N, 77.2766\({}^{\circ}\) E. It is the oldest village situated near the bank of the River Yamuna. The settlements of the Okhla Industrial Area Phase I and Phase II are mostly covered with slums, and the conditions in the area are characterized by a poor to very poor lifestyle under a huge number of industrial settlements. Tekhand is one of the oldest villages in the Okhla Industrial Area. The demography and economic profile are bleak, whereas the environmental and behavioral factors in the area suggest that the groundwater quality is very poor.  
Karol Bagh--This area is situated in the central district of Delhi. It is mostly a settlement of commerce and residence where colonial neighborhood and shopping streets are situated, featuring a population of approximately 3.75 lakh persons. The residential areas in Karol Bagh are W.E.A, Beadon Pura, Reghar Pura, Dev Nagar, and Bapa Nagar. The coordinates of this area are 28.6550\({}^{\circ}\) N, 77.1888\({}^{\circ}\) E.  
Nangloi--The coordinates of the Nangloi area are 28.6841\({}^{\circ}\) N, 77.0633\({}^{\circ}\) E. Nangloi is located in the western district of Delhi, surrounded by the Delhi Outer Ring Road, Bahadurgarh, Najafgarh, and Paschim Vihar. National Highway 10 (Rothak Road) passes through Nangloi, and it is linked to the Green Line of the Delhi Metro. Nangloi, part of the Western District of Delhi, has a population of approximately sixteen lakh. The population density mainly consists of Jat and Muslim communities. A water treatment plant has also been established in Nangloi. However, due to small-scale industries such as battery manufacturing, plastic production, and iron processing, the groundwater is highly contaminated. The THM (Total Trihalomethanes) parameter is prominently visible in many areas of Nangloi, particularly in the water pipelines supplying households.