Assessing water quality in Ayad River Basin

The Ayad River Basin, located in the semi-arid region of Udaipur, Rajasthan, is characterised by complex geological features and significant urbanisation and agricultural activity. Groundwater, the primary source of drinking and irrigation water in the region, faces contamination threats due to both natural and anthropogenic factors. In this region, groundwater quality is compromised by contaminants such as fluoride, nitrate, chloride, and calcium.

This paper focusses on assessing groundwater quality in the Ayad River Basin using the Groundwater Quality Index (GWQI) and identifying its suitability for drinking and irrigation purposes. The findings are crucial for developing strategies for groundwater management in the region.

The Ayad River Basin spans an area of 1207 sqkm, extending across five tehsils in Udaipur and Rajsamand districts. The river originates from the Gogunda hills and flows southeast for 68 km, eventually draining into the Vallabh Nagar reservoir. The basin has a tropical semi-arid climate with average temperatures ranging from 29°C to 42°C in the summer and 3°C to 19°C during winter.

The region experiences an average annual rainfall of 640 mm, most of which occurs during the monsoon season. The high evaporation rates, exceeding 600 mm annually, coupled with irregular rainfall patterns, exacerbate the challenges of groundwater recharge.

Geologically, the Ayad River Basin exhibits diverse formations ranging from the Archaean to the Upper Proterozoic eras. The region's terrain is hilly and undulating, with elevations between 500 and 700 metres above sea level. The geological makeup significantly influences groundwater availability and quality, with crystalline rocks forming low-yield aquifers.

Data collection and methodology

This study utilised groundwater data from multiple sources, including the Ground Water Department (GWD), Central Ground Water Board (CGWB), and a citizen science initiative from 2022-2023. Data were collected from 41 groundwater monitoring wells, 25 within the Ayad River Basin and 16 on its periphery, to ensure robust interpolation of groundwater quality parameters.

The persistently poor groundwater quality in areas underlain by the migmatite complex suggests geogenic factors, such as the dissolution of minerals from the bedrock, contributing to contamination. This highlights the need for region-specific groundwater management strategies that take geological characteristics into account​.

Results and discussion

Groundwater quality for drinking purposes

The GWQI showed significant spatial variation across the Ayad River Basin, with groundwater quality generally decreasing from west to east. The southern and western regions, including Ramgiri, Umarda, and Hariyab, exhibited good groundwater quality, with GWQI values below 50. In contrast, areas in the eastern and northeastern parts, such as Bhoyana and Sisarma, displayed poor to unsuitable groundwater quality, with GWQI values exceeding 100.

Total Dissolved Solids (TDS) and Fluoride: TDS concentrations ranged from 434 mg/L at Umarda to 2545 mg/L at Bhoyana, with an average of 1122 mg/L. While most areas were within permissible limits, some locations in the eastern part of the basin exceeded the TDS threshold, indicating poor water quality. Fluoride concentrations averaged 0.56 mg/L, with most areas falling below the permissible limit of 1.5 mg/L. However, elevated fluoride levels were found near Bhoyana and Lakhawali​.

Nitrate and Chloride: Nitrate levels varied widely, with concentrations ranging from 4 mg/L at Sukher to 370 mg/L at Bhoyana. The average nitrate concentration was 63 mg/L, with many locations exceeding the permissible limit of 45 mg/L, raising concerns for drinking water safety. Chloride concentrations similarly exceeded permissible limits in some locations, particularly around Kewra and Bhoyana​.

Groundwater quality for irrigation purposes

For irrigation suitability, groundwater quality was assessed based on salinity (EC), total hardness (TH), sodium percentage (%Na), and sodium adsorption ratio (SAR). The salinity hazard test indicated that groundwater in most locations was classified as “doubtful” for irrigation, particularly in areas with high EC values. However, the SAR values across the basin were relatively low, suggesting that groundwater posed a low sodium hazard and was suitable for irrigation.

Groundwater quality linked to geology

The geological composition of the Ayad River Basin played a significant role in groundwater quality. Monitoring wells located in areas underlain by the Migmatite Complex and Meta-Volcanics-Basics lithologies exhibited poor groundwater quality, with elevated levels of EC, total hardness, magnesium, potassium, and fluoride. In contrast, areas underlain by marble phyllite and quartzite exhibited better groundwater quality, though some parameters still exceeded permissible limits.

The persistent poor groundwater quality in areas underlain by the migmatite complex suggests geogenic factors, such as the dissolution of minerals from the bedrock, contributing to contamination. This highlights the need for region-specific groundwater management strategies that take geological characteristics into account​.

Conclusion

The groundwater quality in the Ayad River Basin varies significantly across the region, with a clear west-to-east gradient in contamination levels. While areas in the southern and western parts of the basin exhibit good groundwater quality, the eastern and northeastern regions face serious groundwater contamination issues, particularly due to high EC, nitrate, and chloride levels. The GWQI has proven to be a valuable tool for assessing groundwater quality and providing a comprehensive overview of its spatial and temporal variations.

The findings of this study underscore the need for enhanced groundwater monitoring, particularly in regions with poor water quality. Further research should focus on increasing the frequency of monitoring and expanding the use of advanced modelling techniques to predict future trends. Effective groundwater management policies, informed by geological and land-use considerations, will be critical in ensuring the sustainable use and protection of groundwater resources in the Ayad River Basin.

The full paper is available here