Groundwater Dynamics in the Middle Brahmaputra River Basin: A Case Study of Shallow Aquifers in Inner Guwahati City, Assam, India

Abstract

This study investigated the hydrogeological characteristics and groundwater dynamics in the shallow aquifer zones of inner Guwahati city, Assam, India. Sixteen dug wells spread across the city, specifically used for domestic purposes, were selected for this study. Additionally, ten wells were selected for trend analysis. The borehole lithology reveals predominant compositions of clay, sand, and granules, with thin clay cappings indicating significant groundwater potential. Depth-to-water level analysis revealed varying water levels across the study area, with shallow levels in the northern and western regions and gradual deepening toward the eastern and southern parts. The groundwater flow directions show nonuniform patterns and reflect the influence of topography and domestic pumping in urban residential zones. The general groundwater flow direction is toward the Brahmaputra River. Trends in groundwater level, assessed using the Mann–Kendall test and Sen’s slope, suggest both falling and rising trends across different locations, indicating complex groundwater dynamics influenced by factors such as recharge, extraction, and topography. However, the long-term rainfall data indicate no significant trend over the studied period, suggesting limited natural influence on groundwater level trends. These findings may contribute to a comprehensive understanding of groundwater dynamics in the study area and are essential for sustainable water resource management and mitigation of groundwater depletion risks.

1. Introduction

Groundwater is an integral natural resource essential for sustaining life, supporting ecosystems, and driving socioeconomic development [1]. Rapid urbanization, population growth, and climate change have led to an increased demand for this vital resource, creating concerns regarding its sustainable development and management [2]. The quantitative assessment of groundwater resources and studies on water level fluctuations are imperative for effective water resource management, particularly in the context of shallow aquifers, where water level fluctuations can have significant implications on domestic water needs [3]. The complex hydrogeological processes governing these aquifers, coupled with external factors such as climate variability and human activities, can influence water level dynamics and ultimately impact groundwater availability [4].

The investigation of water level fluctuations in shallow aquifers encompasses various dimensions, including hydrogeological characterization, temporal analysis of water level data, assessment of influencing factors, and implications for groundwater management. Groundwater level trend analyses using statistical methods have become popular in recent years [5]. Trend analysis facilitates understanding of aquifer behavior in response to various climatic and anthropogenic factors [5,6]. Linear regression, the Mann–Kendall (MK) test, Sen’s slope (SS), the modified Mann–Kendall (MMK) test, and innovative trend analysis (ITA) are some of the most widely used techniques adopted for understanding trends in groundwater levels [5,7]. These techniques have proven to be effective for better groundwater resource management.

Song et al. (2019) [8] observed the impact of urbanization and precipitation on water level change in the Yangtze River delta. Wang et al. (2023) [9] indicated that the groundwater table showed periodic responses to precipitation. They observed the lag time of groundwater to precipitation, which is linked to aquifer lithology, precipitation intensity, and groundwater exploitation intensity. Swain et al. (2022) [10] illustrated the impact of climate change on groundwater storage. Similar effects have been seen in the Brahmaputra River basin [11]. Along with the seasonal changes in basin dynamics, the Brahmaputra River basin is reported to have the highest rate of groundwater extraction, along with the Ganges River basin [12]. As a result, the region has documented significant groundwater depletion over the years. From in situ and satellite-based estimates of usable groundwater storage in India, Bhanja and Mukherjee (2019) [11] reported that although the Brahmaputra River basin receives ample monsoon rainfall, there is still a high rate of depletion of groundwater storage (>5 km³/year), making it highly vulnerable to water shortages in the near future.

As India battles acute water shortages in different regions, the state of Assam is also under pressure from groundwater depletion. In Assam, which has a population of just over 30 million [13], groundwater is solely relied upon as a source of potable water for both rural and urban areas. According to the Central Ground Water Board (CGWB), the current stage of groundwater extraction is 12.38%, which has slightly increased from 11.73% in 2020, but Assam still maintains a ‘safe category’ status concerning districtwise extraction of groundwater [14]. However, there are certain localized pockets where the depletion of the groundwater level has continued over the years, resulting in a loss of usable groundwater at a rate faster than it can be replenished [11,15].

Despite being located on the southern bank of the Brahmaputra River, Guwahati city primarily depends on groundwater for its water requirements [16]. On the northern bank of the Brahmaputra River, water-bearing formations exist from shallow levels (<30 m) to depths greater than 200 m below ground level [17]. Conversely, on the southern bank, the groundwater potential varies based on the depth of the weathering profile. In the western part, water-bearing formations exist at shallow levels (<30 m) to depths exceeding 300 m [17]. Although surface water is available for utilization, approximately 70% of households use groundwater, 27% depend on the piped water supply, and the remainder use surface water obtained mainly from streams [15,18]. The annual extractable groundwater resource availability in Guwahati city is 5000 ham, with a groundwater development rate of 57.26% [14]. Over the last few years, due to unplanned urbanization coupled with excessive population growth and subsequent exploitation of groundwater, the water table in some parts of the city has shown a declining trend [15,19,20]. The depletion of groundwater around Guwahati city reached a critical level, necessitating proper monitoring and assessment of the groundwater aquifer.

The primary objective of this study was to investigate the pattern of water level fluctuations in the shallow aquifers of inner Guwahati city. An attempt has been made to study the extent of variation in the water level and/or water table during wet and dry periods for nearly 12 years to determine the factors influencing groundwater recharge and discharge processes. Trend analysis was also conducted to determine the temporal variations in water levels across different seasons.

2. Study Area

2.1. Geological Setting of the Study Area

Guwahati city is located in the southern part of the state of Assam, India, between 26°5′ and 26°12′ N and 91°24′ and 91°51′ E (Figure 1). The city forms a part of the Great Brahmaputra valley, which is composed of thick alluvium from the Quaternary Group. The plains are covered by the alluvium brought by the Brahmaputra River and its tributaries [21]. The climate is subtropical and humid with heavy rainfall, hot summers, and high humidity [21]. The average annual rainfall is 1752 mm, and the average temperature is 16.5 °C in winter and 26 °C in summer. The population of the area is approximately 1 million [13].

2.2. Hydrogeology

The study area consists of two broad units: (1) Pre-Cambrian consolidated rocks and (2) Quaternary alluvium consisting of unconsolidated sediments. Pre-Cambrian consolidated rocks are confined to hilly areas and inselbergs, marked by elevated terrain (Figure 1). The groundwater in these areas occurs in the shallow weathered zone and can be tapped through open wells. The joints and fractures that developed due to tectonic activity formed potential water-bearing zones and were suitable for development through the construction of borehole wells [21]. In alluvial plains, groundwater occurs in regionally extensive aquifers down to a depth of 305 m, with good yield prospects. The aquifers consist of sands of various grades with gravel and are suitable for the construction of both shallow and deep tube wells [21]. In the study area, groundwater occurs under unconfined to semiconfined conditions. In different parts, the water level lies between 2 and 5 m below ground level (bgl) during the pre-monsoon period.

3. Materials and Methods

3.1. Data Collection

Sixteen dugwells, primarily used for domestic purposes, were selected to study the variation in groundwater level from August 2018 to March 2019 (Figure 1). The depth to the water level and the height of the measuring points were measured during the well inventory using a 50 m measuring tape. The depth to the water level was calculated by deducting the length of the well height from the total height of the measuring point to the ground level.

Secondary data, such as reduced-level and aquifer lithology, were collected from the Directorate of Geology and Mining (DGM), Guwahati, Government of Assam, India. To evaluate the long-term variability in groundwater levels, data on water levels monitored from 1998 to 2014 were also collected from DGM, Guwahati, for an additional 10 wells. Additionally, rainfall data were collected from the Indian Meteorological Department (IMD), Regional Centre Guwahati, Assam.

3.2. Groundwater Level Trend Analysis

To determine the trend in hydrological and climatological data, the nonparametric Mann–Kendall test is extensively used, as it is less affected by the presence of any outliers [22]. For this study, a trend analysis of groundwater levels over 10 consecutive years was carried out. Water level data were collected from 1998 to 2014 for the months of March (pre-monsoon) and November (post-monsoon) from DGM, Guwahati. Along with the Mann–Kendall test, Sen’s slope estimator is used to determine the magnitude of the trend [23,24]. In the Mann–Kendall test, the ranks of the values replace the n times series value [25].

The Mann–Kendall statistics S are given as follows:

$$S=\sum _{i=1}^{n-1}\sum _{j=i+1}^{n}sgn\left(x_j-x_i\right)$$

The trend test is performed for a time series of X_i, where i = 1, 2, …, n − 1, and X_j, where j = i + 1, 2, …, n. Each of the data points (X_i) is taken as a reference point that is compared with the rest of the data points.

$$Sgn\left(x_j-x_i\right)=\left\{\begin{array}{c}+1 if\left(x_j-x_i\right)>0\\ 0 if\left(x_j-x_i\right)=0\\ -1 if \left(x_j-x_i\right)<0\end{array}\right.$$

The variance is given as follows:

$$\mathit{var}(S)={\displaystyle \frac{ n\left(n-1\right)\left(2n+5\right)-\sum _{i=1}^m{t}_i(t_i-1)(2t_i+5)}{18}}$$

where t_i is the number of ties up to sample i. The Z-statistic is computed as:

$$Z=\left\{\begin{array}{c}{\displaystyle \frac{S-1}{\sqrt{Var}\left(S\right)}}, S>0\\ 0, S=0\\ {\displaystyle \frac{S+1}{\sqrt{Var}\left(S\right)}}, S<0\end{array}\right.$$

Here, Z follows a standard normal distribution and can be used to determine the trend. Positive values of Z (Z > 0) indicate a decreasing/increasing trend, negative values of Z (Z < 0) indicate a rising/decreasing trend, and Z = 0 indicates no trend. We used p values to determine the significance of the trend.

4. Results and Discussion

4.1. Aquifer Lithology

The borehole lithology reveals that the aquifer mostly consists of clay, sand, and granules with thin clay cappings (Figure 2). The thickness of the clay layer is greater in the borehole located in Kamakhya (BH5), which is in proximity to the Brahmaputra River. The sandy fractions dominated the rest of the borehole lithologies. The lithological profiles indicate that the study area is mostly unconfined. The region is a part of the Brahmaputra River alluvial province and has significant groundwater potential [26]. Three distinct physiographic zones—Bhabar, Terai, and Axial—hold most of the groundwater in this region [27]. The flat axial belt, which comprises the bulk of the meander belt and back-swamp deposits of the Brahmaputra River and its tributaries, encompasses the study area. The sediments of the axial belt are composed of well-stratified fine gravel, sand, silt, and clay. In the greater part of the axial belt, the water table occurs at shallow depths of less than 10 m [27]. The younger alluvium in the floodplain of the Brahmaputra River has high groundwater potential [26]. However, alluvial groundwater systems are highly vulnerable to the fluctuation of precipitation amount, hence any variation in precipitation amount is likely to impact the groundwater level [28].

4.2. Depth-to-Water Level and Flow Direction

The depth-to-water level data collected during 2018–2019 (see Supplement Table S1) indicate that the water levels are at shallow depths in the northern and western regions of the study area. This could be due to continuous recharge of the aquifers by the Brahmaputra River. Toward the eastern and southern parts of the study area, the water level gradually deepens (Figure 3a–c). The variation in water level is prominent in all three seasons, i.e., monsoon (August 2018), post-monsoon (November 2018), and pre-monsoon (March 2019), as aquifers in the study area are mostly unconfined, as observed from the lithological profiles (Figure 2). Significant water level fluctuations occurred during the pre-monsoon season (Figure 3c), especially toward the southeastern part of the study area, while the minimum variation was observed during the monsoon and post-monsoon periods (Figure 3b,c).

The water level data are referenced to a common datum, and the contour map indicates the groundwater flow direction (Figure 4). Water table maps indicate the nonuniform groundwater flow patterns caused by the undulatory and rugged topography, i.e., the combination of hillocks and flat areas (Figure 4a,b). In the northern part of the study area, the flow direction is from north to south, while in the northeastern part, the groundwater flow direction is from northeast to southwest; in the southwestern part, the flow direction is from southwest to northeast (Figure 4a,b). The water table data also indicate a possible perched aquifer in the southwest (well S5). Additionally, based on the report by the Water Resources Department of the Government of Assam, the Brahmaputra River’s water level ranges between 48 and 52 m. The highest water table contour of 60 m passes through the northeastern part of the study area, while the lowest water table contour of <48 m passes through the eastern and southwestern parts. As the study area is marked by hills, the water table is discontinuous, and its configuration is nonuniform. It can be inferred from the water table contours that several cones of depression formed in the study area across different seasons. Such local variations in the water table may be attributed to widespread urban pumping [29].

4.3. Water Level Fluctuations

Changes in groundwater level over a specific period of time are primarily governed by rainfall and land use changes [30]. However, Waco and Taylor (2010) [31] and Brauman et al. (2012) [32] specified that precipitation is mainly influencing the seasonal changes in groundwater level. Faridatul (2018) [33] highlighted the direct relationship between groundwater level and precipitation, where higher precipitation has led to shallower groundwater level. However, in highly urbanized areas, a similar trend was not seen, which was attributed to overexploitation for human activities. Similarly, Rahmawati et al. (2024) [34] depicted that change in the rate of precipitation has dominantly impacted the changes in water level for a certain period of study in Yogyakarta city, Indonesia.

To obtain a holistic view of the water level variation in the study area, the water level data were compared from 2007 to 2019, although no data were available for the period from 2015 to 2017 (see Supplement Table S2). The hydrograph for the period from 2007 to 2019 showed a clear correlation between precipitation and water level depth (Figure 5). A comparison of the water level data indicated that the minimum water level during the pre-monsoon season was 0.63 m, while during the post-monsoon season it was 0.20 m below ground level (bgl), and the maximum water levels were 18.64 m and 17.12 m bgl, respectively. With increased precipitation, the water levels rise to shallow depths, while decreased precipitation leads to a fall in water levels to greater depths. However, some anomalies have been observed where high precipitation coincides with deeper water levels, indicating delayed or no recharge to the aquifers (Figure 5). A slight decrease in water levels was observed over the years in specific regions of the study area. A larger fluctuation in the water level is typically associated with low porosity [35].

Seasonal water level fluctuations are clearly observed in each of the 16 studied well locations. For S1, the average water level for the last 12 years was 1.25 m bgl, with a maximum water level of 2.64 m bgl occurring during the pre-monsoon season in 2010 and a minimum water level of 0.3 m bgl occurring during the monsoon period in 2018. Overall, the water level is shallow, and it remains stable during the study period. The average water levels in S2 and S3 are 7.16 m and 5.43 m bgl, respectively. For well S4, the average water level is 14.15 m bgl, with a maximum depth of 18.64 m bgl occurring during the pre-monsoon season of 2010 and a minimum depth of 2.36 m bgl occurring during the monsoon season of 2018. Surprisingly, well S4 showed a significant increase in water level from 16.15 m bgl in 2008 to 8.88 m bgl in 2019. The water level in well S5, located at the foothills of the Nilanchal Hills, almost remained steady between 2007 and 2019, with an average water level of 1.81 m bgl and maximum and minimum water levels of 5.09 m and 0.95 m bgl, respectively. For wells S6 and S7, located near the Brahmaputra River, the average water levels were 7.4 m and 2.8 m, respectively. Wells S8 and S10 have shallow water levels with averages of 1.8 m and 3.06 m, respectively, with some seasonal fluctuations. In well S9, the water level has gone deeper at an alarming rate, with a net decrease of 9.7 m. In 2007, the water level was 8.17 m bgl, which increased to 17.87 m bgl in 2019, with an average depth of 8.7 m. Falling trends in groundwater levels are often attributed to unabated extraction from aquifers [36], further aggravated by urbanization-induced imperviousness inhibiting aquifer recharge [19,37].

In wells S11 and S12, the average water levels were 1.14 m and 3.69 m bgl, respectively. The average water level for S13, located at the foothills of the Kherghuli Hills, was 2.82 m bgl, and it has remained between 3.61 m and 1.44 m bgl in the last 12 years. Bearing close proximity to the Brahmaputra River, the well may have been recharged by the river during dry seasons, leading to persistently shallow water level conditions. Well S14 has an average water level of 6.46 m bgl, with maximum and minimum water levels of 10.34 m and 3.74 m bgl, respectively. The average water level for well S15 was 6.29 m. The minimum water level during the monsoon season in 2018 was 1.6 m bgl, and during the post-monsoon season in 2018, it decreased to 4.8 m. A sharp decrease, from 4.8 m bgl to 16.3 m bgl, in the water level was observed between November 2018 and March 2019. This decrease could be attributed to insufficient recharge and overexploitation of groundwater. In well S16, the average water level was 3.46 m bgl, with maximum and minimum water levels of 6.59 m and 1.54 m bgl, respectively. Based on the long-term analysis of groundwater level data, Nath et al. (2021) [19] reported decreasing trends in the depth-to-water level across Guwahati city and attributed these trends to continuous development and population growth.

4.4. Trends in Groundwater Level

The Mann–Kendall test and Sen’s slope were applied to evaluate the trend of groundwater levels in the study area (see Supplement Table S3). A positive Z value and Sen’s slope indicate a positive trend, which indicates that the depth to the water level increases from the ground surface, thereby indicating that the water level decreases or increases. Conversely, a negative Z value and Sen’s slope indicate a negative trend, indicating that the depth-to-water level is increasing or decreasing.

During the pre-monsoon season (March), the Z value and Sen’s slope indicated that 5 out of the 10 wells experienced a significant positive trend, indicating a decrease or increase in the depth to the water level, which could be due to the overexploitation of groundwater in these areas (

Table 1. Mann–Kendall statistics (Z scores) and Sen’s slope estimator values for pre- and post-monsoon groundwater level of 10 observation wells (1998–2014).

ID	Station	Pre-Monsoon (1998–2014)			Post-Monsoon (1998–2012)
		Z Score	Sen’s Slope	Trend	Z Score	Sen’s Slope	Trend
T1	Amingaon	1.4	0.02	No	−0.84	−0.01	No
T2	Charabbhat Chariali	3.17	0.35	Falling	−0.3	−0.03	No
T3	Garchuk	3.42	0.11	Falling	−1.98	−0.07	Rising
T4	Jonali Path, Zoo Road	3.01	0.35	Falling	3.47	0.24	Falling
T5	Jyotinagar, Durga Namghar	3.17	0.13	Falling	−0.3	−0.01	No
T6	Kharguli, North of Kharguli Hill	−0.87	−0.02	No	−4.01	−0.14	Rising
T7	Krishna Nagar Cemetery	2.39	0.16	Falling	−0.89	−0.03	No
T8	Maligaon Colony	1.44	0.14	No	0.1	0.03	No
T9	Maligaon, Gosala	−1.03	−0.08	No	−1.19	−0.07	No
T10	Noonmati	−2.76	−0.07	Rising	−2.47	−0.10	Rising

Note: Italic and bold font indicate 95% confidence level.

). Previous studies have shown that an increase in the depth-to-water level is linked to an increase in built-up areas [19,38]. However, only one well exhibited a negative trend, indicating a rising or decreasing depth to the water level. On the other hand, 4 out of the 10 wells did not show any significant trend. During the post-monsoon season (in November), the Z value and Sen’s slope indicated that only one well exhibited a positive trend, i.e., falling or increasing depth to the water level. However, three wells exhibited a negative trend, i.e., increasing or decreasing depth to the water level. The decreasing depth-to-water level could be associated with sufficient recharge of groundwater in these regions during monsoon periods. Most of these wells are located near the Brahmaputra River and are possibly recharged by the river, leading to a rise in the water level. In total, in 6 out of the 10 wells, the depth-to-water level did not significantly change.

Mann–Kendall statistics and Sen’s slope estimator have also been applied to rainfall data for the last 12 consecutive years to assess the impact of changes in depth on water level. The data showed no significant trend in precipitation over the years in Guwahati city. The average Z score of 0.046 and average Sen’s slope of 1.23 suggest that there was no natural influence on the depth-to-water level trends in the study area (

Table 2. Mann–Kendall statistics (Z scores) and Sen’s slope estimator values of rainfall data for 12 consecutive years (2007–2018).

Year	Z Score	Sen’s Slope	p-Values	Trend
2007	0.41	−1.56	0.68	No
2008	−0.62	−9.35	0.54	No
2009	1.24	21.34	0.22	No
2010	−0.21	−5.67	0.84	No
2011	0.07	1.53	0.95	No
2012	−0.62	−1.05	0.54	No
2013	0	0	1	No
2014	0	1.65	1	No
2015	0.48	5.81	0.63	No
2016	−0.75	−1.86	0.45	No
2017	−0.07	−0.82	0.95	No
2018	0.62	1.61	0.54	No

).

Previous researchers have observed similar findings in the context of the present study. Saikia et al. (2023) [15] examined the groundwater level trend over 20 consecutive years and reported a depleting groundwater level in the Kamrup metropolitan district of the Brahmaputra River basin. They further observed no significant change (Z values of −1.5 and p-values of 0.13) in annual rainfall patterns from 1980 to 2019. The results of the present study were also found to be consistent with the findings of Sarmah et al. (2022) [39]. Nath et al. (2021) [19] also indicated groundwater depletion in Greater Guwahati, particularly in and around the urbanized area with a highly dense population, which holds true for the present study as well. A recent study observed a declining trend in the amount of groundwater recharge in Greater Guwahati city [40]. These studies indicate the significance of changing groundwater dynamics over time, which requires further research to address sustainable development.

5. Conclusions

This study provides valuable insights into the hydrogeological dynamics of the Brahmaputra River alluvial province. The analysis of water level fluctuations reveals complex interactions between precipitation, recharge dynamics, and anthropogenic influences, with both positive and negative trends observed in the depth-to-water levels across different wells. These hotspot areas, primarily zones of recent urban development, face increasing groundwater demand. The Mann–Kendall test and Sen’s slope analysis offer valuable insights into long-term trends in the depth-to-water levels, emphasizing the need for sustainable groundwater management practices to prevent depletion. Additionally, nonuniform groundwater flow patterns reflect the intricate relationship between topography, urban pumping, and groundwater movement. To prevent further stress on groundwater availability, interventions such as rainwater harvesting, legislative actions—including restrictions on groundwater extraction—and public awareness campaigns on water conservation are already being advocated.

This study further emphasizes the need for regular monitoring of groundwater levels to ensure a sustainable water supply in areas experiencing significant declines. The findings also contribute to climate resilience strategies by addressing the relationship between natural recharge processes and human activities. These insights have broader applicability, informing water management policies in other regions facing similar challenges.