Introduction:
Groundwater, as a vital resource for drinking water, agriculture, and industry, plays a crucial role in sustainable development, especially in arid and semi-arid regions. In recent years, factors such as climate change, reduced precipitation, population growth, and over-extraction have led to significant declines in groundwater levels and quality in various parts of the country. In this context, underground dams have emerged as innovative water management structures that reduce surface evaporation, increase water residence time, and enhance artificial recharge, thereby improving both quantity and quality of groundwater. Global studies have demonstrated that underground dams can raise groundwater levels, reduce salinity, and control saltwater intrusion; however, improper design or operation may increase nitrate levels and electrical conductivity (EC), posing risks for drinking and agricultural uses. Chaharmahal and Bakhtiari Province, characterized by semi-arid conditions and over 20 meters decline in groundwater levels during the past three decades, provides a suitable setting for underground dam implementation. The first underground dam in the province was constructed in 2015 in the Yancheshmeh watershed. This study aims to comprehensively evaluate the effects of this dam on groundwater quantity, focusing on water level changes and quality, with special emphasis on EC and nitrate concentrations as key indicators of water suitability and potential health risks.
Material and Methods:
This study was conducted in five main phases: desk studies, field operations, laboratory testing, statistical analysis, and result interpretation. The research focused on an underground dam located on the Zayandehrood watershed, with a reservoir length of approximately 5800 meters and a surface area of 3.5 hectares. (1) Desk Studies: Baseline data related to geology, meteorology, hydrology, hydrogeology, geography, and climate of the study basin were collected. A monitoring plan was developed to define sampling points and measurement locations for groundwater level and quality in wells both upstream (outside the reservoir) and within the reservoir. )2( Field Operations: Groundwater levels were measured monthly and weekly during rainy periods in wells within a one-kilometer radius of the dam from spring 2020 to spring 2024. Measurements were taken using a tape and meter. Water samples were collected seasonally and monthly during rainy periods from two points: a well at the reservoir inlet (furthest from the dam) and a well behind the underground dam. Sampling employed mechanical pumping assisted by a tractor. (3) Laboratory Testing: The collected samples were analyzed at the Agricultural and Natural Resources Research and Education Center laboratory for water quality parameters including pH, salinity, cations (Ca, Mg, Na), anions (Cl, CO3, HCO3, SO4), insoluble salts (lime, gypsum), and nitrate concentrations. This aimed to evaluate suitability for drinking and agricultural use. (4) Statistical Analysis: Data were analyzed using SPSS software and descriptive statistics (mean, standard deviation, range, skewness, coefficient of variation) were calculated. Student’s t-test (with three replicates) assessed significant changes over time, while Duncan’s test compared means at a 95% confidence level. (5) Result Interpretation: Annual groundwater level changes and yield variations in two selected wells were analyzed. Correlations between groundwater level and physical-chemical parameters were evaluated, especially nitrate and salts. The Schuler and Wilcox diagrams were used to assess the underground dam’s impact on water quality for drinking and agricultural purposes.
Results and Discussion:
Groundwater level monitoring was conducted regularly from spring 2020 to spring 2024 with 46 measurement events. These events included post-rainfall sampling . The water level fluctuations in Well 1 (inflow area) and Well 2 (within the underground dam reservoir) showed strong correlation, confirming the direct impact of surface water inflow on the stored groundwater volume. Well 2 had an average water level depth of 6.32 m, with fluctuations between 3.4 and 7.8 m, while Well 1 had an average of 5.81 m with variations from 3.9 to 9 m. The  coarse alluvial aquifer of the reservoir was estimated to have the volume of 700,000 m³ and an effective porosity of 20%, yielding a potential water storage capacity of about 140,000 m³. Considering an average saturated thickness of 12 m, the usable groundwater volume was calculated to be approximately 100,000 m³. Despite continuous seasonal withdrawal, the reservoir volume replenished annually, which contrasted with pre-dam conditions, under which water was absent during dry seasons. This stable water supply contributed to a 49.3% increase in cultivated land area between 2019 and 2022. Physicochemical analyses of 47 water samples revealed mostly stable concentrations of chemical parameters, except for bicarbonate, carbonate, and nitrate ions which showed significant seasonal changes. Electrical conductivity (EC) increased during dry periods due to water volume reduction but decreased with recharge, indicating natural dilution effects. Nitrate concentrations peaked during irrigation seasons due to fertilizer use but were diluted during wet periods; nitrate levels were lower inside the reservoir, demonstrating the dam’s mitigating effect on contamination. Water quality classification using Scholler, Wilcox, and Piper diagrams confirmed that the water was suitable for drinking and moderate agricultural use. The water type was predominantly calcium-bicarbonate, consistent with local geology.
Conclusion:
The present study demonstrated that the Yan-Cheshmeh underground dam, strategically located in a suitable geological setting and designed based on hydrogeological principles, has effectively contributed to groundwater storage and management. The impermeable foundation, large volume of coarse alluvial deposits, and considerable reservoir depth enable the storage of approximately 100,000 m³ of usable water. This volume remains sustainably available despite seasonal withdrawals, successfully supporting agricultural demands in the region. Water quality analyses revealed that seasonal water retention in the underground reservoir does not degrade water quality; instead, natural dilution reduces nitrate concentrations. Scholler, Wilcox, and Piper diagrams classified the water quality as ranging from good to acceptable for drinking and from moderate to good for agricultural use. A key strength of this research was the integrated approach combining fieldwork, laboratory analysis, and statistical evaluation over multiple years, which enhanced result reliability. However, limitations included the temporal coverage and sample size, which could be expanded for more detailed seasonal analysis. Overall, the Yan-Cheshmeh underground dam exemplifies a successful nature-based infrastructure for sustainable water resource management in semi-arid areas. It is recommended that similar projects be considered by policymakers and water resource