The increasing concentration of nitrogen compounds in the groundwater is of a growing concern in terms of human health and groundwater quality. Although an excess of nitrogen compounds in the groundwater of the Kathmandu Valley has been reported, the seasonal variations of the fate of the nitrogen compounds and their relationships to the subsurface sediments are unknown. In this study, spatially distributed shallow dug well samples were collected during both the dry and wet seasons of 2016, and the nitrogen compound, chloride (Cl⁻), and iron (Fe²⁺) concentrations were analyzed. Two shallow dug wells and one deep tube well were monitored monthly for 2 years. Although NH₄-N concentrations were similar in the clay-dominated areas during both seasons (1 and 0.9 mg-N/L), they were lower in the gravel-dominated areas during wet season (1.8 > 0.6 mg-N/L). The NO₃-N concentration differed depending upon the soil type which increased during the wet season (clay 4.9 < 13.6 mg-N/L and gravel 2.5 < 6.8 mg-N/L). The Fe²⁺ concentration, however, was low during the wet season (clay 2.7 > 0.4 mg/L and gravel 2.8 > 0.3 mg/L). Long-term analysis showed higher fluctuation of nitrogen compounds in the gravel-bearing areas than in the clay-bearing areas.

Decentralized tap water systems are an important drinking water source worldwide. A good quality, high-pressure continuous water supply (CWS) is always the target of any urban settlement. However, tap water in some areas are reported with deteriorated water quality even though treated well before supplying. Such deterioration of tap water quality is reported widely from areas with low water availability and in economically poor countries where water are supplied intermittently (IWS). This study focuses in identifying tap water quality in IWS and causes of water quality degradation using nitrate-nitrogen (NO₃-N) as an indicator and stable isotopes of hydrogen (δD) as tracer. Nine water reservoirs and ninety municipal tap water (ten per reservoir) samples were collected during the wet (June–September) and dry (November–February) seasons in the Kathmandu Valley (KV), Nepal. Ten percent of the tap water samples exhibited higher NO₃-N than those of their respective reservoirs during the wet season, while 16% exhibited higher concentrations during the dry season. Similarly, the isotopic signatures of tap water exhibited 3% and 23% higher concentrations than those of their respective reservoirs during the wet and dry seasons, respectively. Coupling analysis between NO₃-N and δD demonstrates close connection of groundwater and tap water. The results indicate groundwater intrusion as the primary component in controlling tap water quality variations within the same distribution networks during IWS. Meanwhile, the obtained results also indicate probable areas of intrusion in the KV as well as usefulness of δD as a tool in the assessment of tap water systems.

Interconnection between river water and groundwater plays an important role in maintaining water quantity and quality in hydrological systems. Furthermore, the exact interconnection is often difficult to observe and measure. This study attempts to explain river and shallow groundwater interconnection in urbanized areas of the Kathmandu Valley, Nepal. Isotopic (δD and δ¹⁸O) and chemical analyses were performed on river and groundwater samples, and the results were analyzed using statistical methods to identify areas of interconnection between river water and groundwater. Higher concentrations and positive strong correlations of Na⁺ with K⁺, NH₄⁺-N, Cl⁻, HCO₃⁻, and PO₄⁻-P, and a change of water type from Ca-HCO₃ during the wet season to Na-K-HCO₃ during the dry season indicate higher contamination in river water during the dry season. Hierarchical cluster analysis was used in grouping water samples into clusters on the basis of isotopic and chemical (Na+ and Cl⁻) composition. Grouping of river and groundwater samples in one–one clusters from wet and dry seasons shows the presence of interconnection, indicating the contribution of river water in recharging shallow groundwater. These results imply that shallow groundwater found near rivers is chemically contaminated by polluted river water through bank infiltration, in both wet and dry seasons.

This study investigates the contribution ratios of different groundwater recharge sources and influence of a dual monsoon system in Kofu basin, central Japan, through the hydrogen and oxygen isotopic analysis of precipitation, river water, and groundwater. The study is focused on the area of the Kamanashigawa and Midaigawa alluvial fans, which are formed by two main rivers. Precipitation isotopic content exhibits significant seasonal variability. Also, river water presents d-excess values higher than annual precipitation at plain areas (9 and 10‰), suggesting that two different air-masses contribute to precipitation, corresponding to the monsoon and pre-monsoon periods. The results of this study allow estimation of relative contributions of different sources to groundwater and influence of a dual monsoon system. The mass-balance analysis of the δ18O to identify the groundwater source indicates the river water contributes 38–100% of the recharge, while precipitation contributes 29–62% in Kamanashigawa alluvial fan. In the case of Midaigawa alluvial fan, river water contributes 77–99% in the northern part; in the southern side, 30–93% of contribution comes from precipitation. The mass-balance analysis of the d-excess indicates pre-monsoon precipitation contributes 46–68% and 39–65% to groundwater of the Kamanashigawa and Midaigawa alluvial fans, respectively.