Nitrate transport process driven by groundwater-surface water interactions in the Langat River Basin, Malaysia

Rapid population growth and agricultural expansion have increased nitrogen input in tropical regions, increasing the risk of nitrate contamination of water resources in tropical catchments. Most catchment-scale studies of nitrate contamination have focused on either surface water or groundwater. However, because nitrate is highly soluble and mobile, investigating interactions between groundwater and surface water systems is essential for tracking nitrate contamination. In this study, a multi-isotope tracer approach (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-) was applied to investigate three-dimensional nitrate transport processes through groundwater–surface water interactions. The study area is the Langat River Basin (2350 km²), in Selangor, Malaysia, which includes diverse land uses such as tropical rainforest, urban areas, peatlands, and oil palm plantations. Water sampling was carried out in wet and dry seasons at a spatial network of 44 groundwater, 17 river water, and 4 irrigation drainage channel sites. Results in the dry season showed that the mean nitrate concentration in river water was 13.4 mg/L (range 1.38 to 96.2 mg/L), while much lower concentrations were observed in groundwater (0.69 mg/L) and irrigation drainage (0.80 mg/L). Spatial stable water isotope compositions showed that groundwater near the river had similar signatures to river water, whereas groundwater farther from the river was more depleted, indicating river water recharge into the aquifer is important in floodplain areas. Nitrate isotope data revealed that sewage and manure were the dominant nitrate sources in the floodplain, affecting both river water and groundwater. In contrast, downstream areas dominated by oil palm plantations showed nitrate sources derived from fertilizer were dominant. These results demonstrate that land use and groundwater–surface water interactions control nitrate sources and transport pathways, emphasizing the importance of integrated water flow for assessing nitrate pollution in tropical catchments.