Influence of lateral recharge in mountainous areas on groundwater recharge and nitrate dynamics in the Hutuo River alluvial-pluvial fan

Lateral recharge in mountainous areas is an important recharge source for the alluvial-pluvial fans in the Piedmont Plain of North China, which affects the dynamics of the quantity and quality of groundwater in plain areas. In recent years, the mechanism of lateral recharge in mountainous areas of groundwater in alluvial-pluvial fans and its influence on the dynamics of nitrate in groundwater in plain areas have remained unknown due to extreme climate and human activities. Through hydrological observation and hydrochemical and isotope tracer methods across the Hutuo River alluvial-pluvial fan, we estimated the lateral recharge flux and nitrate transport flux in mountainous areas, revealed the recharge relationship between lateral recharge in mountainous areas and groundwater in plain areas, and analyzed the influence of lateral recharge in mountainous areas on the distribution of groundwater nitrate in plain areas. The sampling sites were divided into four sub-regions according to their hydrogeological conditions: the northern top part (Zone Ⅰ) and the middle (Zone Ⅱ) of the Hutuo River alluvial-pluvial fan, the fan margin area near the Hutuo River (Zone Ⅲ), and the southern part of the Hutuo River alluvial-pluvial fan (Zone Ⅳ). The monitoring data of groundwater nitrate dynamics showed that the nitrate concentration of groundwater in the mountainous section of the northern Hutuo River was higher than that in the southern section, and the nitrate concentration of groundwater in Zones Ⅰ and Ⅱ of the northern alluvial-pluvial fan plain was higher than that in Zone Ⅳ. Along the direction of groundwater flow, the mean concentration of groundwater nitrate showed a spatial distribution pattern of Zone Ⅰ (105.28 mg·L⁻¹) > Zone Ⅱ (99.22 mg·L⁻¹) > Zone Ⅳ (37.10 mg·L⁻¹) > Zone Ⅲ (23.08 mg·L⁻¹). The results revealed that the influenced areas by lateral recharge in mountainous areas in the northern part of the alluvial fan were the top and middle of the fan. However, the influenced area in the southern part of the alluvial fan was mainly the top of the fan because the groundwater flow field was changed by overexploitation. Darcy’s law was used to calculate the amount of lateral recharge in the mountainous areas of the Hutuo River alluvial-pluvial fan from March 2022 to February 2023. The results showed that amount of lateral recharge was 2.10×10⁸ m³, and the nitrate flux was 239.56×10⁵ kg. Moreover, the water and nitrate fluxes of lateral recharge in the northern mountainous areas were greater than those in the south, which was also an important factor affecting the spatial distribution of groundwater nitrate in the plain area. The impact of lateral recharge in mountainous areas on the quantity and quality of groundwater in the alluvial-pluvial fan cannot be ignored. Therefore, it is of great significance to achieve comprehensive treatment of groundwater sources and reduce the concentration of groundwater pollutants in mountainous areas for non-point source pollution management and groundwater nitrate pollution prevention in the downstream plain area.