岩溶地下水补给的地表河流溶解无机碳昼夜变化与钙沉降

Diel Changes of Dissolved Inorganic Carbon and Calcite Precipitation in a Typical Karst Spring-fed Stream

溶解无机碳昼夜动态变化对河流水体碳通量的估算有重要影响。本文选择由地下水补给且富含水生植物的典型河流,开展高分辨率水文地球化学监测和高频率水样取样工作,分析了水化学的昼夜动态变化特征,钙与无机碳昼夜循环产生的生物地球化学控制机理,估算了钙与无机碳昼夜通量。结果表明,白天pH、SIC上升,产生钙沉降和水体无机碳(主要为HCO-3)含量的下降,夜间得到地下水的补给,Ca2+和HCO-3浓度回升。监测期间,官村地下河出口地表河流钙与溶解有机碳的流失或沉降量分别为69.04kg/d和168.68kg/d,即51.14g/(m·d)和124.95g/(m·d),分别占输入量的6.2%和4.7%。受水生植物光合作用和钙化作用控制,沿流程发生无机碳向有机碳转化,是真正意义上的自然碳汇。

Study on diel variation of dissolved inorganic carbon can help to estimate carbon flux in river water more precisely. Five-day monitoring with high resolution data logger and high frequency sampling with 1-hour interval were conducted in a typical karst spring-fed stream with abundant aquatic vegetation. Daily changes of hydrochemistry, especially diel cycling of calcium and dissolved inorganic carbon （DIC） were discussed and the influence of biogeochemical processes on diel cycling was analyzed. The diel fluxes of calcium and inorganic carbon were also estimated. Results show that the changes in aqueous chemistry are closely associated with biogeochemical processes and featured as diel cycling. During the daytime, pH value of water and the saturation index of calcite increase, thereby causing calcite to precipitate and removing Ca2＋ and HCO;- from water. During the nighttime, concentrations of Ca2＋ and DIC gradually increase due to the input of Ca and HCOa by groundwater exchange. The daily loss of Ca2＋ and DIC are estimated to be about 69.04 kg/d and 168. 68 kg/d, respectively which represent about 51.14 g/（d.m） and 124.95 g/（d.m）, respectively along the 1350m of the Guancun River. The study clearly shows that photosynthesis and calcite precipitation have strong influence on the chemistry of the spring-fed river. Diel DIC cycling downstream and their changes along the stream flow indicate that the river is losing inorganic carbon along its flowpath, and converting it into organic carbon, thus acts as an important DIC sink in small productive streams.