西南喀斯特小流域地表、地下河流细粒泥沙来源的137Cs和磁化率双指纹示踪研究

在黔中喀斯特高原农林复合生态系统峰丛谷地小流域,利用¹³⁷Cs和磁化率双指纹因子对碳酸盐岩表层土壤(坡地和坝地)、深层土壤(沟道和洞穴/裂隙)以及碎屑岩夹层岩屑3类来源开展指纹特征分析,通过多元混合模型对流域暴雨过程侵蚀悬移质泥沙进行指纹复合示踪,并计算小流域地表及地下出口输出泥沙的主要来源及相对贡献率。结果表明:¹³⁷Cs和磁化率在碳酸盐岩表层土壤、深层土壤以及碎屑岩夹层岩屑3类来源存在显著差异,流域内表层土壤、深层土壤和碎屑岩夹层岩屑的¹³⁷Cs和磁化率平均含量分别为3.39 Bq/kg和310.07×10^-8 m^3/kg、0.50 Bq/kg和180.69×10^-8 m^3/kg、0 Bq/kg和7.02×10^-8 m^3/kg。表层土壤、深层土壤和碎屑岩夹层岩屑对流域地表出口泥沙的相对贡献分别为16.2%,4.3%和79.5%,拟合优度为99.97%;表层土壤、深层土壤和碎屑岩夹层岩屑对流域地下出口泥沙的相对贡献分别为<0.1%,37.9%和62.1%,拟合优度为83.80%。喀斯特流域泥沙来源区别于其他碎屑岩流域具有的特点为:(1)碳酸盐岩风化表层土壤来源少,对河流泥沙贡献小,主要贡献于地表河流。(2)无论流域地表出口还是地下出口,河流泥沙来源主要为流域中碳酸盐岩所夹的少量(地面物质组成<10%)的碎屑岩夹层岩屑。(3)深层土壤略有贡献,地下河流贡献比例高于地表河流。另外,¹³⁷Cs和磁化率可作为双指纹示踪物较好地示踪喀斯特小流域地表、地下河流泥沙来源和确定相对贡献率。

Tracing Fine Sediment Sources in the Surface and Subsurface Rivers of a Karst Watershed Using Compound Fingerprinting with 137Cs and Magnetic Susceptibility in Southwest China

In a small watershed with a typical agro-forest complex ecosystem and peak-cluster valley landform of the central Guizhou,¹³⁷Cs and magnetic susceptibility were used as compound fingerprinting factors to analyze the fingerprint characteristics of carbonate rock surface soil (from hill slopes and depressions), deep soil (from river bank or fissures and grikes), and clastic rock pieces. A multivariate mixing model was used to trace the probable sources of suspended sediment in discharge generated during rainstorm events, and calculate the main source and the relative contribution of the sediment from surface and underground outlets of small watershed. The results showed that the concentration of ¹³⁷Cs and magnetic susceptibility were significantly different in the three types of sources. The average ¹³⁷Cs and magnetic susceptibility values of surface soil, deep soil and clastic rock pieces were 3.39 Bq/kg and 310.07×10^-8 m³/kg, 0.50 Bq/kg and 180.69×10^-8 m³/kg, 0 Bq/kg and 7.90×10^-8 m³/kg, respectively. The relative contributions of surface soil, deep soil and clastic rock pieces to the sediments of the surface outlet were 16.2%, 4.3% and 79.5%, respectively, and the goodness of model fit was 99.97%. The relative contributions of surface soil, deep soil and clastic rock pieces to the sediments of the underground outlet were less than 0.1%, 37.9% and 62.1%, respectively, and the goodness of model fit was 83.80%. The sediment sources in this karst watershed differed from those in typical non-karst terrain, the specific characteristics were:(1) The sediment from carbonate surface soil was few, and carbonate surface soil contributed mainly to the surface water system, while contributed little to river sediment; (2) Clastic rock pieces were the primary source of both surface and underground river sediments; (3) Deep soil contributed slightly, and it contributed more to underground rivers than to surface rivers. In addition,¹³⁷Cs and magnetic susceptibility could be used as compound fingerprinting tracers to trace the sediment source of the surface and subsurface rivers in this karst area, and also could be applied to determine the relative contribution rate of sediment sources.