运城盆地地下水同位素年龄特征及其演化过程和可持续利用

Distribution of isotopic ages of groundwater and its evolution and sustainable utilization in the Yuncheng Basin

通过分析运城盆地地下水的碳同位素组成,结合水化学特征,揭示了盆地深层承压地下水的补给期为22～3 ka BP(现代碳百分比(a14C)6～38 pmC)。浅层地下水(71～89 pmC)由现代水或现代水和老水混合组成。深层地下水氢氧同位素组成特征(δ18O～-10‰;δ2H～-70‰)表明地下老水在气候较冷的环境下受到补给,而浅层地下水的氢氧同位素组成(δ18O～-8‰;δ2H～-51‰)特征与现代西安降水组成相似。浅层地下水NO3-平均含量(31mg/L)比深层地下水(1.8 mg/L)高,硝酸盐的δ15N-δ18ONO3组成(0‰～5‰)揭示了硝酸盐的主要来源为综合肥料。此外,浅层地下水的TDS由于蒸散发、矿物溶解,可达8.5 g/L(平均2.0 g/L),深层地下老水TDS可达1.8 g/L(平均1.1g/L)水质相对较好。研究区目前主要开采深层地下水,受断裂带影响,浅层地下水已经侵入中深层地下水并与之发生混合,严重影响了中深层地下水的水质。如果发生大规模的浅层地下水与中深层地下水混合,会造成中深层地下老水的NO3-、TDS等含量越来越高。

Through analyses of radiocarbon and other isotopes/geochemical indicators of groundwater in the Yuncheng Basin, it is found that groundwater in the middle and deep layers of the basin was recharged approximately 22 to 3 k.a. before present （radiocarbon activities between 6 and 38 percent modern carbon）. The shallow groundwater （71 to 89 percent modern carbon） comprises modern water （e. g. recharged post- 1950s）, or has a significant mixing component of modern water. Stable isotope compositions of deep groundwater （δ^18O, -10‰; δD, -70‰） indicate that the paleo-waters in deep layers was recharged under a cooler climate than the present, while the values of shallow groundwater （δ^18O , -8‰; δD, -51‰） are similar to the average stable isotope compositions of modern rainwater from the East Asian monsoon. The average nitrate concentration of the shallow groundwater （31 mg/L） is much higher than in the deep groundwater （ 1.8 mg/L） ; and stable isotope values （δ^15N -δ^18 ONO3） indicate that nitrate is largely derived from synthetic fertilizers. In addition, the TDS of the shallow groundwater reaches much higher values （up to 8.5 g/L） , compared to that of the deep groundwater （average TDS of 1. 1 g/L） , due to evapotranspiration and mineral dissolution. In the study area, the deep groundwater （which has a relatively good water quality） is exploited intensively. High levels of drawdown coupled with the influence of fractures （that provide preferential migration pathways） have resulted in leakage of the shallow, poor water quality groundwater into the intermediate and deep layers. This has begun to affect the quality of the deeper groundwater. If it occurs on a large scale, the water quality in the intermediate and deep layers is likely to deteriorate, with increasing concentrations of nitrate and total dissolved solids.