南水北调中线水源林水质年际变化及趋势--以秦岭火地塘天然林为例

Annual Changes and Trends in the Water Quality of the Forest Water Supply Region for the Middle Line of the South-to-North Water Transfer Project——A Case Study from the Huoditang Natural Forest Area

在南水北调中线水源地天然林分布区——秦岭南坡中山地带,选择火地塘林区2个小流域及2个支沟集水区,对8年间径流水化学成分年际变化及其影响因素进行分析。结果表明:火地塘林区径流pH值年际变化总体上呈降低趋势,NO3-、NH4+、Ca、Mg及Cd浓度呈上升趋势,主要与当地汽车尾气排放等交通污染有关;Pb浓度上升为土壤吸附Pb解吸或含Pb矿物的溶解所致;K、PO43-、Na、Mn及Zn浓度呈降低趋势缘于森林植被的逐渐恢复。据此对南水北调中线水源地天然林水质发展趋势进行初步预测,结果表明:中线水源地天然林径流pH值将呈降低趋势,NO3-、NH4+、Ca及Mg浓度会上升,但均不会对水质造成较大的不利影响;K、PO43-、Na、Mn、Zn浓度亦将呈降低趋势,其中PO34-浓度降低有利于缓解丹江口水库水质富营养化的威胁;Cd、Pb浓度可能上升,应引起重视。

This study was conducted in the Huoditang natural forest area, which is a part of the forest water supply region for the Middle Line of the South-to-North Water Transfer Project. The area is located at a medium altitude across the southern-slope of the Qinling Mountain range. Stream water samples were collected at the outlets of two small watersheds and two tributaries in the area over an eight year period. Annual changes in stream water chemistry were analyzed and possible causes for these changes were proposed. The results showed that the pH value of the stream water in the Huoditang natural forest area tended to decrease during the sampling period. In contrast, NO3^- , NH4^＋ , Ca, Mg, Cd and Pb concentrations tended to increase. The changes in the pH value and NO3^- , Ca, Mg, Cd concentrations could mainly be attributed to pollution from local vehicular traffic. The concentration of Pb in stream water increased during this period. We attribute this to an increase in the release of Pb from soil and minerals as the pH value of rainwater declined. The K, PO4^3- , Na, Mn and Zn concentrations in stream water tended to decrease during the observation period. These changes are probably related to the revegetation of the forest. According to the results of this case study, we predict that the stream water pH value in the forest water supply region for the Middle Line of the South-to-North Water Transfer Project will tend to decrease while the concentration of NO3^- , Ca, and Mg will tend to increase. The changes in these factors will not be large enough to have a significant adverse impact on water quality however. At the same time, K, PO4^3- , Na, Mn and Zn concentrations will tend to decrease. That decrease in PO4^3- concentrations will help to reduce the probability of water eutrophication in the Danjiangkou Reservoir, the main water storage area for the the Middle Line. The Cd, Pb concentration in stream water will tend to increase, and this problem must be addressed, even if the change is very small.