北京市再生水灌溉对地下水的重金属污染风险

Assessing the effect of reclaimed water irrigation on groundwater pollution of heavy metals in Beijing

通过野外取样分析再生水灌溉、长期污水灌溉条件下土壤剖面和地下水中重金属含量,预测再生水灌溉对地下水的重金属污染风险。结果表明:污染严重的Cr、Cu和Zn等3种元素主要在土壤表层0～20cm累积,0～180cm土层呈先降低后升高,之后再降低的变化趋势。污染较轻的As、Ni和Pb在0～60cm土层含量差异不大,0～180cm土层大致呈先升高后降低的变化趋势。所有重金属在土壤剖面90cm左右的土层中均存在高值区,该层含量高于上、下土层,高值区的分布位置与北京剖面土壤发生层的分布特征一致,与成土母质自然发育有关,并非表层土壤重金属向下淋溶沉积而成。综合土壤剖面重金属分布特征和调查区地下水重金属浓度情况,重金属向下层土壤迁移的趋势很小,即使凉水河灌区污染严重的Cr、Cu、Zn,主要在土壤表层累积,并未导致地下水重金属污染。在当前的水质条件下,农田再生水灌溉输入的重金属量低于大气沉降和有机肥施用,再生水灌溉导致地下水的污染风险小于大气沉降和有机肥施用。因此相对于大气沉降、有机肥施用等输入途径,再生水灌溉导致地下水重金属污染的可能性不大。

It is difficult to assess the risk of heavy metals pollution in groundwater resulting from soil pollution caused by reclaimed water irrigation. The primary reason is the shortage of area with long-term reclaimed water irrigation. The heavy metals concentrations in reclaimed water were generally low and thus they can hardly be accumulated in topsoil with a short history of re- claimed water irrigation. This study investigated the distribution of heavy metals in the soil profile sampled from several irrigation areas with a long history of reclaimed water irrigation, aiming to assess the risk of heavy metals pollution in groundwater caused by reclaimed water irrigation. The result showed that heavy metals were accumulated mainly in topsoil （0-20 cm）. With an in- crease in soil depth （0-180 cm）, the concentrations of Cr, Cu and Zn in the reclaimed water irrigated soil first decreased and then increased, and then decreased again. The concentrations of other heavy metals first increased and then decreased with the increase in soil depth （0-180 cm）. A distinct layer in the soil profile with high heavy metals concentrations were found not only in the polluted areas irrigated with wastewater but also in the areas irrigated with clean ground- water. This pattern was similar to the distribution of soil genetic horizon, implying that it is not the result of migration of heavy metals from topsoil. According to these results, the downward movement of heavy metals was not evident, indicating that the early wastewater irrigation did not result in heavy metals pollution in groundwater. The total input of heavy metals from reclaimed water irrigation was lower than the input by atmospheric deposition and organic fertilizer application, which demonstrates that the pollution risk of heavy metals in groundwater is low in the process of reclaimed water irrigation.