2. 中国科学院大学, 北京 100049;
3. 中国科学院南京地理与湖泊研究所, 南京 210008
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China
赣南地区现存900多座废弃的离子型稀土矿[1],近年来采用原地浸矿工艺开采的稀土矿约占总面积30%[2]。原地浸矿尾矿山易发生水土流失、土壤酸化,尾矿山周围水体氨氮严重超标,硝酸盐污染严重[3-4]。已有研究发现赣江流域干流硝态氮(
地表水
尾矿区水体中具有的高含量
本研究选择赣南一座采用原地浸矿工艺开采后的离子型稀土矿山作为研究区,其所处区域属于亚热带湿润季风气候,年均温为19.5℃,年均降水量为1 479.1 mm[19],降雨主要集中在4至6月[20]。矿山位于赣江上游,赣江流域多年平均年径流总量为283.3×108 m3 [21]。从地质构造来看,研究区地处环太平洋边缘,位于后加里东隆起区南部,在多期地质活动后呈棋盘格式构造形迹[22]。在地质历史时期中,活跃的构造岩浆活动创造了良好的离子型稀土矿形成条件[23]。
采样矿山位于江西省赣州市龙南县城东南方向约10 km(图 1),富含中重稀土[24]。龙南县一半以上矿山采用硫酸铵作为浸矿剂进行原地浸矿开采[2],较为典型。该矿山属于三南矿集区[22],主要岩体为黑云母钾长花岗岩[25],自然植被下的地带性土壤类型为湿润富铁土。矿山高程为352 m,坡度达37°,整体植被覆盖度高,以马尾松、杉木等为优势物种。矿山在山顶修筑有浸矿液池存放硫酸铵溶液,山坡上按0.5~2 m网格修建注液井(直达含矿层1 m左右),山底开挖集液沟。开采过程中硫酸铵溶液通过注液井直接注入矿体内部,沿土壤孔隙进行扩散,与黏土矿物表面吸附的稀土离子充分交换,形成的稀土母液沿基岩面进入集液沟[26]。该矿山自2015年开采完成后,无人为扰动,但保留部分原有采矿设施。采样矿山的地质、地貌和植被类型均与该区域的矿山一致,具有一定的代表性。
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图 1 研究区位置和采样点位置 Fig. 1 Study area and sampling sites |
本研究于2019年11月进行野外采样。在已开采矿山的坡顶、坡中及坡底分别选择一个点位(图 1),使用洛阳铲由表土采至基岩面。1 m深以上的土壤按发生层取样,1 m深以下间隔50 cm采集一份土壤样品,当50 cm范围内土壤颜色、结构、紧实度等存在明显差异时,需要分段取样。坡顶、坡中及坡底处的土体深度分别为9.7 m、8.0 m及5.5 m。坡顶、坡中及坡底表层(腐殖质层)厚度为10 cm、8 cm及10 cm[12],腐殖层以下至半风化体间的土壤层分别为10~300 cm、8~400 cm、10~250 cm,半风化层为300~970 cm、400~800 cm及250~550cm,稀土开采过程中直接向该土层注入浸矿剂。取样过程中,现场直接将小部分土壤样品低温保存,用以测定土壤
土壤
尾矿山整体土壤
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图 2 研究区土壤硝态氮含量统计图 Fig. 2 Statistical diagram of soil nitrate nitrogen content in the study area |
从土体内部垂直方向来看,稀土尾矿山不同土层间土壤
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图 3 土壤硝态氮含量剖面分布图 Fig. 3 Distribution of nitrate nitrogen content in the soil profiles |
整体来看,浸矿区坡顶到坡底的表层土壤
坡顶处土壤
研究区土壤
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注:* P<0. 05,** P<0. 01(双尾检验);所有参数样本量n =56。 Note: * P < 0. 05, ** P < 0. 01(Two-tailed test)Number of samples: 56
图 4 硝态氮与其他土壤理化特征之间的相关性 Fig. 4 Correlation between |
矿山坡顶、坡中及坡底
尾矿山土壤
土壤含水量影响硝化及反硝化过程[32]。但本研究发现土壤
渗流是土壤
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图 5 土壤硝态氮与铵态氮含量随土体的变化趋势 Fig. 5 The variation trend of soil nitrate nitrogen and ammonium nitrogen in the soil profile |
土壤中残留的
土壤中
该矿山于2015年开采完成,矿山开采结束4年后尾矿山土壤仍富集大量
离子型稀土原地浸矿尾矿山土壤
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