Spatial-temporal distribution, genesis and environmental impact of the shallow groundwater pH values in the Fangchenggang, Guangxi
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摘要:研究目的
查明防城港地区浅层偏酸性地下水时空分布、成因及环境影响。
研究方法于2013—2015年间进行了浅层地下水pH值现场测试，枯水期214组，丰水期168组；讨论了其时空分布特征、成因和生态影响。
研究结果结果表明，丰水期和枯水期偏酸性（pH < 6.5）地下水样分别占79.3%和64.3%，pH值总体上丰水期低于枯水期。地下水pH值在低山区（均在5≤pH＜5.5范围，平均值5.18，n=4）＜丘陵区（大多数5≤pH＜6.5，平均值5.18，n=202）＜平原区（大多数6.5≤pH＜8.5，平均值6.77，n=8）。
结论偏酸性地下水成因主要与偏酸性大气降水有关，其次与酸性的包气带介质及硫化物矿物的氧化有关。偏酸性大气降水入渗补给是丰水期pH值低于枯水期的主要原因。相比较而言，潮汐作用使得江平地区地下水pH值升高。偏酸性地下水影响饮水安全（研究区枯水期仅20.6%的样品pH值符合生活饮用水标准），促进某些有害组分的释放，腐蚀地下管网和建筑桩基等设施。偏酸性地下水向地表水排泄亦可对地表水环境和地表生态系统产生影响。本研究有助于研究区水资源管理。
创新点：本研究利用地质调查项目数据优势，系统阐述了防城港地区浅层偏酸性地下水时空分布特征；偏酸性地下水的成因与偏酸性大气降水、酸性包气带介质和硫化物矿物氧化有关；偏酸性地下水对人类健康、地下设施和生态环境具有潜在影响。
Abstract:This paper is the result of hydrogeological survey engineering.
ObjectiveThe purpose of this study is to uncover the spatial-temporal distribution, genesis, and environmental impacts of shallow acidic groundwater in the Fangchenggang area.
MethodsIn-situ pH values of shallow groundwater were determined in the study area during 2013 to 2015, including 214 samples in the dry season and 168 samples in the wet season. Spatial and temporal distribution, genesis, and ecological influences of the pH values were discussed.
ResultsResults show that the acidic groundwater samples (pH < 6.5) account for 79.3% and 64.3%, respectively, in the wet and dry seasons. In general, pH values in the wet season are lower than in the dry season. Values are generally lower in the low mountain areas (pH 5.0-5.5, mean 5.18, n=4) relative to the hillys (pH mostly 5.0- 6.5, mean 5.97, n=202), and the plain areas (pH typically 6.5- 8.5, mean 6.77, n=8).
ConclusionsOccurrence of acidic shallow groundwater can be contributed primarily to the acid rain, and partly to the acidic unsaturated zone, and oxidation of sulfide minerals. Accordingly, seasonal changes of acidic rain should principally responsible for the spatial variations of groundwater pHs. By contrast, however, tides could increase groundwater pH values in the Jiangping area. Acidic groundwater threatens safe drinking water supply, because only 20.6% samples in the dry season having pHs within the drinking water standard thresholds. Also, it may promote release of some harmful elements from aquifer sediments, and erode underground pipe networks and building pile foundations. In addition, due to discharge, acidic groundwater could influence surface water environment and surface ecosystems. This study contributes to water resources management in the study area.
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1. 研究目的(Objective)
通过实施2020年四川省自然资源厅省政府性投资地质勘查项目，对沐川地区上二叠统宣威组底部泥岩中赋存的稀有、稀土、稀散元素进行调查评价，了解“三稀”元素含量与找矿前景，希望在“三稀”找矿方面取得突破性进展，为乌蒙山贫困地区脱贫提供技术服务和支撑产业规划、扶贫开发提供资源保障。
2. 研究方法(Method)
主要采用了1∶10000地质填图、1∶500矿化带剖面测量、探槽工程、钻探工程、采样与测试分析等方法，对上二叠统宣威组底部的富镓泥岩进行了初步研究。
3. 研究结果(Results)
沐川地区位于扬子陆块西缘(图 1a)，峨眉山大火成岩省中带(图 1b)。研究区位于五指山背斜核部，其核部地层为上二叠统峨眉山玄武岩(P3e)，两翼向两侧依次出露上二叠统宣威组(P3x)、三叠系(T)、侏罗系(J)等(图 1c)。镓矿层产出于峨眉山玄武岩组(P3e)顶部、宣威组(P3x)底部的紫红色铁质泥岩、灰白色铝质泥岩、浅灰绿色泥岩、灰色泥岩、浅灰绿色泥岩及深灰色炭质泥岩中，呈层状分布，层位稳定(图 1d)；找矿标志：峨眉山玄武岩组与宣威组(P3x)的平行不整合界线之上，具“成矿界面”特征，颜色上有明显的紫红色、灰白色、灰色及深灰色等，特别是具有特征的紫红色，宏观上易识别；根据地球化学图解判别显示，其成矿物质来源有可能来源于峨眉山玄武岩及峨眉山地幔柱演化末期喷发的火山灰。
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图 1 沐川地区构造位置图(a)，峨眉山玄武岩分布简图(b)，区域地质简图(c)及镓矿层产出层位示意图(d)Figure 1. Structural location map of Muchuan area(a), Simplified geological map showing distribution of the Emeishan basalts(b), Regional geological map of Muchuan area(c) and Schematic diagram of gallium ore occurrence horizon(d)采集钻孔岩心1/2切分样品，送样至自然资源部成都矿产资源监督检测中心采用电感耦合等离子体质谱仪(ICP-MS)进行测试分析，结果显示ZK01钻孔镓(Ga)平均品位104 μg/g，矿层厚度6.27 m；ZK02钻孔镓(Ga)平均品位68.1 μg/g，矿层厚度8.67 m；ZK03钻孔镓(Ga)平均品位55.3 μg/g，矿层厚度13.87 m；三个工程的镓(Ga)平均品位为75.8 μg/g，平均厚度9.60 m。可以看出，沐川地区宣威组底部的泥岩中镓(Ga)品位较高，厚度较大，达到了中国现行的Ga矿资源工业指标要求(30 μg/g)。
4. 结论(Conclusion)
经初步估算沐川地区镓(Ga)资源量可达数万吨到数十万吨，有望找到超大型镓矿床(＞2000 t) 的潜力。沐川地区峨眉山玄武岩分布面积较广，为镓的富集成矿提供了丰富的物源。经地质填图及工程取样显示，宣威组底部富镓泥岩层厚度大，分布面积广泛，镓元素含量较高，具有巨大的找矿潜力。研究宣威组底部“三稀”元素成矿特征有助于中国在战略性关键矿产找矿方面取得重大突破，对地方国民经济发展具有重要意义。
5. 基金项目(Fund support)
本文为四川省自然资源厅2020年省政府性投资地质勘查项目(DZ20 2002)和四川省自然资源厅科技项目(kj-2022-6)资助成果。
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图 1 防城港地区地貌分区及浅层地下水采样点分布图
Figure 1. Geomorphic zoning and Location of the sampling points in shallow groundwater of Fangchenggang area
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图 3 防城港地区浅层地下水枯水期pH值分布特征图
Figure 3. Distribution characteristics of the pH in shallow groundwater of Fangchenggang area
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图 4 浅层地下水pH值与主要水化学参数及ANC（酸中和能力）的关系
Figure 4. Relationship between pH and ion concentrations (TDS, Na+, Ca2+, Mg2+, Cl-, Na+, SO42-, HCO3-, total Fe), and ANC of shallow
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图 6 研究区地表出露地层中的褐铁矿化
a—滩营乡恒尚村二叠系砾岩中褐铁矿; b—防城区志留系砂岩中褐铁矿化
Figure 6. Limonitizations in the strata of the study area
a-Limonitization in Permian conglomerate in Hengshang Village, Yingtan Township; b-Limonitization in Silurian sandstone in Fangcheng District
表 2 丰、枯水期地下水pH值对比统计
Table 2 Comparison of groundwater pH values in wet season and dry season
下载: 导出CSV
表 4 研究区地下水主要离子及pH值相关性
Table 4 Correlation coefficients of ionic concentration and pH of groundwaterin study area
下载: 导出CSV
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