常规污染场地是多年积累的污染造成，污染源相对容易确定，但天津港爆炸污染物的污染规律不易掌握，修复更加困难。图片来源：weibo
天津港爆炸事发地邻近住宅区，土壤污染的治理成为社会各界尤其是附近居民关注的核心问题之一。
8月12日，天津港一个堆放危险品的仓库发生火灾。由于不清楚仓库内存放的危险品种类，消防员采取了不恰当的灭火方式，造成爆炸，近170人遇难，周边居民事后才知道自己与危险品为邻。
据官方公布的数据，事故场地曾存包括硝酸铵、硝酸钾在内的氧化物1300吨左右，金属钠、金属镁等易燃物约500吨。
与常规土壤污染项目相比，天津爆炸事故的遗留场地复杂得多。在应急处置结束之后，后续土壤修复难度大。
中国环境科学院固体废物污染控制技术研究所所长王琪表示，常规污染场地是多年积累的污染造成，污染源相对容易确定，污染物迁移途径与范围可根据扩散模型测算出大致规律，而天津事故麻烦之处是，污染物的污染规律不易掌握。
比如，事故现场是多种危化品仓储库，遗留污染物不明，而且爆炸过程中不排除发生二次化学反应生成新污染物；污染扩散途径多，爆炸过后除了空气和水、土层等自然条件以外，还可以通过喷溅、挥发、沉积以及自然散落等途径造成环境污染。
在清华大学环境学院教授刘建国看来，突发环境事故在各界高度关注之下，现场应对处置的紧迫性更强，制定修复方案时不可能做到常规污染场地项目的长时间论证、比较。
爆炸前，现场存有700吨氰化钠，事故后，工作人员在核心区已清理收集近200吨,并将它们安全运出。也就是说，可能有500吨氰化钠被散播遗留在环境中。
氰化物是冶金、化工、制药、纤维制造等工业排放的主要污染物，中国有成熟的治理方法。通过处理受氰化物污染的土壤，可以使氰化物分解成低毒物或无毒物。因此，与重金属和有机物的污染土壤相比，氰化物的处理难度相对较低。
处理突发环境事故污染场地的原则，一般是按轻重缓急进行分期治理，国外也是如此。位于美国新泽西州的Chemical Control Corporation场地，曾长期作为危废储存、处理场地，接收过不同类型的化学品。这块场地在1980年发生火灾及爆炸事故，引发了土壤、水污染等严重后果。
该场地彻底治理，分三个步骤：首先，是历时一年的应急处置；之后，是全面调查和评估，经多轮论证，7年后，美国国家环保局才确定最终修复方案，选择了固化稳定化技术。最终，直到1993年12月修复工程竣工。
但竣工并不代表修复工程结束。按美国国家环保局规定，该场地每5年要进行一次评估，经三次评估来验证是否达到修复目标。因此，直到2009年，整个修复工作才彻底完成，最终的评估结果显示，该地块能满足对人体健康和环保的要求。
在天津港事故现场，污染土采取了转运、在别处存放的异位修复法。其实，在常规的污染场地项目中，更受业界推崇的是原位修复法，此法可以减少外运污土带来的环境风险。
自8月20日起，中铁十八局五公司在天津港海铁一号路，建设高1.2米的存放池，该池有防渗结构，能存放2.4万方污土。
环保部专家告诉记者，该存放池只是短期封存污土，最终还要原地或再转运出去修复。第一步要对现场含危废物质的土壤进行简单洗消处理，下一步就会转运到临时存放池中，应急处置结束后再进行彻底修复。
全面评估并不容易
大水坑内的水处理完后，坑内底泥该如何处理？这给环保专家出了一道难题。污染场地的治理对象通常还包括含高危、有害物质的渣和屑，以及附有大量有毒物质的底泥等。
2005年11月13日中石油吉林石化公司双苯厂爆炸，事故污染场地涉及约3公里长的地下排水涵管，其中沉积物的清除属于地下密闭空间作业，且其中积水较多，是修复工程的难点。
刘建国教授参与了该事故应急处置和场地调查修复。“当时一个有利条件是正处于冬天，零下20摄氏度的低温把管网中的底泥给冻住了。我们采取机械和人工结合的方法，一点点抠出来，再送到危废处置焚烧炉中处理掉。”他回忆，方案确定后，整个处置过程耗时四个多月才完成。针对不同污染物，上述爆炸后的善后处置采取了安全填埋、高温焚烧以及生化处理等技术结合应用。
污染场地修复技术虽有多种可供选用，但完成天津港事故后场地的风险评估报告并不容易。
自8月16日起，天津市环境监测站在事故区5公里内，布设73个土壤环境监测点。8月22日，官方公布，测出16个土样有氰化物，但均不超标，大致方位是事故点南侧和东部偏北。
多位受访土壤修复专家指出，5公里范围内布设73个土壤点位远远不够，无法全面、系统掌握场地污染实情。何况，深层土壤采样分析仍在进行中。
一位中科院土壤修复专家对记者说，按规范要求，这块场地应网格状布设上万个点位，考虑到场地类型单一、事故刚发生土壤污染迁移慢，至少也得布设上千个点位。“73个点位只能做到大概的摸底。”
现场工作人员的难处是，上述建议受现场客观条件限制很难实施，现场周边并非空地，而是建成的住宅区。一位到过现场的专家说：“如按采样规范，采样目标只能是土壤，但网格点位恰好可能是一栋房子，无法采集。”
由于现场工作量庞大，且要在统一调度下协助应急处置和监测决策，要得出一个评估结果并不容易。针对突发灾难善后，刘建国建议，长期治理千万不能留下隐患，应借此事故，建立一套突发环境事故善后处置长期监控、评估机制，实行分级管理、及时公布。如此，才能让民众放心。
原文刊发于《财经》杂志
高胜科，《财经》杂志记者，2013年中外对话“最佳环境报道奖”的最佳调查报道奖得主
贺涛，《财经》杂志记者
The clean-up following the August 12 Tianjin warehouse explosion is proving more complex than previous industrial accidents in China, mainly because of the hazardous chemicals involved and the disaster’s close proximity to residential areas. Various options are available to restore the site, with action taken at past accident sites helping to inform the work, but there is still widespread concern about the effectiveness of the current clean-up – not least from local residents.
Official reports say the site was storing about 1,300 tonnes of oxidizing agents, such as ammonium nitrate and potassium nitrate, and 500 tonnes of flammable materials, including metallic sodium and magnesium. But it is still unclear which materials have polluted the site and whether chemical reactions have led to secondary pollution, said Wang Qi, head of the Institute for Solid Waste Pollution Control in the Chinese Research Academy of Environmental Sciences.
In previous cases, pollution occurred over a long period of time, enabling easier identification of sources of contamination and the extent and geographical spread of pollution. In Tianjin, it is much more difficult to calculate the environmental impact of chemicals that were dispersed violently by the explosion into the air, soil and water.
Since August 16, the city has been monitoring soil at 73 points within a five kilometre radius of the explosion. On August 22, cyanide compounds were identified in 16 samples to the south and north-east of the site, although authorities said these were not at dangerous levels. Analysis of samples taken from deeper underground is continuing.
But some soil restoration experts say 73 monitoring points is far too small a number to provide a full picture of the pollution at Tianjin. One specialist, who is a member of the Chinese Academy of Sciences, says the normal number of monitoring points would be as many as 10,000. Even given the complex aftermath of the disaster, the short space of time since the blasts and the slow migration of soil pollution, there should be more than 1,000 monitoring points, Wang added. “Just 73 points is only enough to get a general idea,” he adds.
However, ramping up the number of monitoring points isn’t practical because the site is surrounded by residential buildings, rather than empty land. One expert who has visited the site says “you’re meant to be taking soil samples, but it’s likely the site you’re meant to be sampling will actually be a building.”
Liu Jianguo, a professor at Tsinghua University’s School of the Environment, says the high profile nature of the clean-up means there is no time for the lengthy planning process that usually takes place when cleaning up pollution.
Some 700 tonnes of sodium cyanide were stored at the site prior to the explosion. Since then 200 tonnes have been collected and removed – but this means 500 tonnes are still unaccounted for. Yet China has the experience to deal with cyanide pollution. Treatment of the affected soil can break the cyanide compounds down into safer or non-toxic chemicals. Because of this, cyanide is easier to deal with than pollution from heavy metals or organic materials.
Both within China and in other countries, a clean-up is usually determined according to the urgency and severity of the pollution.
The restoration following an accident in 1980 at a chemical storage site in New Jersey, which caused serious soil and water pollution, is cited as an example. A initial 13-year clean up was followed by rigorous checks by US authorities every five years, and only ceased when the site passed three assessments, the last of which was in 2009, when the site was finally deemed to have met environmental and public health requirements.
In Tianjin, the soil will be removed for treatment elsewhere. However, in the opinion of some experts, the preferred option would be to treat the soil on site, as this reduces environmental risks associated with soil transportation.
On August 20, China Railway engineers started building a storage pond, which is sealed to prevent seepage and can hold 24,000 cubic metres of polluted soil.
The first step is to rinse contaminated soil, then move it to the temporary storage. Once the emergency phase is over full restoration will be carried out.
Liu Jianguo warned that care must be taken with the long-term management of the site not to leave hidden hazards. There is an opportunity at Tianjin, he says, to create mechanisms for the long-term monitoring and assessment of polluted sites. Only when that is achieved will public concern be put at ease.
This story was published originally on the website of Caijing magazine
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