河流看守員手冊－流域所面臨的問題 (二)

Threats to Catchments

　　濕地的破壞：濕地是極其有價值的生態系統，能保護流域免受污染和洪水襲擊，孕育許多物種，並能補充地下水供給。在美國的馬里蘭大學生態經濟學學會做了一項年度評估，全球濕地的總價值將近有5兆美元，包括: 防洪抑澇，濕地中能處理污染物質的植群，野生動物棲息地，還有漁場及遊憩價值。美國伊利諾斯州水資源調查局的一項研究發現：濕地的面積每擴大1% 能使得洪水時的洪峰下降4 ％。

　　全世界流域中的濕地都遭受了嚴重的污染或被排水填平。喪失濕地的一個主要原因是導於以排水填平法來建造新都市或農業用地。很多動物（包括遷徙性動物）都依賴於濕地而生存，並因它們遭受破壞而受苦。挖掘河道和攔阻河流也使得濕地裡水量的減少而枯竭。舉例而言，農業和城市發展的水資源調節與排放系統導致了尼日，查得和坦尚尼亞等地區的濕地面積減少了50%以上。南部非洲的濕地也深受其害。包括南非的Rietspruit濕地（受Vaal河附近SASOL開採造成的污染的影響）；莫三比克的尚比西河三角洲（受到因Cahora Bassa和Kariba兩水壩的攔截造成水流減少而枯竭）；Okavango三角洲（因那米比亞政府抽取Okavango河水為供水計劃的危害），坦尚尼亞的Rufiji三角洲（受到一個大型蝦農場的威脅，產生污染物，那將會改變該濕地的水文環境，破壞紅樹林。）

　　過度配給：非洲的大部分河流都遭受著水資源"過度配給"的問題，那就意味著許多河水因各種目的而被運送轉移，那是河流的生態系統無法支撐的。在大多數國家的罪魁禍首是農業。據1995年世界銀行的報告，農作物灌溉用去的水占世界貧困國家用水總量的90%，而且，大約一半並沒有用在植物上，而是被浪費的。在南非，農業灌溉用水占水總供給量的50%~70%，很多河流，包括Orange, Vaal, Oliphants和其他一些河流，都因此呈現出嚴重的後果。地下水也受到了過度使用的影響，很多地方的地下水被抽取的速度都要遠快於其自身填補的速度。當海岸地區的地下水被過分抽取時，鹽水就會進入，並污染整個蓄水層。

　　因將尼羅河的水引出，且由於河中淤泥被大壩和攔沙壩阻擋，使得肥沃的尼羅河三角洲的面積減少。曾經能在尼羅河三角洲發現的47種商業魚種中，有30種已經絕種或瀕臨絕種。曾經養活了一百萬以上人口的三角洲漁業幾乎消失殆盡。在過去的30年裏，因大量的水資源轉移和周期性乾旱，使Sahel地區的查德湖已從原來的25,000平方公里縮小到2000平方公里。該湖曾經豐富的漁業已經完全癱瘓。

　　農業行為：除了過多使用水資源以外，對於流域的健康發展還有另一方面的巨大影響。沒有做好水土保持的農場會影響泥土的滲透率， 從而影響到地下水的填充。不良的農業行為也會導致土壤被侵蝕而污染水流。種植日期，作物類型，耕作系統（從淺耕到深耕），殺蟲劑和肥料的使用量，耕作方向和作物輪耕等農業行為都會對水資源造成影響。

　　污染在發展中國家，被各種污染物污染的水造成了極高的致病率。水受到人類產生的廢物的污染（這在衛生系統極不完善的窮人聚居地是個大問題），工業污染，農業和城市廢水污染，空氣污染，和有毒物質外洩的污染。有毒物質通過以下兩種途徑進入水源：第一是所謂的"單點式污染源"，意思是已被確認的污染源（如工廠的污水排水管），二是"非單點式污染源"(或譯為多源式污染源)，意思是指某些難以確定污染來源，如的農地牧場和城市街道。在世界上一些國家（如美國）最大的"單點式污染源"大多已被清除，非點源污染現在成了最大且最危險的河流和溪流污染來源。南部非洲流域中被嚴重污染的例子中包括了南非的Oliphants湖 和辛巴威的Chivero湖。

　　在非洲，農藥和控制攜帶病菌的蚊子的殺蟲劑是一個嚴重水污染威脅。有許多替代方案或改變農耕系統能至少減少殺蟲劑和人工化肥的使用仍然能維持ㄧ定的生產率。至於控制蚊子，目前尚無很好的方法。殺蟲劑的使用會使昆蟲產生抗體，從而令問題變得更加嚴重。然而，在某些地方，人們使用更多的農藥與殺蟲劑，有時候他們甚至使用已被在世界其他地方禁用的危險化學物品。例如，1998年3月，辛巴威的健康機構公佈了一項計劃，他們拿禁用的DDT殺蟲劑來消滅瘧疾。噴撒10,000噸有毒殺蟲劑的計劃引起了廣泛的爭論。這是因?30年以前DDT因其在環境中存在的持久性，和能透過食物鏈累積在人體內，被禁用於當地的農業。辛巴威大學和2000年環境保護小組的生物學家發出警告，在食物鏈中增加DDT將會形成長期的健康危機，那將比瘧疾帶來得危害更加嚴重。

　　在貧困的社會裡，缺少充分的汙水處理是水污染的泉源，另外，"水資源分佈不均"也增加了水源疾病的問題。因為沒有足夠供水，人們就不會經常洗手，或保持餐具的清潔，從而廣泛傳播了這些病菌。

　　採礦業是該地區河流和集水區的主要污染源。採礦污染的主要類型包括採礦中產生的重金屬污染，採礦加工過程中的化學物質（如氰化物）污染，和來自於礦物廢渣的酸液排放污染。另一個污染源是儲存礦物廢水水壩的外洩（稱為終點壩），從而將大量的污染礦物廢水排入河流。這種外洩能夠立即殺害下游幾千公里河道裏的所有生物。另外，遺棄的礦井也會繼續造成污染。一個例子：Oliphants河的支流流經20世紀50年代遺棄的，到現在尚未恢復的煤礦，水利部認為長期性解決這一問題需耗資南非幣200百萬。新興採礦方式已被證實能夠大量減少這些污染物質，但是，政府必須首先制定出嚴格的採礦標準，並將之付諸實施。

　　採礦業也是用水大戶。採礦業用水處理礦物品，同時，通過礦井脫水使得地下工作變得容易，礦井脫水能使水位降低到使井水和泉水耗竭的程度。採礦業對於流域的另一個影響是沈澱物污染。雖然不是有毒物質，但是採礦場的岩石和泥土的侵蝕物進入水流後對於河流和溪流環境造成了嚴重的危害，使得沿岸的植物窒息，並使水生物喪生。沈澱物還會使下游泛濫。

版權歸屬　國際河網IRN　環境信託基金會(金霞 譯，李堅、陳維立審校)

中英對照全文：http://news.ngo.org.tw/issue/water/issue-water-irn00112001.htm

Wetlands have been badly polluted or dewatered in catchments all around the world. One major cause of wetlands loss has been draining and filling of wetlands to create new urban or agricultural land. Many species of animal life (including migrating species) depend upon wetlands, and have suffered from their destruction. Channelizing and damming rivers can also decrease water available to wetlands, thus dessicating them. For example, water system regulation and drainage for agriculture and urban development have been the major causes of the loss of over 50 percent of the wetlands in Niger, Chad and Tanzania. Recent examples of wetlands in trouble in Southern Africa include the Rietspruit wetlands in South Africa (threatened by pollution from the proposed SASOL strip mine, near the Vaal River); the Zambezi delta in Mozambique (damaged by reduced flows because of Cahora Bassa and Kariba dams), the Okavango Delta (which could suffer from reduced flows under a plan by the Namibian government to tap the Okavango River for water supply), and the Rufiji Delta in Tanzania (threatened by a huge prawn farm that would alter its hydrology, produce pollutants and destroy mangrove forests).

Over-allocation: Many of Africa's rivers suffer from "over- allocation" of their waters, meaning that so much water is being diverted for various purposes that the river ecosystem cannot be sustained. The biggest culprit in most countries is agriculture. Crop irrigation consumes 90 percent of all water used in the world's poor countries, according to a 1995 World Bank report, and nearly half of that is wasted and does not get to the plants. In South Africa, irrigated agriculture uses 50-70 percent of the water supply, and many rivers are in dire shape because of it, including the Orange, Vaal, Oliphants and others. Groundwater, too, is affected by over-use, and in many areas groundwater is being withdrawn far faster than it can be recharged. When groundwater is over-tapped near coastal areas, saltwater can intrude and may eventually contaminate the entire aquifer.
Diverting water from the Nile River, along with build-up of sediments trapped behind dams and barrages, has caused the fertile Nile delta to shrink. Of 47 commercial species of fish once found in the Nile delta, about 30 have become extinct or virtually extinct. Delta fisheries that once supported over a million people have been wiped out. And Lake Chad, in the Sahel region, has shrunk from 25,000 square kilometres to just 2,000 sq.km. in the last three decades from massive diversions of water for irrigation and periodic droughts. The lake's once rich fisheries have entirely collapsed.

Agricultural practices can have huge impacts on the health of a catchment in other ways besides using too much water. Farms that do not practice good soil conservation can affect the soil's infiltration rates, and consequently groundwater recharge. Poor farm practices can also result in soil erosion, which pollutes waterways. Date of planting, type of crop, tillage system (from zero tillage to deep plough), amount of pesticides and fertilizers, plough direction and crop rotations are some of the agricultural practices which can affect water resources.

Pollution: Contaminated water of all kinds causes a huge proportion of disease in the developing world. Water can become contaminated by human waste (a major problem where the poor have no access to sanitary systems), industrial pollution, agricultural and urban runoff, air pollution and toxic spills. There are two ways that pollutants enter the water system: from what is known as a "point source", which means the source of the contaminant is easily identified (like a factory pipe), or from a "non-point source" which refers to polluted runoff from sources that are difficult to pinpoint - such as farm- land and urban streets. In some parts of the world where the biggest "point sources" have been cleaned up (such as the US), nonpoint source pollution is now the biggest, potentially most dangerous source of pollution for rivers and streams. Examples of Southern African catchments which are particularly polluted include South Africa's Oliphants and Zimbabwe's Lake Chivero.

Pesticide use by agriculture and to control disease-bearing mosquitoes is a serious water-pollution threat in Africa. For agricultural use, there are numerous alternatives and farm systems that can replace or at least minimize pesticides and artificial fertilizers without harming production. As for mosquito control, there are as yet no perfect solutions. Pesticide use can even make the problem worse, by leading to resistance in insects. Still, pesticide use is increasing in some places, sometimes even involving dangerous chemicals that are banned elsewhere in the world. For example, in March 1998, health authorities in Zimbabwe announced a plan to fight malaria with the banned pesticide DDT The plan to spray 10,000 tons of the poisonous pesticide was very controversial, especially since DDT was banned for local agriculture three decades ago because of its persistence in the environment and its ability to move through the food chain. Biologists at the Zimbabwe University and the conservation group Environment 2000 warned that a new build-up of DDT in the food chain posed long-term health risks which outweighed the dangers from malaria.

In addition to a lack of adequate sewage treatment in poor communities as a source of water pollution, "water inequity" in poor communities can also increase the incidence of water- borne diseases overall, because people without adequate water supply may not have enough water to wash their hands regularly or keep food-preparation utensils clean, therefore contributing to the spread of these diseases.

Mining is a major pollution source for the region's rivers and catchments. The main types of mining pollution include contamination from heavy metals produced by mining, from mining processing-chemicals like cyanide, and acid mine drainage from mining waste. Another source is the failure of dams used to contain mine wastewater (called tailings dams), which dump large quantities of contaminated mine waste into rivers. Such failures have been known to immediately kill all life for many kilometres downstream. Abandoned mines also continue to cause pollution. One example: tributaries feeding the Oliphants pass over unrehabilitated coal mines abandoned in the 1950s; the Department of Water Affairs believes the longterm solution to this problem will cost R200 million. Newer methods of mining have been shown to dramatically reduce such pollutants, but governments must first develop tough standards for mining and then enforce them.

Mining is also a very heavy water-user. Mining uses water for processing mining products and also by de-watering mine shafts to facilitate underground work, which can lower water tables to the point of drying up local wells and springs. Another mining:' impact on catchments is sedimentation pollution. Although not toxic, the erosion of rock and soil into waterways from mine sites can cause severe environmental damage to streams and rivers, smothering riverside vegetation and killing aquatic life. Sedimentation can also increase flooding downstream.