臺灣大漢溪底泥重金屬含量調查與可能污染源之探討

Abstract

伴隨著經濟快速成長和人民生活水準的提高，其背後所產生的廢水，包含生活廢水、工業廢水和農牧業廢水等因素，使河川成為重金屬直接或間接承受之水體，而重金屬隨著河水流動的過程又因重力沉降至水體底層，進而保存在河川底泥中，因此導致河川底泥重金屬濃度持續增加。 大漢溪底泥也因廢水的排放而受到重金屬污染，大漢溪流域主要以生活廢水和事業廢水排放為主，而事業廢水的排放又集中於土城工業區和樹林工業區。大漢溪底泥重金屬在乾季的污染濃度較雨季來的嚴重，其污染較嚴重的河段為城林橋 (土城工業區聯合廢水排放) 至浮洲橋 (土城抽水站，包含土城區生活廢水和事業廢水)，其中又以金屬銅和鋅最為嚴重，在浮洲橋的底泥重金屬濃度分別超過臺灣底泥重金屬品質指標上限值的6.7倍和4.2倍之多。 分別從不同污染指數來評估大漢溪底泥污染的程度，本研究之評估的結果為 (1) 銅：嚴重污染；(2) 鋅：嚴重污染；(3) 鉻：高度污染；(4) 鎳：高度污染和 (5) 鉛：中度污染，表示大漢溪底泥主要受到金屬銅和鋅的污染，而大漢溪底泥品質受到重金屬惡化影響的程度分別為銅＞鋅＞鎳＞鉻＞鉛。 為了進一步瞭解大漢溪底泥高濃度重金屬銅和鋅的移動性，分析金屬銅和鋅的化學型態劃分以評估銅和鋅在底泥中的潛在污染風險。金屬銅在移動性較高的交換態 (Exchangeable, EXC) 比例為0.7-15 ％而金屬鋅更高達5.8-35 ％，表示重金屬銅和鋅在污染最嚴重的浮洲橋底泥濃度不但遠高於臺灣底泥重金屬品質指標的上限值，且大部分容易因底泥受到擾動而再次釋出至水體環境中。以不同風險指數評估的結果為依據， 顯示重金屬銅和鋅分別為中度和高度潛在風險污染，而潛在污染風險最高的河段為城林橋至浮洲橋之間。

從因素分析 (Factor analysis, FA) 的結果亦可以得知乾季污染較嚴重的重金屬鋅 (全量、交換態和碳酸鹽鍵結態 (Bound to Carbonates, CA))、銅 (全量、交換態和碳酸鹽鍵結態)、鉻和鎳可以分成同一類，而雨季重金屬鋅、銅、鉻和鎳亦分成同一類，表示金屬銅、鋅和鉻的污染來源可能為土城工業區和土城區廢水，而金屬鎳則為土城工業區廢水和柑園大橋上游的瀝青拌合場。

Along with the rapid economic growth and promotion of living standard, causing rivers are sink of heavy metals from domestic, industrial, agricultural and dust depositional sources directly and indirectly. Heavy metals will be finally stored in sediment by gravity through the flow of rivers and hence increase the concentration of heavy metals in sediment persistently. Dahan Creek, one of the polluted rivers in Taiwan, is also polluted it’s sediment by the discharge of domestic region and industrial park. The main discharge of Dahan Creek are domestic and industrial wastewater and their industrial wastewater are mainly discharged from Tucheng Industrial Park and Shulin Industrial Park. The most severe polluted discharged entrance is from Chenglin bridge (CLB, wastewater of Tucheng Industrial Park) to Fuchou bridge (FCB, domestic and industrial wastewater of Tucheng region) while it’s most severe pollution were produced by copper (Cu) and Zinc (Zn) in dry season higher than in rainy season. The total concentration of Cu and Zn are at the level of 6.7 and 4.2 times of upper limit of sediment quality of heavy metals announced by Taiwan EPA. The degree of sediment contamination are evaluated into different pollution classes based on the ratio values of different indicators, respectively. The assessed results are (1) Cu: severe polluted; (2) Zn: severe polluted; (3) Cr: high polluted; (4) Ni: high polluted; and (5) Pb: moderate polluted, indicating that sediment of Dahan creek are mainly polluted by Cu and Zn and the sediment quality of the study area can be classified into high to low level in sequences as Cu＞Zn＞Ni＞Cr＞Pb. In order to further understand the potential risk of contamination in the high concentration of Cu and Zn in sediment from CLB to Hsin region of Dahan Creek, the fractionations analysis of Cu and Zn to understand the mobility and potential bioavailability in the sediments. The proportion of high mobility (exchangeable fraction, EXC) for Cu is 1 to 15 % of total content and for Zn is up to 6 to 35 % of total content, which indicated that the content of Cu and Zn are not only over the upper limit of sediment quality of heavy metals in Taiwan, but also easily soluble into the water phase when the sediment be disturbed under natural condition. The potential risk of contamination of Cu and Zn of sediment are classified as medium pollution degree and high pollution degree, respectively, and the highest contamination potential risk region can be evaluated and located at the region from CLB to FCB sampling region based on different indicators ratios. Based on the results from factor analysis (FA) of heavy metals concentration of sediments sampled from different sites, the contaminated metals of Zn (total, EXC and (Bound to Carbonates, CA) fractions), Cu (total, EXC and CA fractions), Cr (total) and Ni (total) can be classified into same group in both dry season and rainy season, which indicated that the pollution of Cu, Zn and Cr in sediments are from the same sources of Tucheng Industrial Park and wastewater of Tucheng region, while the pollution of Ni in sediments is from Tucheng Industrial Park and asphalt plant located at the upstream of KYB presumably.