台灣南部高屏溪流域山區地下水與溪水溶解氣和水化學之分析

Abstract

台灣的年降雨量約為2500mm，為全球年降雨量平均的2.5倍，中央山脈南部的高屏溪流域山區支流較少、流域面積大。本研究首次利用高屏溪山區地下水與地表水中的溶解氣體與水化學的分析探討該區含水層的特性，以利未來水資源的利用﹔共採集9個地下水和19個地表水(溪水)以及3個溫泉水，另在平原區共採集6個地下水和2個地表水以作比對。全部的樣品均分析其氫氧同位素、溶解氣(氮氣、氧氣、氬氣、甲烷和二氧化碳)、惰性氣體(氦氣、氡氣) 和主要離子成分。 地表水氫氧穩定同位素值(δ18O與δD)的投影均落於區域天水線上，顯示為天水來源。溶解氣體成分主要為氮氣(>90%)和氧氣(>5%)，餘皆<1% ﹔氦同位素比值接近1RA(RA為空氣中氦-3/氦-4比值)﹔大多數水氡值皆小於偵測極限(0.2 Bq/L) ，只有寶來和廣林測站測得0.4和0.6 Bq/L，顯示附近有地下水的顯著補注 ﹔主要離子濃度皆低(Na+ <20 mg/L, Ca2+ < 60 mg/L, Cl- <2 mg/L)，支持地表水補注來源主要為天水。 地下水氫氧同位素值也皆落於區域天水線上﹔氦同位素比值(0.9~0.23 RA)比地表水低，而水氡(0.3~19.6 Bq/L)則明顯高於地表水。部分地下水溶解氣有高含量甲烷(>20%)或二氧化碳(>10%)。有些地下水比其他樣品均溫高約5℃，且含有較高濃度的離子成分(Na+ >500 mg/L, Ca2+ >150 mg/L, Cl- >80 mg/L)，呈現和高屏溪山區溫泉相似的水質，但溫度尚未達法定的54℃溫泉標準。 分析結果顯示高屏溪山區許多深層地下水地球化學特徵呈現流動緩慢的滯留性含水層特性，使深層地下水不易流出。山區淺層岩盤因較為破碎或是含有厚層岩屑層，地下水流動性較好，顯示屏東平原地下水主要的補注源除了當地的雨水和溪水外，還有鄰近山區淺層地下水的直接補注。

Due to high topographic relief, groundwater is critical for water supply in Taiwan. The mountain region of the Goaping river in southern Taiwan is a large catchment with few tributaries, that makes it a suitable area to understand the potential of groundwater recharge in the mountain area. The aim of this study is to interpret the characteristic of groundwater in the study area using dissolved gas and water chemistry. Nine groundwater, 19 surface water and three hot spring samples were collected from the mountain region﹔6 groundwater and 2 surface water samples were collected from the plain area. All samples were analyzed for stable hydrogen and oxygen isotopes, dissolved gases (including N2, O2, Ar, CH4 and CO2), noble gases (helium and radon) and major ions dissolved in the water samples. For surface water samples, results of stable hydrogen and oxygen isotopic ratios are plotted on the local meteoric water line. Dissolved gases are dominated by N2 (>90%) and O2 (>5%). Helium isotopic ratio is approximately equal to 1 RA (RA is 3He/4He ratio of air). Radon-222 concentration is below the detection limit (<0.2 Bq/L); and concentrations of major anions and cations are low (Na+ <20 mg/L, Ca2+ < 60 mg/L, Cl- <2 mg/L). All these features indicate that surface water is predominately recharged by local precipitation. For groundwater samples, results of stable hydrogen and oxygen isotopic ratios are also aligned along the local meteoric water line. Helium isotopic ratios (0.90-0.23 RA) are lower, and radon-222 concentrations (0.30-19.6 Bq/L) are much higher than the surface water samples. Some samples show that CH4 (>20%) or CO2 (>10%) are dominated in dissolved gases other than N2, most likely contributed by biogenic or terrigenic sources. On the other hand, samples that have temperature 5℃ higher than the average of other samples, show ion concentration similar to hot springs. In the mountain region of the Goaping river catchment, shallow bedrocks have well developed fractures or thick layer of regolith, which make groundwater discharge more easily to the Pingtung plain. This study suggests that groundwater in the Pingtung plain are recharged not only by local rainfall and river waters, but also by significant amount of shallow groundwater in the mountain area.