森林集水區對酸性與非酸性沈降之水化學變化－－以台灣北部文山林場為例

Changes of Water Chemistry for Acid and Non-Acid Precipitation at A Forested Watershed–A Case Study of The Wenshan Forest Station in Northern Taiwan

本試驗於文山林場將2009年3月至2012年2月三年期間所收集的雨水，分為酸性沈降（pH≦5.0）與非酸性沈降（pH>5.0），以探討二種沈降雨水經過樹冠層（穿落水）、土壤層（土壤水）至溪流水的水化學變化。經由47場有效降雨之分析結果顯示，非酸性沈降的雨水含有較高濃度的Na+ 、 Ca2+、Mg2+、HCO3 - 等鹼性離子，但雨水沈降至穿落水及15、30 cm深土壤水時，二沈降的離子濃度除了F以外並無明顯差異，說明此森林冠層對酸性沈降有極大的緩衝效能。然而，60 cm深之土壤水，非酸性沈降者含有較高濃度的Ca2+、HCO3 - 及總陰、陽離子。非酸性沈降者之溪水其Mg2+ 濃度高於酸性沈降者，Cl- 和NO3 - 之濃度則以酸性沈降者較大，推測造成溪水離子的差異可能為溪水流量及大氣沈降所致。由上述結果顯示，此區60 cm深土壤水及溪流水的水化學性質與沈降的水化學性質有很大的相關性。 This study grouped rainfall and water samples which were collected from March 2009 to February 2012 at Wenshan Forest Station into acid precipitation (pH≦5.0) and non-acid precipitation (pH>5.0) two categories for investigating changes of water chemistry after it passing through canopy (throughfall), soil layers (soil solution) and draining into stream. Results from 47 valid events indicated that samples from non-acidic precipitation category contained a higher concentration of Na+ , Ca2+, Mg2+, HCO3 - and other alkaline ions. Except for concentration of F, there was no significant differences of ions concentrations between these two categories in throughfall, soil solution of 15, 30 cm depths and this phenomenon illustrated that this forest ecosystem had great buffering capacity for acid precipitation. However, soil solution at 60cm depth had higher concentrations of Ca2+, HCO3 - , anions and cations for non-acid precipitation. Samples of streamwater of non-acid precipitation category contained higher Mg2+ concentration but lower Cl- and NO3 - concentrations in comparison with those of acid precipitation category. We speculated that differences in ions of streamwater may be affected by discharge of streamflow and characteristics of atmospheric precipitation. Results from above indicated that water chemical properties of soil solution at 60cm depth and streamwater have close relationship with those of gross rainfall in this area.