彰化地區地下水之水化學特徵

Abstract

經濟部爲經營台灣地區之水資源，自1992年起開始進行「台灣地區地下水觀測網整體計畫」，以建立台灣地區地下水之基本資料庫。本研究即利用此資料庫來研究彰化地區地下水之主要化學組成，描述各含水層地下水之化學反應特徵，並解釋水 化學特徵之環境意涵。結果顯示(Ca + Mg)-HCO3爲彰化地區地下水之基本水化學 相，在含水層下游地帶另因發生陽離子交換作用而形成Na-HCO3之水化學相。(Ca + Mg)-SO4型態地下水之形成可能是受地表入滲高濃度SO42-進入之影響所致，主要分佈在台糖農場地區。混合型水化學相之地下水出現在扇尾地區，可能是不同水化學相水體之混合所造成。比較各含水層地下水中之離子濃度和Ca2++ Mg2+與HC03-及SO42-之關係顯示，含水層中有不尋常高濃度之Ca2++Mg2+及SO42-之情況，並且影 響到地下水的水化學反應。幾乎所有(Ca + Mg)-SO4型態之地下水，因S042-濃度過高而表現出較低Ca2++ Mg2+/ S042-比値，此過高之SO42-可能主要去自雨水和肥料。以台糖之農場爲例，透過肥料施用量、雨水之SOZ濃度與雨量資料，估算農場地區平均每年每公頃約有1,260公斤之SO42-進入地下水中，此源自肥料之量至少佔全部入滲 通量的55%。大部分含水層1之地下水因Ca2+ + Mg2+濃度較高，而顯示高Ca2+ + Mg2+ / HCO3-之比値，推測其原因也應與肥料之施用有關。 Groundwater Monitoring Network Plan in Taiwan, sponsored by the Ministry of Economic Affairs (MOEA), has been implemented to establish a basic database ot groundwater for efficient management of water resource in Taiwan since 1992. This study employs MOEA's groundwater database to study major chemical compositions of groundwaters, characterize hydrochemical reactions of groundwaters among aquifers and interpret the environmental implication of hydrochemical features on groundwater in Changhua area. The results indicate that (Ca + Mg)-HCO3 is the basic type of hydrochemical facies in Changhua area. Na-HCO3 was formed due to the cation exchange in the downstream part of aquifers. (Ca + Mg)-SO4 groundwater may be caused by infiltrating high concentration of SO42- through soil profile, particularly in the area of Taiwan Sugar Corporation (TSC). Mixing different facies water probably causes a mixed type existing in the distal fan. Comparing ion concentrations and the relation of Ca2+ + Mg2+ with HCO3- and SO42- in each aquifer indicate that unexpected high concentrations of Ca2+ + Mg2+ and SO42- are found. Those ions strongly affect hydrochemical reactions in the aquifers. Almost the (Ca + Mg) - SO4 groundwater exhibits lower Ca2+ + Mg2+ / SO42- ratio due to the high concentration of SO42- which may be derived mainly from rainwater and fertilizer. About 1260 kg / ha per year of SO42- would infiltrate into groundwater estimated on the basis of SO42- concentration in rainwater, the precipitation and the fertilizer used in the farms of TSC in Changhua area. This estimation also shows that at least 55% of the SO42- flux was derived from fertilizer. The application of fertilizer probably contributed to the higher Ca2+ + Mg2+ concentration and Ca2+ + Mg2+ / HCO3- ratio in most groundwaters in aquifer 1.