扇頂區除了是地下水資源豐富的區域，也是地下水的源頭，若有污染滲入，將對地下水資源產生很大的衝擊。許多國家已設立地下水水源保護區，並進行地下水水質變化的基礎調查研究，以提供地下水水質保護區劃定及水資源保育管理的重要參考。近年之研究已對濁水溪扇頂區之水質地化背景、地下水來源、地化特性、時空之演變及其機制，有初步的結果。彰雲扇頂區歷年導電度的變化顯示，有多口井有鹹化的趨勢，推測來自人類活動的影響，例如：以地下水灌溉、化肥、生活污水、工廠廢水等（陳文福等，2009：陳文福等，2010）。研究也發現，濁水溪扇頂的觀測井，井內水的導電度有明顯的垂向差異。同口井之上下層水的導電度可相差一至三倍。井內上下層水導電度不同的原因，可能有：（1）上層井管有破漏，淺層水經由破漏處入滲；（2）流入井內水為不同密度，因此低密度往上浮、高密度者往下沉；（3）井管內的停滯水發生化學變化。但是，確實的原因仍不清處，有待進一步研究。本次研究工作，將探討觀測井井內垂向水質分層現象，經由地下水分層採樣及化驗，井內水質垂向量測，及同一井上下兩點的導電度連續監測，應可解決下列問題，例如：（1）分層是否發生變化？（2）為何有分層之現象？其控制因素？是井管破漏、地化作用或是密度流？ 其結果將對扇頂區地下水水質演化有重要的參考價值。 Upper fan is the recharge zone for aquifers of an alluvial fan system. It is worth to protect the vulnerable unconfined aquifer because of its important for water resources and the groundwater is easily contaminated by pollutions. Many countries had provided regulations for protecting the important recharge area, e.g. the well head protection area in Issaquah, Washington, USA. A lot of investigation and research about groundwater quality had to been done in a variety of unconfined aquifers to provide information for groundwater resources protection and management. In recent years, some interesting results have been uncovered by researches related to the geochemistry of the Choshui upper fan aquifer, i.e. groundwater composition, recharge sources, geochemistry characteristics variation in time and space (Chen et al., 2009, 2010). The previous studies showed some of groundwaters in the Choshui upper fan aquifer, the electrical conductivities have an increasing trend. Over a half of groundwaters in monitoring wells the electrical conductivity are higher than 750 uS/cm that is exceeding the irrigation water quality standard of Taiwan. The mechanisms that cause groundwater more salty may be by human activity i.e. irrigation of using groundwater, chemical fertilizer, sewage and waste water etc. Study shows the electrical conductivity (EC) values vary 1 to 3 times along the vertical direction in the monitoring wells of the Choshui upper fan. The reasons that cause the difference may be (1) well casing leakage, (2) groundwaters in different density with upward flow, and (3) changed by geochemistry reactions. In this study, we will sampled groundwaters in different depth of monitoring wells to reveal its ions composition. We will apply vertical EC logging in monitoring wells and install EC probes with recorders for 2 points of different depth in monitoring wells. Our purpose is to find out the mechanism that control the vertical EC variation in those monitoring wells.
