進入檢視： 2022-2023年英語週報九年級第22期答案彙總

US and Canadian researchers recently calculated the total amount of the world's groundwater and estimated that it is equal to a lake 180 metres deep covering the entire Earth. This makes groundwater the largest active freshwater resource on the planet. The team also investigated its age. How did they do this?

Humans introduced a convenient time signature on the Earth when we started testing atomic(原子的) weapons. This leaves a time stamp which can be found in water, rocks and living organisms. By seeing if radioactivity(放射性) from atomic bomb testing was present in the groundwater, the researchers could estimate that about 6% of this groundwater is younger than50 years old. If this water was spread evenly over the continents, then there is just three metres depth of "modern" groundwater spread equally over the continents.

It could be argued that if we use only this "modem" groundwater, then this is sustainable, as we know that it has been replaced ( or recharged ) by natural processes over the last 50years. But how quickly groundwater recharges changes over time.