英译汉,分析下句型.

题目

英译汉,分析下句型.
When the water level was at -2.2 cm depth,the soil CH4 concentration rose 10-fold at 10 cm depth until it reached a stable concentration of ca.3,500 ppm down to a depth of 60 cm where concentrations increased to ca.15,000 ppm.
直接谷歌、yahoo的别费劲了,它翻译的看不懂.
麻烦分析下句型,主要是down to a depth of 60 cm是什么成分.
ca.是大约的意思，ppm是单位不用翻译。麻烦分析下结构，那个3500和15000是什么地方的浓度？

答案

When the water level was at -2.2 cm depth,固态甲烷浓度上升十倍到10cm深,直到它（固态甲烷）达到大约3,500 ppm这样一个稳定的浓度,而到达60cm深（指的应该还是固态甲烷）.在这个状态下,浓度上升至大约15,000 ppm....