Seepage free surface issue exists in analysis of the seepage concerning geotechnical slope, earth dam, underground cave and groundwater movement.

在岩土边坡、土坝、地下洞室及地下水运动等渗流分析中，均存在有渗流自由面问题。

youdao

The passages of the tax code to which you refer are but the surface of the issue.

FORBES: Relinquishing U.S. Citizenship

On the surface, the issue is what to do about the budget deficit.

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Defusing local opposition to "fracking" to extract gas from rocks deep below the surface is an issue many of the more tactically-adroit MPs are increasingly concerned about.

BBC: Week ahead