In porous aquifers groundwater flows as slow seepage in pores between sand grains. Groundwater is mainly recharged by rainfall and seepage streams. All water that enters the lake is eventually lost from evaporation or subsurface seepage. This makes such a dam impervious to surface or seepage erosion. It is also almost waterproof, resulting in virtually no seepage of groundwater into the tunnel. The release of water is dependent upon evaporation and seepage. There are Tertiary and Quaternary aquifers which are recharged by precipitation and seepage from streams. Properly waterproofing foundation walls is required to prevent deterioration and seepage. The verb "Halih" in Mongolian means leakage, seepage or overpour. It has no outlet, with water leaving only by evaporation, or sub-surface seepage. Production was limited to bailing seepage from shallow wells dug manually. The loss in strength due to upward seepage is a common contributor to levee failures. Today's cliff slip problems are largely due to inland water seepage. Acidic flushes (areas of water seepage) with their carpets of sphagnum mosses are common. Pumping stations remove water in the tunnels from rain, seepage, and so on.