Natural arsenic levels in groundwater and soils vary. Water pollution can be classified as surface water or groundwater pollution. They can also occur from the overpumping and extraction of groundwater and subsurface fluids. Many landfills now require countermeasures against groundwater contamination. Spills also take out crops and aquacultures through contamination of groundwater and soils. Often, improper fecal disposal leads to contamination of groundwater. Fresh water is found in rivers, lakes, wetlands, groundwater, and frozen glaciers. Cleanup of contaminated groundwater tends to be very costly. Interactions between groundwater and surface water are complex. Several nations estimate sectoral distribution of use of water withdrawn from surface and groundwater sources. Rising damp from groundwater or drainage issues is especially problematic. The dry ash is disposed in landfills, which typically include liners and groundwater monitoring systems.. For example, mining affects groundwater and water table levels and acidity. Perchlorate salts, when in solid form, dissolve and move rapidly in groundwater and surface water. Tucson's city water currently consists of CAP water mixed with groundwater.