Another problem with the use of groundwater today is its potential contamination with hazardous chemicals.
Until the 1970s, it was thought that soil would filter agricultural and industrial byproducts from water as it percolated down to underground aquifers. However, groundwater in some places is now known to be contaminated with pesticides, chemicals such as perchlorate from defense-related industrial plants, solvents from computer makers and benzene from leaky gas storage tanks. Other sources of groundwater contamination include landfills, leaking septic systems, de-icing salts used on roads, and hazardous waste sites.
It is difficult to assess the extent of the problem in California. Remediation of contaminated groundwater sites is possible (for example, by removing soil and pumping and treating the water) but expensive. Prevention of contamination is critical.
In recent decades, substantial progress has been made in artificially recharging groundwater supplies to use them as underground reservoirs.