Aging underground storage tanks can leak gasoline and other contaminants into groundwater Groundwater also plays a key ecological role by replenishing rivers, streams and wetlands. It provides much of the flow for the Mississippi, the Niger, the Yangtze and many other great rivers - some of which would otherwise not run year round. Some of this waste has a half-life of 250,000 years. 15 percent of shallow groundwater sampled below cities and farms in the U.S. in the mid-1990s had nitrate concentrations that exceeded the guideline for drinking water. One third of the wells tested in California's San Joaquin Valley in 1988 contained the pesticide DBCP at levels 10 times higher than the maximum allowed for drinking water - more than a decade after its use was banned.
Water recycles very slowly underground, too slowly to flush out or dilute toxic chemicals.
Water that enters an aquifer remains there for an average of 1,400 years, compared to only 16 days for rivers. Thus Londoners, for example, may be drinking water that fell as rain as long ago as the last ice age. The urgency of preventing groundwater contamination is highlighted by the costs of cleanup efforts. To preserve this valuable resource, we need to make systematic changes in the way we grow our food, manufacture goods and dispose of waste. The report proposes retooling industrial agriculture to reduce farm runoff, a leading source of groundwater pollution.