Silver is a naturally occurring element that is found in natural bodies of water and soils.
According to the Agency for Toxic Substances & Disease Registry (ATSDR), silver is found in several locations:
“Silver levels of less than 0.000001 mg silver per cubic meter of air (mg/m³), 0.2-2.0 parts silver per billion parts water (ppb) in surface waters, such as lakes and rivers, and 0.20-0.30 parts silver per million parts soil (ppm) in soils are found from naturally occurring sources. Silver compounds are also found in groundwater and at hazardous waste sites throughout the United States. Drinking water supplies in the United States have been found to contain silver levels of up to 80 ppb. Surveys show that one-tenth to one third of samples taken from drinking water supplies (both groundwater and surface water) contain silver at levels greater than 30 ppb.”
The full source document can be found here.
While silver levels will vary from one location to another, this ATSDR information provides a valuable framework for understanding the concentration of silver in the environment.
For a related topic, including recommended limits for safe levels of silver in drinking water, see Silver in Drinking Water.