What is the concentration of silver in tap or freshwater in the United kingdom
Monitoring results from UK groundwaters 1996-2007, Silver:

Sites 2929

Samples 19420

Min &lt;0.04 µg/l

Max 79.6 µg/l

Average 0.457 µg/l

Source - Defra


  Routine monitoring for trace elements in drinking water is only
  conducted for the elements specified in the Water Supply (Water
  Quality) Regulations. Specified elements include antimony, arsenic,
  boron, cadmium, chloride, chromium, copper, fluoride, iron, lead,
  manganese, mercury, nickel, selenium and sodium. Monitoring for other
  elements will only normally be conducted in response to specific
  identified risk. Recently DWI has funded investigations of uranium1
  and molybdenum2 in drinking water in response to recent developments
  in WHO guideline values. Both these studies showed a good level of
  compliance should the WHO guideline value be adopted as a standard.


Also...


  Water companies also conduct raw water monitoring and these results
  are reported to Drinking Water Inspectorate. Preliminary analysis suggests most of the 
  raw water monitoring relates to elements already covered by the
  regulations but a few companies monitor for additional elements in raw
  water. A summary of the additional elements and (number of companies
  monitoring). Silver (4).


Source - Defra 2


  Average silver concentrations in natural waters are 0.2-0.3 µg/litre.
  Silver levels in drinking-water in the USA that had not been treated
  with silver for disinfection purposes varied between "non-detectable"
  and 5 µg/litre. In a survey of Canadian tapwater, only 0.1% of the
  samples contained more than 1-5 µg of silver per litre. Water treated
  with silver may have levels of 50 µg/litre or higher; most of the
  silver will be present as nondissociated silver chloride.


Source - WHO