2.2.1 rocks
Silver is a valuable, naturally occurring, metallic element. It is rarely found as elemental silver but commonly occurs as a nitrate, chloride, sulphide or oxide in rocks and soils, and is often associated with other metals such as copper, zinc, iron and antimony (Taylor et al. 1980). In rocks, the mean silver concentration ranges from about 0.04 to 0.12 mg/kg and the mean crustal abundance is about 0.07 mg/kg. Granitic igneous rock in Nevada contains up to 50 mg/kg of silver (Seiler et al. 1988). Coal contains between 40 and 2000 µg/kg of silver but the level in crude oil is much lower, about 100 ng/kg.
2.2.2 air
Volcanic activity emits silver sulphides, sulphates, chlorides and ammonium salts. The background concentration of silver in air is estimated at <0.1 ng/m3 (Boyle 1968, Mulvey 1978).
2.2.3 soils
Normal soils contain silver levels of about 0.3 mg/kg, with a range of 0.1 to 5 mg/L, but near lead, zinc, and silver deposits this may rise to 10 mg/kg. Well-drained New Brunswick podzols contained 0.1 to 7.8 mg/kg of silver. Silver tends to concentrate in the surface layers of soils at a pH greater than 4, particularly in organic soils with high levels of humic colloids. Silver also binds to clay minerals. Adding 0.5 to 5 mg/L silver iodide solutions to soils resulted in surface concentrations of ten times the solution strength and equal concentrations were reached at 5 cm. Soils amended with sewage sludge can have up to 10 times as much silver as normal soils (USEPA 1980a).
2.2.4 water
2.2.4.1-rain water
Rainfall in a remote area of Ontario contained 2.8 to 30 ng/L of silver compared to rainfall in Germany that ranged from 70 to 9000 ng/L (Merritt 1976). In the USA, silver in rain, snow and hail ranged from 0 to 1200 ng/L (Mulvey 1978). The normal silver level in rainfall in Alberta is 2.5 to 5 ng/L but it rose to 46 to 62 ng/L after cloud seeding with 240 g of AgCl. These effects could be detected 100 km from the site of the seeding; the peak silver levels occurred an hour after seeding (Warburton 1973, Zacharuk et al. 1977).
2.2.4.2-drinking water
Silver levels in 380 samples of finished drinking water in the USA ranged from <1 to 5 µg/L (Nordberg et al. 1988). Finished water from the 100 largest USA cities had a median silver level of 2.3 µg/L with a maximum of 7 µg/L (Durfor et al. 1964). Other set of 380 samples of finished drinking water had silver levels of 0.3 to 5 µg/L, with a mean of 2.2 µg/L (Kopp et al. 1967). Only 6% of the 380 samples contained any silver and data from 1577 samples of well and surface waters from 130 points in the USA indicated silver levels above the detection limit of 0.1 µg/L in only 104 of the samples. The levels ranged from 0.1 to 38 µg/L with a median of 2.6 µg/L (Kopp et al. 1967). A later report on drinking water supplies in the US, both ground water and surface water, showed silver at levels up to 80 µg/L with up to one-third of these sources containing over 30 µg/L (Anon. 1990).
2.2.4.3-fresh water
Acidic waters tend to contain more silver than neutral or alkaline waters and silver is often associated with chloride, sulphate, bicarbonate, organic material, manganese and iron. Uncontaminated lakes and rivers in non-industrialized areas may contain about 0.2 to 2.0 µg/L from natural sources (Anon. 1990, Boyle 1968). Raw water from Lake Ontario contained 1 µg/L (Bard et al. 1976). In the Environment Canada computerized data storage system, NAQUADAT, over 90% of the silver records were below the various detection limits which ranged between 4 and 10 µg/L. However, in heavily industrialized regions, or areas where heavy-metal mining and smelting occur, Sudbury and Trail for example, contamination by silver can result in stream concentrations up to 38 µg/L (Stokes et al. 1973, MRI 1975).
The seasonal fluctuation of silver in various components of a small lake in Colorado was monitored by Freeman in 1975. In the summer, silver is associated with terrestrial and aquatic communities, which increases the silver concentration in the soil and lowers its concentration in the water. At the end of the growing season, when massive die-off occurs in terrestrial organisms and chemical changes occur in the soil, silver concentrations in surface and ground water rise. In the lake, die-off resulted in higher silver levels in the sediment which remained high over the winter under the ice.
Redox reactions with iron and biological fallout contributed to high sediment silver levels. Plankton levels were low in the winter. In the spring, surface and ground waters contained about the same level of silver as at freeze-up. During spring thaw, as biological activities intensified, the cycle repeated and silver moved from the sediment into the biotic communities.
2.2.4.4-marine water
The natural erosion of rocks and soils releases a large amount of silver into the environment. Weathering processes are estimated to result in 11 000 tonnes of silver per year entering the oceans. Silver is apparently delivered to the oceans primarily (up to 90%) as dissolved species. Estimates of silver in seawater vary from 0.15 to 2.9 µg/L and the concentration tends to increase with depth in highly productive areas (Boyle 1968, MRI 1975, Thompson 1973, Bowen 1966, Weast 1977).