Iron in excessive concentrations can limit severely the use of water for public supply, domestic, and recreational purposes. Consequently, most water supply criteria contain recommended maximum limits for dissolved iron; the recommended maximum concentration of iron in drinking water is 300 micrograms per liter (U. S. Environmental Protection Agency, 1976). The Act specifies 7,000 micrograms per liter as the maximum allowable concentration of total iron in effluents from mining operations. Total recoverable (dissolved plus suspended) and dissolved concentrations of iron in water have been determined at 31 sites in Area 19 (fig. 5.6-1).

Dissolved iron is only a small part of the total recoverable iron transported by streams in Area 19; concentrations generally were less than 200 tig/L. Although dissolved iron concentrations ranged from 0 to 1,300 ug/L, only eight of approximately 500 determinations exceeded 300 micrograms per liter. Dissolved iron did not vary significantly with large changes in streamflow. Although pH is an important factor affecting metal solubilities, no statistically significant relation between pH and dissolved iron has been established areawide.

The maximum total recoverable iron in water from most streams occurred during high flows because large amounts of suspended iron were transported with suspended sediment. The increase in iron load correlates significantly with increasing suspended-sediment concentrations (section 5.9). Because of this relation and because most of the suspended sediment in a particular stream is transported during storms, neither suspended-sediment nor iron yields can be defined with data obtained by random sampling. These yields from a basin can be determined only by more comprehensive sampling.