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-26- The effect mining has had on the naturally occurring levels of these chemical parameters is not easily determined, due to the lack of pre-mining data in the basin. Leaching through the regraded spoil piles will undoubtedly produce some increased levels of total dissolved solids and B, Mn, and Fe. This will be a short term impact which will decrease through time as the readily dissolvable elements are leached from the spoils. A monitoring well program has been stipulated for certain pits in order to more fully quantify the leachate quality and quantity in the spoils. This monitoring program can be found in Section VI of this document. C. Surface Water The Grassy Gap Mine area lies in the headwaters of Grassy Creek. Grassy Creek is a tributary to the Yampa River in the mid to upper section of the Yampa Valley. The small drainages feeding the headwaters of Grassy Creek are ephemeral in nature and flow only in response to snowmelt or direct precipitation. Grassy Creek, through the mine area, flows perennially only during wet years. The numerous beaver dams located on the stream extend the duration of the flow period during dry months by percolation through the dams. The active pit, Pit 4, is the furthest downstream extent of mining at the Grassy Gap Mine. Four reclaimed pits exist upstream. The potential effects of mining on Grassy Creek surface water quality are. 1. Increased levels of total suspended solids (TSS), 2. Increased levels of iron (Fe), Manganese (Mn), and boron (B), and 3. Increased levels of total dissolved solids (TDS). Table 1 gives the mean concentration of selected chemical parameters in Grassy Creek. Relatively high levels of TSS have been noted, especially during high flows, in the upper reaches of Grassy Creek. Surface water monitoring station 3 (SW3) is located on the eastern fork of Grassy Creek and during a high flow event had a TSS level of 9090 mgJl. No disturbed sites drain into this fork upstream of SW3 so the sample should depict undisturbed or baseline conditions. This high level of TSS is typical of ephemeral streams during high flow events. All drainage from unreclaimed active pits flows through diversion ditches into sediment ponds. This greatly mitigates the effects of surface mining on TSS levels in the natural drainage. Grassy Creek also has numerous beaver dams along its course through the mine site. The effects of these dams on settling of suspended solids can be seen in the following TSS levels sampled in April, 1980.