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The impact of subsidence cracks on numerous ponds <br />within the area is much less important. The probability of a <br />subsidence crack disrupting the pond is remote. Most ponds <br />are located in areas covered by alluvial or colluvial soils; <br />the occurrence of cracks is less probable in these <br />formations and healing of the cracks would be faster in them <br />than in bedrock formations or in colluvial soils covering <br />the ridges. Any pond disrupted by subsidence cracks could <br />easily be repaired at low costs and in a short period of <br />time. <br />The occurrence of the subsidence cracks on the surface <br />may not necessarily result in direct inflows into the mine <br />when a surface water body was disrupted. Studies by various <br />authors such as Orchard (1969, 1975)'in Great Britain and <br />Williamson (1978) in Australia indicated that the <br />subsidence cracks may be limited in depth by tt,e development <br />of compressive stresses replacing the tensile stresses that <br />cause surface fracturing. <br />Subsidence impacts may also influence the quality of <br />surface water for the following reasons: <br />o changes in land surface within the drainage area and <br />changes in stream channel slope <br />• o changes in ground water quality in cases of gaining <br />streams <br />Changes in land surface and stream slope can affect the <br />water quality mainly by increasing the sediment load. The <br />increase of stream slope results in an increase of flow <br />velocity and, therefore, increases the erosion potential. <br />The potential for impact on the surface water quality <br />due to changes in the ground water quality is considered <br />insignificant, especially in cases where the stream is being <br />recharged from shallow (colluvial and alluvial) aquifers, <br />which is the case in the lease area. <br />In summary, we believe that the occurence of the <br />surface subsidence cracks comprises the most important <br />hazard to the surface water regime. Their occurrence is <br />predicted principally ~+ithin the areas where the overburden <br />thickness does not exceed some 800 feet. Such areas include: <br />o southern part of East Roatcap Creek <br />o south=rn part of Stevans Gulch <br /> <br />_ 44 _ <br />