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~\ <br />SENECA II <br />RESPONSE TO COMMENTS <br />1986 ANNNUAL HYDROLOGY REPORT <br />C• <br />1. Peabody Coal Company has requested that the monthly monitoring of Wells GW-S31-A2 and <br />GW-533 be reduced ~to semiannual monitoring of field parameters and sampling for water <br />quality. Civen the water quality correlations rvith NPDES 002 and Site SW-S2-2, this <br />request appears reasonable. <br />RESPONSE: <br />Peabody requests MLRD's approval to reduce the water quality field parameter monitoring <br />frequency at sites CW-S31-A2 and GW-533 from monthly to a semiannual basis. This request <br />is being made based on the following results and conclusions: <br />A. The alluvial aquifer canductivi ty values appear to show the same seasonal <br />fluctuations as the streamflow. <br />B. The range of alluvial aquifer conductivities generally falls within the range of <br />conductivities for adjacent streamflow. The greatest deviation in conductivities <br />corresponds to spring snommelt where the streamflow is greatly diluted in comparison <br />to alluvial aquifer flow. Large volumes of streamflow can be moved and replaced <br />rapidly during this time period where as ground water flow and recharge occurs <br />several orders of magnitude slower. <br />C. Five years of study indicates that the stream is the TDS conveyance source and, <br />because of its fast rate of movement, conductivity or TDS changes are dispersed <br />rapidly, Thus, the streamflow creates a series of potential iDS point sources along <br />the entire reach of flow rather than one, concentrated point source entering the <br />alluvial aquifer only immediately below the NPDES outf all. <br />D. Spring snommelt causes a lagged dilution effect on alluvial aquifer conductivities. <br />This is followed by alluvial aquifer conductivity or TDS increases due to ET and the <br />additional leaching of concentrated salts during the growl nq season. <br /> <br />E. Based on the data submitted in previous AMR's, it is apparent that any detrimental <br />alluvial water quality changes will be reflected in the surface water quality along <br />