6.6. SR 03-15-2010 - Laserfiche WebLink

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Pale 2 of ~ Le~~er to N~~•, ~pli~i E[l~s Mr. N~~t#t Ledvina RE; Revised SDA ~ydro~ealo~ic Evalu~~ian Re~art Elk River La~nd~IC, ~~ic. cha~acteri~zatian of the lower outwash sand is needed, The direction ar~d rate of groundwater flow of the rower autwash sand'shall ~~ dacurnented, Two soil borings and four pie7ometers wore installed between November ~~04 and April 200 to evaluate the hydraulic relationship between the upper and lower outwash units. The four pie~orneters were completed in the lower outwash unit as deeper pairs to existing monitoring wells 3 l 5®~wA, 316~OwA, 31 ~~nwA, and l'p31$~. hydraulic head. data from the well pairs yields the following inforn~atian: ~ This add.itianal worl~ indicates tla.at the SU. e~~ior till u~.ts for~n a canti~auous ~on~irtin la e~• betwee~~ th.e u er and lower o~~,twash~ ~ 1"h~ dower autwash unit l.s full ~atua at~~.~ ~ The potentiometric surface in the lower outwash is approximately four feet lower than the hydraulic head in the upper outwash indicating a downward potential through tlae till that separates the two outwash deposits; The direction of groundwater flow in the lower outwash is to the southwest; and ® The ave~•age linear groundwater velocity through the lower outwash unit is approximately l3 fact per year, 1]iscussian regarding the hydraulic relationship between the upper and lower outw~h units is included in Section 4.2,5, 2, how was the till isapach rnap generated? Please provide, in the narrative, a description of how the thicl~ness of the till unit was determined, and give e~an~pl es from representative borings. Can the cross sections, pleas Shaw dashed contacts were its thicl~ness is estimated, additionally, is the till unit continuous across the site? ~f not, the upper and lower outwash could be hydraulically connected, particularly in the area of sort borings 5.~~9, SP~~4 and S~~,ZS, Geologic data from the two recently advanced soil borings, SAS 1 and 552, suggest the till is continuous across the S>~A. A discussion of how the till thickness map was generated. is included in S ection 4,1.2. Additionally, the estimated till thickness is illustrated on the Cross sectiDn f gUI'eS. 3. .how do current groundwater elevations at the South ~]evelopment Area compare historically with groundwater elevations measured at the existing facility? Elsa, please document the relationship Uetween current groundwater elevations and the proposed liner elevation, A, hydrograph illustrating water levels from the SI~A and the existing facility is attached. as figure 1 ~. The hydrograph indicates that the S1]A water level data has been collected during a period of average to high water table elevations at the existing facility. The water levels measured at the existing facility fluxing 244 are consistent with those measured. from 199 to 1 ~9~ and are apprax,im.atelyfeur feet higher than