Laserfiche WebLink
Table 2: Hydraulic Conductivity Values for Selected Rock Units at the Site. <br />Rock Type Hydraulic Conductivity (cm/s) <br />Dakota /Burro Canyon (Kdbc) <br />Mudstone /Shales (Jms,Js) <br />Salt Wash Sandstones <br />Entrada Sandstone (Je) <br />Navajo Sandstone (Jn) <br />Wingate Sandstone (Jw) <br />1.0E -04 <br />1.6E -07 <br />1.0E -06 <br />5.0E -05 <br />1.0E -06 to 3.0E -03 <br />3.0E -05 to 5.0E -04 <br />With saturated conditions in the Salt Wash Sandstone, it is important to estimate the rate <br />of groundwater migration in order to evaluate monitoring requirements and assess <br />potential impacts to the environment. Sandstones in the Salt Wash are not used as a <br />water resource on the Colorado Plateau due to poor water quality and limited water yields. <br />Without wells or hydraulic testing to quantify permeability, estimates based on published <br />values are required to estimate groundwater transport rates. Freeze and Cherry (1979) <br />provide a range of hydraulic conductivity values for sandstones from 1.0E -08 to 1.0E -04. <br />Characteristics such as sorting, grain size and the degree of cementation determine actual <br />permeability values. Well- sorted, coarse -grain poorly- cemented sandstones have the <br />highest hydraulic conductivity values. The Salt Wash sandstones are poorly -sorted fine - <br />grain sandstones that are well cemented suggesting that hydraulic conductivity values <br />tend toward the lower end of the range. The value of 1.0E -06 presented in Table 2 <br />represents the mid -range of hydraulic conductivity values and is considered a <br />conservative estimate. <br />Using the hydraulic conductivity values of 1.0E -06, a porosity value of 0.2, and a <br />hydraulic gradient of 0.008, equal to the regional dip of the strata, then calculated <br />groundwater transport rates for uranium and related constituents from ore bodies would <br />be 0.04 feet per year in a south - southwest direction. Increasing the hydraulic <br />conductivity value by a factor of ten still yields groundwater transport rates of less than a <br />foot per year. The low transport rates have important implications on the location of <br />monitoring wells, interpretation of existing groundwater quality data, and the potential <br />impact of mining activities on the surrounding area as discussed in the following sections. <br />Average transport rates in the upper -most aquifer for the area which is assumed to be the <br />Navajo Sandstone Formation can range as high as 120 feet per year in the same <br />northwesterly direction. Groundwater in the Navajo Formations would eventually reach <br />the axis of the syncline and then migrate to the southeast following the plunging syncline. <br />It is unlikely that groundwater would ever discharge to the surface environment. <br />Groundwater in the Salt Wash would eventually be discharge in Bull Canyon in small <br />seeps such as Fawn Springs. Considering Fawn Springs is 4000 feet from the JD -9 portal, <br />it would take 100,000 years for groundwater from the mining area to reach Fawn Springs. <br />This estimate assumes that no retardation due to sorption of uranium related constituents. <br />Regardless, groundwater hydraulic indicates that mining will have minimal impact on the <br />surrounding environment. <br />O'Connor Design Group Inc. U - 15 <br />