_REVISION - M1977208 (3) - Laserfiche WebLink

Home Browse Search

is a computer program that performs speciation, mixing, reaction-path, advective- transport, and inverse geochemical calculations. PHREEQC uses ion-association model and Debye Huckel expressions to account for the non-ideality of aqueous solutions. The speciation and mixing modules of PHREEQC were used in this analysis. fn the mixing module, each solution is multiplied by its mixing fraction and a new solution is calculated by summing over all of the fractional solutions. The mixing ratio of the groundwaters and the CKD leachate were determined using the annual leachate volumes calculated by the HELP model and the volume of each aquifer beneath the CKD disposal site. The volume of water in the alluvium and Dakota Sandstone aquifers is estimated at 28,575,360 cubic feet and 85,726,080 cubic feet, respectively. The CKD leachate volumes estimated using HELP for the alluvium and Dakota Sandstone simulations are 53,898 cubic feet and 366 cubic feet, respectively. The initial PHREEQC calculations were performed using a mixing ratio of one part aquifer water volume to one part of the annual CKD leachate volume. Sensitivity runs were also performed for 10 and 100 parts annual CKD leachate volume to one part aquifer water volume. The results of the PHREEQC simulations are summarized in Table 7.5. The results of the groundwater-leachate mixing simulations indicate that at reasonably expected volumes of CKD leachate, the alluvium and Dakota Sandstone aquifer pH will not be adversely impacted by an hypothetical CKD leachate release. At higher CKD leachate volumes, the alluvial water quality may be adversely impacted. Closure of the CKD pit could be designed to minimize the potential volume of leachate formed by limiting precipitation infiltration into the disposed CKD. 7.3.6 Summary The results of the solute transport analyses suggest that a CKD leachate release will not adversely impact groundwater quality in either the St. Vrain Creek alluvium or the underlying Dakota Sandstone. The solute transport model predicts that the hypothetical leachate release will be sufficiently dispersed during transport so that the existing groundwater quality in the alluvium or the Dakota Sandstone is not adversely affected. The solute transport simulations were performed using conservative model parameters.. Under these conservative conditions and assuming no constituent attenuation, the concentration of leachate constituents released to groundwater do not cause the Colorado primary or secondary drinking water standards or agricultural water quality standards to be exceeded at the hypothetical receptor wells completed in the alluvium or the Dakota Sandstone. 19