2023-05-25_PERMIT FILE - M2022048 (16)

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L.G. Everist West Farm Groundwater Modeling GEOTECHNICAL INVESTIGATIONS The general subsurface lithology at the West Farm site consists of one to four feet of overburden at the surface, underlain by alluvial sand and gravel deposits, followed by weathered Laramie Formation bedrock measuring about 2 feet thick, and finally less weathered Laramie Formation bedrock. The total depth to bedrock for this model was estimated to vary from about 27 to 74 feet thick. Subsurface lithology data was obtained from three separate geotechnical investigations. The first investigation on the property, consisting of 31 borings, was performed by L.G. Everist in October of 2018. In July of 2019, Drilling Engineers installed seven monitoring wells. The third investigation, consisting of four borings, was conducted by Deere&Ault in January of 2022. The information from these three investigations was used to create the bedrock contours used in the groundwater model. These boring locations are shown on Figure 1. ANALYTICAL PARAMETERS Conceptual Model The conceptual model of the aquifer system analyzed for this investigation consists of one layer, the unconfined sand and gravel of the South Platte River alluvial aquifer. Hydrologically the aquifer is bounded on the bottom by bedrock consisting of low permeability Laramie Formation. The bedrock acts as no-flow boundaries because the permeability of the rock is much lower than that of the alluvial aquifer. The aquifer is comprised of alluvial deposits, which consist primarily of well graded to poorly graded gravelly sand. The hydraulic conductivity (K) of the aquifer is generally around 500 feet per day (Colorado's Decision Support Systems GIS map). The saturated thickness ranges from 25 to 30 feet. The principal source of water for the aquifer is the South Platte River. Other minor sources include precipitation and seepage from irrigation ditches. In the project area, the South Platte River acts as a drain (sink) because it is a gaining stream, that is, the adjacent groundwater is higher than the water in the river. Steady-state conditions were simulated because the maximum water level rise is of principal interest and the time required to reach steady state is not of concern. Groundwater Modeling Software The MODFLOW 2005 computer code was used to simulate ground water flow by solving the 3-dimensional groundwater flow equation using a finite-difference method where the model domain is subdivided into a grid of cells, and the hydraulic head is calculated at the centroid of each cell(Harbaugh, et al., 2000). Groundwater flows into and out of the model via constant head, head-dependent flux, and constant flux boundaries. These flows are calculated in the same manner for each simulation. Pre-and post-processing of MODFLOW 2005 files were completed using Groundwater Vistas Version 8.03 Build 3 (Rumbaugh & Rumbaugh, 2017), a graphical design system for MODFLOW. Groundwater Vistas facilitates model construction, data analysis and data presentation. It summarizes results as contours, shaded contours, velocity vectors and detailed mass balance analyses. This section discusses the modeling assumptions, limitations, solution techniques, and the way that they affect the models. When analyzing the groundwater flows in the model, as implemented, MODFLOW 2005 simulates the system as unconfined with isotropic hydraulic conductivity. One limitation is that cells can go"dry" or"flood". If the calculated head is above the top of the aquifer(ground surface) at any model cell, then that cell is flooded and will be treated as if the aquifer is confined (i.e., the saturated thickness will equal the top-elevation minus the bottom-elevation). If the calculated head falls below the bottom of the aquifer, that cell is dry and will be assigned a zero value for hydraulic conductivity. Project 20C26026.02/May 19,2023 Page 2 Deere&Ault,a Schnabel Engineering Company