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,, RECEIVED J~BBA MEMORANDUM APR 28 ypp~~-~- water consultants pivisiooolMinerelsa~~g~~p_gROCDEN ASSOCIATES, rNC. To: Stephanie Haenny, File 9641.01 From: Timothy A. Crawford Subject: Loveland Ready-Mix Property - Larimer Pits - Drnwdown Modeling Summary Job: 964 LO1 Date: April 21, 2006 This memorandum summazizes the modeling of drnwdown resulting from the pumping of two gravel pits at our client's (Loveland Ready-Mix) Larmier Pits property (property). The pits and property are generally located in Sections I5, 16 and 17, Township 5 North, Range 69 West of the 6'" P.M., as presented in Figure 1. This effort was completed to provide Loveland Ready-Mix with the information they need to satisfy a request from the Division of Minerals and Geology (DMG) for an analysis of the "expected cone of depression resulting from the dewatering of cells 8 and ]0." Accordingly, we modeled the azea and the dewatering of the pits using the MODFLOW 6.0 program to provide this information, as described below. This effort benefited from information in our files and imformation provided by the client. Summary • Our naming of the pits references Pit 8 as Pit 18 and Pit 10 as Pit 20. There was some discrepancy between the pit names provide to our office and to the DMG. • The MODFLOW 6.0 program was used to model the alluvial aquifer and the dewatering of the gravel pit at the property. The model was developed with very simplified inputs. The model was operated in steady state mode, a flat earth water table was used and a flat aquifer bottom was assumed. • The boundaries of the model were the boundaries of the alluvial aquifer azound the property determined based on a review of USGS topographic mapping. The boundaries of the model were set as no flow boundaries. • The no flow boundaries modeled the aquifer with no upstream inflow or downstream outflow, but the modeled drnwdown did not extend to the upstream or downstream boundaries, indicating that dewatering impacts do not extend to the upstream and downstream boundaries. The no flow boundaries also eliminated interaction with bedrock units beneath and adjacent to the alluvial aquifer. • The alluvial aquifer was modeled with cells of equal thickness and elevation with a beginning saturated thickness of 10.0 feet based on test bore log data provided by the client and well information available from the State's Master Wel] List. • Three distinct hydraulic conductivities were used in the model to represent three distinct features of the alluvial aquifer at the property. A high hydraulic conductivity (9999 gpd/ft) was used for the aeeas of the existing unlined pits. A low hydraulic conductivity (1 gpd/ft) was used for the areas of the existing lined pits. These low hydraulic conductivity areas acted as rto flow boundaries within the model domain. A moderate hydrnulic conductivity (250 gpd/ft) was used for the undisturbed alluvial aquifer material aeeas.