Gilcrest-LaSalle Hydrogeologic Characterization Report

Prepared for: The Colorado Division of Water Resources and The Colorado Water Conservation Board
By: Peter E. Barkmann, Andy Horn, Annette Moore, Jeremy Pike, and William Curtiss
September 30, 2014

Executive Summary (pdf file)
Complete Report (pdf file)

NOTE: In 2015, an addendum technical memorandum was prepared using water level data in 2013 and 2014, which is available for free download: 2015_TechMemo_2013-2014 mapping_FINAL

Study area location with topographic relief

Study area location with topographic relief

The Gilcrest/LaSalle Hydrogeologic Characterization is a component of the Gilcrest/LaSalle Pilot Project conducted by the Colorado Division of Water Resources (DWR) and funded by the Colorado Water Conservation Board (CWCB). The Gilcrest/LaSalle Pilot Project goal is to evaluate high groundwater conditions in the South Platte alluvial aquifer near Gilcrest and LaSalle, Colorado. For this study the CGS compiled, analyzed, and reinterpreted existing hydrogeologic data from previously published regional studies.

The integration of previous investigations along with new analysis added significantly to the Study Area hydrogeologic characterization, and provided insight into the South Platte alluvial aquifer depositional model, particularly with respect to discontinuous low permeability beds within the central portion of the alluvial aquifer, and lower permeability beds on the aquifer flanks.

Specifically, the CGS hydrogeologic characterization presented herein has:

  1. Developed a hydrograph template to facilitate HydroBase data analysis;
  2. Refined alluvial aquifer extent mapping;
  3. Combined surficial geology from different sources to revise surficial mapping;
  4. Refined the bedrock and alluvial surface geologic mapping;
  5. Refined the alluvial aquifer lithologic and depositional conceptual models;
  6. Compiled driller’s logs from 448 boreholes into a digital lithologic dataset;
  7. Developed updated localized time-series water table contour and depth-to-groundwater maps;
  8. Evaluated groundwater flow in detail;
  9. Identified local-scale water level data gaps;
  10. Identified existing candidate wells for additional water level monitoring and aquifer testing.

Additionally, this study has determined that, despite a large number and distribution of wells monitored, the overall data set contains significant gaps in data continuity and location. These data gaps limit the ability to analyze detailed long-term groundwater level trends throughout the entire study area and to map detailed water table surfaces across portions of the Study Area during long periods.