This report provides a regional overview with the general public in mind: it also contains detailed background that will benefit more technical users. It is a compilation of the most recent geologic mapping and interpretations focusing on groundwater occurrences in the various geologic formations found in the area. This project was funded by a grant from the Colorado Water Conservation Board (CWCB) matched by the CGS through its Severance Tax and Colorado General Operational Funds. Digital PDF/GIS/ZIP download. OF-16-04D
An online map — ON-16-04 Geology and Groundwater Resources of Chaffee County, Colorado — is also available.
Zip file contents
Includes report with text, tables, figures, and appendices
Geologic map plates:
Water quality and type map plates:
GIS Data folder
Contains OF-16-04_ChaffeeCo.mpk map-package file
To view GIS files
GIS map-package (.mpk) files may be viewed using ESRI’s ArcGIS software. The map-package was generated in ArcGIS 10.4.
Disclaimer: These data are intended for use at 1:50,000 scale and are checked for accuracy accordingly. The CGS does not assume responsibility for the use of these data.
Chaffee County, located in the central portion of the Rocky Mountains, has experienced considerable population growth in recent decades with development becoming increasingly reliant on local groundwater resources. This product compiles the most recent geologic mapping and interpretations, focusing on groundwater occurrences in the various geologic formations found in the area. Chaffee County straddles two contrasting geologic terrains: mountain ranges surrounding a central valley. The Arkansas River flows to the south through a deep, structurally-controlled valley that is part of the Rio Grande rift system. The valley is flanked by the Sawatch Range on the west, the Mosquito Range on the east, and the Sangre de Cristo Range on the southern end. As part of the southern Mosquito Range, the Arkansas Hills trend east on the north side of the Arkansas River.
Based on differences in hydrologic properties, hydrogeologic units in Chaffee County can be grouped into three general categories: 1) crystalline-rock aquifers, 2) sedimentary bedrock aquifers and confining units, and 3) unconsolidated Quaternary deposits. The crystalline-rock aquifers are subdivided into Precambrian igneous and metamorphic bedrock and the Cretaceous-Tertiary igneous rocks. The sedimentary bedrock aquifers and confining units consist of the older Paleozoic formations, Pennsylvanian-Permian formations, the Dry Union Formation and other Tertiary sediments. The unconsolidated Quaternary deposits consist of the alluvial aquifer, terrace alluvium, glacial deposits, and other Quaternary deposits that include colluvium, sheetwash, and landslide deposits.
There were 3,355 completed water wells in the Colorado Division of Water Resources database, inventoried in Chaffee County as of February 9, 2016. Permitted uses include: domestic or household use only, commercial, industrial, municipal, livestock, irrigation, monitoring, geothermal, and other (unspecified). Wells and springs were assigned a hydrogeologic unit. Due to various sources of uncertainty, hydrogeologic unit designations were assigned a confidence level value of 1, 2, or 3, with 3 representing the least confidence. The majority of wells in Chaffee County are located in the Arkansas River valley where they tap either the unconsolidated Quaternary deposits or the Tertiary Dry Union Formation.
Chaffee County has a higher than average geothermal gradient (heat flow). Fault patterns bounding the northern Rio Grande rift create conduits for deep groundwater flow systems which tap the greater heat flow; as a result, hot springs and some hot wells are common in the area.
A Chaffee County groundwater quality database was compiled from publicly available databases and reports. The primary electronic data source was the Water Quality Portal (WQP) from the National Water Quality Monitoring Council. Groundwater data was also manually compiled from other publicly available publications not in the WQP. A total of 130 wells and 36 springs had groundwater quality data through 2016. A limited number of sample locations have data from multiple sampling events at the same well or spring. Data analysis for these locations used the maximum value detected, unless it was identified as an anomalous outlier. Where feasible, dissolved water quality data was used preferentially over total data.
Water quality in sampled Chaffee County wells and springs is variable and dependent on local geology, geography, and seasonal influences. It is emphasized that the existing database spans a time period of multiple decades and the data do not represent a synoptic view of water quality conditions. The water quality database was evaluated for water type in each of the hydrogeologic units. Cations (sodium, potassium, calcium, and magnesium) and anions (chloride, bicarbonate, carbonate, and sulfate) data were used to generate Stiff diagrams and/or pie charts. Pie charts were scaled in size relative to the total dissolved solids (TDS) concentration, with larger pies having higher measured TDS. The most common water type was a sodium- or calcium-bicarbonate water, followed by a sodium- or calcium-sulfate water.
Groundwater quality was also evaluated against the federal Environmental Protection Agency/Colorado maximum contaminant level (MCL) primary and secondary standards for drinking water. Constituents having primary and/or secondary MCL exceedances were mapped using circles of varying sizes that were blue for concentrations at or below the MCL and red if they exceeded the MCL. Parameters with concentrations that exceed primary or secondary standards include pH, total dissolved solids (TDS), fluoride, nitrate/nitrite, iron, manganese, and uranium. Results of the water quality evaluation showed few exceedances throughout the county, except for fluoride which is present throughout the county.