This open-file report provides a regional overview with the general public in mind, although it contains detailed background that may be beneficial to more technical users. It is a compilation o the most recent geologic mapping and interpretations focusing on groundwater occurrences in the various geologic formations found in the area. It was funded by the CGS through its Severance Tax and Colorado General Operational Funds. Digital PDF/GIS/ZIP download. OF-17-01D
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-17-01_MesaCo.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.
Mesa County is the fourth largest county in Colorado and has the largest population of the 28 counties on Colorado’s Western Slope region. Census figures reveal that the population has doubled between 1990 and 2010. A majority of the county (72%) is public land, with the remainder generally housing the population in 15 communities. There are two cities and four towns with municipal water systems, though recent growth tends to be in unincorporated areas that are reliant on groundwater (wells) for domestic water.
The county is located within the Colorado Plateau physiographic province, characterized by high mesas cut by river canyons, dry gullies and washes, and the beds of intermittent streams. Down cutting by the Colorado and Gunnison Rivers has created the Grand Valley, in which many of the cities and towns are located. There is about a 5,000 foot elevation difference between the Grand Valley and the top of the basalt capped Grand Mesa, located to the east.
The county can be structurally divided into three primary geologic features: the Paradox basin to the southwest, the Piceance basin to the northeast, and the Uncompahgre Plateau separating the two. The Uncompahgre Plateau, also referred to as the Uncompahgre Uplift, is a northwest-southeast trending structural arch with bounding faults on both sides. It is cored with Precambrian igneous and metamorphic rocks, typically only exposed in canyons, the largest of which is the Unaweep Canyon. The Grand Valley abuts the north flank of the Uncompahgre Plateau, along the southern edge of the Piceance basin.
The geology of Mesa County encompasses nearly the entire geologic time scale. Most time periods have rock exposures at the land surface, except for the Cambrian through Mississippian sedimentary rocks. These are only present in deeper portions of the Paradox and Piceance basins where they truncate against the sides of the Uncompahgre Uplift. Overlying much of the bedrock are Quaternary unconsolidated sediments, including alluvial aquifers and terraces, glacial deposits near Grand Mesa, and mass wasting deposits. Based on differences in hydrologic properties, aquifers and confining units in Mesa County can be grouped into three general categories: 1) crystalline-rock aquifers, 2) sedimentary bedrock aquifers and confining units, and 3) unconsolidated Quaternary deposits. Within these categories, there were 15 mapped hydrogeologic units.
There were 2,433 completed water wells in the Colorado Division of Water Resources database, inventoried in Mesa County as of November, 14, 2016. Permitted uses include: domestic or household use only, commercial, industrial, municipal, monitoring, geothermal, irrigation, livestock, and other (such as evaporative, fire, augmentation, or 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, 3, or 4, with 4 representing the least confidence.
A Mesa 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 available in the WQP. A total of 365 sample sites had groundwater quality data through 2017. 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 Mesa 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 majority of the groundwater samples were collected from wells that had a sodium- or calcium-bicarbonate dominated chemical signature. This was followed by a sodium- or calcium-sulfate water. This water was often classified as hard water. The highest sulfate-containing water occurred within the Quaternary Mass Wasting units, which is likely influenced by the underlying bedrock.
Groundwater quality was 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. Exceedances of the MCLs were sparse for fluoride, few for nitrate and arsenic, and significant for selenium and uranium. Similarly, exceedances of the unenforceable secondary MCLs (SMCLs) were few for fluoride and aluminum; a little more for pH; and significant for TDS, chloride, iron, manganese, and sulfate. These types of exceedances are not unusual for marine derived sedimentary geologic formations, which constitute the majority of bedrock where sampled wells were located.