Aquifer Characteristics

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Geologic Properties

The Dawson Aquifer rests atop a double-plunging syncline with high-angle dips along the western basin margin and low-angle dips in the central and eastern parts of the basin.

The upper section of the Dawson aquifer represents the top of the bedrock units within the Denver Basin. It is covered by a very shallow layer of alluvial aquifer, and most of the outcrop is exposed at the surface. (Musgrove et al., 2005)

Table 1. Stratigraphic, hydrogeologic, and lithologic characteristics of the Dawson aquifer, Colorado (adapted from Paschke, 2011, and Robson and others, 1998). (Musgrove et al., 2005)

Hydrologic Properties

The Dawson aquifer is divided into upper and lower aquifers due to an intervening 'confining' unit composed of clay and shale at the top of the lower aquifer sequence. Note that the aquifer is not subdivided but is considered a single aquifer unit, while the 'confining' unit serves as an administrative boundary for designation purposes. The aquifer is regarded as unconfined. (Paschke, 2011)

Thickness: 100 - 1100 ft
Base Altitude (Lower Aquifer): 5400 (North) - 6800 (South) ft
Base: 1100 ft below the land surface

Figure 5. Conceptual block diagram illustrating hydrogeologic features of the Denver Basin aquifer system, Colorado (Paschke, 2011)

Table 2. Simulated groundwater budget for individual bedrock aquifers and confining units in the Denver Basin, 2003. (Paschke, 2011)

**[In some cases, values do not sum because of rounding; all flow rates are in cubic feet per second (ft3/s); <, less than; blank, not applicable]

Aquifer Parameters

Hydraulic Conductivity

- Median: 0.80 ft/d
- Geometric Mean: 0.7 ft/d

Transmissitvity

1200 ft2/day

Storage Coefficient Values

0.0002 - 0.0006

Mean Specific Yield

15.2%

(Paschke, 2011)

Table 3. Summary of hydraulic-conductivity values calculated from pumping tests (adapted from Colorado Division of Water Resources [2005]). [ft/d, feet per day] (Paschke, 2011)

Table 4. Estimated pumping rates assigned to wells in the Denver Basin model (Paschke, 2011)

Chemical Character of the Groundwater

Table 5. Summary statistics for well completion information, physicochemical properties, and selected geochemical constituents for groundwater samples from the Denver Basin aquifer system, Colorado, 2003–5.
[MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; HBSL, health-based screening level; AMCL, alternative maximum contaminant level; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter; FNU, formazin nephelometric units; °C, degrees Celsius; μg/L, micrograms per liter; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13; ‰, per mil; pCi/L, picocuries per liter; Ca, calcium; Mg, magnesium; HCO3, bicarbonate; Na, sodium; Cl, chloride; Br, bromide; CaCO3, calcium carbonate; N, nitrogen; P, phosphorus; min., minimum; max., maximum; na, no standard or not applicable; %, percent; <, less than; --, not measured or not determined; n, number of samples; =, equals; E, estimated] (Musgrove et al., 2005)

Relationship to Unsaturated Zone

Transport

An increase in urban and agricultural recharge can alter the groundwater flow system and affect the water quality of the Denver Basin aquifers. It can also transport constituents naturally and human-sourced contaminants from the unsaturated zone to the water table.

Temperature

In areas where the unsaturated zone is a few feet thick, the mean annual surface soil temperature is generally within 1°C of the recharge temperature, which is the temperature at which water first enters the groundwater system.

Chemical Analysis

In the Denver Basin, manganese (Mn) is found in the unconsolidated sediments of the unsaturated zone and in the sedimentary rocks of the bedrock aquifers. Selenium (Se), uranium (U), and arsenic (As), which are naturally present in the sediments and rocks of the Denver Basin, have established Maximum Contaminant Levels (MCLs) for drinking water of 50, 30, and 10 μg/L, respectively.

For the bedrock aquifers, a few samples exceeded the MCLs for Se, U, or As. The concentrations of these elements were generally transitional for the Dawson aquifer, with higher values found in water table wells and lower values in deeper bedrock aquifers. The median concentrations of these trace elements in the Dawson aquifer were typically at least ten times greater than those in the deeper bedrock aquifers. This suggests that the Dawson aquifer, being the shallowest, is more susceptible to contamination, likely due in part to the downward movement of surface water into the groundwater. Irrigation water increases, and evaporation in the subsurface are the possible culprits in mobilizing these trace elements from rocks and soils in the unsaturated zone and transporting them to the water table.

Results from groundwater samples from the water-table wells show that shallow groundwater quality is degraded. The causes are likely due to natural factors and human activities. Land use changes in agriculture and urban development have impacted the recharge patterns and composition within the basin. The additional recharge from irrigation water leads to the movement of soluble salts and nutrients built up in the unsaturated zone under the region's natural, semiarid conditions.

(Musgrove et al., 2005)

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