3.2 Aquifers

3.2.1 Description

Three types of aquifers occur in the Planning Area: fractured crystalline-rock, glacial fill and alluvial deposit aquifers. Well yields from crystalline rock aquifers are typically small to inadequate, from glacial fill, medium to small (inadequate <1 gal/minute, small <15 gal/min., medium 15-100 gal/min.). Crystalline rock of igneous or metamorphic origin (mostly Boulder Creek granodiorite) is generally impermeable, and functions as an aquifer only where the rock is fractured as a result of jointing or faulting. Over 95 percent of the Planning Area is underlain with crystalline-rock aquifers from which the vast majority of water is drawn for domestic use. The regional direction of water movement in the aquifers is along the fractures and principally to the east (Hall, Hillier et al, 1980). These fractures typically decease in size with increasing depth and may be reduced significantly below 300 feet depth. Water may be excluded from these fractures by small particle soils, or conversely, the fractures may act as conduits for the movement of water and any contaminants contained therein.

Glacial fill aquifers consist of poorly- to well-sorted material of various sizes, from silt to boulders, deposited by glaciers. The glacial fill measures approximately 0-50 feet deep where it occurs. No more than 5 percent of the Planning Area is covered with this glacial fill (some sources state that no glacial fill is found in the Planning Area, but instead is alluvial fill), although it is likely the cause of the most significant natural body of water in the Magnolia area, Giggey Lake on Reynolds Ranch (Figure 8.5). This lake may have been enhanced by dam construction (Dave Hallock, personal communication). Other small areas may retain remnants of this once more widely spread soil. Water yield and quality from glacial alluvium is generally sufficient for domestic use. Depth to water averages half that in the crystalline-rock aquifers and yield is generally greater (average 30 percent) than from crystalline aquifers (Hall, Hillier et al, 1980). Riparian alluvial deposits exist only in very restricted zones along the few small streams (Forsythe Creek and Winiger Gulch) in the Planning Area.

Bedrock is overlain with soil that permits precipitation to seep into underlying fractures. Soils studies in the Planning Area listed three predominant types: Fern Cliff, Juget, and Peyton (Moreland, USDA Soil Conservation Service, 1975)¹. These soils were rated for suitability and limitations under several uses, summarized in Table 3.1. Water capacity is similar in all three soil types present in the Planning Area, with one soil type somewhat lower than the other two in available water-storage capacity. From these data it is reasonable to assume that less than one-third of the land in the Planning Area is suitable (without engineered solutions) for septic tank leach fields.

3.2.2 Aquifer Recharge/Discharge

Precipitation is the primary means of recharge of aquifers in the Planning Area; streamflow contributes very little. Recharge is greatest in spring and early summer during snowmelt, however, late summer storms contribute a small (almost insignificant) amount. Recharge occurs by the simple process of infiltration, which is dependant on the permeability of soils in the area and the localized fracture systems in the bedrock. The thin layer of soil in this mountainous area plays a significant role in temporary storage of water as it percolates into the underlying aquifers. Once the soils are saturated, the remaining snowmelt quickly runs off, following stream courses toward the plains. Because of variations in winter weather, from - 40 to + 50 degrees F, it is likely that recharge occurs throughout the year, especially along south-facing slopes where snow rarely is permanent and the ground may remain largely unfrozen.

Discharge from aquifers occurs by evapo-transpiration, seepage, spring flow, pumping, and exchange between aquifers. Significant evaporation can take place during Chinook winds, removing snow cover at rates exceeding 1 inch/hour. Colorado's intense sunlight can further evaporate snow and rain fall, especially from south-facing slopes. Significant evapo-transpiration occurs from vegetation cover.

3.2.3 Aquifer Depth, Flow Rates, Rate of Draw-Down

Parallel fractures that act as aquifers in the region may or may not have direct hydraulic connection. Therefore, water levels in adjacent aquifers (and wells) may not be interdependent.

The amount and pattern of precipitation does not appear to have a direct effect on the water levels from month to month (Hall and Johnson, 1979). However, studies in Evergreen, Colorado, a location similar to the Planning Area with respect to the underlying crystalline-rock aquifers, pointed to a long-term relationship, perhaps as long or longer than a year, between changes in precipitation and water levels in wells. Water levels in the Planning Area do follow a predictable annual trend, lowest in February. Drawdown of water from the crystalline-rock aquifer then is dependent on fairly rapid recharge. It is possible that over-pumping could affect the water storage in the fracture system.

Hall, Hillier et al. in their 1980 report on Water Resources of Boulder County, studied water-levels during 1958 and 1959 in several wells. These well levels were re-measured in 1976. Only two sample wells were obtained in the crystalline-rock aquifer; both were relatively shallow and both showed a decrease in water level, one a significant loss of 28 feet over the period of approximately 18 years. The well of greatest loss is located in a region topographically and geologically similar to the Planning Area. Local factors were probably the cause of the change in well levels.

Data from the State Engineer's records of wells drilled in the Planning Area (data on over 600 wells were available) is presented in Table 3. 2.