Hydrology of Area 61, Northern Great Plains and Rocky Mountain Coal Provinces, Colorado and New Mexico

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7.0 GROUND WATER -- Continued 7.4 Aquifer Tests Aquifer Tests Reveal Differences in Aquifer Permeability The alluvium is a more permeable aquifer than the bedrock. Aquifer tests (fig. 7.4 -1) measure the ability of a formation to transmit water. The results of aquifer tests in Area 61 are summarized in table 7.4 -1. The specific capacity of a well is the discharge divided by the drawdown of the water level in the well during pumping, as expressed in gallons per minute per foot of drawdown. Where wells are constructed similarly, aquifers with greater permea- bility, such as the alluvium, have greater specific capacities than aquifers with less permeability, such as the Raton - Vermejo- Trinidad aquifer. The small specific capacity of the Cuchara- Poison Canyon aquifer indicated in table 7.4 -1 results because the formations comprising this aquifer are saturated only near the base in the area where the tests were per- formed. Such aquifers have small specific capacities because they have little water in them and are rapidly .drained by pumping, even if they are known to be highly permeable, as in the case of the Cuchara- Poison Canyon aquifer. The hydraulic conductivity is the rate at which ground water moves through a unit cross- sectional area of an aquifer -under a unit hydraulic gradient, as expressed in feet per day. The data in table 7.4 -1 indicate that ground water may move more easily in the alluvium than in the bedrock, and that ground water in the Raton - Vermejo- Trinidad aquifer moves 70 at about the same rate through coal and sandstone. The apparently large rate of ground -water movement through siltstone and shale in the Raton- Vermejo- Trinidad aquifer is misleading because of a dispro- portionately large hydraulic- conductivity measure- ment in one 2 -foot- thick zone of fractured siltstone. Tests of shale and siltstone in the Raton- Vermejo- Trinidad aquifer usually show these rocks to be nearly impermeable. r The transmissivity is the hydraulic conductivity of an aquifer multiplied by its saturated thickness and is an indication of the volume of water that an aquifer can transmit, as expressed in feet squared per day. Equal transmissivities can result from a thick aquifer with small hydraulic conductivity or a thin aquifer with large. hydraulic conductivity. The data in table 7.4 -1 indicate that the alluvium can transmit much more water than bedrock, that fracturing can increase the transmissivity of a bedrock aquifer, and that in the Raton - Vermejo- Trinidad aquifer, coal and sandstone have about the same transmissivity. Aquifer test data are contained in Griggs (1948), Dames and Moore (1978), Water, Waste, and Land, Ltd. (1980), Howard (1982), Colorado Water Re- sources Division drillers' logs, and unpublished U.S. Geological Survey data.