Geology of the Santa Fe Region
The availability of good ground water from well drilling is influenced by a number of factors. This part of our page will address some of those factors in order to give you a better understanding of the process.
Hydrologic Cycle
The availability of water in any region is dependent upon the storage capacity of the geological make up of the region and the dynamic activity of water replentishment. The "hydrologic cycle" is a term used to describe the evaporation of water from larger sources -- such as the oceans and seas -- into the atmosphere in the form of rainclouds, which then drop moisture onto the land. This dynamic also includes the porousity of the rock and soil and its ability to allow water to flow and be retained as ground water for recovery by a well system.
Ground Water
Ground water is that part of precipitation that infiltrates through the soil to the water table. The unsaturated material above the water table contains air and water in the spaces between the rock particles and supports vegetation. In the saturated zone below the water table, ground water fillsin the spaces beetween rock particles and winin bedrock fractures.
Where Ground Water Occurs
Rock materials may be classified as consolidated rock, such as sandstone, limestone, granite, etc. Two characteristics of all rocks that affect the presence and movement of ground water are porosity (size and amound of void spaces) and permeability (the relative ease with which water can move through spaces in the rock).
Consolidated rock may contain fractures, small cracks, pore spaces, spaces between layers, and solution openings which are usually connected and can hold water. Bedded sedimentary rock contains spaces between the layers that can transmit water great distances. Most bedrock contains verticle fractures that may interstect other fractures, enabling water to move from one layer to another. Depending upon the size and number of connected openings, bedrock may yield plentiful water to individual wells or be a poor water-bearing system.
Unconsolidated material overlies bedrock and may consist of rock debris transported by glaciers or deposited by streams. Well-sorted unconsolidated material can store large quanitities of water: the coarser materials -- sand and gravel -- readily yield water to wells.
Aquifers
Most of the void spaces in the rocks below the water table are filled with water. Wherever these water-bearing rocks readily transmit water to wells or springs, they are called aquifers.
The Movement of Ground Water
Ground water is constantly moving by the force of gravity from recharge areas (such as glaciers and heavy snow and rain areas in the mountains) to discharge areas. Groundwater movement in most areas is slow...a few feet per year. But in more permeable zones, such as solution channels in limestone, movement can be as much as a few feet per day.
Effects of Long Term Climatic Trends on Ground Water Storage
In latitudes where freezing is common, there is less recharge from rain or snowmelt during the winter, which causes the water table to fall. Freezing, either long-term or temorary, inhibits water saturation and recharge of the water table. The saturated zone beneath the water table is recharged by the excess water that is not discharged into streams. In late spring, summer, and early fall, evaporation and transpiration by plants capture most of the water that would otherwise recharge the aquifer. A seasonal decrease in ground-water storage results, as indicated by declining water levels in wells.
In addition to seasonal fluctuations in permeability and refreshing of the water table, long term trends result from variations in precipitation. Several years of below normal precipitation causes a progressive decline in ground-water levels.
Types of Wells
Most modern wells are drilled by truck-mounted percussion (cable-tool) or rotary (air or hydraulic) drill rigs. Dug wells are still constructed in some areas, either by power equipment or by hand, but most hand-dug wells are the "relics" of older homes and were dug before drilling equipment was readily available or because drilling was too expensive.
Wells and Pumpage
Even though water is present at some depth at almost any location, the success of obtaining an adequate domestic supply (usually 5 gallons per minute) of water from a well depends upon the permeability of the rock. Where permeable materials are near land surface, a shallow well may be adequate. Elsewhere, such as where clayey material overlies bedrock, a deep well extending into bedrock may be needed.
Pumping a well lowers teh water level around the well to form a cone of depression in the water table. If the cone of depression extends to other wells, the water level of those wells will be lowered. For a given rate of withdrawal, the cone of depression extends deeper in low-yielding aquifers than high-yielding ones.
Water-Level Declines
Few wells ever actually go dry. Instead, what occurs most often is that the water table had dropped to near or below the pump intake because the pump intake is not set deep enough to allow for a potential decline in water levels. Another possibility is that the strainer which covers the end of the pump could be partl clogged so that it takes longer to pump the same amount of water.
Increased Pumping in Immediate Areas
Another reason that wells "go dry" is the lowering of the water table by increased pumpage in the immediate area. Housing developments with small lots and individual wells have a higher demand than larger, more widely spaced lots with a centeral water system. Also, if there are industrial or agricultural businesses that are drawing large amounts of water from an aquifer, the water level will drop accordingly.
For more information look for Ground Water and the Rural Homeowner, by Roger M. Waller or refer to our reference page.