| GROUNDWATER
AND SURFACE WATER Introduction Water is vital to all living things on the Earth and though there appears to be an abundance of water on the globe (about 1.46 trillion cubic kilometers) the amount of water usable to humans is diminishing. Change in the location and state of water on the Earth is part of a natural process that is called the hydrologic cycle  Although the hydrologic cycle is operating constantly and will never end, human activities constitute a major interference in the natural process. Human activity that has impacted the natural cycle include:
In this section we will review some of the basic science behind ground and surface water with some focus on issue particular to New Mexico. Web links for activities and more information related to ground and surface water are also included. Groundwater Groundwater is water that infiltrates through the surface of the Earth and resides in pores or cracks in the subsurface. Underground waters constitute approximately 1.05% of the total water in the Earth. Groundwater resides in small open spaces such as pores or fractures below the Earth’s surface except where large caves (Carlsbad Link here) have formed due to dissolution of the surrounding rocks. The amount of open space available for water is referred to as the porosity. Rocks with a high porosity include sandstone where the volume of pore space can be as high as 30%. In unfractured bedrock, porosity can be below 10%. Permeability is another important characteristic of materials and refers to the ability of a solid to allow fluids to pass through. A rock or sediment with high (or good) permeability has many pores that are well connected so that water can pass easily through the material. A good water reservoir has both high permeability and porosity. The groundwater table is the boundary that separates an upper zone where air and water occupy pores from a lower zone where all pores are saturated with water. The upper zone is referred to as the unsaturated zone; the lower zone is the saturated zone. The depth to the groundwater table depends on several factors including the rate of discharge and recharge, time of year, the topography and the local geology. Dramatic changes in the groundwater table due to over pumping of water wells can have many serious negative effects such as ground subsidence, formation of fissures and sinkholes, reduction in groundwater quality and, near the ocean’s edge, incursion of salt water into freshwater aquifers. In areas where withdrawal of groundwater exceeds recharge of the source, a cone of depression (link to photo or site) that has a shape of an inverted cone forms and the water level in the well is depressed below the water table. Administering to problems of mining groundwater (that is, when withdrawal of water exceeds recharge) is very difficult and, as in the case of ground subsidence, the effects may be irreversible. Decreasing withdrawal is the clearest solution to problems in balancing recharge and discharge of groundwater. Injecting water into source areas has also been proposed for some areas. Caves and Karst Topography Most groundwater exists between pores and spaces in sediments or within fractures of bedrock. However, in areas where limestone is abundant and rainfall is sufficient groundwater may be a significant erosion agent producing large cavities and caves. The dissolution of limestone to form caves is enhanced in part due to the mixing of water and atmospheric carbon dioxide to form a weak acid called carbonic acid. Limestone begins to dissolve as water seeps into cracks and fissures resulting in formation of larger and larger cavities. The erosive power of the water may also be enhanced by combination of water with carbon dioxide form organisms and plants within the soil. Formation of stronger and more corrosive sulfuric acid may also occur where sulfur-producing organisms exist. The amount of limestone removed may be immense considering that some caves are kilometers in length. More information on caves and cave formation can be found at: National Caves Association Welcomes You! Carlsbad Caverns National Park: Official Home Page Karst topography is found in areas where limestone occurs near the Earth’s surface and is characterized by sinkholes, caverns and paucity of surface streams. Dissolution of limestone leading to formation of karst topography is facilitated in areas with high-rainfall, abundant vegetation, and fractured limestone. Karst topography is found in Indiana, Kentucky and Florida. The following links show images and more information on karst topography. Karst Topography Florida Karst Topography Karst Topography Paper Model Groundwater in New Mexico In 1990, groundwater supplies 47% of all water supply in New Mexico with surface water from reservoirs supplying the remaining 53% (Garrabrant, 1994, Water Use in New Mexico, USGS, Water-Resources Investigations Report 93-4199). Water use in New Mexico is dominated by agriculture with 80% of all resources used for irrigation of crops. Public water supply including drinking water and other municipal uses required 7.2% of resources whereas evaporation from reservoirs contributed 7.7% of the total water supply. The remainder of the total supply was used for power generation and mining. The city of Albuquerque serves as an example of a municipality that relies solely on groundwater for the water supply. Prior to 1993 the aquifer beneath Albuquerque that supplies all of the water supply was thought to be limitless; the size of Lake Superior and replenished by infiltration from the Rio Grande and other sources. A United States Geological Survey report released in 1993 showed this model to be in error. Instead of a large, easily replenished underground reservoir the aquifer is actually limited to a small portion of the valley within the sediments of the Santa Fe group. Also, data suggests that infiltration from the Rio Grande into the aquifer is negligible. There is much less freshwater then previously thought and it is being depleted at an alarming rate – water levels have dropped near 160 feet in 33 years and, recently, more rapidly in some areas. Although deeper drilling may tap new resources the quality of the water is in question. As a result of this finding and the recent explosive population growth, Albuquerque is faced with some daunting challenges regarding careful use of this valuable resource. Water conservation, waste treatment, injecting water into the aquifer and supplementing ground water with surface supplies are some of the efforts that are underway to work towards maintaining a clean and ample supply of water for the Albuquerque area. |