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Ground Water and Surface Water GROUNDWATER & SURFACE WATER: UNDERSTANDING THE INTERACTION A GUIDE FOR WATERSHED PARTNERSHIPS SECOND EDITION GROUNDWATER: A HIDDEN RESOURCE. About half of irrigated cropland uses INTRODUCTION. groundwater. Water. It’s vital for all of us. We depend on Approximately one third of industrial its good quality—and quantity—for drink- water needs are fulfilled by using ing, recreation, use in industry and growing groundwater. crops. It’s also vital to sustaining the natural systems on and under the earth’s About 40% of river flow nationwide surface. (on average) depends on groundwater. Groundwater is a hidden resource. At one Thus groundwater is a critical component time, its purity and availability were taken of management plans developed by an for granted. Now, contamination and increasing number of watershed partner- availability are serious issues. ships. Some facts to consider... GROUNDWATER ABC’S. Groundwater is the water that saturates Scientists estimate groundwater the tiny spaces between alluvial material accounts for more than 95% of all (sand, gravel, silt, clay) or the crevices of fresh water available for use. fractures in rocks. (See illustration of Approximately 50% of Americans groundwater profile.) obtain all or part of their drinking water from groundwater. Aeration zone: The zone above the water Nearly 95% of rural residents rely on table is known as the zone of aeration groundwater for their drinking supply. (unsaturated or vadose zone). Water in the soil (in the ground but above the water table) is referred to as soil moisture. Spaces between soil, gravel and rock are filled with water (suspended) and air. Aquifer: Most groundwater is found in aquifers—underground layers of porous rock saturated from above or from structures sloping toward it. Aquifer 2 capacity is determined by the porosity of Infiltration: Movement of water into subsurface material and its area. Under and through soil. most of the United States, there are two major types of aquifers: confined and Permeability: The capacity of rock or unconfined. soil to transmit fluid, usually water. Capillary water: Just above the water Saturation zone: The portion that’s table, in the aeration zone, is capillary saturated with water is called the zone water that moves upward from the water of saturation. The upper surface of this table by capillary action. This water can zone, open to atmospheric pressure, is move slowly in any direction, from a wet known as the water table (phreatic particle to a dry one. While most plants surface). rely on moisture from precipitation in the unsaturated zone, their roots may also tap Unconfined aquifers: More common into capillary water or the saturated zone. than confined aquifers, unconfined aquifers have a permeable deposit that Confined aquifers: (also known as leads into the aquifert. Water may have artesian or pressure aquifers) exist where arrived by percolating through the land the groundwater system is between layers surface. This is why water in an of clay, dense rock, or other materials with unconfined aquifer is often very young, very low permeability. in geologic time. The top layer of the aquifer is also the water table. Thus, it’s Water in confined aquifers may be very affected by atmospheric pressure and old, even millions of years old. This water changing hydrologic conditions. is under more pressure than water in unconfined aquifers. Thus, when tapped by Vadose zone: The area of soil and rock a well, water is forced up, sometimes just above the water table. above the soil surface. This is how a flowing artesian well is formed. Water-bearing rocks: Several types of rocks can hold water, including... Evapotranspiration: Water returned to the atmosphere by evaporation from water Sedimentary deposits (i.e. sand and and land surfaces, and by the activity of gravel), living plants. Channels in carbonate rocks (i.e. Hydrologic cycle: Complete cycle through limestone), which water passes from the atmosphere Lava tubes or cooling igneous and, eventually, returns to the atmosphere fractures in igneous rocks, (See illustration on page 4). Fractures in hard rocks. Impermeable: Having texture that does not permit water to move through quickly. 3 THE HYDROLOGIC CYCLE. It’s crystal clear. Groundwater and surface As rain or snow fall to the earth’s surface: water are fundamentally interconnected. In Some water runs off the land to rivers, fact, it’s often difficult to separate the two lakes, streams, and oceans (surface because they ‘feed’ each other. This is water). why one can contaminate the other. Some water returns to the atmosphere by evaporation or transporation. A CLOSER LOOK. Some water infiltrates the soil To better understand the connection, take a where it can... closer look at the various zones and - Be absorbed by plant roots, or actions. A logical way to study this is by understanding how water recycles, the - Continue to move down to become hydrologic (water) cycle. (See illustra- groundwater, or tion.) - Move down and then sideways (laterally) or backup (via capillary action) to become surface water CLOUDS FORM PRECIPITATION RUNOFF TRANSPIRATION 4 INFILTRATION PLANT INTAKE GROUND WATER ACCESSED THROUGH WELL through wells, natural springs, marshes, streams, etc. This is why successful watershed partner- Although its rate of movement varies, ships take a special interest in the groundwater very slowly moves unconfined aquifer adjacent to the stream, toward low areas (including streams lake or other surface waterbody. and lakes) where it is discharged. Here it becomes surface water again. HOW SURFACE WATER CAN And, upon evaporation, it completes the cycle. ‘FEED’ AN UNCONFIED AQUIFER. This movement of water between the earth The source of groundwater (recharge) is and the atmosphere through runoff and through precipitation or surface water that infiltration, evaporation, transpiration, and percolates downward. Approximately 5- precipitation is continuous. 50% (depending on climate, land use and many other factors) of annual precipitation HOW UNCONFINED results in groundwater recharge. AQUIFERS CAN ‘FEED’ Losing streams. In some areas, streams literally recharge SURFACE WATER. the aquifer through stream bed infiltration. One of the most commonly used forms of These are called losing streams. groundwater comes from unconfined shallow water table aquifers. ACHIEVING BALANCE. These aquifers are major sources of Left untouched, groundwater naturally drinking and irrigation water. They also achieves balance— interact closely with streams, sometimes discharging and recharging— flowing (discharging) water into a stream depending on hydrologic conditions. In fact, or lake and sometimes receiving water some streams are gaining streams at times from the stream or lake. and losing streams at other times. Gaining streams. An unconfined aquifer that feeds streams provides the stream’s baseflow. This is called a gaining stream. In fact, groundwater can be responsible for maintaining the hydrologic balance of surface streams, springs, lakes, wetlands, and marshes. 5 INTEGRATING DELINEATIONS*. THE QUESTION OF COMMON BOUNDARIES. The boundaries of aquifers are often BOUNDARIES. difficult to map. It requires someone with Partnerships using the watershed approach an understanding of the geology and to protect natural resources identify and surface drainage above the aquifer. understand the individual resources— water, soil, air, plants, animals and An unconfined aquifer area often ex- people—early in the process. tends to the surface waterbody’s (i.e. lake, river, estuary) watershed. When determin- This is why watershed partnerships ing an aquifer protection area, pumping delineate boundaries to address all natural (working) wells are not considered. resources—not just one. They realize that groundwater, surface water, air quality, The biggest risk to an unconfined aquifer is wildlife and human activities all interact the water, potentially carrying contami- with each other. nants, moving through the permeable materials directly above it. This area is Occasionally watershed partnerships run known as the primary recharge area. into difficulty with differing boundaries of Depending on the depth and overlying watersheds and groundwater. If this geologic characteristics, travel time from occurs, consider combining surface and the surface to the aquifer can be rapid. *A deliniation is the groundwater into a single, larger area. In process of determin- other situations—like the transfer of water Less permeable deposits located at higher ing the boundary of to reach distant users—there can, and elevation than the aquifer form a second- an area. In the guide should be, two distinct areas. ary recharge area. These areas also it generally refers recharge the aquifer through both overland how you differentiate Thus, watershed partnerships’ boundaries runoff and groundwater flow. Because the direction that may combine the wellhead area, aquifer, they are less permeable and tend to be a water travels. watershed, and many other areas into one. greater distance from the aquifer, the deposits often lessen contaminants. Additional recharge areas to consider include an adjacent stream that poten- tially contributes to the aquifer through infiltration. When pumping wells are close to a stream, infiltration can be increased. Infiltrating streams typically 6 provide an aquifer with large quantities of water and a pathway for bacteria and When selecting the best method, consider viruses. available funds and the level of concern.
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