Research and Analysis Techniques
Follow these links to learn about the Ground Water Management Section's research methods. Some or all of these analytical techniques may be used during a particular study. The needs of each investigation or the questions to be answered dictate which procedures are used.
With the advent of Global Positioning System (GPS) equipment and software, well location surveys have become easier and more accurate. Using the GPS receiver, employees can locate wells and other objects within 2 to 5 meters of their actual position. The latitude and longitude collected by the GPS receiver is corrected to this high accuracy using data from one of the three NC Geodetic Survey's GPS base stations. Well locations are used in maps, cross-sections, and ground water flow models produced by the Section.
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Ground water levels obtained through wells allow Division personnel to monitor water pressures in aquifers throughout the State. Those levels are used to produce two types of charts that show existing or historical conditions in a particular aquifer. A hydrograph shows the pressure level over time and potentiometric surface map illustrates the geographic distribution of pressure conditions for an aquifer at a particular time. Ground water levels are measured using an electric or steel tape that is lowered down the well. Levels are referred to as a depth below land surface or an elevation (depth above [+] or below [-] mean sea level). Initially, the level is recorded as a depth below the top of the well casing. That measurement combined with the height of the stickup (distance between top of well casing and land surface) and the elevation of land surface (referenced to mean sea level) gives enough information to create hydrographs or potentiometric surface maps. |
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| Hydrographs allow comparison of water levels over time. In the hydrograph below, several wells are pictured (levels are in feet above or below mean sea level). This type of graph can show how ground water levels vary between wells screened in the same aquifer or different aquifers. This example shows three wells from Clarks Research Station. The topmost hydrograph is from a well screened in the surficial aquifer, the next lower is from the Castle Hayne aquifer, and the lowest is from the Black Creek aquifer. Notice that surficial and Castle Hayne ground water levels show seasonal effects of rainfall (higher in the winter and lower in the summer). The Castle Hayne aquifer shows only minimal confinement (lower water levels than the surficial aquifer). Both of these wells are affected by a nearby quarry beginning in 1988. The Black Creek aquifer shows complete confinement (the screen is located at -690 feet). The water level decline indicated is typical for the Black Creek and upper Cape Fear aquifers in the central coastal plain. |  |
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| The potentiometric surface maps pictured below not only delineate the shape and position of the 1986 pressure surface for the Black Creek aquifer, but illustrate the critical need for more ground water level monitoring. With any confined aquifer, the water level in a properly constructed well will reflect the pressure on the water in the aquifer. So, the levels from wells in each map have been contoured (lines of equal potential) to form a surface. That surface clearly shows a cone of depression -- an area of reduced water pressures associated with the ground water users in that area (down to -120 feet msl). The two maps demonstrate that the fifteen wells presently monitored do not define that potential surface adequately. |  |
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Geophysical logs such as the spontaneous potential, gamma, and single point resistance curves above are common measurements taken from an uncased well (click on image above to receive full-size diagram). Using these curves, along with the driller's log allows interpretation of the subsurface aquifer system. In this example, the logs straddle aquifer/confining unit and lithology interpretations. Strong peaks in the gamma curve are indicative of phosphate-rich sediments. The gamma curve (radioactivity) is generally lower (on the left side of the graph) in sands and limestone and higher in clays. Both the spontaneous potential and single point resistance curves respond to the different electrical properties of subsurface materials. In general, the single point resistance curve responds in opposite directions to the spontaneous potential curve. |
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The chloride concentration in samples of ground water are very useful to Ground Water studies. The source of chlorides is almost always the ocean, therefore, the chloride concentration indicates the extent of fresh water recharge (fresh water is derived from landward sources). Chloride values can range from about zero to almost 20,000 milligrams/liter. Usually there is a wide interface zone between fresh and salt water. In North Carolina that zone can extend for tens of miles or be only a few tens of feet thick. Before desalinization procedures, communities looked for fresh ground water sources (those equal to or less than 250 mg/l chloride). Although reverse osmosis and other methods of de-salting are widely available their cost may still be prohibitive and water suppliers still search for fresh sources. We examine chloride levels to help establish confining properties of a clay (discussed under aquifer framework development). Chlorides are used to help us interpret the TDEM response. The location of higher chloride also indicates regions of higher pumping or aquifers with less recharge. Both criteria are critical to understand the ground water flow in an area and provide proper advice to water suppliers and users.
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| Time Domain Electromagnetic surveys involve setting up a transmitter and receiver coil and recording a bulk resistivity profile (see figure 1). Bulk resistivity is a function (see figure 2) of effective porosity (how easily ground water moves through a subsurface material), clay content (clay is electrically conductive), and salinity of the ground water (chlorides make the water more conductive). For a typical North Carolina aquifer TDEM resistivity changes as saltwater and clay replace freshwater (as seen in Figure 3). The example dataset in diagram 4 shows the TDEM response near Scuppernong, N.C. and the resistivity profile model (noise in data is signified by an "x" and not used in calculation). The TDEM resistivity is compared to geophysical logs taken from a nearby research station well in figure 5. Finally, a profile illustrates how logs and interpretations from our research stations can be combined with TDEM resistivity to better understand the subsurface aquifer system. | 1. Setup
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2. Function
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4. Dataset
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It is important to know how much water is being pumped from a particular aquifer during ground water flow modeling efforts or while determining drawdown effects from competing users. Obtaining that data means doing local and regional inventories of users (depending on the aquifer) and accessing databases created to store use information from three Division programs. Those three programs are as follows: 1. Central Coastal Plain Capacity Use Area, 2. Water Use Registration (one hundred thousand gallons per day or more), and 3. Local Water Supply Planning (public entities providing potable water). The two latter programs operate State-wide and CUA #1 regulates water use in a 15 county area in the central coastal plain of NC.
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This procedure creates a three-dimensional picture of an aquifer system. The use of geological descriptions of subsurface material in combination with borehole geophysical logs and TDEM soundings (both described earlier) give us the information with which to form a framework. How are the aquifers (sands and limestones) and confining units or aquitards (silts, clays, and some bedrock) distributed in the subsurface? That framework must satisfy known ground water flow and quality conditions. These constraints are determined by historical and present day ground water levels and water quality analyses from the monitoring well network. Usually, borehole and TDEM sounding interpretations are connected along cross-section lines parallel and perpendicular to the strike of the geologic formations. The hydraulic characteristics of an aquifer vary both vertically and laterally, so many cross-sections are needed to accurately represent the aquifer framework.
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| The source of ground water is rainfall. In the coastal plain two thirds of rainfall is lost via evapotranspiration, and up to a tenth runs off to surface water bodies. The remaining fifth infiltrates the subsurface. Most of that water stays in the shallow ground water system (surficial aquifer) and discharges to surface water. The remaining 2 percent recharges deeper aquifers (see diagram below). Rainfall is not distributed evenly over time or land area. Many rain gauges are set up throughout North Carolina to better assess the geographic distribution of rainfall. At a minimum, precipitation is measured daily to judge the temporal spacing of rainfall events. |
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| The following graph illustrates ground water recharge using rainfall data from Duck, NC. Rainfall amounts are irregularly spaced over time. When evapotranspiration and runoff (assumed to be zero in Duck) are subtracted from rainfall, negative recharge occurs during many of the months on this chart. The ground water recharge curve is computed by tracking the rainfall-evapotranspiration curve until it goes negative -- here, recharge is set to zero. Following a dry spell some of the recharge goes toward soil moisture so the recharge curve falls below the rainfall-evapotranspiration curve. (method is modified from EPA, 1985, The Impacts of Watewater Disposal Practices on the Groundwater of the North Carolina Barrier Islands, Final Report: EPA 904/9-85 139, 332 p.) |
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Mathematical ground water flow modeling using computer based programs allows input of data that describes the hydraulic parameters and physical constraints of the aquifers, aquifer recharge rates, withdrawals via wells and natural sinks, and ground water quality. Ground water levels are extracted during the model calibration process that are compared to actual levels. How well these water levels match determines how well the model simulates the actual ground water system. Often, aquifer parameter sensitivity tests are run to evaluate which variables effect model results the most. Once a model is calibrated, simulations are run that estimate future conditions. In this way, management schemes can be tested to resolve resource issues or conflicts among users. Flow models provide the hydrogeologist a method of understanding a complex aquifer system. In many cases it would not be possible to determine the effects of multiple withdrawals on our complex coastal plain aquifer system any way other than modeling.
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