Groundwater Research Techniques

		Well Location Surveys
	Groundwater Levels	Hydrographs: water levels vs. time	Potentiometric Surface Maps
Geophysical Log Interpretations	Groundwater Quality	Chloride Analyses		TDEM Surveys
	Water Use Surveys	Aquifer Framework Development	Rainfall-Recharge Determinations
		Groundwater Modeling

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 groundwater flow models produced by the branch.

Groundwater 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. Groundwater 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.

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 groundwater 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 groundwater 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.

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 groundwater 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 groundwater users in that area (down to -120 feet msl). The two maps demonstrate that the fifteen wells monitored in the mid to late 1990s do not define that potential surface adequately (current monitoring is substantially improved).

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.

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 groundwater moves through a subsurface material), clay content (clay is electrically conductive), and salinity of the groundwater (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
2. Function	3. Surface
4. Dataset	5. Comparison

The source of groundwater 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 groundwater 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.

The following graph illustrates groundwater 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 groundwater 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 Wastewater Disposal Practices on the Groundwater of the North Carolina Barrier Islands, Final Report: EPA 904/9-85 139, 332 p.)