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Regional Groundwater Availability Study Geospatial Data

Press Release: USGS Assesses Current Groundwater-Quality Conditions in the Williston Basin Oil Production Area

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Water-Table Fluctuation (WTF) Method

Key Assumptions and Critical Issues

The water-table fluctuation method is based on the premise that rises in groundwater levels in unconfined aquifers are due to recharge arriving at the water table. Recharge is calculated as the change in water level over time multiplied by specific yield. This approach is a gross simplification of a very complex phenomenon, namely, movement of water to and from the water table (Healy and Cook, 2002).

Favorable aspects of the WTF method include its simplicity and ease of use: it can be applied for any shallow well that taps the water table, and an abundance of available water-level data exists. The method requires no assumptions on the mechanisms for water movement through the unsaturated zone; hence, the presence of preferential flow paths does not restrict its use. Recharge rates calculated with the WTF method are values that are integrated over areas of several square meters to hundreds or thousands of square meters. This is a distinct advantage relative to point measurement approaches, such as methods that rely on measurements within the unsaturated zone. Wells should be located so that the water levels they monitor are representative of the aquifer as a whole (Healy and Cook, 2002).

Specific-Yield Determination

Although simple in concept, the WTF method has drawbacks in its application. The method requires an estimate of specific yield and assumes this value is constant with time. Sophocleous (1985) challenged the validity of this assumption on a theoretical basis, and Sloto (1990, p. 25) showed that specific yield decreased with water-table depth in an aquifer in southeastern Pennsylvania, USA.

The hydrograph depticts only fluctuations caused by recharge

This method is most applicable in areas with shallow water tables that display sharp rises and declines following rainfall events. The method should work best for wells that show a relatively rapid water-level rise in relation to the rate that water moves away from the water table. Errors associated with this method relate to ensuring that water-table fluctuations are related to recharge and not to other factors, such as pumping, evapotranspiration, changes in atmospheric pressure, the presence of entrapped air, and earth tides. There can also be some source of error in the WTF method with closer proximity of a well to a stream because the nearby groundwater levels may respond to changes in stream stage (bank storage effects) (A. Rutledge, U.S. Geological Survey, written commun., 2005).

Extrapolation of the pre-recharge recession

The graphical application of the WTF method is only capable of estimating recharge when water is arriving at the water table at a greater rate than it is leaving, a condition that produces a water-level rise. Recharge can still be occurring even when a well hydrograph shows that water levels are declining. Such an occurrence simply indicates that the rate of recharge is less than the rate of water movement away from the water table (Healy and Cook, 2002) . If water movement away from the water table were equal to the steady recharge rate, no change in water level would occur, and the WTF method would predict no recharge. Use of the new master recession curve approaches greatly mitigate this issue.

Other Issues – Fractured Rock

Application of the WTF method to fractured-rock aquifers offers some unique challenges (Healy and Cook, 2002; Risser and others, 2005) . Fractures usually serve as the primary conduits for water movement, but they account for a small percentage of the total storage available in the aquifer. Therefore, care must be exercised in selecting a value for specific yield and in analyzing water-level fluctuations.