During a laboratory investigation using radiotracers, empirical formulas were developed to replace the ambiguous or incorrect Dupuit‐Forchheimer assumption used in relationships governing the flow of water through porous, unconsolidated materials ranging from clay to coarse sands or fine gravels. The Dupuit‐Forchheimer assumption states that the ratio of specific flow rate to average flow velocity is equal to porosity, but it is not specified whether true porosity or void ratio or specific yield, also called effective porosity, is meant. The hydraulic conductivity shows a good correlation with the maximum 10% grain size, and the ratio of specific flow rate to average flow velocity shows good correlation with functions of specific yield and percentile sorting coefficient. From field measurements of specific flow rate and average flow velocity the specific yield can be computed.

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The radioisotopes iodine 131 and tritium were used to determine the specific yield and permeability in an aquifer near a pumping borehole. The direction and rate of flow of natural groundwater was also determined by means of a free flow test, using these isotopes.