Hardpan in the Central Valley – Its Effect on Groundwater Model

Hardpan exists on many type of soil but the challenging one is the red or brownish red hardpan of the San Joaquin soil series. The depth of this hardpan varies within 6 inches to 6 foot of the surface. The hardpan is composed of a mass of soil grains firmly cemented by iron-silica, and is so dense that it could only be broken by blasting. This impervious layer serves as a barrier to water percolating down from the surface.- The origin of hardpanHardpan can be found in area with semiarid to subhumid Mediterranean climate type, as in the Central Valley (the summer half of the year is hot and dry and the winter half is cool). Average annual precipitation ranges from 5 to 16 inches in the San Joaquin Valley. About 85 percent of the annual precipitation occurs in the six months from November to April. Summers are hot, and winters are moderate (Williamson et. al., 1985). The mean January temperature varies between 45 and 52F. Many days during July, August, and September are having a maximum temperature as high as 110F. The mean annual temperature is 56 to 63F. (Harradine, 1963).

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Harradine (1963) hypothesized the genesis of this iron-silica hardpan. During the early spring months chemical and biological activity is favored by a warming soil and the moisture from the late rains. This promotes the release of bases, the solution of silica and sesquioxides, and their general movement downward in the profile. As the soil is rapidly dried during late spring, iron and silica are irreversibly precipitated and a small increment of the less permeable subsoil gradually becomes cemented. Also, subsoil stratification gives a perched moisture condition and thus determines the depth of hardpan formation. In summary, existence of hardpan shows that on this type of soil (loam) and climate (Mediterranean type), the infiltration after precipitation does not percolate further down to aquifer. Because of the high temperature, the infiltrated water would evaporate early on before reaching the groundwater table.

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This impervious hardpan, 1 to 6 feet in depth, is a barrier for any infiltration that follows the precipitation on the surface. Thus, in calculating a water balance, no recharge to groundwater from precipitation should be included on areas covered by iron-silica hardpan. Otherwise it would overestimate the recharge. In the City of Fresno, this hardpan of the San Joaquin series prevents the percolation of nitrate to groundwater (Schmidt, 1972).