Hardpan in the Central Valley: Its effect on groundwater model
Some groundwater models for Central Valley and their cities in California use deep percolation of precipitation on the valley floor as a significant source of recharge. One important aspect that these models somehow neglect is the present of hardpan soil layer.
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.
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Figure 1. General distribution of San Joaquin series in California.
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About San Joaquin series
The San Joaquin soil occupies large areas of the low alluvial terraces of the San Joaquin River and Kings River, as well as those of the larger creeks draining the foothills of western flank of the Sierra Nevada (Figure 1). They are at elevations of 250 to 500 feet. The dominant classes of native vegetation presently existing in the alluvial fan and terrace regions are grasses, forbs, and shrubs.
In a typical soil profile, they have thin clay subsoil, about 8 inches thick, which rests abruptly on a strongly cemented iron-silica hardpan (Harradine, 1963). The hardpan is 6 to 36 inches thick and overly several feet of weakly cemented sandy or silty material. In Fresno County area, other types of soils that have hardpan are the Exeter and Madera series. Soil profile of San Joaquin series is (Figure 2):
| A1 horizon | 0-6 in. | Brown loam |
| A3 horizon | 6-15 in. | Yellowish brown loam |
| B1 horizon | 15-24 in. | Reddish yellow loam |
| B2 horizon | 24-30 in. | Reddish brown clay loam |
| Cm horizon | 30-38 in. | Strong brown indurated iron-silica hardpan |
| C horizon | 38-72 in. | Light yellowish gritty loam |
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Figure 2. Typical San Joaquin soil profile.
Description of Cm horizon (hardpan): strongly cemented iron-silica hardpan; dark stains of manganese dioxide on surface; reddish yellow to strong-brown seams of cementing materials; pale-brown sandy matrix; extremely hard; nonporous but occasionally fractured.
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The origin of hardpan
Hardpan 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 52˚F. Many days during July, August, and September are having a maximum temperature as high as 110˚F. The mean annual temperature is 56˚ to 63˚F. (Harradine, 1963).
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.
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).
In preparing a groundwater model, a modeler needs to understand the soil physics of the area. Also, a modeler needs to understand the relation between percolation, temperature and evaporation of the modeled area to accurately calculate the amount of recharge and discharge.
References:
- Harradine, Frank. 1963. Morphology and genesis of noncalcic brown soils in California. Soil Science. Vol. 96(4), pp: 277-287.
- Huntington, G.L. 1971. Soil survey, eastern Fresno area, California: U. S. Department of Agriculture Soil Conservation Service, U. S. Government Printing Office, Washington, DC, 323 p.
- Schmidt, K.D. 1972. Nitrate in ground water of the Fresno-Clovis metropolitan area. Ground Water v. 10, no. 1, pp: 50–64.
- Williamson, A. K.; Prudic, D. E.; Swain, Li. A. 1985. Ground-water flow in the Central Valley, California. USGS Series Open-File Report Number 85-345 .
Love your pic
I’m working on a model that has the Sacramento Valley hardpan as dried marshland, with natural gas and oil beneath. The usual earthquakes around the Valley is my study, and whether the Sacramento Valley isn’t a filled, emptied marshland over an old volcanic caldera, and whether it’s still a potentially explosive event.. Hmmmm… I live in Foresthill, CA.. Please reply, if you will.. -daqueen