Locating high-yield well in alluvial aquifer: Fresno-Clovis area
Study Case: Fresno-Clovis area, San Joaquin Valley, California
In Fresno-Clovis area, wells that tap water on relatively similar depth varies greatly in their yield; from about 500 gpm to 2,000 gpm. This article shows a practical method to locate an area, where high yield water well can be expected.
Background Geology
Below the Fresno-Clovis area, unconsolidated deposits overlie the basement complex at more than 2,000 feet deep (Page and LeBlanc, 1969). These deposits are the Continental deposit of Tertiary and Quaternary age and the Older Alluvium of Quaternary age; derived from the weathered crystalline bedrock of the Sierra Nevada on the east. Stream deposit from the San Joaquin River covers the shallow surface of northern area; known as younger alluvium (Qya).
The depth and thickness of the transported deposits vary from east to west (Figure 1). The depth of Continental deposit (QTc) ranges from 150 feet on the eastern side to about 500 feet deep on the western of Fresno-Clovis area, and overlain by the Older Alluvium (Qoao) to the surface.
Contact between these two deposits can be inferred from the electric and lithologic logs (Figure 2). The contact shows abrupt change in resistivity from low at the bottom (continental deposit) to high on upper part (older alluvium). On lithologic log, the low resistivity represent zone with about 25 percent of coarse-grained material; whereas, about 65 percent of coarse-grained material makes up for the zone of high resistivity.
Wells and Pumping Test
Wells with high discharge rate perforate zones of high transmissivity, and vice versa.
On this article, the selected wells are mostly supply wells for the cities of Fresno and Clovis. Generally, top of the well perforated zone is between 200 and 300 feet deep from the land surface. Most well depth is from 500 to 700 feet below land surface. Duration of the pumping test to get transmissivity is about 10 hours in average, at rate between 300 and 3,000 gpm.
Method and Discussion
Based on geologic cross-section, the western and eastern sides of Fresno-Clovis area show distinct subsurface geologic condition. Thus, these two sides are discussed individually (Figure 3).
Western Area
Figure 4 shows the vertical transmissivity distribution on the older alluvium deposit (Quaternary age), on western side of Fresno-Clovis area. Refer to the geologic cross-section on Figure 1, the older alluvium deposit presents from land surface to a depth about 500 feet.
Nine wells that perforated in this deposit give transmissivity of 510,000 gpd/ft at the first 200 feet of depth from land surface, and about 220,000 gpd/ft from depth of 200 to 500 feet. Zone at the base of this deposit, near the contact with the continental deposit beneath it, has transmissivity around 110,000 gpd/ft.
Eastern Area
On this area, the older alluvium deposit (Qoao) is only 150 feet thick from the land surface. Thereby, wells in eastern side of Fresno-Clovis perforate mostly at the continental deposit (QTc) below Qoao unit.
Figure 5 shows vertical transmissivity distribution on the continental deposit (QTc, Tertiary and Quaternary age) on eastern side of Fresno-Clovis area. Pumping test at twelve wells that perforate on this deposit, revealed the average transmissivity of about 40,000 gpd/ft on the first top 100 feet of continental deposit, and 30,000 gpd/ft on the next 400 feet deep below.
CONCLUSION
- In the Fresno-Clovis area, wells with similar perforation depth interval have significant varied discharge rate. By combining geologic cross section and pumping test data, the cause is found. The older alluvium deposit of Quaternary age (Qoao) is thickening to the west and has high transmissivity of around 220,000 gpd/ft.
- Therefore, a well sitting on western part of the Fresno-Clovis area and perforating on the Older Alluvium deposit would yield relatively high discharge rate.
- Simplifying these two deposits, with distinct transmissivity values, into one layer unit (unconfined aquifer) generates inaccurate conceptual model of groundwater flow in the Fresno-Clovis area (Fig. 6).
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Reference
Page, R. W., and R. A. LeBlanc. 1969. Geology, Hydrology, and Water Quality in the Fresno Area, California. USGS. Open-File Report.
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