A-10 Buck Creek Well Report.pdf

Well Feasibility Report Prepared for: Modoc National Forest – Buck Creek Well

Prepared by: Edward Gazzetti and Joe Gurrieri, Minerals & Geology Management – Groundwater Program

Date: March 27, 2020

Well Purpose

The Modoc NF plans to drill a well on a FS administrative site this year. The site is at the Buck Creek Guard Station in the north Warner Mountains, five miles off Hwy 395 on county road 9 (Sec 7, T 46 N, R 15E).

Geologic Setting

The Preliminary Geologic Map of the Cedarville 30’ x 60’ Quadrangle, California, indicates the proposed well site is near an “approximately located” contact, which separates two geologic units: the Basalt of the Vya Group and the Pyroclastics of Fandango Valley. It is currently unclear which unit the proposed well would be completed in. However, there are several nearby wells with data that can be used to estimate aquifer properties and well depth and yield.

Reference Wells

Well completion reports from the California Department of Water Resources’ online database provide data for three existing wells located near the proposed well site. The wells are located in Section 36 of Township 47N, Range 14E, which is west of the area of interest. While exact locations of the wells are confidential and, as a result, unknown, sketches from the reports indicate they are relatively close to the proposed site. Total well depths of reference wells range between 105 and 170 ft. Groundwater is first encountered between 30 and 140 ft. Water table depths range from 8 to 100 ft. Two well completion reports include well test data used in the calculations below.

Discharge and drawdown data from the well tests were used to calculate specific capacity, an expression of productivity, for both wells using the following equation:

𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐶𝐶𝐶𝐶𝑆𝑆𝐶𝐶𝑆𝑆𝑆𝑆𝐶𝐶𝐶𝐶 =

𝐷𝐷𝑆𝑆𝐷𝐷𝑆𝑆ℎ𝐶𝐶𝑎𝑎𝑎𝑎𝑆𝑆

𝐷𝐷𝑎𝑎𝐶𝐶𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷

Known specific capacities can be used to estimate potential yield of a well by rearranging the equation and inputting a value for available drawdown, which is the difference between the depth at which a pump is set and the depth to the non-pumping water level in the well. For example, a well drilled to a depth of 400 feet would have approximately 250 feet of available drawdown assuming a depth to water table of ~100 ft and the pump set at ~50 feet above the well bottom (400 ft – 100 ft – 50 ft = 250 ft).

Estimated Depth and Yield

The following estimates assume aquifer properties at the proposed site will be similar to the reference wells utilized in this report. In addition, the estimates assume transmissivity is the same throughout aquifer thickness.

Based on well tests from two reference wells, an average specific capacity of 32 ft3/day/ft was calculated and used to determine potential discharge rates for wells constructed to depths of 300, 400, and 500 feet. A well drilled to a depth of 300 feet will likely produce ~25 gallons per minute (gpm); a well drilled to a depth of 400 feet will likely produce 40-50 gallons per minute and; a well drilled to 500 feet will likely produce 50-60 gpm.

To achieve the proposed discharge rate of 200 gpm using the specific capacity calculated above, a well would need to be drilled to a depth of roughly 1300 feet, which is costly. Instead, a shallower well coupled with a storage tank can be used to provide intermittent high flow rates.

Well Location

As proposed (see location map).