Fresh Groundwater Supplies Tapped in Water-scarce California

A study has found 32,000 AFY of potentially recoverable water from the Fenner and northern Bristol Valley area in California. Terry Foreman outlines how the water conservation and storage project has measured the scale and recharge rate of the much needed aquifer.

Arecent water supply and storage program has the potential to add to the portfolio of Southern California's sustainable water resources mix. Cadiz, Inc. owns 34,000 acres of largely contiguous land in the Cadiz and Fenner valleys, located in the eastern Mojave Desert, California, where they have farmed successfully for more than 15 years. Along with agricultural operations, Cadiz, Inc. has been working to expand its water supply and storage resources.

In 2009, Cadiz partnered with CH2M HILL to assess the potential recoverable water that could be conserved over the long-term as well as assess the groundwater in storage in the Fenner Valley and northern Bristol Valley area. The objectives of the study were to assess the volume of groundwater in storage upgradient and in the project area, assess the potential recoverable water that could be conserved through capturing water that would otherwise flow to and evaporate from the dry lakes.

The objectives did not include developing an operations plan to extract or store water in the project area, or an environmental impact assessment. However, the next steps will include the development of an operations plan, together with an environmental assessment. The former will include specification of recharge facilities, extraction wells, monitoring, and put (recharge) and take (extraction) operations to safely implement a water supply and storage project.

The following activities were included as part of CH2M HILL's study:

• Geologic reconnaissance to directly observe geologic, hydrologic, and geomorphic features and conditions in the field

• Drilling of four boreholes to better delineate the subsurface geology and hydrogeology

• Three aquifer tests and one packer test to provide estimates of hydrogeologic properties

• Survey of wells and measurements of groundwater levels to define hydraulic gradients and groundwater level fluctuations

• Collection of water samples from new wells to assess groundwater quality

CH2M HILL's assessment included a collection of field data, development of a watershed soil-moisture budget model based on the USGS INFIL3.0 model, and development of a three-dimensional groundwater flow model, based on the USGS MODFLOW-2000 computer code, of the Fenner Gap area.

The study indicates a reasonable estimate of potential recoverable water of 32,000 acre-feet per year (AFY) and the volume of groundwater in storage is reasonably estimated to be between about 17 million to 34 million acre-feet in the alluvium of the Fenner Valley and northern Bristol Valley area. Cadiz intends to manage groundwater levels in the basin by withdrawing a temporary surplus from storage, that is, by withdrawing 50,000 acre-feet per year. It will then use this available storage to store imported water, such as surplus Colorado River flows. This conjunctive use management of groundwater and surface water supplies creates a long-term sustainable water supply resource.

The primary purpose of well TW-1 was to assess the hydrogeologic properties of the carbonate rock units in the Fenner Gap. This test indicated that hydraulic conductivity values of the carbonate rock units can be in excess of 1,000 feet per day.

The purpose of well TW-2 was to assess the thickness and hydrogeologic properties of the alluvium in its thicker section through the Fenner Gap. Wells TW-2 and TW-2B confirm a thickness of about 860 to 810 feet of alluvium, respectively.

An aquifer test conducted for three days at 1,130 gpm on TW-2 indicated an average hydraulic conductivity value of about 600 feet per day, demonstrating substantial water transmitting capacity of the younger alluvium in the Fenner Gap.

Well TW-3 indicated that the eastern side of the Fenner Gap is underlain by older alluvium (fanglomerates) that are consolidated and less permeable than the younger alluvium. Packer tests in this hydrogeologic unit indicate an average hydraulic conductivity of 3.1 x 10-3 feet per day.

A recent well survey and groundwater-level measurements confirm a steep hydraulic gradient upstream of the Fenner Gap and a flattening of the gradient in and downstream of the gap. This is consistent with an increase in water transmitting properties of those hydrogologic units through and downstream of the gap.

Summary of recoverable water

The study also evaluated recoverable water; water that would otherwise discharge to the Bristol and Cadiz dry lakes and then evaporate. The estimates were developed using the USGS INFIL3.0 watershed soil moisture budget model and then tested through application of the USGS MODFLOW-2000 model of groundwater flow through the Fenner Gap. Groundwater originates as precipitation falling on the surrounding mountains. A portion of this precipitation infiltrates into the groundwater system as recharge, then flows, principally through alluvial and carbonate rock units and to a lesser extent through volcanic deposits, toward and through the Fenner Gap on its way to the Bristol and Cadiz dry lakes. Here, it ultimately evapotranspires, leaving behind salts that are carried with the groundwater.

Total recoverable water, therefore, is equal to the amount of recharge to the groundwater system in the Fenner Watershed, which is approximately equal to the amount of groundwater flow through Fenner Gap through the alluvial and carbonate rock units (flow through other rock units is expected to be substantially less than through these two hydrogeologic units). By intercepting this groundwater flow through the gap, a reduction of evapotranspiration from Bristol and Cadiz dry lakes is expected, but there would be no reduction in groundwater storage.

Validation of recoverable water estimate

Fenner Gap is the path of groundwater flow through alluvial and bedrock aquifers (such as carbonate units) from Fenner Valley into Bristol and Cadiz valleys. The long-term steady-state flow of groundwater through the gap is expected to be similar to long-term groundwater recharge in the Fenner Watershed. A three-dimensional groundwater flow model of the Fenner Gap area was developed for the purposes of validating the 30,000 AFY estimate of steady-state groundwater flow through Fenner Gap, previously described. The model is used to solve the inverse problem, that is, given a boundary inflow of groundwater at the north end of the gap of 30,000 AFY, and measured steady-state groundwater levels, what distribution of aquifer properties (specifically hydraulic conductivity) is required to allow for this flow? Also, is this distribution likely given available information on aquifer properties? The question was answered using a software program called PEST, which is often used in inverse modeling to aid in calibrating groundwater flow models. PEST calculates values of hydraulic conductivity that makes the groundwater flow model "calibrate" to the measured values. PEST makes many (often thousands) model simulation runs to find the best set of parameter values that minimises the residuals (differences) in simulated and observed measurements (e.g., groundwater levels).

PEST results produced two distributions of hydraulic conductivity that are both reasonable and consistent with observed data from aquifer tests, while maintaining 30,000 AFY of groundwater flow through the gap and matching observed groundwater levels in monitoring wells. Because of this match, it is reasonable to assume that 30,000 AFY is flowing through the gap and, therefore, that 30,000 AFY is a reasonable estimate of potentially recoverable water. In total, data obtained from field investigations, INFIL3.0 watershed soil-moisture budget assessments, and Fenner Gap three-dimensional groundwater flow model simulations support a 32,000 AFY estimate of potentially recoverable water from the Fenner and northern Bristol Valley area.

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