Task 4 — Geochemistry

Proposed geochemical studies are designed to provide:

1. A description of the chemical quality of ground water based on existing, and to a limited extent, new data
2. Support evaluations of the magnitude and distribution of recharge, and ground-water flow direction and travel times.

Ground-water quality samples will be analyzed for major ions and trace metals. Recharge and ground-water flow evaluations primarily will be supported by sampling for environmental tracers (noble gasses, carbon-13, carbon-14, helium-4, helium-3, tritium, and stable isotopes of hydrogen and oxygen).

The following principal geochemical tasks will be performed:

Data Compilation: Existing water chemistry and isotopic data will be compiled during the study and entered into the USGS National Water Information System (NWIS). New data collected during the study will be added to NWIS after it is quality assured.

Water Sampling: Water sampling will focus on providing geochemical information to help constrain hydrologic evaluations of principal ground-water flow paths and rates of ground-water movement. As a result, analyses to determine the chemical quality of water will be based almost entirely on existing data, with limited data collection for major ions, trace metals, or pesticides.

Sampling target areas for geochemical analyses include

1. Basin-fill aquifers in selected valleys,
2. Basin boundaries that appear to have significant interbasin flow,
3. Upland springs, and
4. Existing precipitation stations operated by the USGS.

Wells in the study area that are completed in the carbonate aquifer are sparse, and for most of these wells, geochemical samples have been collected in previous studies (Thomas and others, 1991; Thomas and others, 1996). Therefore, water samples from the carbonate aquifer will be restricted to those areas where significant interbasin flow is thought to occur or where previous samples were not analyzed for environmental tracers. Analysis of selected tracers (most likely deuterium) will be used to help constrain ground-water flow path interpretations.

Dissolved-gas tracers (noble gases He, Ne, Ar, Kr, Xe; and tritium) and in-situ total dissolved-gas pressure measurements in ground water are useful tools for evaluating elevations where recharge to the aquifer occurs, dating of the modern (< 50 years old) component of ground water, and possibly as an alternative age-dating technique to carbon-14. Approximately 20 samples will be collected from wells or springs; sample collection will be coordinated with other environmental tracer, isotopic, and geochemical ground-water collection activities. Results of gas-pressure measurements and tracer analyses will aid in evaluating recharge elevations, potential ground-water flow paths, ground-water residence times, and recharge rates.

Modeling: Geochemical modeling of water-rock interactions will be applied using NETPATH or PHREEQC to validate or constrain interpretations of regional ground-water flow paths. These models also will be used to determine corrected ground-water ages along flow paths with carbon isotope data. The range in ground-water ages determined from carbon isotope values and modeling will be used to help constrain estimates of recharge-area to discharge-area travel times. The optimization code PEST may be used to quantify uncertainty and sensitivity in the geochemical reactions.

Ground-water recharge and interbasin flow will be evaluated using a discrete-state compartment model (DSC). This accounting-type model requires the input of water-budget and environmental-tracer values to produce an isotopic mass balance. Simulated concentrations of the selected environmental tracer, such as deuterium, are compared to measured values to calibrate the mass-balance model. The DSC model will be applied to each basin in the primary study area.