SOLUTE CHEMISTRY AND ISOTOPIC INVESTIGATION OF THE GROUNDWATER FLOW PATHS IN HONEY LAKE BASIN, LASSEN COUNTY, CALIFORNIA AND WASHOE COUNTY, NEVADA

Honey Lake Basin is a large, hydrologically closed valley with two playa lakes separated by a low elevation divide. The Basin has a complex hydrogeologic setting, with numerous groundwater flow paths that interact with surface waters and three aquifers; shallow, deep, and geothermal. Thirteen physical flow paths; 11 cold and 2 thermal, are identified and organized into 7 chemically similar flow paths. The chemical evolution of those paths is characterized by integrating solute chemistry and isotopic data. The chemical flow paths include recharge in either volcanic or granitoid terrains in the Sierra Nevada and the Modoc Plateau. Groundwater then flows through alluvial fan and stream sedimentary environments and eventually through pluvial and playa sediments in the basin.

Fifty-seven samples from springs, wells, streams, and rivers were collected and analyzed for major cation and anion concentrations, the stable isotopes δD and δ¹⁸O, and the radiogenic isotopes δ¹³C, ¹⁴C, and ³H. These data were combined with data from six other studies for a complete analysis of 312 sample locations.

Temperature data reveal that thermal waters circulate to 1.6-3.0 km and 2.8-3.8 km along two major fault zones. Shallow groundwaters above 17°C mix with thermal water and mixing ratios are presented.

δ¹⁸O and δD data show that deep groundwater was recharged by cooler, more humid precipitation from the last ice age, while shallow groundwaters reflect current meteoric recharge and extensive evaporation. The two thermal flow paths show exchange with silicate minerals at high temperatures.

Tritium concentrations and ¹⁴C ages show that deep groundwaters and shallower groundwaters in the center of the basin are not greatly affected by post-1952 recharge. Mean ¹⁴C ages range from modern to 25,500 years old.

NETPATH was used to model chemical evolution along the flow paths. Groundwater on the west side of the basin is typically low TDS (~170 mg/L) calcium-bicarbonate water and evolves into higher TDS (~550 mg/L) sodium-bicarbonate water. Groundwater on the east side of the basin is typically low TDS (~320 mg/L) sodium-bicarbonate and evolves into extremely high TDS (~14,300 mg/L) sodium-chloride groundwaters. Dissolution of silicate minerals, calcite, halite, and gypsum and ion exchange with clays is responsible for major chemistry changes.