THE IMPACT OF POST-PLUVIAL ISOSTATIC REBOUND ON SHALLOW GROUNDWATER CHEMICAL EVOLUTION IN A CLOSED BASIN

The 1,750 km² endorheic Honey Lake basin, located along the California-Nevada border, was part of the 22,000 km² Pleistocene Lake Lahontan pluvial lake system which existed between 5,000 and 40,000 years bp. The basin consists of two subbasins separated by a low elevation divide. Groundwater in the western subbasin, which is located adjacent to the Sierra Nevada Range and which receives most of the surface runoff, has a maximum TDS of only ~ 1,300 mg/L. The eastern subbasin, which receives only minimal surface inflows and hence minimal solute loading has groundwater with a maximum TDS of 46,000 mg/L.

In-situ reactions and upwelling thermal water contribute to groundwater chemistry. NETPATH was used to model chemical evolution. Chemical evolution includes dissolution of aluminosilicate minerals, calcite, gypsum, and halite, and ion exchange. Thermal waters, which circulate to depths of 1.6-3.8 km, locally account for up to 40% of total shallow groundwater recharge. Thermal waters have calculated ¹⁴C ages ~ 13,000 years. Basin margin groundwaters contain abundant ³H and anthropogenic ¹⁴C, however, groundwater ages as great as 25,000 years occur in the basins.

The large TDS difference between subbasins is attributed to post-Lake Lahontan isostatic rebound about 13 ka. Prior to rebound the subbasins did not exist and the low point of the basin was in the eastern area where hydraulic isolation from the larger Lake Lahontan and frequent desiccation of the basin surface water resulted in evaporite mineral accumulation.