Big Soda Lake, located near Fallon, Nevada, occupies an explosion crater rimmed by basaltic debris; volcanic activity apparently ceased within the last 10,000 years. This lake has been selected for a detailed multidisciplinary study that will ultimately cover the organic and inorganic hydrogeochemistry of water and sediments because the time at which chemical stratification was initiated is known (~1920) and chemical analyses are available for a period of more than 100 years.

Detailed chemical analyses of the waters show that the lake is at present alkaline (pH = 9.7), chemically stratified (meromictic) and is extremely anoxic (total reduced sulfur—410 mg/L as H₂S) below a depth of about 35 m. The average concentrations (in mg/L) of Na, K, Mg, Ca, NH₃, H₂S, alkalinity (as HCO₃), Cl, SO₄, and dissolved organics (as C) in waters of the upper layer (depth 0 to 32 m) are 8,100, 320, 150, 5.0, < 0.1, < 0.5, 4,100, 7,100, 5,800, and 20 respectively; in the deeper layer (depth 37 to 64 m) they are 27,000, 1,200, 5.6, 0.8, 45, 410, 24,000, 27,500, 6,800, and 60, respectively.

Chemical and stable isotope analyses of the waters, δ13C and Δ14C values of dissolved total carbonate from this lake and surface and ground waters in the area together with mineral-water equilibrium computations indicate that the waters in the lake are primarily meteoric in origin with the present chemical composition resulting from the following geochemical processes:

• 1.

  (1) evaporation and exchange with atmosphere, the dominant processes,

• 2.

  (2) mineral-water interactions, including dissolution, precipitation and ion exchange,

• 3.

  (3) inflow and outflow of ground water and

• 4.

  (4) biological activity of macro- and microorganisms, including sulfate reduction in the water column of the deeper layer at a very high rate of 6.6 μmol L⁻¹ day⁻¹.