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SELENIUM IN SELENIFEROUS ENVIRONMENTS 43 Table 2-6. Water chemical-data for samples taken from the Cretaceous-aged Colorado or Montana Group formations in central Montana (Donavan et al., 1981-. Component Colorado Montana F ratiot pH 7.6 7.9 7.1 mg/L Ca 275 120 37 Mg 866 71 41 Na 1317 670 12 K 17.7 6.1 NS Fe 0.28 0.26 NS Si02 12.1 11.6 NS HC03 534 722 8 Cl 141 177 NS S04 6041 1143 38 N03 57 6.1 32 Sr 4.6 2.0 26 TDS 9262 2928 32 Se 0.308 0.0276 35 f F ratios for chemical constituents calculated in a one-way analysis of variance between data of the two groups. F = 6.75 or 11.1 at P = 0.01 or 0.001, respectively. NS =not significant, all F values <1.0. unweathered shale in areas where saline-seep discharges occur. The hydrol- ogy of these local hydrologic systems, including both contributing and seep area, is described by Brown et al. (1983) and Donovan et al. (1981). Water chemistries were determined on 81 samples collected from domes- tic, livestock, or research wells drilled into the cretaceous formations of central Montana (Donovan et al., 1981). The chemistry of each sample was closely related to the particular geological source (Table 2-6). Water samples from the Colorado group have higher proportions of Mg, SO4, N03, and Se when compared with water obtained from the Montana formations. Selenium con- centrations in these waters ranged from <2 to 1400 µg Se/L. Irrigation Versus Seleniferous Cretaceous Materials Humans have also impacted the Se cycle through the use of leaching fractions to remove excess salts from the root zone of irrigated agricultural crops. In the western part of the San Joaquin Valley, these leaching frac- tions have intercepted soluble Se initially derived from the Cretaceous Moreno shales (Tanji et al., 1986). Drains were installed to lower the groundwater table underlying the farmland. The drainage water contained soluble salts including Se, which were then drained to and concentrated in the evapora- tion ponds at the Kesterson Reservoir. The Se was now accessible to aquatic life and sufficiently available to alter the spectrum of bacterial genera present (Burton et al., 1987) and to cause chronic and acute toxicosis of the aquatic wildlife (see Chapter 8 of this publication). SUMMARY Selenium is of biological importance because of (i) its essentiality in animal and possibly plant metabolism, (ii) its insufficiency for animal re-