Buffering of silica and phosphate in a turbid river1

The buffering of riverine dissolved silica and phosphate by sorption reactions between the aqueous phase and suspended secliment is examined with the turbid Colorado River system as a model. Concentrations are found to lie in a range predicted from laboratory sorption experiments with natural sediments and waters. Phosphate is probably highly buffered by suspended sediment during river flow while silica is not. Silica appreciably affects phosphate sorption reactions but not vice versa. Increased temperature results in higher silica but lower phosphate concentrations as a result of sorption. The buffering action of suspended sediments is largely complete within a few hours and is approximately proportional to the concentration of suspended sediment. Of prime concern in the study of nutrient dynamics in natural waters are the factors governing the availability of various nutrients to biotic uptake. This availability is often related to the concentrations of nutrients, which in certain systems are strongly affected by chemical interplay between their dissolved forms and mineral surfaces. Adsorption/desorption reactions involving common soil and sediment materials can exert a strong influence on the partitioning of silica and phosphate between solid and aqueous phases in periods of hours to days (e.g. Edzwald et al. 1976; Kennedy 1971). Such rapid partitioning could exert significant influence on the nutrient flux through a river system with high concentrations of suspended materials. The presence of labile silica and phosphate sorption sites on the suspended sediment should therefore provide a buffer capacity for the dissolved forms of these species. Our study focuses on the turbid Colorado River system above present-day 1 This work was supported in part by grants NSFRANN AEN 72-03469 AO3 and GI-34831 Al. z Present address: New York Cooperative Fishery Unit, Department of Natural Resources, Fernow Hall, Cornell University, Ithaca, New York 14853. Lake Powell (Fig. 1). The reservoir (occupying about 300 km of the former Colorado River) was formed by Glen Canyon Dam in 1963. Most of the water delivered to this region derives from snowmelt in the mountains of Colorado and Wyoming during spring runoff. The seasonal variation of flow has been partially damped by the construction of reservoirs upstream of Lake Powell in the early 1960s. Historically, average water flows through this region are of the order of 1-2~ lOlo rn”. yr-I. Suspended sediment concentrations in the Colorado River system commonly reach several thousand milligrams per liter. The bulk of the sediment load in the system originates in the Colorado Plateau rather than in the mountains from which the water comes (Iorns et al. 1965). The suspended sediment consists primarily of illite, kaolinite, smectite, quartz, feldspar, calcite, and dolomite, reflecting the largely Mesozoic sedimentary rocks which are their provenance (Mayer 1973; Kennedy 1965). Evidence for the buffering of silica in river systems has been summarized by Edwards and Liss (1973). That this phenomenon has occurred in the Colorado River is shown in Fig. 2, in which weighted mean annual concentrations of silica and chloride in the Colorado River