CHAPTER 3. PHYSICOCHEMICAL ENVIRONMENT 



The physicochetnical environment of 

 floodplains (including both aquatic and 

 soil environments) is a function of the 

 interactions or processes occurring in the 

 water column, in soil, and at the soil- 

 water interface. These processes are 

 facilitated by the prolonged periods of 

 flooding (inundation) which saturate the 

 soils and the subsequent periodic inter- 

 vals of drydown which de-water the soils. 

 This cyclic wet/dry regime imparts a 

 unique chemical environment that has pro- 

 found effects on nutrient cycling and the 

 character and adaptations of the flood- 

 plain biotic communities. 



CHEMICAL CHARACTERISTICS OF RIVERS 



The chemical composition of flood- 

 plain rivers and streams reflects water 

 sources, headwater origin, and the compo- 

 sition of geological formations through 

 which rivers flow to the coast. Of the 

 three major chemical classes of world 

 rivers (rock-dominated, precipitation- 

 dominated, and evaporation-dominated; 

 Gibbs 1970), floodplain rivers fall into 

 two: rock-dominated and precipitation- 

 dominated. Alluvial rivers are rock- 

 dominated rivers whose inorganic chemical 

 load is derived from the products of 

 weathering and leaching of the parent 

 rocks and soil in the mountains and 

 Piedmont. Concentrations of inorganic 

 ions are typically higher than total 

 organic carbon (TOC) concentrations 

 (Table 4). Blackwater rivers arising in 

 the Coastal Plain, on the other hand, are 

 precipitation-dominated. Rainfall, which 

 represents most of the water input to 

 these streams, contains relatively low 

 concentrations of dissolved inorganic 

 solids (specific conductance). A compari- 

 son of river data in Georgia (Wharton and 

 Brinson 1979a) indicated that alluvial 

 rivers usually were higher than blackwater 

 rivers in nitrogen, phosphorus, calcium, 

 and magnesium (the latter two constituents 

 increasing water hardness) (Table 4). 

 Blackwater rivers were more acidic (lower 

 pH) and characterized by high concentra- 



tions of total organic carbon and low con- 

 centrations of dissolved inorganics. 



Distinctions among blackwater streams 

 can be explained by their different ori- 

 gins within the Coastal Plain (Wharton and 

 Brinson 1979a) (Table 4). The waters of 

 the Satilla River, arising in the lower 

 coastal plain of Georgia, were soft, 

 acidic and highly organic, while the chem- 

 istry of the Ogeechee, Canoochee, and 

 Ochlockonee Rivers reflect the input 

 (increased hardness, pH, and nutrients) 

 from geological formations at their head- 

 waters. In Florida many rivers clear dur- 

 ing low flows, and pH approximates that of 

 subsurface aquifers (pH = 7.7). During 

 high flows, however, surface leachates add 

 organic acids and lower the pH to 4.0. 

 The blackwater Santa Fe River typifies a 

 phenomenon especially evident in many 

 Florida rivers. In its swampy headwaters 

 the Santa Fe has a pH of 5.3. In the cen- 

 tral section, with swamp drainage during 

 high flow and alkaline ground water drain- 

 age during low flow, the pH is 6.4; in the 

 lower river, fed by artesian springs, the 

 pH rises to 7.4. 



The distinction between blackwater 

 and alluvial river water chemistry is best 

 reflected in the difference in the ratios 

 of inorganic to organic constituents. The 

 high concentrations of organic matter in 

 'blackwater rivers result in a 1:1 ratio of 

 dissolved inorganics to total organics 

 whereas the predominance of inorganic com- 

 ponents in alluvial rivers leads typically 

 to a 1C:1 ratio (Beck et al. 1974). The 

 magnitude of the organic load affects the 

 concentrations of some of the inorganic 

 load constituents. For example, only 

 those inorganic ions such as iron and 

 aluminum, which form complexes with the 

 dissolved organic matter (COM), are pres- 

 ent in greater concentrations in black- 

 water streams (Table 5). Additionally, 

 since the bulk of the dissolved organic 

 constituents are organic acids (humic and 

 fulvic), the waters of blackwater streams 

 are considerably more acidic (low pH) and 

 highly colored than alluvial streams. 



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