A transition from undifferentiated mud to estuarine sediment occurs in the bottom of the Rappahannock River estuary between the sediment-water interface and 100 centimeters in depth. Differentiation produces characteristic sequences of horizons that are analogous to those in terrestrial soil profiles. The sediment profiles and their chemical properties respond to a dynamic equilibrium between sedimentation rate, depositional rate of organic detritus, and microbiological activity. The equilibrium is influenced by the general bathymetry and intensity of physical processes near the bottom, the salinity and ventilation of the bottom water, the composition of organic detritus, and sediment compaction processes such as dewatering and gas ebullition.
Typical chemical properties in low salinity sediments from the upper estuary are: pH, 6.6; Eh, –50 millivolts; pS, 12 to 14; NH3, 0.5 milliequivalents per 100 grams of sediment; interstitial Si, 300 to 400 microgram atoms per liter; and interstitial PO4, 0 microgram atoms per liter. Below 50 centimeters in depth, Eh typically becomes positive.
In the middle estuary, where salinity ranges from 3 to 15 parts per thousand and where sedimentation is rapid, the following are typical: pH, 7.0 to 7.3; Eh, –100 to –200 millivolts; pS, 9 to 13; NH3, 0.5 to 1.0 milliequivalents per 100 grams; interstitial Si, 400 to 600 microgram atoms per liter; and interstitial PO4, 5 to 20 microgram atoms per liter. A short distance below the sediment surface, concentrations of these variables reach maxima that reflect the attempt of microorganisms to adjust the overwhelming sediment supply to a new set of chemical conditions.
In the lower estuary, less intense physical processes in greater water depths and higher salinities cause higher rates of microbial activity, and the following are typical: pH, 7.2 to 7.5; Eh, –200 to –250 millivolts; pS, 8 to 9; NH3, 1.0 to 2.0 milliequivalents per 100 grams; Si, 600 to 700 microgram atoms per liter; and PO4, 100 to 200 microgram atoms per liter. Variations in the degree of bottom ventilation cause some extreme values, such as: pH, 8.7; Eh, —350 millivolts; pS, 4.0; NH3, 4.0 milliequivalents per 100 grams; interstitial Si, 1,000 microgram atoms per liter; and interstitial PO4, 380 microgram atoms per liter. The chemical properties of estuarine sediment respond to the biological, chemical, and physical forces in their environment, and the particular expression of profile development at any location tends to reflect a dynamic equilibrium between these forces.