USGS Publications Warehouse

Abstract

The characteristics and distributions of near-surface bottom sediments and of nutrients in the sediments provide information on modern sediment and nutrient sources, sedimentation environments, and geochemical reactions in the tidal Potomac system, Maryland and Virginia. This information is fundamental to an improved understanding of sedimentation and eutrophication problems in the tidal Potomac system. The tidal Potomac system consists of 1,230 square kilometers of intertidal to subtidal Potomac mainstem and tributary streambed from the heads-of-tides to Chesapeake Bay. Tidal Potomac sediments are dominantly silt and clay except in local areas. An average sediment sample is about two-thirds silt and clay (fine) particles and one-third sand (coarse) particles. The mean of the median size of all samples is 6.60 phi, or 0.010 millimeters. Sorting generally is poor and the average sediment is skewed toward the fine tail of the size-distribution curve. Mean particle-size measures have large standard deviations. Among geomorphic units, two distinctly different size populations are found; fine (median phi about 9), and poorly sorted (sorting about 3) sediments in the channel and the smooth flat, and coarse (median phi about 2), and well sorted (sorting about 1) sediments in the shoreline flat and the irregular slope. Among mainstem hydrologic divisions, an average sediment from the river and the estuary division is coarser and more variable than an average sediment from the transition division. Substantial concentrations of total carbon, total nitrogen, and total phosphorus, and limited amounts of inorganic carbon, ammonia nitrogen and nitrite plus nitrate nitrogen occur in tidal Potomac sediments. An average tidal Potomac sediment sample weighing 1 kilogram contains about 21,000 milligrams of total carbon, 2,400 milligrams of total nitrogen, 1,200 milligrams of total phosphorus, 600 milligrams of inorganic carbon, 170 milligrams of ammonia nitrogen, and 2 milligrams of nitrite plus nitrate nitrogen. Total carbon, nitrogen, and phosphorus have an average ratio by weight of 18:2:1 and an average ratio by atoms of 94:8:1. Nutrient concentrations and nutrient ratios have large ranges and standard deviations. Nutrient concentrations usually are closely related to particle size; large concentrations are characteristic of fine sediments in the channel and the smooth flat, and small concentrations are typical of coarse sediments in the shoreline flat and the irregular slope. Concentrations typically decrease from the river division to the estuary division. Mainstem and tributaries show no statistically significant difference in mean particle-size measures or mean nutrient concentrations. Tributaries do not contribute large quantities of sediment with diverse texture or nutrient content to the Potomac mainstem. Particle-size measures and nutrient concentrations in the mainstem are significantly related to hydrologic divisions and geomorphic units; that is, particle size and nutrients vary significantly along and across the Potomac mainstem. Lateral variations in particle size and nutrient content are more pronounced and contribute more to significant relations than longitudinal variations contribute. The mean values for the median particle size and for the percentage of sand indicate significant variations among hydrologic divisions for samples from a geomorphic unit, and among geomorphic units, for samples from a hydrologic division. Sediments of channels and smooth flats in the river division commonly are coarser than sediments of channels and smooth flats in the transition and the estuary divisions. Shoreline flats in the estuary division are coarser than shoreline flats in the river division. Shoreline flats and irregular slopes in each hydrologic division generally are significantly coarser than channels and smooth flats. Relations between particle-size measures and geomorphic units show progressively larger correlation coefficients from the river division to the estuary division. Significant differences in mean values of total carbon and of total phosphorus show typical variations in nutrient concentrations of sediment samples from hydrologic divisions and geomorphic units. The river division channel and irregular slope contain greater carbon concentrations than the transition division and the estuary division channel and irregular slope contain. Phosphorus concentrations in the channel and the irregular slope in the transition division are relatively greater than phosphorus concentrations in the channel and irregular slope in the river division and significantly greater than phosphorus concentrations in the channel and irregular slope in the estuary division. For both carbon and phosphorus, the channel and the smooth flat in each hydrologic division usually contain larger concentrations than the shoreline flat and the irregular slope. The distribution of coarse sediments is indicative of sources of sediments in the tidal Potomac system. Variable coarse and fine sediments in channels near the heads-of-tides and the relative absence of shoreline flats indicative of a shore erosion source, identify variable river inflows as sediment sources. Extensive shoreline flats underlain by coarse sediments indicate that contributions from the shoreline source increase toward the Potomac mouth. Tributaries do not contribute coarse sediments to the Potomac mainstem. Coarse sediments of irregular slopes occur in moderate to deep waters where modern currents are inadequate to transport sand and gravel. These sediments and the associated geomorphic features are 'relicts' from a pre-modern phase of Potomac River erosion and deposition. The present characteristics and the distribution of irregular slopes and relict sediments are indicative of modern sedimentation patterns. The increase in the relative extent of irregular slopes in a seaward direction in the Potomac mainstem is primarily an indication of rapid deposition and burial of relict sediments and features in the river and the transition divisions by modern sediments. The absence of irregular slopes and relict deposits in most tributaries is evidence for rapid deposition in the tributaries. Isolated occurrences of fine sediments on irregular slopes indicate that modern sediments are being deposited over relict sediments and geomorphic features in some locations. Changes in sediment textures within the Potomac mainstem reflect the relative influences of contributions from the river and shoreline sources and (or) changing hydrologic conditions. Decreasing particle size in sediments of the channel and the smooth flat from the river division through the transition division is primarily an indication of decreased competence of currents and the development of a two-layer estuarine circulation pattern. In the estuary division, shoreline flats increase in extent, and nearby channel sediments are coarser and more variable than channel sediments in the transition division. Because nearby tributary and Chesapeake Bay sediments are uniformly fine grained, these changes indicate increased contributions from the shoreline source or from erosion of nearby relict deposits. Large and variable nutrient concentrations and poor relations of concentrations to textures and to geomorphic units in the river division indicate nearby sources for most nutrients. Organic carbon to nitrogen ratios indicate that terrestrial organic matter from the Potomac River is the dominant source. Ratios do not vary significantly near sewage treatment plants or along the Potomac mainstem. Smaller ratios and larger and more variable nutrient concentrations in the estuary division may indicate a new source of organic matter, presumably in situ phytoplankton production. Changes in nutrient concentrations seaward from the river division are a complex function of physical, chemical, and biological processes. General seaward decreases in the concentrations of most nutrients occur because nutrients associated with particulate material are deposited, diluted, or dispersed with distance from sources in the river division, and some nutrients sorbed by sediments may be released to the water column. An increase in sediment phosphorus concentration from the river division to the transition division and a large decrease from the transition division to the estuary division probably indicates uptake of phosphorus by generally aerobic river and transition sediments and release in periodically anaerobic estuary sediments. Nutrient concentrations are significantly related to particle size. Trends in nutrient concentrations among geomorphic units are primarily due to trends in particle size. Trends in nutrients along the Potomac mainstem mostly are independent of particle size and are due to changes in nutrient sources or to changes in hydrologic conditions that promote uptake or release of nutrients associated with sediments.