Bulk sediment

As is the case with assessments of the toxicity of dissolved trace metals, the development of sediment quality criteria (SQC) must be based on the fraction of sediment-associated metal that is bioavailable. Bulk sediments consist of a variety of phases including sediment solids in the silt and clay size fractions, and sediment pore water. Swartz et al. (1985) demonstrated that the bioavailable fraction of cadmium in sediments is correlated with interstitial water cadmium concentrations. More recent work (e.g., Di Toro et al, 1990 Allen et al., 1993 Hansen et al, 1996 Ankley et ai, 1996, and references therein) has demonstrated that the interstitial water concentrations of a suite of trace metals is regulated by an extractable fraction of iron sulfides. 

This D value is IJbAwZ4, where UB, the sediment burial rate, is 2.0 x 10-7 m/h. It can be viewed as GBZB4, where GB is the total burial rate specified as Vs/tB where tB (residence time) is 50,000 h, and Vs (the sediment volume) is the product of sediment depth (0.01 cm) and area Aw. Z4, ZB4 are the Z values of the sediment solids and of the bulk sediment, respectively. Since there are 20% solids, ZB4 is about 0.2 Z4. There is a slight difference between these approaches because in the advection approach (which is used here) there is burial of water as well as solids. 

Bulk sediment samples ( 30 kg) of till (n=63) and glaciofluvial sediments (n=7) were collected in C soil-horizon (>1 m depth) from hand-dug pits, natural bluffs, and man-made exposures. Samples underwent heavy mineral separation (SG >3.2 g/cm3) using a shaking table and heavy liquid separation. Heavy indicator minerals were visually identified and picked from the sand-sized fraction (0.25 - 2.00 mm). 

Comparison of the relative sediment toxicity of different SPs can be difficult as there are a variety of different test methods and endpoints evaluated, in addition to other confounding factors relating to sediment quality. Amweg et al. [28] determined the toxicity of six SPs to //. azteca in 10-day studies at 23 °C in natural sediments containing 1-6% OC. Toxicity data were reported as bulk sediment concentrations and normalized to the organic carbon content (Table 5). The results indicated that normalization removed some, but not all, of the variability between sediments. Other factors such as sediment texture may also affect bioavailability and hence apparent toxicity in sediment studies. 

Booij et al. (2003b) made an effort to model contaminant uptake by buried passive samplers. The major assumptions underlying this model are that the sampler can be regarded as an infinite sink for target contaminants, that the depletion of the bulk sediment phase is insignificant, and that the contaminant desorption kinetics are not rate-limiting. 

Figure 11. (a) Late Pleistocene 5 Ca record based on measiwe-ments of G. sacculifer (from Nagler et al. 2000). The inferred variations of temperature are similar to diose inferred from variations of Mg/Ca in die same sediment core, (b) Plio-Pleistocene record of seawater b Ca based on bulk coccolith ooze from DSDP Site 590B in the soudiwestem Pacific (Tasman Sea). The 2.5 m.y. record shows only small variations of 5 Ca. The seawater curve is constructed assuming that the fractionation between seawater and bulk sediment remained constant. The decrease of S Ca at ca. 0.7 Ma could reflect cooling rather than a change in the seawater 5 Ca. 

A global map of quartz abundance is given in Figure 14.12. In this case, the contribution of quartz is presented as the contribution to the bulk sediment from which biogenic carbonate and silica have been removed. This map is very similar to the global distribution of dust presented in Figure 11.4, reflecting the importance of aeolian transport for this detrital silicate. 

Net sedimentation is defined as the flux of material incorporated into the permanent sediment record. 210Pb and 137Cs geochronologies indicate a mass sedimentation rate of 103 g/m2 per year for profundal sediments in Little Rock Lake. By using the mean Hg concentration (118 ng/g) in the top 1-cm slice of our bulk sediment profile, we estimated an annual net sedimentation of 12 xg of HgT/m2 per year. This net accumulation rate is similar to the calculated atmospheric input rate of about 10 xg/m2 per year (18, 19). Additionally, gross deposition rates (from sediment traps) exceeded these estimates by about a factor of 3 this rate suggests substantial internal recycling of material deposited at the sediment-water interface in this lake. 

The estimated volume of hydrate via chlorinity had a mean value of 2.4 2.7 vol% ranging as high as 13.6 vol% again these are minimum values, perhaps caused by a low baseline. Logging tools indicated that hydrate occupied approximately 4 vol% of bulk sediments, ranging as high as approximately 11 vol%. 

Data from the reconnaissance survey indicated that bulk sediment concentrations of all COPCs were greater than apparent effect levels (AELs Environment Canada, 1995), indicating that contamination was widespread. A 20% reduction in toxicity test endpoint performance (relative to the negative control) was used to evaluate toxicity data. Such a reduction typically indicates real differences from the control. All samples demonstrated greater than a 20% reduction in bivalve normal development, however, similar reductions in amphipod survival were not observed, with the exception of one marginal hit for a single sample. As a result of the bivalve toxicity and elevated COPC concentrations observed in the reconnaissance survey, a full SQT was considered necessary for the site. 

Chemistry LOE received the lowest weight since elevated concentrations of bulk sediment and porewater COPC concentrations do not necessarily translate into adverse effects in the other two LOE. 

Quartz typically yields low concentrations of the elements when measured by this procedure, and therefore does not contribute significantly to the overall trace element composition of the bulk sediment samples. Thus, the presence of quartz in sediments acts as a dilutant to their overall trace element content. Unfortunately, Si cannot be measured by this INAA pro-... 

Table II. Average Concentrations (by Weight) of Some Elements in Bulk Sediment Samples from Hierakonpolis ...

Valuable information wiU continue to arise from measurements of the isotopic composition of bulk sediments, especially in margin environments. Expanding the network of high-resolution d N uik records is crucial to map out patterns of changing in space and in time, without which mechanistic hypotheses... 

In each case, changes in the elemental ratios are caused by relative changes in bulk sedimentation— dilution or condensation results from changes in the terrigenous clay flux relative to eolian or other inputs. Correct interpretation of proxies depends on distinguishing elemental sources and transport... 

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