Tillage erosion and its effect on soil properties and crop yield in Denmark

Tillage erosion had been identified as a major process of soil redistribution on sloping arable land. The objectives of our study were to investigate the extent of tillage erosion and its effect on soil quality and productivity under Danish conditions. Soil samples were collected to a 0.45-m depth on a regular grid from a 1.9-ha site and analyzed for 137Cs inventories, as a measure of soil redistribution, soil texture, soil organic carbon (SOC) contents, and phosphorus (P) contents. Grain yield was determined at the same sampling points. Substantial soil redistribution had occurred during the past decades, mainly due to tillage. Average tillage erosion rates of 2.7 kg m(-2) yr(-1) occurred on the shoulderslopes, while deposition amounted to 1.2 kg m(-2) yr(-1) on foot- and toeslopes. The pattern of soil redistribution could not be explained by water erosion. Soil organic carbon and P contents in soil profiles increased from the shoulder- toward the toeslopes. Tillage translocation rates were strongly correlated with SOC contents, A-horizon depth, and P contents. Thus, tillage erosion had led to truncated soils on shoulderslopes and deep, colluvial soils on the foot- and toeslopes, substantially affecting within-field variability of soil properties. We concluded that tillage erosion has important implications for SOC dynamics on hummocky land and increases the risk for nutrient losses by overland flow and leaching. Despite the occurrence of deep soils across the study area, evidence suggested that crop productivity was affected by tillage-induced soil redistribution. However, tillage erosion effects on crop yield were confounded by topography-yield relationships.     

Assessment of spatial variation of cesium-137 in small catchments

Surface contamination by bomb-derived and Chernobyl-derived 137Cs has been subject to changes due to physical decay and lateral transport of contaminated soil particles, which have resulted in an on-going transfer of radionuclides from terrestrial ecosystems to surface water, river bed sediments, and flood plains. Knowledge of the different sources of spatial variation of 137Cs is particularly essential for estimating 137Cs transfer to fluvial systems and for successfully applying 137Cs as an environmental tracer in soil erosion studies. This study combined a straightforward sediment redistribution model and geostatistical interpolation of point samples of 137Cs activities in soil to distinguish the effects of sediment erosion and deposition from other sources of variation in 137Cs in the small Mochovce catchment in Slovakia. These other sources of variation could then be interpreted. Besides erosion and deposition processes, the initial pattern of 137Cs deposition, floodplain sedimentation, and short-range spatial variation were identified as the major sources of spatial variation of the 137Cs inventory.     

Determining long-term (decadal) deep drainage rate using multiple tracers

The deep drainage rate is a critical hydrological parameter in understanding contamination mechanisms of soil and groundwater. Little research has been conducted on the temporal variations in deep drainage rate during the last century. The objective of this study was to determine the long-term deep drainage rate on a cultivated loamy soil in the Canadian Prairies. Three tracers were used: KCl applied in 1971, fallout tritium in 1963, and NO3* released during the initial cultivation of the field (1923). Two soil cores to a depth of 3.6 m were taken along a flat portion of the field, and soil Cl(-), 3H, and NO3* concentrations were measured as a function of depth. An additional four cores were taken for soil water content measurements between 2000 and 2003. Distinct peaks in the depth distribution of these three tracers were located at 1.27 m for Cl(-), 1.31 m for 3H, and 1.52 m for NO3*, 32, 40, and 80 yr after the application of Cl(-), 3H, and NO3*, respectively. The average deep drainage rates, calculated as the product of the estimated tracer velocity and volumetric soil water content below the active root zone, were 2.0 mm yr(-1) from the Cl(-) tracer, 2.2 mm yr(-1) from 3H, and 2.5 mm yr(-1) from the NO3* tracer. Therefore, there was little temporal variability in the groundwater recharge over the eight decades that the field has been cultivated. The recharge rates are less than 1% of the mean annual precipitation (333 mm).     

Radionuclide transport above a near-surface water table: III. Soil migration and crop uptake of three gamma-emitting radionuclides, 1990 to 1993

This paper summarizes the vertical distributions of 22Na, 137Cs, and 60Co above controlled water tables in deep and shallow lysimeters during a four-year experiment. The activity concentration profiles were all determined at the time of harvest of a winter wheat (Triticum aestivum L. cv. Pastiche) crop. Activity concentrations in different crop tissues were determined and crop uptake expressed as both an inventory ratio (IR) and a transfer factor (TFw), weighted to account for root and radionuclide distributions within the soil profile. Experimental variates were subjected to analysis of variance to determine the single and combined effects of the soil depth and the year of the experiment on the results obtained. Each radionuclide showed significant variations in activity concentration with soil depth, but the significance of these variations from year to year was dependent on radionuclide. A distinction in the behavior of weakly sorbed (22Na) and more highly sorbed (137Cs and 60Co) radionuclides was observed. The former exhibited significant variations in its distribution in the soil profile from year-to-year whereas the latter did not. Relatively high TF, values for 22Na were maintained throughout the experiment, whereas for 137Cs and 60Co, the highest TFw values were recorded in 1990 followed by a significant decline in 1991, with TFw remaining low in 1992 and 1993. The TFw values were, in general, significantly higher for deep lysimeters than for shallow lysimeters. This is thought to provide evidence of enhanced radionuclide absorption by the relatively small fraction of roots in the vicinity of the deeper water table.     

Uptake of cesium-137 and strontium-90 from contaminated soil by three plant species; application to phytoremediation

A field test was conducted to determine the ability of three plant species to extract 137Cs and 90Sr from contaminated soil. Redroot pigweed (Amaranthus retroflexus L.), Indian mustard [Brassica juncea (L.) Czern.], and tepary bean (Phaseolus acutifolius A. Gray) were planted in a series of spatially randomized cells in soil that was contaminated in the 1950s and 1960s. We examined the potential for phytoextraction of 90Sr and 137Cs by these three species. Concentration ratios (CR) for 137Cs for redroot pigweed, Indian mustard, and tepary bean were 2.58, 0.46, and 0.17, respectively. For 90Sr they were substantially higher: 6.5, 8.2, and 15.2, respectively. The greatest accumulation of both radionuclides was obtained with redroot pigweed, even though its CR for 90Sr was the lowest, because of its relatively large biomass. There was a linear relationship between the 137Cs concentration in plants and its concentration in soil only for redroot pigweed. Uptake of 90Sr exhibits no relationship to 90Sr concentrations in the soil. Estimates of time required for removal of 50% of the two contaminants, assuming two crops of redroot pigweed per year, are 7 yr for 90Sr and 18 yr for 137Cs.     

Pulsed redistribution of a contaminant following forest fire: cesium-137 in runoff

Of the natural processes that concentrate dispersed environmental contaminants, landscape fire stands out as having potential to rapidly concentrate contaminants and accelerate their redistribution. This study used rainfall simulation methods to quantify changes in concentration of a widely dispersed environmental contaminant (global fallout 137Cs) in soils and surface water runoff following a major forest fire at Los Alamos, New Mexico, USA. The 137Cs concentrations at the ground surface increased up to 40 times higher in ash deposits and three times higher for the topmost 50 mm of soil compared with pre-fire soils. Average redistribution rates were about one order of magnitude greater for burned plots, 5.96 KBq ha(-1) mm(-1) rainfall, compared with unburned plots, 0.55 KBq ha(-1) mm(-1) rainfall. The greatest surface water transport of 137Cs, 11.6 KBq ha(-1) mm(-1), occurred at the plot with the greatest amount of ground cover removal (80% bare soil) following fire. Concentration increases of 137Cs occurred during surface water erosion, resulting in enrichment of 137Cs levels in sediments by factors of 1.4 to 2.9 compared with parent soils. The elevated concentrations in runoff declined rapidly with time and cumulative precipitation occurrence and approached pre-fire levels after approximately 240 mm of rainfall. Our results provide evidence of order-of-magnitude concentration increases of a fallout radionuclide as a result of forest fire and rapid transport of radionuclides following fire that may have important implications for a wide range of geophysical, ecosystem, fire management, and risk-based issues.     

The use of 137Cs to establish longer-term soil erosion rates on footpaths in the UK

There is increasing awareness of the damage caused to valuable and often unique sensitive habitats by people pressure as degradation causes a loss of plant species, disturbance to wildlife, on-site and off-site impacts of soil movement and loss, and visual destruction of pristine environments. This research developed a new perspective on the problem of recreational induced environmental degradation by assessing the physical aspects of soil erosion using the fallout radionuclide caesium-137 (¹³⁷Cs). Temporal sampling problems have not successfully been overcome by traditional research methods monitoring footpath erosion and, to date, the ¹³⁷Cs technique has not been used to estimate longer-term soil erosion in regard to sensitive recreational habitats. The research was based on-sites within Dartmoor National Park (DNP) and the South West Coast Path (SWCP) in south-west England. ¹³⁷Cs inventories were reduced on the paths relative to the reference inventory (control), indicating loss of soil from the path areas. The Profile Distribution Model estimated longer-term erosion rates (ca. 40 years) based on the ¹³⁷Cs data and showed that the combined mean soil loss for all the sites on ‘paths’ was 1.41 kg m^?2 yr^?1 whereas the combined ‘off path’ soil loss was 0.79 kg m^?2 yr^?1, where natural (non-recreational) soil redistribution processes occur. Recreational pressure was shown to increase erosion in the long-term, as greater soil erosion occurred on the paths, especially where there was higher visitor pressure.     

Soil erosion and sediment sources in an Ohio watershed using beryllium-7, cesium-137, and lead-210

Soil cores and suspended sediments were collected within the Old Woman Creek, Ohio (OWC) watershed following a thunderstorm and analyzed for 7Be, 137Cs, and 210Pb activities to compare the effects of till vs. no-till management on soil erosion and sediment yield. The upper reaches of the watershed draining tilled agricultural fields were disproportionately responsible for the majority of the suspended sediment load compared with lower in the watershed (2.0-7.0 metric tons/km2 [Mg/km2] vs. 1.2-2.6 Mg/km2). About 6 to 10 times more sediment was derived from the subbasins that are predominantly tilled (6.8-12.4 Mg/km2) compared with the subbasins undergoing no-till practices (0.5-1.1 Mg/km2). In undisturbed soils the 210Pb activities decreased with movement toward the bottom of the cores to the constant supported 210Pb value at a depth of about 10 cm. There was a subsurface maximum in 137Cs activity within the top 10 cm. In contrast, the 210Pb and 137Cs distributions in soils that are currently or were previously tilled were nearly homogeneous with depth, reflecting continuing or previous mixing by plowing. The activities of 210Pb and 7Be were linearly correlated and were higher in suspended sediments derived from no-till subbasins than those derived from tilled subbasins, indicating that the soil surface is the source of suspended sediment. This study demonstrates that no-till farming results in decreases in soil erosion and decreases in suspended sediment discharges and that those eroded sediments have a radionuclide signature corresponding to the tillage practice and the depth of erosion.     

90Sr、137Cs在某种包气带土壤中的迁移研究

本文叙述了放射性废物中具有代表性的裂变核素^90Sr、^137Cs在某种包气带土壤中的迁移情况研究。使用小型土柱的氚水淋洗实验研究土壤水力学性质，最后进行大型土柱实验研究核素在该包气带土壤中的迁移，并对实验情况进行了数学模拟。用该土壤原状土进行小型土柱的氚水淋洗试验，测得了土壤的水力弥散度为0．32cm，土壤有效孔隙度为0．35。经过290天的大型柱迁移试验表明，土壤对^90Sr的阻滞系数为220．4，在模拟实际降雨量的情况下，^90Sr的平均迁移速度为0．63cm/y，^137Cs在大型柱试验中没有明显迁移。数值模拟^90Sr、^137Cs迁移，得出经过上述大型柱试验相同的条件下，^90Sr、^137Cs迁移的峰位置基本和大型柱试验结果相同。     

RESERVOIR SEDIMENTATION RATES LINKED TO LONG-TERM CHANGES IN AGRICULTURAL LANI) USE1

ABSTRACT: Long-term land use and reservoir sedimentation were quantified and linked in a small agricultural reservoir-watershed system without having historical data. Land use was determined from a time sequence of aerial photographs, and reservoir sedimentation was determined from cores with ¹³⁷Cs dating techniques. They were linked by relating sediment deposition to potential sediment production which was determined by the Universal Soil Loss Equation and by SCS estimates for gullied land. Sediment cores were collected from Tecumseh Lake, a 55-ha reservoir with a 1,189-ha agricultural watershed, constructed in 1934 in central Oklahoma. Reservoir sediment deposition decreased from an average of 5,933 Mg/yr from 1934 to 1954, to 3,179 Mg/yr from 1954 to 1962, and finally to 1,017 Mg/yr from 1962 to 1987. Potential sediment production decreased from an average of 29,892 to 11,122 and then to 3,589 Mg/yr for the same time periods as above, respectively. Reductions in deposition and sediment production corresponded to reductions in cultivated and abandoned cropland which became perennial pasture. Together, cultivated and abandoned cropland accounted for 59 percent of the watershed in 1937, 24 percent in 1954, and 10 percent in 1962. Roadway erosion, stream bank erosion, stored stream channel sediment, and long-term precipitation were considered, but none seemed to play a significant role in changing sediment deposition rates. Instead, the dominant factor was the conversion of fields to perennial pastures. The effect of conservation measures on reservoir sedimentation can now be quantified for many reservoirs where historical data is not available.     

SEDIMENT DEPOSITION ESTIMATION FROM CESIUM-137 MEASUREMENTS1

ABSTRACT The movement of fallout ¹³⁷Cs carried by soil particles was studied as an indicator of erosion and sedimentation in the Allerton watersheds and 4-H Memorial Lake located near Monticello, Illinois. Sediment deposition was greater in the waterway draining from watershed IB than in the waterway from watershed IA. At the average rate of 2.3 cm/yr of sediment deposition in the lake (from 1954 to 1979), there will be a loss of over 2 meters of water depth in the next century. However, there appears to be a decreasing rate of sediment deposition in the 4-H Memorial Lake as a result of improved conservation practices on the watersheds and the increased effectiveness of vegetated waterways and buffers for retaining sediment.     

Time-dependent distribution of surface-applied radionuclides and their recovery in maize during the growing season

The spatial and temporal heterogeneity of field soils influences the fate and behavior of strongly sorbing pollutants and their entry into the food chain. We studied the redistribution of surface-applied 54Mn, 65Zn, 57Co, and 134Cs in the soil profile and their recovery in the aerial parts of maize grown on an untilled agricultural soil during the growing season. Radionuclides were more concentrated in the preferential flow paths (PFP) than in the soil matrix and their concentration decreased with time. The recovery of 54Mn in the aerial plant parts increased between pollen shed and maturity, while the recovery of 65Zn and 57Co did not show any significant difference, and the recovery of 134Cs decreased with time. The amount and distribution of rainfall, and the chemical, physical, and microbiological soil characteristics are the major factors influencing the variation of radionuclide recovery with time.     

Soil Organic Carbon Stock and Distribution in Cultivated Land Converted to Grassland in a Subtropical Region of China

Land-use change from one type to another affects soil carbon (C) stocks which is associated with fluxes of CO₂ to the atmosphere. The 10-years converted land selected from previously cultivated land in hilly areas of Sichuan, China was studied to understand the effects of land-use conversion on soil organic casrbon (SOC) sequestration under landscape position influences in a subtropical region of China. The SOC concentrations of the surface soil were greater (P < 0.001) for converted soils than those for cultivated soils but lower (P < 0.001) than those for original uncultivated soils. The SOC inventories (1.90–1.95 kg m^?2) in the 0–15 cm surface soils were similar among upper, middle, and lower slope positions on the converted land, while the SOC inventories (1.41–1.65 kg m^?2) in this soil layer tended to increase from upper to lower slope positions on the cultivated slope. On the whole, SOC inventories in this soil layer significantly increased following the conversion from cultivated land to grassland (P < 0.001). In the upper slope positions, converted soils (especially in 0–5 cm surface soil) exhibited a higher C/N ratio than cultivated soils (P = 0.012), implying that strong SOC sequestration characteristics exist in upper slope areas where severe soil erosion occurred before land conversion. It is suggested that landscape position impacts on the SOC spatial distribution become insignificant after the conversion of cultivated land to grassland, which is conducive to the immobilization of organic C. We speculate that the conversion of cultivated land to grassland would markedly increase SOC stocks in soil and would especially improve the potential for SOC sequestration in the surface soil over a moderate period of time (10 years).     

Uptake and release of cesium-137 by five plant species as influenced by soil amendments in field experiments

Phytoextraction field experiments were conducted on soil contaminated with 0.39 to 8.7 Bq/g of 137Cs to determine the capacity of five plant species to accumulate 137Cs and the effects of three soil treatments on uptake. The plants tested were redroot pigweed (Amaranthus retroflexus L. var. aureus); a mixture of redroot pigweed and spreading pigweed (A. graecizans L.); purple amaranth (A. cruteus L.) x Powell's amaranth (A. powellii S. Watson), referred to here as the amaranth hybrid; Indian mustard [Brassica juncea (L.) Czern.]; and cabbage (Brassica oleracea L. var. capitata). For control plants, the concentration ratios (CR) of 137Cs were greatest for redroot pigweed and the amaranth hybrid, with average CR values of 1.0 +/- 0.24 and 0.95 +/- 0.14, respectively. The lowest value was for Indian mustard at 0.36 +/- 0.10. The soil treatments included (i) application of NH4NO3 solution to the soil after plants had matured, (ii) addition of composted manure to increase organic matter content of the soil, (iii) combination of the manure and ammonium solution treatments, and (iv) controls. The ammonium solution gave little overall increase in accumulation of 137Cs. The use of composted manure also had little influence, but the combination of the composted manure with application of ammonium solutions had a distinctly negative effect on plant uptake of 137Cs. On average the fraction of 137Cs taken up from the soil was reduced by 57.4 +/- 1.2% compared with controls. This was the result of release of competing ions, primarily Ca, from the manure and was observed across all five plant species tested. The application of ammonium solution took place in the last two weeks before harvest. The reduction of plant 137Cs content, by addition of the ammonium solution, as it interacted with the manure, indicates that substantial quantities 137Cs can be released from the shoots of plants as a result of sudden changes in soil solution chemistry.     

ROLE OF EROSION PROCESSES IN TRANSFER OF RADIONUCLIDES: RESULTS OF FIELD EXPERIMENTS1

ABSTRACT: An experimental one-year fieldwork has been conducted in the vicinity of the Chernobyl NPP, within an agricultural watershed, to study the transfer of radionuclides brought into the environment by the disaster of 1986. Presented are results of observation of the washout of ¹³⁷Cs from the runoff plot both in natural conditions and under artificial rainfalls. Beside traditional hydro-logical methods, new techniques were used allowing to consider microtopographical peculiarities of the runoff plot and their role in the redistribution of radionuclides. The estimate of the annual mass balance for the soil and the radionuclides within the runoff plot has shown that, regardless of significant areal variation of their concentration, the ¹³⁷Cs washout with the solid runoff resulted from artificial rainfalls amounts to some 1 percent of its reserves in the uppermost 5 cm of the topsoil. The same parameter for the natural runoff is lower by an order of magnitude. Both these factors of self-purification are about two times less than natural radioactive decay of ¹³⁷Cs.     

Pesticide fate and transport throughout unsaturated zones in five agricultural settings, USA

Pesticide transport through the unsaturated zone is a function of chemical and soil characteristics, application, and water recharge rate. The fate and transport of 82 pesticides and degradates were investigated at five different agricultural sites. Atrazine and metolachlor, as well as several of the degradates of atrazine, metolachlor, acetochlor, and alachlor, were frequently detected in soil water during the 2004 growing season, and degradates were generally more abundant than parent compounds. Metolachlor and atrazine were applied at a Nebraska site the same year as sampling, and focused recharge coupled with the short time since application resulted in their movement in the unsaturated zone 9 m below the surface. At other sites where the herbicides were applied 1 to 2 yr before sampling, only degradates were found in soil water. Transformations of herbicides were evident with depth and during the 4-mo sampling time and reflected the faster degradation of metolachlor oxanilic acid and persistence of metolachor ethanesulfonic acid. The fraction of metolachlor ethanesulfonic acid relative to metolachlor and metolachlor oxanilic acid increased from 0.3 to >0.9 at a site in Maryland where the unsaturated zone was 5 m deep and from 0.3 to 0.5 at the shallowest depth. The flux of pesticide degradates from the deepest sites to the shallow ground water was greatest (3.0-4.9 micromol m(-2) yr(-1)) where upland recharge or focused flow moved the most water through the unsaturated zone. Flux estimates based on estimated recharge rates and measured concentrations were in agreement with fluxes estimated using an unsaturated-zone computer model (LEACHM).     

Cesium-137 monitoring using mosses from W. Macedonia,N. Greece

¹³⁷Cs activities in mosses and substrate (soil, bark) collected from W. Macedonia, Greece were measured 20 years after the Chernobyl reactor accident. Archive material from previous studies was also used for comparison and diachronic estimation of the radio-contamination status. A gradual decrease was detected which depended on various factors such as the collected species, location, growth rate and substrate. Maximum accumulation capacity of ¹³⁷Cs was observed in the epilithic mosses in comparison to the epiphytic ones. The ¹³⁷Cs content in the bark of the two broad-leaved species (oak and fagus) was higher than that of the conifer (pinus). Bark specimens of about 50 cm height were in general more contaminated than those of 200 cm. Autoradiography revealed an amount of ¹³⁷Cs distributed more or less uniformly in moss thalli. The high ¹³⁷Cs activities found in mosses 20 years after Chernobyl suggest that these primitive plants are effective, suitable and inexpensive biological detectors of the distribution and burden of radionuclide fallout pattern.     

Radiocaesium accumulation in stemwood: integrated approach at the scale of forest stands for contaminated Scots pine in Belarus

Twenty years after the Chernobyl accident, root uptake from the surface layers of contaminated forest soils plays a major role in radiocaesium ((137)Cs) transfer to the trees and accumulation in perennial compartments, including stemwood. Trustworthy long-term predictions (modelling) of stemwood contamination with (137)Cs should accordingly be based on a reliable picture of this source-sink relationship. Considering the complexity of the processes involved in (137)Cs cycling in forest stands, elementary ratios like transfer factors (TF) were shown to be not very relevant for that purpose. At the tree level, alternatives like the wood immobilisation potential (WIP) have therefore been proposed in order to quantify the current net (137)Cs accumulation in stemwood. Our objective was here to compare WIP values determined for a series of contaminated forest stands in Belarus with the corresponding pools of (137)Cs available in the soil for root uptake. The comparison reveals that both indices are quite proportional, whatever the forest ecosystem features. This corroborates the relevancy of WIP as an indicator of the current (137)Cs root uptake by the trees, which could accordingly help to improve the existing models of (137)Cs cycling and the long-term management of contaminated forest ecosystems.     

Immobilization of cesium-137 and uranium in contaminated sediments using soil amendments

Batch and dynamic leaching methods were used to evaluate the effectiveness of hydroxyapatite (HA), illite, and zeolite, alone and in combination, as soil additives for reducing the migration of cesium-137 (137Cs+) and uranium (U) from contaminated sediments. Amendment treatments ranging from 0 to 50 g kg(-1) were added to the sediment and equilibrated in 0.001 M CaCl2. After equilibration, the treatment supernatants were analyzed for 137Cs+, U, PO4, and other metals. The residual sediments were then extracted overnight using one of the following: 1.0 M NH4Cl, 0.5 M CaCl2, or the Toxicity Characteristic Leaching Procedure (TCLP) extractant. Cesium was strongly sorbed to the contaminated sediments, presumably due to interlayer fixation within native illitic clays. In fact, 137Cs+ was below detection limits in the initial equilibration solutions, the CaCl2 extract, and the TCLP solution, regardless of amendment. Extractants selective for interlayer cations (1.0 M NH4Cl) were necessary to extract measurable levels of 137Cs+. Addition of illitic clays further reduced Cs+ extractability, even when subjected to the aggressive extractants. Zeolite, however, was ineffective in reducing Cs+ mobility when subjected to the aggressive extractants. Hydroxyapatite was less effective than illite at reducing NH4+-extractable Cs+. Hydroxyapatite, and mixtures of HA with illite or zeolite, were highly effective in reducing U extractability in both batch and leaching tests. Uranium immobilization by HA was rapid with similar final U concentrations observed for equilibration times ranging from 1 h to 30 d. The current results demonstrate the effectiveness of soil amendments in reducing the mobility of U and Cs+, which makes in-place immobilization an effective remediation alternative.     

RATES OF RECENT SEDIMENTATION IN LAKE PEPIN1

ABSTRACT: Sedimentation rates since 1954 in Lake Pepin, as determined from the content of fallout cesium-137 in the sediment profile, have exceeded 2.5 cm/yr in the upper part of the lake. These rates, although somewhat less than those of the previous half century (1895–1954), are sufficiently large that the upstream portion of Lake Pepin is threatened with conversion to a marsh within a century. The density of the sediments measured increased with depth in the sampled profile from 1.1 to 1.2 g/ml at the sediment surface to 1.4 to 1.5 g/ml at 2- or 3-m depth. There was little or no change in the patterns of textural composition or density of the sediment profile with depth and age over the past 80 to 150 years.