Stream Sediment Concentration Research Articles

Causal inference approaches reveal both positive and negative unintended effects of agricultural and urban management practices on instream biological condition

Agricultural and urban management practices (MPs) are primarily designed and implemented to reduce nutrient and sediment concentrations in streams. However, there is growing interest in determining if MPs produce any unintended positive effects, or co-benefits, to instream biological and habitat conditions. Identifying co-benefits is challenging though because of confounding variables (i.e., those that affect both where MPs are applied and stream biota), which can be accounted for in novel causal inference approaches. Here, we used two causal inference approaches, propensity score matching (PSM) and Bayesian network learning (BNL), to identify potential MP co-benefits in the Chesapeake Bay watershed portion of Maryland, USA. Specifically, we examined how MPs may modify instream conditions that impact fish and macroinvertebrate indices of biotic integrity (IBI) and functional and taxonomic endpoints. We found evidence of positive unintended effects of MPs for both benthic macroinvertebrates and fish indicated by higher IBI scores and specific endpoints like the number of scraper macroinvertebrate taxa and lithophilic spawning fish taxa in a subset of regions. However, our results also suggest MPs have negative unintended effects, especially on sensitive benthic macroinvertebrate taxa and key instream habitat and water quality metrics like specific conductivity. Overall, our results suggest MPs offer co-benefits in some regions and catchments with largely degraded conditions but can have negative unintended effects in some regions, especially in catchments with good biological conditions. We suggest the number and types of MPs drove these mixed results and highlight carefully designed MP implementation that incorporates instream biological data at the catchment scale could facilitate co-benefits to instream biological conditions. Our study underscores the need for more research on identifying effects of individual MP types on instream biological and habitat conditions.

Environmental assessment of heavy metal contamination at Chekfa aggregate quarry (Jijel, North‐East Algeria)

AbstractIt is fascinating to observe the potential hazards associated with mining and quarrying activities, despite the limited number of comprehensive studies conducted in Algeria to evaluate their environmental impact. This particular investigation focuses on El M'cif quarry in Chekfa, located within the Nil watershed in Jijel, a region situated in the North‐East of Algeria. The primary objective is to assess the concentrations of lead (Pb), zinc (Zn), copper (Cu), and cadmium (Cd) in surface water, groundwater, stream sediments, and soil. Consequently, a suite of petrographic and geochemical analyses is undertaken to discern the origins of these heavy metals. Employing statistical methodologies, notably the Geo‐accumulation Index (Igeo), Pollution Index (PI+PI), and Ecological Risk Assessment (ER), this study is further fortified to quantify the levels of hazardous contaminants. The findings reveal that the concentrations of Cd and Pb in surface water are recorded at 22 and 654.3 μg/L, respectively, surpassing both Algerian and WHO standards. Meanwhile, the heavy metal concentrations in stream sediments are ranked in the order of Zn > Cu > Cd, with respectively mean concentrations of 1085.30, 14.60, and 1.49 (mg/kg). The mean concentrations of Zn, Pb, Cd, and Cu in soil are measured respectively at 219.78, 128.86, 20.76, and 2.42 (mg/kg). Based on the statistical methods such as I‐geo, PI, IPI, and RI, the research reveals that the sediments of the Aftis River are uncontaminated by Cu, moderately to heavily contaminated with Zn and heavily contaminated by Cd. Besides the soil samples which are found to be extremely contaminated with Cd, moderately contaminated with Pb, and uncontaminated with Cu and Zn. Additionally, the X‐ray fluorescence analysis reveals that the limestone extracted from the quarry contains 0.7% cadmium, suggesting that the quarry could potentially contribute to cadmium contamination. The surrounding rocks exhibit significant concentrations of iron, lead, and zinc. Therefore, the elevated levels of these heavy metals can primarily be attributed to geological factors rather than human activities, highlighting the combined influence of the extractive industry and the geological environment.

Significant Enrichment of Rare Earth Element Concentrations in Stream Sediments of Sharm El-Sheikh Area, Southern Sinai-Egypt: Geochemical Prospecting and Heavy Mineral Survey

Economic rare earth element bearing-heavy mineral accommodation in alluvial deposits (stream sediments) is a well-known process caused by varying rates of weathering and transportation of heavy minerals and sediments, which is significant in geochemical exploration. In the present work, stream sediment samples from Wadi Lethi, Sharm El-Sheikh, were systematically collected. The collected stream sediments were investigated mineralogically and geochemically using collaborative techniques. The separated heavy fractions have been analyzed for trace elements and Rare Earth Element. Important heavy minerals identified are monazite, sphene, apatite, garnet, xenotime, magnetite, ilmenite, and hematite, with subordinate riebeckite, epidote, and chlorite. Geochemical analyses of the separated heavy minerals revealed an abnormal concentration of total Rare Earth Element that was more than eleven times higher than in the studied area's granitic rocks. The Rare Earth Elements trends of these minerals depict the enrichment of light rare earth elements and the depletion of heavy rare earth elements, as well as the typical negative Eu anomaly found in granitic magma.

Characteristics of B, Zn and Se in stream sediments of Tanzania and their agricultural applications: Based on 1:1000000 geochemical survey

Tanzania is located in eastern Africa with a predominantly agricultural ecomomy, the potential for developing and utilizing cultivated land are promising, but scientific guidance is required. B, Zn and Se are essential micronutrients for plants and human body with crucial biological functions, in particular, Se is significant for human health and considered as “the king of anti-cancer”. As these elements required by human or plants are mainly absorbed from soil directly or indirectly, therefore, it is important to understand the contents and distributions of them in the soil of cultivated land for guiding agricultural production. In this work, low-density geochemical survey at the scale of 1:1000000 was carried out in Tanzania, and the results show that the concentrations of B, Zn and Se in stream sediments are low and their distributions are heterogeneous. According to the distributions of geological units, the existing cultivated land resources can be divided into five regions in Tanzania. Compared with the national background values, the concentrations of B, Zn and Se are insufficient overall but enriched locally in these regions. In general, element concentrations in stream sediments and soil have a positive correlation because of their similar sources, which is essential in agriculture application. Based on the information provided by low-density geochemical data and maps, the Se-sufficient and Se-rich regions were delineated in Tanzania, where can be used to develop Se-rich industries. Finally, this paper believes that geochemical survey is a powerful tool for cultivated land evaluation, agriculture management and land development.©2023 China Geology Editorial Office.

Interpretation of geochemical anomalies and domains using Gaussian mixture models

The element's concentrations in stream sediments are frequently the product of complex geological processes and a variety of lithological sources. This variety of processes dilutions or concentrates the trace elements in the sediments concerning the original concentration presented in the source rock. When analyzing the geochemical data of stream sediments, it is common to find skewed distributions. This skewness can be attributed to multiplicative effects involved in dilution processes that result in lognormal distributions. It also can imply a mixture of populations that results from the variety of geological processes and/or the diversity of lithological sources. In this study, a method is proposed to classify geochemical samples of sediments into data sets or geochemical domains that follow normal, lognormal or log-ratio normal distributions. This methodology can be based on the classification of samples according to geological features or use Gaussian mixture models to determine the geochemical domains. To mixture modeling, CLR transformation and the expectation-maximization (EM) algorithm were used. The results obtained include mixture models of 2–3 populations associated with main geochemical processes and/or dominant lithological sources. The proposed methodology defines threshold values for each domain and allows to set a more appropriate background. An analysis carried out with data from northern Mexico identified anomalies that would not be defined without the separation of previous domains.

Petrology of Precambrian metagranites and heavy mineral chemistry of the Olounou area within the Cameroon mineral belt (Ntem complex, South Cameroon): relationships with the Fe mineralization

The petrology of Achaean biotite- and amphibole-rich metagranites, the geochemistry of Fe occurrences and heavy mineral concentrations in stream sediments from Olounou (Ntem complex) help to constrain their inter-relationships. These metagranites display granoblastic textures outlined by microcline, biotite, amphibole and oxides. Ilmenite (60 to 80%), zircon (15 to 20%) and rutile (1 to 2%) are the mineral fractions in pan concentrates. Two types of Fe mineralization as silicified veins cross-cut the above rocks: hematite ± magnetite quartzites with primary hematite enclosing magnetite relicts contain high average Fe2O3 (69.05 wt%) and TiO2 (0.73 wt%); banded magnetite-rich quartzites with magnetite partially replaced by hematite are characterized by Fe2O3 (52.15 wt%), TiO2 (0.27 wt%), relatively high W (286 ppm) and Ni (108 ppm). Both types have Zn, Cu, Ga contents below 10 ppm, positive Eu anomalies, HREE enrichment over LREEs and low ΣREE (9.76 to 12.07). Iron and SiO2 were derived from weathering of Archean biotite and amphibole metagranites, deposited in existing Post-Archean intracratonic basins under greenschist facies and impacted by submarine hydrothermal solutions. These results are genetically comparable to other Precambrian BIFs and underline the iron potential of Olounou. They equally contribute to iron ore exploration in Cameroon and worldwide.Supplementary material: Table S1 is available at https://doi.org/10.6084/m9.figshare.c.5426547

Modelling the Transference of Trace Elements between Environmental Compartments in Abandoned Mining Areas.

An openly accessible cellular automaton has been developed to predict the preferential migration pathways of contaminants by surface runoff in abandoned mining areas. The site where the validation of the results of the Contaminant Mass Transfer Cellular Automaton (CMTCA) has been carried out is situated on the steep flank of a valley in the Spanish northwestern region of Asturias, at the foot of which there is a village with 400 inhabitants, bordered by a stream that flows into a larger river just outside the village. Soil samples were collected from the steep valley flank where the mine adits and spoil heaps are situated, at the foot of the valley, and in the village, including private orchards. Water and sediment samples were also collected from both surface water courses. The concentration of 12 elements, including those associated with the Cu-Co-Ni ore, were analyzed by ICP-OES (Perkin Elmer Optima 3300DV, Waltham, MA, USA) and ICP-MS (Perkin Elmer NexION 2000, Waltham, MA, USA). The spatial representation of the model’s results revealed that those areas most likely to be crossed by soil material coming from source zones according to the CMTCA exhibited higher pollution indexes than the rest. The model also predicted where the probabilities of soil mass transfer into the stream were highest. The accuracy of this prediction was corroborated by the results of trace element concentrations in stream sediments, which, for elements associated with the mineral paragenesis (i.e., Cu, Co, Ni, and also As), increased between five- and nine-fold downstream from the predicted main transfer point. Lastly, the river into which the stream discharges is also affected by the mobilization of mined materials, as evidenced by an increase of up to 700% (in the case of Cu), between dissolved concentrations of those same elements upstream and downstream of the confluence of the river and the stream.

Probabilistic Estimation of Stream Turbidity and Application under Climate Change Scenarios.

Streamflow-based rating curves are widely used to estimate turbidity or suspended sediment concentrations in streams. However, such estimates are often inaccurate at the event scale due to inter- and intra-event variability in sediment-streamflow relationships. In this study, we use a quantile regression approach to derive a probabilistic distribution of turbidity predictions for Esopus Creek, a major stream in one of the watersheds that supply drinking water to New York City, using measured daily mean streamflow-turbidity data pairs for 2003 to 2016. Although a single regression curve can underpredict or overpredict the actual observation, quantile regression can estimate a range of possible turbidity values for a given value of streamflow. Regression relationships for various quantiles were applied to streamflows simulated by a watershed model to predict stream turbidity under: (i) the observed historical climate, and (ii) a future climate derived from 20 global climate model (GCM) scenarios. Future scenarios using quantile regression in combination with these GCMs and a stochastic weather generator indicated an increase in the frequency and magnitude of hydrological events that may generate high stream turbidity and cause potential water quality challenges to the water supply. The methods outlined in this study can be used for probabilistic estimation of stream turbidity for operational decisions and can be part of a vulnerability-based method to explore climate impacts on water resources.

Short communication: Increasing vertical attenuation length of cosmogenic nuclide production on steep slopes negates topographic shielding corrections for catchment erosion rates

Abstract. Interpreting catchment-mean erosion rates from in situ produced cosmogenic 10Be concentrations in stream sediments requires calculating the catchment-mean 10Be surface production rate and effective mass attenuation length, both of which can vary locally due to topographic shielding and slope effects. The most common method for calculating topographic shielding accounts only for the reduction of nuclide production rates due to shielding at the surface, leading to catchment-mean corrections of up to 20 % in steep landscapes, and makes the simplifying assumption that the effective mass attenuation length for a given nuclide production mechanism is spatially uniform. Here I evaluate the validity of this assumption using a simplified catchment geometry with mean slopes ranging from 0 to 80∘ to calculate the spatial variation in surface skyline shielding, effective mass attenuation length, and the total effective shielding factor, defined as the ratio of the shielded surface nuclide concentration to that of an unshielded horizontal surface. For flat catchments (i.e., uniform elevation of bounding ridgelines), the effect of increasing vertical attenuation length as a function of hillslope angle and skyline shielding exactly offsets the effect of decreasing surface production rate, indicating that no topographic shielding correction is needed when calculating catchment-mean vertical erosion rates. For dipping catchments (as characterized by a plane fit to the bounding ridgelines), the catchment-mean surface nuclide concentrations are also equal to that of an unshielded horizontal surface, except for cases of extremely steep range-front catchments, where the surface nuclide concentrations are counterintuitively higher than the unshielded case due to added production from oblique cosmic ray paths at depth. These results indicate that in most cases topographic shielding corrections are inappropriate for calculating catchment-mean erosion rates, and are only needed for steep catchments with nonuniform distributions of quartz and/or erosion rate. By only accounting for shielding of surface production, existing shielding approaches introduce a slope-dependent systematic error that could lead to spurious interpretations of relationships between topography and erosion rate.

The Origin of Fine Sediment Determines the Observations of Suspended Sediment Fluxes Under Unsteady Flow Conditions

AbstractField observations in a wide range of environments have shown that sediment availability is a major control on the suspended sediment observations in streams. Here we examine, via laboratory experiments, how the amount of proximal in‐channel fine sediment storage relative to the upstream fine sediment distal supply influences the observations of suspended sediment concentrations in streams. Experiments under idealized conditions in a laboratory flume with different ratios of proximal and distal sediment supplies were conducted under a varying flow regime. In addition, the role of the sediment particle size of the supplied sediment on suspended sediment observations was explored. The combinations of proximal and distal sediment supply result in multiple responses of the channel bed and sediment quantity within the channel bed, and the responses adjust through aggradation and degradation. The signature of sediment concentration observed at the upstream section of the channel, given by the distal supply, differs from the downstream observations of the total conveyed sediment (distal and proximal), as shown by an in‐phase analysis of sediment concentration‐discharge plots. Furthermore, we show that nonuniform sediment mixtures may result in a change in the direction of the hysteresis observed between sediment concentration and discharge (i.e., from a clockwise hysteresis to a counterclockwise hysteresis). We also demonstrate that the ratio between sediment distal supply and proximal sediment availability modulates the magnitude of the aggradation/degradation processes in the channel reach and thus the joint observations of sediment concentration and discharge.

Geostatistical prediction of heavy metal concentrations in stream sediments considering the stream networks

Heavy metals in mine wastes can considerably influence surrounding surface waters, soils, and human health. To estimate environmental impact, heavy metal concentrations in stream sediments can be utilized because they are indicators of contamination and change negligibly with time. This study proposes a new Kriging method to predict heavy metal concentrations in stream sediments. The proposed methods compensate for the drawbacks of Kriging based on Euclidean distance because they utilize the stream distance for the prediction by analyzing the stream path and networks using digital elevation models. Moreover, the developed method reduces the exaggeration problem in predicting the concentration of an uncontaminated stream segment by considering the catchment basin area in Kriging. Application of these methods to synthetic and real-world datasets proves that they exhibit improvement in terms of overall error reduction, and they provide reasonable predictions at stream junctions, rather than Kriging based on Euclidean distance.

Assessment of several nonlinear methods in forecasting suspended sediment concentration in streams

In this paper, the use of nonlinear nearest trajectory based on phase space reconstruction along with several data-driven methods, including two types of perceptron artificial neural networks with Levenberg–Marquardt (ANN-LM) and particle swarm optimization learning algorithms (ANN-PSO), adaptive neuro-fuzzy inference system (ANFIS) and gene expression programming for forecasting suspended sediment concentration (SSC) dynamics in streamflow is studied. The nonlinearity of the series was tested using the method of surrogate data at 0.01 significance level as well as correlation exponent method. The proper time delay is calculated using the average mutual information function. Obtained results of different models are compared using root mean square error (RMSE), Pearson's correlation coefficient (PCC) and Nash–Sutcliffe efficiency with logarithmic values (Eln). Of the applied nonlinear methods, ANFIS generates a slightly better fit under whole daily SSC values (the least amount of RMSE = 10.5 mg/l), whereas ANN-PSO shows superiority based on the Eln criterion (the highest amount of Eln = 0.885). According to the non-parametric Mann–Whitney test, all data-driven models represent the same forecasted results and are significantly superior to the nearest trajectory-based model at the 99% confidence level.

Effects of mechanical macrophyte control on suspended sediment concentrations in streams

ABSTRACTSuspended sediment (SS) is an important pollutant in freshwater ecosystems and can be detrimental to fish communities. Although macrophytes mediate sediment deposition, little effort has been put into determining how their removal affects sediment resuspension. The present study examined the immediate and long-term impacts of mechanical macrophyte removal on SS concentrations in streams. The results of this study suggest that bed disturbance during mechanical excavation of macrophytes significantly increases SS in the short term, and concentrations were found to increase by as much as 15,687 mg L–1 immediately after macrophyte removal. Significant long-term (77 day) increases in SS were also observed, indicating that without macrophytes, disturbed material is continually resuspended after excavation by fluvial processes. These results demonstrate that macrophyte removal can result in SS levels that have previously been shown to harm fish, and indicate that this activity may be more detrimental to fish than previously thought.

Detecting the impact of heavy metal contaminated sediment on benthic macroinvertebrate communities in tropical streams

The effects of heavy metal pollution on benthic macroinvertebrate communities in tropical streams draining ultramafic systems have not been explored, despite a pressing need for ecological risk assessment to protect and manage aquatic ecosystems in these areas. The objective of this study was to examine benthic macroinvertebrate community composition in relation to metal concentrations in stream sediments and other physico-chemical variables in the Manyame River system, which drains part of the Great Dyke of Zimbabwe. Benthic macroinvertebrate sampling and community composition analysis, sediment collection, processing and metal analysis and assessment of other variables in the water column were done once at 55 sampling sites: 12 urban, 30 communal (i.e. sparsely populated rural areas, with livelihoods centred around subsistence agriculture) and 11 Great Dyke sites. Canonical correspondence analysis and partial canonical correspondence analysis (pCCA) were used to determine the importance of sediment heavy metal concentrations in explaining benthic macroinvertebrate community composition in comparison with other factors. Water quality ranged from very poor for urban locations due to sewage pollution, to good in communal locations. Significantly high concentrations of metals (ANOVA, p<0.05) and high magnesium/calcium (Mg/Ca) ratio were recorded in sediments for the Great Dyke site locations. The Mg/Ca ratio, Ca2+, Cr3+ and K+ were found to be important metals structuring benthic macroinvertebrate communities in the study streams, with metals explaining a larger percentage (58.0%) of the total variation explained compared to other variables (35.9%). However, taxa richness, diversity, evenness, percentage of Ephemeroptera, Plecoptera Trichoptera related metrics were higher at some Great Dyke sites than at communal sites. Thus, measures of taxa richness, diversity and %EPT may provide misleading information when assessing heavy metals in moderately polluted environments as in this study. Assessment of tropical streams draining ultramafic systems that are heavy metal-contaminated should also include benthic invertebrate community structure analysis, as it is possible that common endpoints, such as %EPT, may not identify impacts to aquatic communities.

Controls on Holocene denudation rates in mountainous environments under Mediterranean climate

AbstractThe Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional denudation rates in the framework of a source‐to‐sink approach. Indeed, fluvial sedimentation in East‐Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz‐rich crystalline rocks allowing cosmogenic nuclide 10Be measurements. In this paper, Holocene denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced 10Be concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated denudation rates range from 15 to 95 mm ka‐1. When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho‐climatic parameters including sun exposure and the direction of the main wet wind, so‐called ‘Libecciu’. This distribution, as well as the basement rock strength seems to play a significant role in the denudation distribution. We thus suggest that the landscape reached a geomorphic steady‐state due to the specific Mediterranean climate and that Holocene denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport‐limited. Al/K measurements used as a proxy to infer present‐day catchment‐wide chemical weathering patterns might support this assumption. Copyright © 2016 John Wiley &amp; Sons, Ltd.

The role of chronic and episodic disturbances on channel–hillslope coupling: the persistence and legacy of extreme floods

AbstractLandscape form represents the cumulative effects of de‐stabilizing events relative to recovery processes. Most geomorphic research has focused on the role of episodic rare events on landscape form with less attention paid to the role and persistence of chronic inputs. To better establish the interplay between chronic and episodic extreme events at regional scales, we used aerial photography and post‐flood sediment sampling to assess stream and hillslope response and recovery to a 100–300 yr. flood caused by Tropical Storm Irene in New England. Within a 14 000 km2 study area, analysis of aerial photographs indicated that the storm initiated (n = 534) and reactivated (n = 460) a large number of landslides. These landslides dramatically increased overall estimates of regional erosion rates (from 0.0023 mm/yr. without Irene to 0.0072 mm/yr. with Irene). Similarly, Irene‐generated LWD inputs of 0.25–0.5 trees/km exceeded annual background rates in a single event, and these concentrated inputs (101–102 of trees/landslide) are likely to result in large jams and snags that are particularly persistent and geomorphically effective. Finally, we found that landslide scars continue to provide elevated sediment inputs years after the event, as evidenced by sustained higher suspended sediment concentrations in streams with Irene‐generated landslides. Overall, our results indicate that infrequent, high‐magnitude events have a more important geomorphic role in tectonically stable, more moderate‐relief systems than has been previously recognized. Understanding the role of these events has particular relevance in regions such as New England, where the frequency and magnitude of extreme storms is expected to increase. Further, these effects may force reconsideration of conservation and restoration targets (for example in channel form and large wood loading and distribution) in fluvial systems. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Assessing the effect of a major storm on 10BE concentrations and inferred basin-averaged denudation rates

Abstract 10Be concentrations in stream sediments are commonly employed to calculate basin-averaged denudation rates. Such calculations assume that denudation is steady in time and that quartz is uniformly distributed in the watershed. The 10Be concentrations in stream sediments are assumed to represent a spatially and temporally averaged concentration, and therefore should not be affected by a discrete erosion event. The effect of such events on 10Be concentrations has been modeled, but has not been previously field-tested following a large precipitation event. In this study, we resampled stream sediments that had been previously analyzed to deduce basin averaged erosion rates in small, tributary basins in the Colorado Front Range following an extreme precipitation event in 2013. Most of our sample sites show no significant change between pre- and post-flood concentrations, and hence no change in the denudation rates inferred from them. The sample locations include a partially burned site, a debris flow site, and sites with very small drainage areas. A notable exception exists at lower Gordon Gulch, within the Boulder Creek Critical Zone Observatory; here, local 10Be data indicate that a large portion of flood-deposited sediment is likely derived from deep soils in fans, terraces, and fills, that are predominantly located on the north-facing toe slope. Overall, the reproducibility of denudation rates deduced from pre- and post-storm samples indicates that the uncertainty associated with 10Be-derived denudation rates is likely less than 15% in this setting.

Detection of transience in eroding landscapes

AbstractPast variations in climate and tectonics have led to spatially and temporally varying erosion rates across many landscapes. In this contribution I examine methods for detecting and quantifying the nature and timing of transience in eroding landscapes. At a single location, cosmogenic nuclides can detect the instantaneous removal of material or acceleration of erosion rates over millennial timescales using paired nuclides. Detection is possible only if one of the nuclides has a significantly shorter half‐life than the other. Currently, the only practical way of doing this is to use cosmogenic in situ carbon‐14 (14C) alongside a longer lived nuclide, such as beryllium‐10 (10Be). Hillslope information can complement or be used in lieu of cosmogenic information: in soil mantled landscapes, increased erosion rates can be detected for millennia after the increase by comparing relief and ridgetop curvature. This technique will work as long as the final erosion rate is greater than twice the initial rate. On a landscape scale, transience may be detected based upon disequilibria in channel profiles or ridgetops, but transience can be sensitive to the nature of transient forcing. Where forcing is periodic, landscapes display differing behavior if forcing is driven by changes in base level lowering rates versus changes in the efficiency of either channel or hillslope erosion (e.g. driven by climate change). Oscillations in base level lowering lead to basin averaged erosion rates that reflect a long term average erosion rate despite strong spatial heterogeneity in local erosion rates. This averaging is reflected in 10Be concentrations in stream sediments. Changes in hillslope sediment transport coefficients can lead to large fluctuations in basin averaged erosion rates, which again are reflected in 10Be concentrations. The variability of erosion rates in landscapes where both the sediment transport and channel erodibility coefficients vary is dominated by changes to the hillslope transport coefficient. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Insecticide concentrations in stream sediments of soy production regions of South America

Concentrations of 17 insecticides were measured in sediments collected from 53 streams in soy production regions of South America (Argentina in 2011–2014, Paraguay and Brazil in 2013) during peak application periods. Although environmental regulations are quite different in each country, commonly used insecticides were detected at high frequencies in all regions. Maximum concentrations (and detection frequencies) for each sampling event ranged from: 1.2–7.4ng/g dw chlorpyrifos (56–100%); 0.9–8.3ng/g dw cypermethrin (20–100%); 0.42–16.6ng/g dw lambda-cyhalothrin (60–100%); and, 0.49–2.1ng/g dw endosulfan (13–100%). Other pyrethroids were detected less frequently. Banned organochlorines were most frequently detected in Brazil. In all countries, cypermethrin and/or lambda-cyhalothrin toxic units (TUs), based on Hyalella azteca LC50 bioassays, were occasionally>0.5 (indicating likely acute toxicity), while TUs for other insecticides were <0.5. All samples with total insecticide TU>1 were collected from streams with riparian buffer width<20m. A multiple regression analysis that included five landscape and habitat predictor variables for the Brazilian streams examined indicated that buffer width was the most important predictor variable in explaining total insecticide TU values. While Brazil and Paraguay require forested stream buffers, there were no such regulations in the Argentine pampas, where buffer widths were smaller. Multiple insecticides were found in almost all stream sediment samples in intensive soy production regions, with pyrethroids most often occurring at acutely toxic concentrations, and the greatest potential for insecticide toxicity occurring in streams with minimum buffer width<20m.

MERCURY AND ARSENIC CONTAMINATION FROM SMALL SCALE GOLD MINING ACTIVITIES AT SELOGIRI AREA, CENTRAL JAVA, INDONESIA

Small scale gold mines discussed here are located at Selogiri area, Central Java, Indonesia which was mined by local community mainly during gold rush in 1990s. This Selogiri gold deposit genetically is characterized by porphyry mineralization overprinted by epithermal system. The ore minerals assemblage consists of pyrite, sphalerite, chalcopyrite, galena, chalcocite and rare arsenopyrite.&#x0D; &#x0D; Chemical analysis of soil and stream sediment sampled over 1.5 km across at the Selogiri gold extraction site indicates that the site has been contaminated with mercury due to mining activities. The mercury concentrations in soil and stream sediments collected during dry season range from 0.01 to 481 ppm and 0.01 to 139 ppm, respectively, higher than background value of 0.05 ppm. In contrast, mercury concentration in stream sediments collected during rainy season from the same location as dry season sampling ranges from 0.01 to 13.42 ppm, and one sample has anomalous value of 331 ppm. This result show that rain water may disperse and decrease mercury concentration in stream sediments.&#x0D; &#x0D; In case of arsenic, although the ore contains rare arsenic minerals, arsenic concentration in bulk rock and ore is high ranging from 8 to 59 ppm, while the arsenic concentration in tailing is much higher ranging from 5.8 to 385 ppm. Chemical analyses on pyrite reveal that the pyrite grains contain arsenic and might be the source of arsenic in Selogiri mine site. However, analysis of dug-well water demonstrates that the mercury and arsenic content is still lower than the maximum allowable concentration.&#x0D; &#x0D; Keywords: Mercury, arsenic, contamination, Selogiri, gold mine