Silt Clay Research Articles

Characterization of granitoid and gneissic weathering profiles of the Mucone River basin (Calabria, southern Italy)

The paper deals with the development of a multidisciplinary research on weathering profiles of granitoid and gneissic rocks related to tectonic and landscape evolution of the western Sila Grande Massif (Calabria, southern Italy). Field scale observations and petrographical and mineralogical features are used to characterize in detail the weathering processes. The weathering profiles of the granitoid cut slopes are generally simple showing a progressive weathering increase toward the top of the slopes, whereas the weathering profiles of the gneissic cut slopes are generally complex with irregularities in the spatial distribution of weathered horizons. The microfabric and petrographic analyses show that gneissic samples (classes V–VI of weathering grade) are characterized by higher percentage of altered minerals and microfracture and void rather than granitoid samples (classes V–VI of weathering grade). The main mineralogical changes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary minerals (clay minerals and Fe-oxides) and with a substitution of the original rock fabric. Neoformed clay minerals and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Referred as physical changes coupled with chemical variations, the final results of weathering process are a soil-like material characterized by sand–gravel grain-size fraction for the granitoid rocks and by both silt–clay and sand–silt grain-size fraction for the gneissic rocks. This generally produces a greater value of the SGI (Sand Generation Index) for granitoids and explains the great productivity in sandy deposits of this lithology.

Assessment of aggregate resources: an integrated geophysical approach

ABSTRACTFluvio‐glacial sand and gravel provide an important resource for the construction industry. The efficient and economic extraction of such geologically complex deposits requires a detailed understanding of their grade distribution. This is typically undertaken as point observations over the resource using a borehole drilling programme. We investigate the viability of using measurements of the elastic and electrical properties of the subsurface, derived from geophysical profiles, to determine the spatial variability of aggregate quality.A refraction seismic, surface wave seismic and resistivity survey was carried out over a fluvioglacial sand and gravel deposit at Scorton, Yorkshire, UK, on agricultural land adjacent to an active quarry. Two 190 m profiles, with borehole control at their extremes, were acquired with a 2 m source and receiver spacing. The P‐ and S‐wave velocities ( and ), together with resistivity and depth distributions were determined down to c. 20 m. The subsurface was divided into four layers: uncon‐solidated sand and gravel; dry consolidated sand and gravel; saturated sand and gravel; and basal clay.The work flow initially involved processing and interpreting the three geophysical techniques independently. Crossplots of the elastic and electrical parameters were used to distinguish unconsolidated from consolidated material; the water table; increased silt content; and recognize the basal clay. The Vp/Vs ratio proved very powerful at discriminating fine material () from coarse material (Vp/Vs ~ 1.5). Regression analysis of crossplots produced P‐ and S‐wave relationships for the material, while electro‐acoustic relationships were less successful, especially below the water table. The results of the petrophysical analysis were used to set up guided inversions for the resistivity survey. Careful use of the refraction results as an a priori model for the resistivity survey was found to improve the resistivity inversion, though use of a sharp boundary introduced edge effects at the water table.The results from the integrated analysis were combined with the guided inversions into a final interpretation that enabled silt content, particle size and clay lenses to be mapped, which were not identified in the separately processed data.

Coastal evolution of Yancheng, northern Jiangsu, China since the mid-Holocene based on the Landsat MSS imagery

Since the 1970s, remote sensing images have provided new information for the delineation and analysis of coastline changes, especially focusing on the short timescale changes. This paper, based on the Landsat MSS imagery, focuses on the coastline evolution of Yancheng, northern Jiangsu, China since the mid-Holocene. A zebra stripe image, which could reveal the ancient coastal evolution of Yancheng, was extracted from a Landsat MSS image. Based on the extracted black-white stripes, 19 surface sediment samples were recovered and analyzed to recognize the sedimentary characteristics of these stripes. It shows that most sand and silty sand samples appear on the white stripes, while silt and silty clay samples are on the black stripes. Sandy and muddy sediments present an alternating distribution pattern on the Yancheng coastal plain. A historical coastline map was drawn according to the previous research achievements of the paleo-coastal sand barriers and paleo-coastlines, and was superimposed on the zebra stripe image. The trend of the extracted zebra stripes is consistent with the historical coastlines, and it should be the symbol of the Yancheng coastline evolution. On the basis of ten sets of black-white stripes and previous research results, we divided the progression of Yancheng coastal evolution into three stages (i.e., the early stable stage (6500 a BP-AD 1128), the rapid deposition stage (AD 1128–1855) and the adjustment stage (AD 1855-present)). Ten sets of black-white stripes were identified as the characteristic pattern of the coastline evolution on the Yancheng coastal plain.

A coupled carbon, aggregation, and structure turnover (CAST) model for topsoils

In this work, a soil carbon, aggregation, and structure turnover (CAST) model was developed based on the concept suggested by many authors in the scientific literature that macro-aggregates are formed around particulate organic matter, followed by the release of micro-aggregates. A simplified mechanistic Nitrogen model was also developed. The CAST model was evaluated by field data of cropland to set-aside conversions of Critical Zone Observatories in Greece (fine textured Mediterranean) and Iowa (coarse textured humid continental). The model was calibrated successfully to capture the carbon content and the C-to-N ratio content of the pools comprising the three aggregate types (macro-aggregates: >250μm, micro-aggregates: 53–250μm, silt–clay sized aggregates: <53μm) in both sites. The soil system reached maximum macro-aggregation/porosity and minimum bulk density after 7 and 14years of cropland to set aside conversion in Greece and Iowa, respectively, according to the model results. From then onward, macro-aggregate disruption presented a constant inter-annual variability and any further SOC increase was due to micro-aggregation resulting in the increase of bulk density and decrease of porosity towards a stable value. Sensitivity and Monte Carlo uncertainty analysis on model parameters suggested that the most uncertain state variables affected by most model parameters is the carbon mass in the silt–clay size aggregates and the silt–clay mass itself. Higher size aggregates and their carbon content as well as soil structure parameters such as porosity and bulk density do not exhibit significant sensitivity to model parameters. Validation and improvement of the CAST model could assist in revealing the primary factors determining organic matter, aggregation, and structure turnover in different ecosystems. Obtaining this capability should allow better prediction of the response of the soil system to management practices, landuse changes, and climate change in order to design and optimize the appropriate measures/practices.

Stochastic Prediction for Frost Heave Effect of a Subway Tunnel

Horizontal freezing induces ground frost heave and ground displacement happens. Excessive uneven displacement or deformation may be dangerous to the surrounding building. The stratum that a Nanjing subway tunnel will traverse through is the soft and silt clay. The artificial freezing construction method is proposed to help the tunnel to traverse through the worse stratum. To control the ground building deformation and displacement due to the ground frost heave, the frost heave effect for the artificial freezing construction should be predicted before the construction process. The stochastic media theory is proposed to predict the frost heave effect due to the artificial freezing construction in the paper. The basic analysis procedure for analyzing frost heave effect is introduced and ground deformations such as ground vertical displacement, ground curvature are analyzed. The analysis result shows that the ground deformation due to the frost heaves is in the limit of ground displacement control requirement.

Lab-Scale Treatability Tests for the Thermal Desorption of Hydrocarbon-Contaminated Soils

Within an integrated management plan for contaminated site remediation at a given territorial scale, the performance of treatability tests could be useful and/or expressly requested by the control authority on a site-specific basis to evaluate the overall feasibility of a given remedial option. The thermal desorption process appears to be a favorable treatment technology for organic contaminants. In this context, a particular lab-scale, indirectly heated desorber for treatability tests was originally developed and first applied to natural soils with different textures (silty sand, loam, silt clay, and clayey silt) that were “ad hoc” highly contaminated with diesel oil at various desorption process conditions (heating temperature in the range of 300–390°C, and reactor retention time in the range of 40–120 min). The Italian soil threshold level for heavy hydrocarbons (C > 12) of 50 mg kg−1 (dry matter) was assumed to be the successful goal of the treatability studies. In addition to the individuation of the favorable desorption process conditions for each soil, also in terms of a composite evaluation of heating temperature and retention time, the comparative experimental results provided useful indications of the possible influence of soil texture, the reduction of initial soil organic matter, and the evaluation of kinetic rate constants.

Okrzemki środkowego plejstocenu w osadach jeziornych z Krępca (Wyżyna Lubelska)

Osady jeziorne z profilu Krepiec 13 kolo Łecznej zostaly uznane na podstawie analizy pylkowej (Z. Janczyk-Kopikowa, 1980) za rownowiekowe z osadami z Ciechanek Krzesimowskich, ktore wedlug A. Środonia (1969) reprezentują interglacjal lubelski. W nawiązaniu do podzialu palinologicznego opisywanego profilu dokonano korelacji wydzielonych okresow rozwoju roślinności wyzszej z fazami rozwoju okrzemek – diatomofazami (KI, K2, K3). W obrebie diatomofaz wyrozniono ponadto ich cześci oraz subdiatomofazy reprezentujące poszczegolne etapy rozwoju jeziora interglacjalnego. MIDDLE PLEISTOCENE DIATOMS FROM LACUSTRINE DEPOSITS FROM KRĘPIEC (LUBUN UPLAND) A thick packet of lacustrine deposits, about 20 m thick, was found at depth from 21.15 m to 41.50 m in the Krepiec 13 borehole column. They are represented by silts varying in lithology and with intercalations of gyttja, peat and peaty silts. They are underlain by sands with boulders, resting on till, and overlain by sands, peats, clay silts, chalk gravels and sandy silts. The pollen analysis, carried out by Z. Janczyk-Kopikowa (in print), has shown that the Krepiec borehole column comprises Interglacial deposits coeval with those from Ciechanki Krzesimowskie. According to A. Środon (1969), the latter do not correspond to the Masovien I Interglacial but to the Lublin Interglacial. On the basis of palynological subdivision of the studied borehole column, the differentiated periods of development of higher plants were correlated with phases of development of diatoms – diatom phases (K1, K2, K3). The diatom phases were further subdivided into parts and diatom subphases representing individual stages in evolution of the interglacial lake (Fig. 2). The bulk of the studied lacustrie deposits from Krepiec, comprising diatom phases Kl, K2A and K3A, are characterized by predominance of nannoplankton diatoms Stephanodiscus and Cyclotella in the dominant group (Tables I – IV). Fragilaria crotonensis , Asterionella formosa and Synedra ulna and S. acus are fairly common in the subdominant group. Middle part of the studied column (diatom phase K2B) displays a transitional stage in the development of the lake, characterized by decrease of plankton, accompanied by increase in number of littoral, mainly epiphytic, and rheophilous species typical of flowing water. Upper part of the section (diatom phase K3B) is characterized by predominance of Melosira italica and Epithemia zebra and, subsequently, Synedra ulna and Fragilaria construens . The development of these species indicates a marked shallowing, eutrophication of waters in the lake, and its overgrowing by macrophytes. The changes are related to further amelioration of climate, recorded as an Interglacial optimum in the pollen spectrum (III) by Z. Janczyk-Kopikowa (in print). The Krepiec section seems most close to that of the Lichwm Interglacial from Gwoznica in south-western Byelorussia (G.G. Gruzman et al., 1975).

Simulation of advective methane flux and AOM in Shenhu area, the northern South China Sea

Anaerobic oxidation of methane (AOM) occurring in the marine sediment is an important process for methane cycle and methane sequestration. In this work, a one-dimensional numerical model was developed to study the distribution of advective methane flux with the AOM process. The model has been applied to investigate the gas hydrates bearing sediments of Shenhu areas located in the northern South China Sea, where advective methane transport was detected. The modeling results suggest that methane flux will be consumed in the sediment column via dissolution, sorption, and AOM reaction. Only when the methane flux was one order of magnitude higher than current level, then a portion of methane will enter water column and possibly escape to the atmosphere. The numerical simulation also revealed that, due to the lower permeability of the silt–clay sediments, a much thicker sulfate-methane transition zone exists in the Shenhu area, where AOM is able to consume more.

Arsenic anomalies in shallow Venetian Plain (Northeast Italy) groundwater

A pilot area within the Venetian Plain was selected to assess the arsenic (As) contamination of groundwater. The area represents a typical residential, industrial and agricultural organization representative of most western countries, and is also devoid of lithologies with high or anomalous As content. Hydrogeological and chemical data have been collected, the latter spatialized by a geostatistical approach. The unconfined aquifer reservoir varies from a predominantly gravel composition in the north to a sandy and silt–clay composition further south, including peat layers. The hydrochemical features of the waters are rather homogeneous, featuring low mineral content and a Ca-bicarbonate signature. In contrast, the redox state is highly variable; oxidizing conditions are predominant in the northern and coarse parts of the aquifer, whereas reducing potentials prevail in the southern and silt–clay parts. Several well waters contain arsenic in excess of drinkable limits (=10 ppb), and most of these wells are located in the southern area. A large portion of the studied area has a high probability of containing non-potable water (up to 150 ppb As). Remarkably, As “hot spots” (As > 300 ppb, up to 431 ppb) were identified at the transition from gravel to silt–clay sediments. No industrial or agricultural source of As has been found.

Sedimentation processes in silt-rich sediment systems

Sediment found in China’s Yangtze and Yellow River systems is characterized by large silt fractions. In contrast to sand and clay, sedimentation and erosion behaviour of silt and silt–clay–sand mixtures is relatively unknown. Therefore, settling and consolidation behaviour of silt-rich sediment from these river systems is analysed under laboratory conditions in specially designed settling columns. Results show that a transition in consolidation behaviour occurs around clay contents of about 10 %, which is in analogy with the transition from non-cohesive to cohesive erosion behaviour. Above this threshold, sediment mixtures consolidate in a cohesive way, whereas for smaller clay percentages only weak cohesive behaviour occurs. The settling behaviour of silt-rich sediment is found to be in analogy with granular material at concentration below 150 g/l. Above 150–200 g/l, the material settles in a hindered settling regime where segregation is limited or even prevented. The results indicate that for modelling purposes, multiple sediment fractions need to be assessed in order to produce accurate modelling results.

Interrill erosion in soils with different land uses: The kinetic energy wetting effect on temporal particle size distribution

Millions of hectares of cropland have slopes of less than 2%, which can result in an enormous amount of pollutants transported with clay–silt sized particles via interrill erosion. However, few studies have attempted to understand the detachment-transport mechanism of particles with different wetting processes as determined by the surface soil cover of a field. The goal of this study was to determine the effects of different wetting rates on the runoff rate, temporal sediment particle size distribution (PSD), and sediment delivery rate produced via interrill erosion in low slopes under two contrasting tillage systems. Selected sites were under continuous corn rotations on Maury silt loam (Typic Paleudalf) or corn/soybean on Calloway silt (Aquic Fragiudult), both under either moldboard plow (MT) or no-till (NT) soil management. A nozzle-type rain simulator was used to deliver 87.5 mm h− 1 for 1 h on a plot of 1 m2. High kinetic energy wetting (HKE) was evaluated on a bare soil surface. Low kinetic energy wetting (LKE) was accomplished by covering the bare soil surface with layers of plastic mesh. Soil texture and aggregate stability were more important than overland flow in modifying the temporal PSD. Transport capacity appears to be diminished due to soil cover, making all predictions based on settling velocity ineffective. Comparison between HKE and LKE showed that soils can produce different temporal PSD in the sediment without changes in total soil loss. HKE increases both the total and temporal sediment delivery rate (Di) values. Observed differences in Di were consequences not of particle size but of the number of particles released.

Correlation between Compression Index of Silt–Clay Matrices and their Index Properties

Before designing a foundation for a particular soil condition, the interest of a foundation engineer is to determine the total settlement of the structure and to select such contact pressure on soil that would not cause excessive settlement. Geotechnical engineers are, therefore, more interested in coefficient of consolidation (Cv), coefficient of volume compressibility (mv) and compression index (Cc) for design of foundations. The compression index (Cc) is determined through a time consuming and extensive consolidation tests. Attempts have been made to develop the empirical relations which can be used to determine the compression index (Cc) of silt–clay matrices without resorting to conventional geotechnical tests that may save costs as well as time of laboratory tests.

Trend Effect and an Isotropy of Soil Particle Composition in the Chengdu Plain

This paper studies the spatial variation characteristics of soil particle composition on both sides of Qingbaijiang River (Chengdu Xin Du District of Sichuan province) by using the geostatistical component of ArcGIS and GS software. The results have shown that the trends of sand contents in the east-west direction and the north-south direction were first order and second order, respectively. And the trends of both silt particle and clay particle contents were first order in the east-west direction, and second order in the north-south direction. The anisotropy semi variance models of sand particle content showed that the ranges in the long axis direction were 718.77 m and 677.01 m and the ranges in the short axis direction were 273.78 m and 276.63 m with first order and second order. The results showed that the difference between the ranges in the short axis direction and the ranges in long axis direction changed little, while the anisotropy semi variance model had longer ranges than the isotropy model with the trend parameters of first order. From the error analysis and the result that reflect regional and local trend, it indicated that Kriging interpolation method considering the study about soil particle composition along Qingbaijiang River with the second order trend effect was the best, comparing the contour maps of sand particle content under different trend effects and anisotropy parameters.

Relating microbial community structure and environmental variables in mangrove sediments inside Rhizophora mangle L. habitats

This study aimed to access the structure of microbial communities in sediments in the root zone of the red mangrove (Rhizophora mangle) in three sites with an increasing distance from the sea inside a mangrove forest in two distinct periods (dry and wet seasons) using denaturing gradient gel electrophoresis (DGGE). These data were correlated to environmental variables and sediments characteristics by using redundancy analysis (RDA) which revealed that the distribution of the microbial communities is significantly (p<0.05) influenced by the silt–clay percentages for both Bacteria and Archaea and organic matter content significantly influences the distribution of Archaea. The archaeal community also exhibited an annual clustering pattern. The sites had only 30% and 35% of similarity of bacterial and archaeal communities, respectively, and this could be interpreted as being the core microbiome of R. mangle. Thus the spatial distribution of microbial communities inside the red mangrove habitats seems to be primarily controlled by the abiotic variables of each habitat.

Density and habitat dependent effects of crab burrows on sediment erodibility

Despite biological interactions being highlighted as a key process in determining particle fluxes, relatively few studies have attempted to establish the links between burrow building bioturbators and sediment stability. The mud crab Austrohelice crassa, is a key burrowing species in New Zealand estuaries that has shown context-specific interactions with its environment. Here we use annular flumes to test if sediment stability and erodibility were altered as a function of A. crassa burrow density in two contrasting sediment types: a cohesive sandy-mud and a non-cohesive sand. Three burrow density treatments (n=3) reflecting the natural density range in each sediment type (sand; 0–100m−2, sandy-mud; 0–400m−2), were collected from the field and subjected to sequential increases in water flow velocity. Flow profiles were measured and bed shear stresses were calculated for each treatment. Increasing burrow density reduced the mass of sediment eroded at 0.35ms−1(ME-35, gm−2) in cohesive sandy-mud, while in non-cohesive sand a unimodal pattern was observed, whereby erosion rates were greatest at the lowest burrow density (19m−2). In the cohesive sediment, the linear decrease in erodibility with increasing burrow density was likely affected by the sluicing of fine particulates (silt–clay) from burrows when the tide was out creating both a smoothing and consolidating effect on the sediment surface. A reduction in flow velocity due to the increased presence of surficial pellets and greater trapping of bedload transported material was attributed to the reduction in the mass of sediment eroded in sand at high burrow densities. This study demonstrates that burrow builders influence sediment transport by more than just vertical particle mixing and highlights some of the complexities of small-scale sediment processes. Knowledge of different organism–sediment interactions among sediment types and spatial scales will enhance the accuracy of sediment transport models.

Operational Coefficient of Consolidation Around a Pile Group Driven in Clay/Silt

Although finite element packages facilitating coupled consolidation analyses are increasingly in use, many practitioners still favour linear uncoupled analysis out of familiarity with the use of coefficients of consolidation. However, coefficients of consolidation measured by any single means tend to exhibit significant variation, with mean results from different laboratory and field tests also varying widely, leaving uncertainty over the correct values to apply to field problems. In this paper, a finite difference approach is used to back-calculate operational coefficients of consolidation from pore pressure measurements pertinent to a pile group driven in clay–silt. The research shows that this method is capable of successfully capturing the process of pore pressure dissipation, and that the operational coefficient of consolidation around the pile group is higher than that derived from piezocone dissipation tests in the same material.

Pore Pressure Build-up of Silt Soft Clay under Dynamic Loading at Zhoushan Islands

The mechanism of dynamic tri-axial test is introduced in this paper and the dynamic responses of silt soft clay at Zhoushan are studied using a dynamic tri-axial test system. The laws of pore pressure build-up of the silt clay are obtained which are affected by the consolidation pressure and dynamic load. The greater the consolidation pressure and the dynamic loading is, the more the build-up of pore pressure is. However, the dynamic load has minor effect on pore pressure build-up under the anisotropic consolidation.

Integrating Sieving and Laser Data To Obtain Bulk Grain-Size Distributions

Abstract It is not always possible to use a single technique to determine the complete grain-size distribution of a natural sediment, namely when dealing with successions of variable grain size or with sediments or soils that contain silt–clay and sand–gravel populations. In this work we assess the feasibility of using laser diffraction to measure sand-size particles by analyzing the representativeness of laser samples and quantifying the deviations between results of sieving and laser diffraction. Four groups of sediments, with different grain-size distributions and proportion of heavy minerals, were compared. It is demonstrated that the risk of laser sample underrepresentativeness is higher when measuring grains coarser than 1 mm, in particular when clay–silt particles are also frequent. Depending on the size fraction, the divergence between the results of sieving and laser diffraction due to shape and density effects is variable and not easy to predict quantitatively. Given these uncertainties it is proposed to use different computational threshold diameters that separate the fractions considered by sieving and laser diffraction and measure part of the distribution with both techniques. We developed a flexible computer application that allows users to visually compare different possibilities of combined laser-sieving grain-size distribution curves and to choose the best option of threshold diameter. The best combined results can then be used to model the grain-size distribution.

Characterization of thick varved-clayey-silt deposits along the Delaware River by field and laboratory tests

The existence of thick compressible varved clayey silt deposits along the Delaware River presents one major obstacle for design and construction of structures or industrial facilities in this region. To understand the characteristics of this thick weak soil layer and then develop reliable soil parameters for future earthwork design and construction, a comprehensive soil testing program was carried out. The program consisted of both in situ tests [e.g., standard penetration test borings, piezocone penetration test (CPT) soundings, Marchetti flat dilatometer test soundings, seismic shear wave test soundings, CPT pore pressure dissipation tests, and field vane shear tests] and laboratory tests (e.g., soil index tests, consolidation tests, triaxial tests, and chemical tests). Additionally, a field surcharge test program with and without installation of wick drains was carried out to investigate the compressibility of the thick clayey silt deposit and the efficiency of wick drains to help accelerate consolidation. The field and laboratory test results showed that the varved clayey silt deposit was normally consolidated to slightly overconsolidated and moderately to highly compressible. Its physical properties were intermediate between those of Savannah muddy clay and Shanghai soft clay, while much closer to that of Shanghai soft clay. Due to its lamination structure in the horizontal direction, the horizontal consolidation coefficient of the varved clayey silts was significantly greater than that in the vertical direction. The measured undrained shear strength of the varved clay silt almost increased linearly with depth. The installation of wick drains in the thick clayey silt deposits accelerated primary consolidation, but showed no effect on the secondary consolidation under constant loading. The findings obtained from this study will be beneficial for future design and construction of earthworks along the Delaware River.

Nutrient contents in bottom sediment samples from a southern Brazilian microbasin

The nutrients, carbon, nitrogen and phosphorus, significantly affect the quality of aquatic environments, especially when present at concentrations above natural levels. In this context, the sedimentary column can act as an environment for storage or accumulation of these nutrients and for the reprocessing of such substances in the water column and aquatic biota. In this context, this study aimed to estimate the concentration of total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), inorganic phosphorus (IP) and organic phosphorus (OP) in seven sediment samples that were collected from the Marrecas Stream microbasin (Caxias do Sul, Rio Grande do Sul State, Brazil). The relationships among the sediment nutrient concentrations and the levels of organic matter (OM), fine material (silt–clay) and the concentration of the metal species, Al, Fe and Mn, as well as the possible nutrient sources to the stream studied were also evaluated. The data set suggest that the TOC and IP appear to have a direct correlation with the vast riparian vegetation present in areas adjacent to the sampling points and to the physico-chemical properties of the water and sedimentary column. Moreover, the results obtained for TN suggest that its concentration possibly suffers interference from the temperature of the water and its oxygenation, in addition to other biological activities. On the other hand, one cannot rule out human interference, mainly in the levels of OP, possibly due to the inflow of domestic sewage to the stream.