A multivariate analysis of the spatial variations of water quality during high-flow period in the Chaobai River (Beijing, China) restored by reclaimed water

Reclaimed water has demonstrated its broad applications in social construction to alleviate the contradiction of water shortage in Beijing, China. Using multivariate statistical analysis, the current study investigated the spatial variations of water quality in the Chaobai River restored by reclaimed water during the high-flow period. Hierarchical cluster analysis (CA) classified the 11 sampling sites into four clusters, namely most polluted, highly polluted, moderately polluted, and lowly polluted sections. The Kruskal-Wallis test showed that pH, TDS, EC, Ca2+, Mg2+, Cl−, SO42−, NO3−-N, and TN had significant spatial differences among four clusters (p < 0.05). Mean value of total nitrogen (TN) in the most polluted exceeded the guideline (15 mg/L) of the Water Quality Standard for Scenic Environment Use, reaching 22.3 mg/L. Principal component analysis (PCA) extracted three principal components (PCs) accounting for 81.5% of the total variance in the data set of water quality. Three PCs reflected the chemical characteristics of reclaimed water, mineral pollution, and nutrient pollution, respectively. With the ordination biplot of sampling sites defined by the first and second PCs, PCA provided a classification of sampling sites based on the similarity of pollution sources, which supported the results of CA. The results revealed that water quality of the Chaobai River restored by reclaimed water was affected by untreated domestic and agricultural sewage with nitrogen and minerals being the main pollutants along the river basin. This study showed rivers restored by reclaimed water had significant spatial variations of water quality, demonstrating effectiveness of multivariate statistical methods on water quality analysis.

Characterization of Diffuse Groundwater Inflows into Stream Water (Part II: Quantifying Groundwater Inflows by Coupling FO-DTS and Vertical Flow Velocities)

Temperature has been used to characterize groundwater and stream water exchanges for years. One of the many methods used analyzes propagation of the atmosphere-influenced diurnal signal in sediment to infer vertical velocities. However, despite having good accuracy, the method is usually limited by its small spatial coverage. The appearance of fiber optic distributed temperature sensing (FO-DTS) provided new possibilities due to its high spatial and temporal resolution. Methods based on the heat-balance equation, however, cannot quantify diffuse groundwater inflows that do not modify stream temperature. Our research approach consists of coupling groundwater inflow mapping from a previous article (Part I) and deconvolution of thermal profiles in the sediment to obtain vertical velocities along the entire reach. Vertical flows were calculated along a 400 m long reach, and a period of 9 months (October 2016 to June 2017), by coupling a fiber optic cable buried in thalweg sediment and a few thermal lances at the water–sediment interface. When compared to predictions of hyporheic discharge by traditional methods (differential discharge between upstream and downstream of the monitored reach and the mass-balance method), those of our method agreed only for the low-flow period and the end of the high-flow period. Our method underestimated hyporheic discharge during high flow. We hypothesized that the differential discharge and mass-balance methods included lateral inflows that were not detected by the fiber optic cable buried in thalweg sediment. Increasing spatial coverage of the cable as well as automatic and continuous calculation over the reach may improve predictions during the high-flow period. Coupling groundwater inflow mapping and vertical hyporheic flow allows flow to be quantified continuously, which is of great interest for characterizing and modeling fine hyporheic processes over long periods.

DOC sources and DOC transport pathways in a small headwater catchment as revealed by carbon isotope fluctuation during storm events

Monitoring the isotopic composition (δ13CDOC) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative contribution to stream DOC flux. High-resolution (< hourly basis) δ13CDOC data were obtained during six successive storm events occurring during the high-flow period in a small headwater catchment in western France. Intra-storm δ13CDOC values exhibit a marked temporal variability, with some storms showing large variations (> 2 ‰), and others yielding a very restricted range of values (< 1 ‰). Comparison of these results with previously published data shows that the range of intra-storm δ13CDOC values closely reflects the temporal and spatial variation in δ13CDOC observed in the riparian soils of this catchment during the same period. Using δ13CDOC data in conjunction with hydrometric monitoring and an end-member mixing approach (EMMA), we show that (i) > 80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the riparian soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its δ13C fingerprint, we find that the upland DOC contribution decreases from ca.~30% of the stream DOC flux at the beginning of the high-flow period to < 10% later in this period. Overall, upland domains contribute significantly to stream DOC export, but act as a size-limited reservoir, whereas soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that δ13CDOC provides a powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, having a high-resolution assessment of temporal and spatial variability.

New insights from the use of carbon isotopes as tracers of DOC sources and DOC transport processes in headwater catchments

Monitoring the isotopic composition (δ13CDOC) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative contribution to DOC stream flux. High-resolution (< hourly basis) δ13CDOC data were obtained on six successive storm events occurring during the high-flow period in a small headwater catchment from western France. Intra-storm δ13CDOCvalues exhibit a marked temporal variability, with some storms showing large variations (>2‰), and others yielding a very restricted range of values (<1‰). Comparison of these results with previously published data shows that the range of intra-storm δ13CDOC values closely reflects the temporal and spatial variation in δ13CDOC observed in the riparian soils of this catchment during the same period. Using δ13C data in conjunction with hydrometric monitoring and an end-member mixing approach, we show that (i) >80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its δ13C fingerprint, we find that the upland DOC contribution decreases from ca. 30% of the stream DOC flux at the beginning of the high-flow period to <10% later in this period. Overall, upland domains contribute significantly to stream DOC export, but act as a size-limited reservoir, whereas soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that δ13CDOC provides a powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments.

Gypsum amendment of soils reduces phosphorus losses in an agricultural catchment

We estimated the changes in the losses of particulate and dissolved phosphorus (P) after treating 93 ha of agriculturalfields with gypsum (4 t ha–1) in a 245 ha catchment in southern Finland. Runoff was monitored using onlinesensors and manual sampling during one high-flow period before and six periods after the gypsum amendment.Turbidity recorded by the sensors correlated with particulate P analysed in the laboratory, which enabledthe evaluation of changes in particulate P from the online data. Using a covariance model, gypsum amendmentwas estimated to have reduced the loss of particulate P by 64%. The loss of dissolved reactive P appeared to decreaseby one third, but was estimated with less precision. No such changes were found during the same periodin a nearby ‘reference’ catchment, where gypsum was not used. Gypsum did not affect soil test values for P, K, Mgor Ca, but it did increase the ionic strength and soil test SO4. In clayey catchments discharging into the sea, gypsummay provide an efficient means to reduce P losses from field cultivation. The duration of the gypsum effect andimpact of SO4 associated with gypsum amendment on the ecology of rivers and lakes has yet to be determined.

Optimized lamellae settling for urban stormwater waste

Lamellae sedimentation is an efficient and compact solution for handling high stormwater influx at wastewater treatment plants during rainy periods. This paper presents and comments on the results obtained at several different sites using specially adapted lamellae settlers. Surface hydraulic loads above 100 m3/m2.h have been reached using optimized doses of chemical reagents. One key advantage of the reactor is that start-up is practically instantaneous; this means that efficient treatment is obtained right from the start of the high flow period.